Belt Bucket Elevator Design

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Bucket Elevator Design Notes Protecting bearings from dust & water. Bucket elevator experiences. Hazardous areas for dusts and flammables. Electrical Motor Current Protection Saves Your Plant Dust control concepts Belt conveyor tuning Tracking belts on elevators and conveyors.

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BELT BUCKET ELEVATOR DESIGN NOTES DISCLAIMER These notes are intended to assist and provide direction in the process of designing belt bucket elevators. They are not a substitute for conducting a thorough engineering analysis of the design requirements. Because the author and publisher do not know the context in which the notes are to be used and cannot review the resulting design they accept no responsibility for the consequences of using them. The author claims copyright over all the material in these notes – 15 March 2001.

CONTENTS OF DESIGN NOTES 1.0

Overview of belt bucket elevators and their use.

2.0

Determine throughput capacity.

3.0

Determine belt speed and throw.

4.0

Calculate motor power.

5.0

Calculate top and bottom pulley shaft sizing.

6.0

Drive arrangement and design.

7.0

Shaft bearing and seal arrangement.

8.0

Selecting elevator frame structural members.

9.0

Inlet and outlet chute design.

10.0 Considerations in choosing panel materials. 11.0 Selecting belts and buckets. 12.0 Methods to take-up belt tension. 13.0 Protection against bogging the buckets. 14.0 Clean-out considerations. 15.0 Dust extraction requirements. 16.0 Installing the bucket elevator in place. 17.0 Correct operation of belt bucket elevators. 18.0 Maintenance of belt bucket elevators.

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OVERVIEW OF BELT BUCKET ELEVATORS PURPOSE OF BUCKET ELEVATORS Bucket elevators are used to lift bulk materials from one height to another. They are a reliable and well-proven piece of equipment. METHOD OF OPERATION Bucket elevators operate by using an endless belt or chain on which rectangular buckets are mounted. The belt or chain revolves between a top and bottom pulley and the buckets move with it. At the bottom the buckets pick up product fed into the elevator boot and at the top the product is discharged as the bucket turns downward over the head pulley. TYPES OF BUCKET ELEVATORS Bucket elevators come in several standard forms with numerous variations to suit the characteristics of the products being moved. The most common forms of bucket elevator are - centrifugal discharge where the speed of the belt around the top pulley flings the product out of the bucket, - positive discharge, for product requiring slower, less aggressive handling, where a snub pulley below the top pulley orients the buckets downward for emptying, - continuous discharge, for large lumpy products or very friable products, where the buckets are placed in contact with each other, and - pivoted bucket for transporting materials horizontally. Along with each type of elevator, different styles of buckets have been developed which better suit the elevator or the materials to be handled.

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The left-hand column provides descriptive text. NECESSARY INFORMATION Required product parameters. Service use. Material chemical name. Bulk density – mass/volume – kg/m3 Maximum duty – kg/hr or m3/hr Maximum lump size - dimensions average size percentage of lumps in total Height product is to be raised (meters) and angle of incline if any. Provide enough height at the outlet of the discharge chute so the product is always falling following discharge. Product characteristics – abrasiveness flowability – free/cohesive/slug dampness – % moisture friability – firm/breaks/powders particle shape – length/size/volume temperature of product angle of repose corrosiveness Operating environment, location and conditions – corrosive/damp Service required – continuous/intermittent. Open or closed boot design.

The right-hand column provides an example. NECESSARY INFORMATION Product parameters. Raise crushed product from mill outlet to storage silo. Aluminium Sulphate. 1700 kg/m3 5,000 kg/hr 3 mm max 2 mm Nil 5.5 m including length of discharge chute into 4 m high storage silo.

SELECT BUCKET SIZE AND SPACING The size and number of buckets is determined from the required throughput using an iteration process.

SELECT BUCKET SIZE AND SPACING 5,000 kg/hr throughput. Select a bucket 150 mm wide x 100 mm projection with a volume of 0.78 litre. Using 2/3 of the volume give a capacity of 0.5 litre. 0.5 lt. is 0.0005 m3 and holds 0.85 kg of product. (0.0005 m3 x 1700 kg/m3). To move 5000 kg/hr using 150 x 100 buckets requires 6,000 buckets per hour or 100 buckets per minute. Select a bucket spacing of 300 mm.

Select the bucket from the range in the bucket supplier’s catalogue. Only 2/3 (67%) of the bucket’s design capacity is used in calculations. Centrifugal discharge conveys usually have a spacing between buckets that is 2 to 3 times the bucket projection, though the spacing can be greater for free-flowing products. DETERMINE BELT SPEED The bucket spacing times the number of buckets per second determines the required belt speed. The speed for centrifugal bucket elevators is usually in the range of 1 m/s to 2 m/s to insure the product throws into the chute at the head pulley. CALCULATE HEAD PULLEY DIAMETER A simplifying assumption is made that the throw commences at the top of the head pulley. At this point the centrifugal force and gravity force are balanced. cos β where Centrifugal force = m ⋅ v 2 ⋅ r m = mass in kg v = belt speed in m/s = angle from top dead centre r = pulley radius in m Gravity force = m ⋅ g where g = gravity constant 9.8 m/sec2. Putting both forces equal to each other The right-hand column provides an example.

Sharp edges Free Less than 2% Firm Consistent Ambient 30 degrees Corrosive if damp Dry and airy

Intermittent – up to 12 hours per day 6 days a week Open boot bottom, elevator will sit on a concrete floor.

DETERMINE BELT SPEED 100 buckets per minute/60 sec per minute = 1.7 bucket/sec. 1.7 bucket/sec x 0.3 m = 0.5 m/sec. This is too low and will prove to be insufficient for a clearance throw into the discharge chute. The bucket spacing will need to be increased and the calculation repeated. CALCULATE HEAD PULLEY DIAMETER v2 (0.5m) 2 = = 25mm r (radius ) = g 9.8m / sec 2 The head pulley diameter is 50 mm. This size, though accurately calculated, is not practical. It is far too small. The buckets cannot deform sufficiently to go around the pulley without over-stressing both buckets and belt. The solution is to increase the bucket spacing or to use smaller buckets. This then requires a proportionate speed increase to maintain the throughput. The greater velocity needs a larger head pulley revolving at the same RPM. However as we are using the smallest buckets available it is necessary to increase the bucket spacing.

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v 2 = g ⋅ r ⋅ cos β cos β = 1 at top dead centre. Therefore r =

v2 and diameter (d ) = 2 ⋅ r g

CALCULATE THROW INTO CHUTE AND CHUTE SIZE Using the standard trajectory formula s = u ⋅ t + 0.5 ⋅ a ⋅ t 2 Where s = displacement (m) u = initial velocity (m/s) a = acceleration (m/s2) = gravity constant g = 9.8 m/s2 t = time (sec) The trajectory after the product leaves the bucket can be graphed and the chute height determined. The horizontal component at top dead centre of the pulley where acceleration due to gravity in the horizontal direction is zero is given by s h = u ⋅ t meters. The vertical component at top dead centre where velocity in the vertical direction is zero is given by s v = 0.5 ⋅ a ⋅ t 2 meters.

The belt velocity using a bucket spacing of 700 mm with the removal rate of 1.7 bucket/sec x 0.7 m = 1.2 m/sec. The pulley diameter is now d = r ⋅ 2 = (1.2 2 ⋅ 9.8) ⋅ 2 = 300mm . The diameter could be made slightly larger if so desired.

CALCULATE THROW INTO CHUTE AND CHUTE SIZE Calculate the horizontal and vertical position of the product for every 0.1 seconds of flight time. TIME (sec)

HOR. DIST. (mm)

VERT. DIST (mm)

0.1 120 50 0.2 240 195 0.3 360 440 0.4 480 780 0.5 600 1220 From the table it is noted that after 0.2 seconds of flight the product has traveled 240 mm horizontally from top dead centre and 195 mm vertically. The pulley radius is 150 mm which means the product is clear of the pulley by 90 mm. But it is not yet clear of the 270 mm radius circle scribed by the lip of the bucket (allowing for belt thickness).

The distance of the chute from the vertical center of the head pulley must be sufficient to allow the buckets to clear the wall of the elevator on the downward leg.

This distance is reached shortly after 0.2 seconds. A satisfactory chute depth would be 600 mm, with the chute opening starting 350 mm from the vertical centre of the head pulley. This makes the bucket elevator 700 mm deep. Because of the 150 mm width of the buckets a 175 mm wide belt on 200 mm wide head pulley will be used. To provide clearance to the wall the elevator it will be 250 mm wide.

DETERMINE THE DRIVE ARRANGEMENT With the head pulley size determined and the linear belt speed known, the RPM of the head pulley can be calculated.

DETERMINE THE DRIVE ARRANGEMENT 1 .2 RPM = ⋅ 60 = 38 2 ⋅ π ⋅ 0.3

V (m / s) RPM = ⋅ 60 2 ⋅ π ⋅ r ( m) Usually a 4-pole motor at 1450 RPM with a reduction gearbox of suitable ratio is selected to drive the head pulley. The gearbox can be a direct drive or shaft-mounted unit depending on the available space and access.

It will be necessary to select sprocket sizes for the motor and head pulley to produce the required rotational speed. A gearbox can be selected to reduce from 1450 RPM input shaft speed to 38 RPM output shaft speed. Alternatively the sprocket sizes can be used to produce some of the reduction and the gearbox the remainder. Limit reduction via the sprockets to around a 3:1 ratio to not over-stress the chain.

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Chain and sprocket drives are preferred to vee-belt and pulley drives because of their more positive transfer of power. Setting the motor current overload protection to the upper limit of the motor accommodates overload situations. The selection of the gearbox sprocket size is dependent on the maximum allowable torque. This value can be found from the gear motor manufacturer’s catalogue. Once the limiting torque at the gear motor output shaft is known the allowable force for different sprocket sizes can be calculated from the equation T = F ⋅ r where F = force (N) and r = sprocket or pulley radius (m). Select a sprocket size that is well within the torque rating of the gear motor and has more than 20 teeth. A lesser number of teeth cause excessive forces in the chain links as they come around a tight radius. Less torque is needed with a larger sprocket radius on the gearbox output shaft.

CALCULATE POWER REQUIRED 2 ⋅π ⋅ N ⋅ F ⋅ r T ⋅ω Power (kW ) = = 60 ⋅ 1000 ⋅ η 1000 Where N = revs per minute F = force at outer edge of the head pulley (N) r = radius to force (m) T = torque (Nm) = drive efficiency = radians per second The load on the belt results from the weight of product lifted plus the dredging drag as the bucket scoops up the product. A duty factor is used to accommodate start-up loads. Belt Load = (total bucket load + dredge load) The bucket load is the sum of the loaded buckets on the upward side. The dredging load can be estimated either by adding an equivalent length (5m for continuous buckets, 12m for spaced buckets) to the belt or by use of the following formula. Dredge load (N) =

90 ⋅ Wb Ps

where Wb = weight of material in each bucket (kg) Ps = Bucket spacing on belt (m) A quick check on the load can be done using the formulas for work and power. W = F.s (Nm) and P = W/t = F.v (W).

The first iteration gearbox output RPM can be determined from the knowledge of the head pulley RPM and use of the 3:1 sprocket reduction suggested above. The head pulley speed is 38 RPM. A 3:1 reduction produces 114 RPM at the gear motor output shaft. Check the gear motor ratios available from the manufacturer and select the closest next higher shaft speed gearbox. Often it is necessary to choose a sprocket size and number of teeth and then to confirm the selection through an iterative process of checking calculated against allowable torque. Start with a 25-tooth ½” simplex chain 101 mm diameter sprocket on the gearbox output shaft and a 76-tooth 307 mm diameter sprocket on the head pulley. The selection of sprockets and chain will be confirmed later.

CALCULATE POWER REQUIRED The linear height of the bucket elevator is 5.5 m and bucket spacing is 0.7 m. This means there are 16 buckets in total, with 8 buckets on the upward and 8 on the downward legs. The load from the material weight is calculated by multiplying the bulk density of the product by the volume in each bucket by the number of buckets. Bucket load = Lb = 1700kg / m 3 ⋅ 0.0005m 3 ⋅ 8 ⋅ 9.8 = 67 N The dredging load =

90 ⋅ 1700kg / m 3 ⋅ 0.0005m 3 = 109 N 0 .7 m

Total pulley load = (67 N + 109 N) = 176 N This load acts at the centre of the buckets, which have a projection of 100 mm. The radial distance to the bucket centers is 150 mm + 50 mm = 200 mm Power pulley =

2 ⋅ π ⋅ 38 ⋅ 176 ⋅ 0.2 = 147W 60 ⋅ 1000 ⋅ 0.98

As a check on the calculation To lift 5,000 kg/hr to a height of 5.5 m allowing for 50% efficiency overall. 5000 ⋅ 9.81 ⋅ 5.5 = 540kNm W= 0.5 W 540,000 = = 150W (which is close to the previous P= 3600 t answer considering the actual efficiency is unknown).

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CONFIRM DRIVE ARRANGEMENT POWER The load on the motor is transferred through the drive from the head pulley. Simple ratio calculations back to the gear motor shaft will allow determination of the torque at the output shaft. This torque is then compared to the allowable torque to confirm the suitability of the gear motor. The power through the gearbox must be increased in accordance with - The manufacturer’s service factors requirement for intermittent operation and shock loading. - Drive efficiency. With the power through the drive train known, the chain selection can be confirmed by the chain supplier or calculations performed using appropriate formula. DETERMINE PULLEY DRIVE SHAFT SIZE The drive shaft size is calculated to handle the stresses generated by a bogged or jammed conveyor. Allowance is made for stress concentrations causing metal fatigue and service factor corrections are also applied. The diameter of the shaft is selected so that the stresses are well within the shaft material’s metallurgical capacity. By this stage initial dimensioned drawings can be sketched using the information compiled from the previous calculations. Commence by constructing a free-body diagram of the head pulley located in its bearings with the drive sprocket mounted at the drive end. The head pulley will be mounted to the shaft using hubs at each end. This allows the uniform load produced across the pulley by the belt to be drawn as point loads on the shaft at the mid point position of each hub. The position and orientation of the gearbox drive has not yet been determined. It is best to design for the loading arrangement that produces the greatest stresses and size the shaft accordingly. This permits the gearbox to be located in any orientation in future. The loads on the shaft are its own self-weight, the belt and bucket weights, the belt tension load (from product weight and friction drive requirements) and the drive sprocket force generated by a bogged or jammed conveyor. The bearings counter all these forces and keep the shaft in place. Power = (T1 − T2 ) ⋅ v (Watt) where T1= tight side tension (N) T2 = loose side tension (N) v = belt speed (m/sec) Also

T1 = e µθ where e = 2.718 (base of natural logs) T2 = Coefficient of friction = arc of contact in radians

CONFIRM DRIVE ARRANGEMENT POWER Power at the head pulley is 147 W. Torque at the head pulley sprocket is directly proportional to the inverse of the diameters at which the torque acts. In this calculation the gearbox service factor is 2 and chain drive efficiency is 0.98. 147 ⋅ 2 = 300W Gear motor power = 0.98 The logical choice is to select a small 0.55 kW or 1.1 kW 4-pole motor. For the calculation use a 1.1 kW motor, as this will permit altering sprocket sizes if operating duties change in future. DETERMINE PULLEY DRIVE SHAFT SIZE The conceptual sketch for the head pulley is shown below.

The forces are oriented in the vertical, including the drive sprocket force. This arrangement produces the highest loads on the shaft. The gearbox can be oriented in the horizontal. Such an arrangement would not have vertical loads at the sprocket. The sprocket load would then be at 90 degrees to the belt tension. This would produce less overall stress in the shaft and a smaller shaft could be used. However in this design the worst-case orientation will be used. It is necessary to determine the tension in the belt to lift the full buckets and overcome the dredging load. The power through the head pulley is 1.1 kW. P v

1100 1.2 = = 1.68kN T1 = 1 1 1 − 0.25⋅π 1 − µθ e e T2 = 0.77 kN

The coefficient of friction for rubber on steel is 0.25 and for rubber on rubber 0.35. The arc of contact is 180 degrees for bucket elevators provided the bottom pulley is the same diameter as the head pulley. Postal Address: FEED FORWARD PUBLICATIONS, PO Box 578, BENTLEY, West Australia, 6102. E-mail Address: [email protected] Because the authors and publisher do not know the context in which the information presented in the flyer is to be used they accept no responsibility for the consequences of using the information contained or implied in any articles

Combining the above tension equations allows T1 to be determined. P v T1 = Newton 1 1 − µθ e The weight of the rubber belt depends on the ply of the belt, its width and length. Since the belt tension is know the belt can be selected. The belt supplier can advise the belt to be used. Select a belt specifically for bucket elevators and not horizontal conveyors. Those used on bucket elevators contain more weaving than those for horizontal conveyors. Select the belt with a safety factor beyond the calculated loads for long life under intermittent and shock load conditions.

The force at the head pulley drive sprocket is in the proportionate ratio of drive train sprocket diameters. 2 ⋅π ⋅ N ⋅ F ⋅ r P ⋅ 60 ⋅ 1000 ⋅ η Power (kW ) = and F = 60 ⋅ 1000 ⋅ η 2 ⋅π ⋅ N ⋅ r 1.1 ⋅ 60 ⋅ 1000 Fgearbox = = 1.84kN 2 ⋅ π ⋅ 114 ⋅ 0.05 1.84 ⋅ 101 Fheadshaft = = 606 N 307 The load on the bearings is the sum of the belt tension and the drive force considering their direction of action. The pulley and shaft self self-load and the belt material loads will need to estimated and later checked if suitably accurate. The buckets are 150 mm wide so the belt will be 175 mm wide inside a 225 mm wide elevator frame. The elevator height is 5.5 m and the belt length about 12 m. For the purpose of the example a 4-ply 36-oz belt will be used with a mass of 0.0018 kg/mm width/metre length/ply. A 12-metre, 175 mm wide, 4 ply belt weighs 15 kg and produces 150 N downward force.

The shaft undergoes both bending and torsion simultaneously. The bending and torsional stresses are combined into an equivalent stress Se in the formula -

S e = K b ⋅ K cb ⋅ M 2 + K t ⋅ K ct ⋅ T 2 = Fs ⋅ Z p where M = largest bending moment Nm T = shaft torque Kb = shock factor bending Kt = shock factor torsion Kcb = stress concentration factor bending Kct = stress concentration factor torsion Fs = allowable shear stress (MPa) Zp = polar section modulus = d3/16 for a solid bar The maximum allowable shear stress is half the maximum principle stress. In addition a factor of safety of 2 for shock loading and stress raisers is included. For rotating shafts under minor shock loads Kb varies from 1.5 to 2.0 and Kt varies form 1.0 to 1.5. In heavy shock load conditions Kb varies from 2.0 to 3.0 and Kt varies form 1.5 to 3.0. Stress concentration factors can be found from stress concentration graphs for the form of stress raiser involved.

The mass of the head pulley can be estimated by assuming it will be made of 300 mm diameter steel pipe of 12 mm thickness with end plates of 12 mm thick flat plate. The shaft will be assumed to be 50 mm solid bright steel bar 400 mm long. The head pulley mass is calculated at 22 kg and 220 N force. It can be seen that the self-load forces are minor when compared to the product load generated forces. The free-body diagram can now be completed with all the forces acting on the shaft. With the drive and load forces known the reactions at the bearings can be determined by balancing the moments at each bearing. The shear force and bending moment diagrams for the shaft can be drawn to indicate the position of the highest bending stress. The highest bending stress is at the drive-end head pulley hub. At the same time the shaft is undergoing torsional stress from the drive. The worst case under torsion would be if the buckets were bogged and the geared motor applied full power of 1.1 kW. This produces a torque T at the pulley shaft of 1000 ⋅ P 1000 ⋅ 1.1 T = = = 31Nm 2 ⋅ π ⋅ RPM 2 ⋅ π ⋅ 38 60 60

S e = K b ⋅ K cb ⋅ M 2 + K t ⋅ K ct ⋅ T 2 = 1.5 ⋅ 129.5 2 + 1.5 ⋅ 312 = 163 Nm = 163,000 Nmm No allowance was made for stress concentration since the shaft will be mounted to the pulley by taper locks, which do not require the shaft to be machined. If the shaft is stepped it will be necessary to factor in stress concentration effects.

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Se =

Fs F πd 3 ⋅Zp = s ⋅ 2+2 2 + 2 16

d (mm) =

3

16 ⋅ S e ⋅π Fs 4

The allowable stress for the steel selected is found in the steel manufacturer’s catalogue. If a key way is used in the shaft, the allowable stress for the shaft is reduced by 0.75. Se =

16 ⋅ S e Fs Fs F π ⋅d3 ⋅Zp = s ⋅ and d = 3 ⋅ 4 4 16 π 4

For the calculation CS 1030 steel will be used. Fs = 225 MPa d=

3

16 ⋅ 163,000 ⋅ π = 25mm 225 4

This shaft is on the small side and a 40 mm solid shaft will be used for ease of assembly and future maintenance.

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SHAFT BEARING AND SEAL ARRANGEMENT Once the shaft size is determined the bearing size can be selected. Follow the bearing manufacturer’s selection process for calculating the required bearing type and configuration for the equipment design life and service factors. Provide shaft seals for the bearing at the bearing housing and at the penetration into the elevator frame. The bearing must ne ver be exposed to dust or dirt or moisture while in the production environment. Do everything necessary to protect the bearing. The bearing and seal suppliers can advise other ways of mounting and protecting the bearing. The best bearing arrangement design is to stand the bearing off the elevator frame with a clearance of around 25 mm. ELEVATOR FRAME MEMBERS The frame can either be made of an angle iron skeleton to which sheets of steel are attached or from sheets of steel pressed to the required rectangular shape that are flanged and bolted together. The thickness and lengths of section used in the frame must be sufficient to prevent buckling under load. INLET AND DISCHARGE CHUTE DESIGN The inlet chute should be designed to promote product flow and to minimise the amount of bucket drag. Preferably the product feed falls into the buckets as they come around the tail pulley without being dragged through a fully plugged boot. The feed chute should be made with a slightly smaller width than the buckets. It should be sufficiently steep to insure product always flows and does not build back. Test the product’s flowability if possible by putting some on a bent sheet of the elevator chute material shaped into a ‘U’ the same width as the chute. Tilt it to find the angle that produces flow. Insure there are no restrictions or protrusions into the chute that will cause the product to build back. The discharge chute size is known from the initial design. The angle at which it is set must meet the same criteria as the inlet. HEAD AND TAIL PULLEY DESIGN The head pulley dimensions have been determined. For simplicity the tail pulley should be to the same dimensions as the head pulley. This will keep the buckets a constant distance off the elevator wall and aid product pick-up and simplify chute design and fabrication. Both head and tail pulleys need to be crowned to centralise the belt and permit the belt to be tracked if it wanders. The crowning should be 2 degrees both left and right from the center of the drum. The head pulley could be rubber lagged if desired to increase the coefficient of friction and lower the belt tension. This will allow use of a lighter duty belt. But there is always the possibility the lagging will be stripped off during operation. It is best to design for a metal drum and use lagged pulleys only when detection of bogged conditions is installed. Ribbing can also be mounted on the top pulley to increase friction and act by ‘digging’ into the rubber belt and producing a grabbing effect. The ribs are placed across the full axial length of the drum and positioned so that at least two ribs are always in contact with the belt. The rigs should be 3 mm to 4 mm high and contoured into the drum so as not to rip the belt. The tail pulley should be a self-cleaning design. This can be achieved in two ways – -

constructing the pulley drum of 20 mm or 25 mm round bars of length wider than the belt. The bars are spaced around the end plates with gaps for product to fall through. Size the spacing between ribs with sufficient clearance for small product to fall through. Larger product will not fit through the gaps.

-

provide a twin opposed-cone hub with the cone’s base starting at the center and tapering to the shaft at the ends of the pulley. 20 mm or 25 mm round bars are welded to the outer rim of the cones and gussetted back to the cone wall for stiffness.

The pulley can be mounted to the shaft using taper locks fitted to suitably sized hub, interference press fits, retainers screwed and doweled to the shaft, hubs screwed and doweled to the shaft or key way in the hub and shaft.

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CONSIDERATIONS IN CHOOSING FRAME AND PANEL MATERIAL The materials selected for the frame and panels must consider the corrosive nature of the product, the dampness of both product and environment and the protective coating system requirements to be applied to the finished elevator. SELECTING BELTS AND BUCKETS ] The materials selected for the belt and buckets must be compatible with the product. The product’s abrasiveness will influence the choice of bucket material. Plastic and metal buckets are available. It is often a good idea when plastic buckets are used, to install an occasional metal bucket that acts to scrape away solid build-up in the elevator boot. A steel bucket every 6 –8 plastic buckets is a good place to start. Use belts specifically designed for bucket elevators as they constructed in a way to take more stress that similar ply horizontal conveyor belts. METHODS TO TAKE-UP BELT TENSION The belt will stretch when in use after a period of time. Unless there is a method to take-up the stretch the pulleys will eventually start slipping. The take-up moves one pulley further away from the other. The pulley to be moved can be either the top or bottom pulley. If it is the top pulley the drive must also permit the pulley to move. One way is to mount the pulley shaft bearing housing on movable plates running in slides. They are positioned by jacking bolts in both directions. Another method is to have the entire top portion of the elevator on jacking bolts and slide it in and out of the lower section. A rubber skirt is used to seal the gay between the two sections. PROTECTION AGAINST BOGGING THE BUCKETS To protect against bogging install a proximity sensor to detect rotation of the non-drive pulley and stop the motor and in-feed system when motion is no longer present. Signal to the operator that a problem exists. CLEAN-OUT CONSIDERATIONS At times the elevator boot will need to be accessed for cleaning, especially if multiple products are put through the elevator. Whether a floor is required in the boot as part of the elevator or whether the bottom of the boot is open and sits on the plant floor is dependent on the product characteristics and operating environment. With fully sealed boots removable flanged doors, either bolted or wedged in place using restraining bars and retaining hooks, are mounted to one or both ends of the. When open bottom boots are used flanged triangular sliding draws fitted to both sides of the boot at the bottom is the best alternative. Operators and maintainers prefer easy methods of access that do not require large numbers of bolts to be removed. DUST EXTRACTION AND EXPLOSION PROTECTION Dust is generated within the elevator by the bucket loading process. Dust can be extracted from the bucket elevator by dust collection systems where necessary. Use methods of dust removal that allow entrainment air to be drawn through the elevator, else the dust collection fan may create a vacuum within the elevator. Explosive conditions are beyond the scope of these notes. Should this situation arise tt will be necessary to install bursting panels venting to a safe place and possibly pressure and temperature sensing instrumentation to detect high-risk conditions. The selection of materials suitable for an explosive environment will also be required. As will consideration of static charge build-up control.

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INSTALLATION OF THE BUCKET ELEVATOR Consideration will need to be given as to how the elevator will be restrained in position. Methods of fixing the frame to the floor and to attached equipment have to be selected. Installation techniques will need to be designed into the elevator. Lifting points may be necessary. Will installation be as a complete unit or in sections? Is crane access possible at the working site? Are there height restrictions? How will bracketing and ancillary items and equipment be mounted once on-site? CORRECT OPERATION OF BUCKET ELEVATORS Once an elevator is installed operators will need to know how to start and stop it. Local power isolation switches are necessary at both the top and bottom of the bucket for emergency needs. It is useful to have the power supply to the motor on a plug and lead so power can be remove during maintenance access. The feed to the bucket elevator should always be at a lesser rate than the elevator can remove it to protect against bogging and overload conditions. Reversal prevention will be necessary on tall or heavily loaded elevators to protect personal and equipment from damage by a belt running backward during a power failure. Usually a brake option is available with the gear motor. MAINTENANCE OF BELT BUCKET ELEVATORS Access for maintenance will be needed to the motor, gearbox and drive. Motors mounted at the top of the elevator will require access platforms. Inspection and observation openings will be required to the top and bottom pulleys to observe the belt when tracking it. Operators should conduct periodic inspections of the belt and bucket condition and a record made of their observation. If the selected bearings require greasing they will need to be put onto a preventative maintenance route. The bearing manufacturer can advise the greasing frequency. It is preferred to select bearings that are greased for life and remove all greasing points from the bearing housing. When belts have stretched beyond their take-up limits they can be cut shorter and spliced together while a replacement belt is procured. Overlapping the ends of the belt past two or more buckets and bolting the buckets through the overlap produce a belt slice. This lap makes a bump in the belt and stresses the belt as it runs over the pulleys. Another way to splice is to butt the end of the belt together and lay a separate piece of belt spanning two buckets either side of the butt. The overlay piece is put on the same side as the buckets. This lay produces a flat belt against the pulley. The buckets mounted to the overlay piece will stick out further than the buckets mounted to the belt. An alternative splice is an oil well splice where the ends of the belt are brought together and folded outward against each other on the bucket side. The turned-out ends are clamped and bolted together. Authored by Mike Sondalini – Maintenance Engineer.

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PROTECTING BEARINGS FROM DUST AND WATER. ABSTRACT Protecting bearing from dust and water. Protection methods like labyrinth rings, rubber seals, felt seals and shaft mechanical seals are described. Choice of the appropriate shaft seal and seal configurations to protect against dust and water ingress is critical. Numerous shaft seal designs suited to contaminated conditions are reviewed. Keywords: Particles, contamination, bearing, shaft, grease barrier, breather.

Dusty surroundings are one of the most difficult environments for bearings. In equipment handling powders or in processes generating dust the protection of bearings against contamination by fine particles requires special consideration. BEARING HOUSINGS Bearings are contained within a housing from which a shaft extends. The shaft entry into the housing offers opportunity for dust (and moisture) to enter the bearing. The shaft seal performs sealing of the gap between the housing and shaft. Choice of the appropriate shaft seal and seal configurations to protect against dust ingress is critical.

Figure No. 1. Shaft Bearing Housing Seals Bearing housing seals for dusty environments may be either a labyrinth type or a rubbing seal type. The labyrinth type requires a straight shaft running true. Rubbing seals are the more common and allow for some flexing of the shaft. The sketches below are conceptual examples of each type of seal. When setting a lip seal into place to prevent dust ingress insure the sealing lip faces outward. In situations of high dust contamination there may be a need to redesign the shaft seal arrangement for better dust protection than provided in standard housings. Some ideas which can reduce dust ingress into bearing housings are to : i.

provide two or more seals in parallel. Bearing housings can usually be purchased with combination seals as standard.

ii. retain the housing shaft seals but change from a greased bearing in the housing to one which is sealed and greased for life. If contamination were to get past the shaft seals, the bearing’s internal seals would protect it. iii. stand the bearing off the equipment to create a gap between the end of the equipment and the bearing housing while sealing the shaft at the equipment. iv. put in a felt seal wipe between the housing and the wall of the equipment to rub the shaft clean. Install of a mechanical seal in very harsh environments. v.

install a grease barrier chamber sandwiched between two seals. This barrier is separate to the bearing housing and acts as the primary seal for the bearing. Grease pumped into the chamber will flush out past the seals.

vi. replace the grease barrier chamber instead with an air pressurised chamber. vii. shield the bearing housing from dust with use of a specially fabricated rubber shroud encapsulating the housing and wiping the shaft or fit a rubber screen with a hole wiping the shaft over the opening emitting the dust. viii. flush the bearing with grease by pumping excess grease into the housing and allowing the grease to be forced past the shaft seals or through a purposely drilled 15mm hole in the housing. The hole must be on the opposite side of the bearing to the grease nipple, at the bottom of the bearing housing when in service and between the bearing and seal. ix. Mechanical seals can be fitted to the shaft with the stationary seal sitting toward the machine and the rotating seal mounted back along the shaft. Combinations of other seals and wipers can also be used in conjunction with the mechanical seal. Mount the auxiliary seals so they see the dust/water first and keep the mechanical seal as the last line of protection. Some conceptual examples are shown in Figure No. 2. Postal Address: FEED FORWARD PUBLICATIONS, PO Box 578, BENTLEY, West Australia, 6102. E-mail Address: [email protected] Because the authors and publisher do not know the context in which the information presented in the flyer is to beused they accept no responsibility for the consequences of using the information contained or implied in any articles

ASSEMBLY The process of assembling a bearing into the housing must be spotlessly clean. If contamination occurs at the time the housing is assembled no amount of external protection will stop the bearing from premature failure. When assembling bearings into housings make sure that: i. your hands have been washed. ii. the work bench is clear and wiped down clean. iii. no one creates dust or grinds nearby during assembly. iv. fresh, clean grease is used to pack the housing. v. the components are clean and all old grease has been thoroughly removed.

Figure No 2. Conceptual Sketches of Dust Sealing Methods BREATHERS When protecting bearings from dust you want to always consider another important area. A breather is used to let hot air out of a confined space and then to let the air back in when it cools down. Enclosed bearings get hot when operating and cool down to ambient temperature when not in use. The air drawn back into the space needs to be clean of dust and moisture. A breather on a bearing housing or bearing housing enclosure allows ingress of moisture and dust into the bearings causing premature life failure. Often a breather is insufficient and should be replaced with a low micron air filter that removes dust particles two micron and greater in size. Protect the breather or filter from water spray and damp conditions (ban hosing down if possible) with a shroud or by using an extension tube going into a clean, safe environment. Make sure the breather tube cannot be crushed closed by accident. Mike Sondalini - Equipment Longevity Engineer

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Experiences with Bucket Elevators ABSTRACT Experiences with bucket elevators. Bucket elevators lift bulk materials from one level to another. They are used on powders, granules, grain, chip shaped products and lumpy materials. They function well when designed properly for the duty, and used as designed. Problems that can occur and possible remedies are noted. Keywords: belt, pulley, tracking, tension, bucket, housing. The sketch below is of a belt bucket elevator. The buckets are bolted to a belt, driven by a pulley. The frame and housing enclose the belt, buckets and product. The buckets scoop up the material fed into the base or boot of the elevator. At the top it is flung through the outlet chute. Adjustable screws move one of the pulleys to provide belt tensioning and tracking. Inspection doors at the top and bottom allow viewing of the belt when making tracking adjustments

The drive pulley can be either the top or bottom pulley. With a top pulley drive the motor and gearbox are clear of product spills and dust fall-out. The belt tension only needs to be sufficient to provide enough friction between belt and pulley to lift the material. Access platforms to the drive at the top of the elevator is needed for belt tracking and maintenance.

With bottom pulley drive maintenance access is easy but belt tension is doubled to provide the same drive friction. This increases loading on all the moving components. If the bottom drive pulley becomes coated in product or the belt stretches, the belt slips. Top pulley drives have less operating problems.

Where the bucket elevator is used for multiple products, quick cleaning access for operators is required. Flanged and bolted access doors seal well but removal is slow and threads become crusted with dust. Other options on non-hazardous materials are to use doors like those in the drawings below. The bottom pulley ought to be a self-cleaning design and not allow product to build up between belt and pulley. One method is to use round bar to create a grizzly bar design. Postal Address: FEED FORWARD PUBLICATIONS, PO Box 578, BENTLEY, West Australia, 6102. E-mail Address: [email protected] Because the authors and publisher do not know the context in which the information presented in the flyer is to beused they accept no responsibility for the consequences of using the information contained or implied in any articles

Gluing rubber to the drive pulley will increase the drive friction. Cut the rubber splice at an angle of 45 degrees to the pulley axis so the splice gradually feeds into the friction area of the pulley. The belt speed must be sufficient to throw the material clear of the bucket and into the outlet chute. Too slow and the material slides from the upturned bucket as it comes over the top pulley and falls back to the bottom of the elevator. Too fast and the material is flung out too soon and hits the top of the elevator before falling back to the bottom. Formulas are available to determine the right belt speed and throw for the material. The pulley shaft bearings are best mounted on standoff brackets to the outside of the elevator housing in case the shaft seals leak. Shaft sealing should be well designed to stop any leaks. The UP-TIME article on Protecting Bearings in Dusty Places (Code No. 111) can be consulted for some useful shaft sealing ideas. Feeding the product into the elevator boot is done by allowing material to fall through a chute under gravity or by forced methods such as a powered feed screw. Both the feed chute angle and its cross section must be large enough to prevent product hang-up or build-back. A clear passage without obstructions is critical. Similarly the discharge chute angle, size and design must allow product to flow freely. Pressurisation commonly occurs inside the elevator housing as the buckets drag air on the downward run from top to bottom. When the feed rate into the elevator boot is less then the removal rate of the buckets, the flow of air is carried through the filling section and upward with the filled buckets. Dust is raised inside the elevator and the internal air pressure forces the dust out through openings and seals. The problem is worst with powdery or dusty products. If it is important to reduce the amount of dust, the boot should be kept choked without bogging the elevator. Increasing the feed rate into the boot slightly above the bucket removal rate will cause plugging. With such a feeding arrangement it would be necessary to also install build-back detection to periodically stop the feed until the boot was cleared. An alternative, successfully used on powdered products, is to feed the product in from the downward side of the elevator. With this method the product filling the boot moves through with the bucket and both product and bucket act to plug off the bottom of the boot to the flow of air. Quick detection and stoppage of the feed to a bogged bucket elevator is critical. When this is overlooked the belt stops but the drive continues to run. If undetected the rubber on the drive pulley peels off and the belt is eventually worn through. To detect bogging, a proximity detector is fitted to confirm the presence of rotation of the non-drive shaft. A stationary shaft would raise an alarm and stop the elevator and feed system. Mike Sondalini - Maintenance Engineer

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Hazardous areas for dusts and flammables. ABSTRACT Hazardous areas for dusts and flammables. Many explosions in the processing, manufacturing and bulk materials handling industries involve flammable gases or vapours and explosive dusts or fibres. Such chemicals are known as hazardous materials. The article provides a basic overview of the design requirements and maintenance practices for electrical equipment in hazardous areas. Keywords: explosive range, hazard assessment, zone classification, explosion protection, surface temperature, pressure wave, explosive range. WHAT IS A HAZARDOUS AREA? One definition of a hazardous area is “an area in which an explosive atmosphere is present, or may be expected to be present, in quantities such as to require special precautions for the construction, installation and use of potential ignition sources.” Flammables and combustible dusts are dangerous if present at explosive concentrations; in an atmosphere that will support combustion; when exposed to a sufficiently energetic ignition source. An explosion is impossible unless all three requirements are present together. EXPLOSIVE RANGE As with the engine of a motor car not firing if the fuel mixture is too lean or too rich, so must the concentration of a flammable gas or vapour be within a certain range for it to explode. For flammable materials like gasoline, methane or hydrogen to be in explosive concentrations, the atmosphere must be laced, or loaded, with appropriate quantities of the material to support combustion. The bottom of the flammability range is called the lower explosive limit (LEL) and the top of the range the upper explosive limit (UEL). For explosive dusts the criteria for an explosive condition is the amount of dust suspended in the atmosphere. Combustible dust clouds will only explode once a minimum threshold concentration in air is passed and a minimum amount of ignition energy is available. Should an ignition occur when sufficient the dust is suspended then an explosion would result. A combustible dust layer siting on equipment will ignite if the layer ignition temperature is reached for a sufficient length of time. ASSESSING THE HAZARDS It is the responsibility of the User to assess the nature of the hazards present. The persons involved in assessing hazardous areas need to have a strong background in the industry concerned as well as a good appreciation of the nature of the hazards caused by the chemicals present. The chemical properties and explosive nature of a flammable gas or vapour are major factors that influence the extent of the hazard. Other properties for consideration include the flash point temperature, vapour pressure, boiling point, extent of the explosive range, density of the gas or vapour and the ignition temperature to set of an explosion. If the hazardous area involves dusts and fibers a good appreciation of the physical, chemical and bulk material properties is required. The critical factors are the dust layer temperature at which a heated surface can ignite a layer of the dust. And the dust cloud ignition temperature at which a cloud of the dust ignites. Additional factors like fineness of particle size, dilution by inert materials and moisture content also affect the extent of the hazard. The size of the hazardous zone may increase during maintenance and cleaning if dust is lifted off equipment. WHAT IF AN EXPLOSION OCCURS? An important factor to consider is what occurs if a flammable or combustible material is ignited and explodes? Explosions generate a pressure front or shock wave that travel ahead of the flame front. Properties of the shock wave, maximum generated pressure, the speed of pressure rise and the amount of energy liberated by the explosion need to be considered when addressing the hazards. DOCUMENTATION IS CRITICAL The result of a hazard assessment is the classification of an area of plant and equipment into hazard zones. The area classification documents, zone indication drawings and justifications must be compiled in a verification dossier and made available to all persons who work on the plant. An example of a completed hazardous area zone classification drawing is shown in Figure No. 1. The drawing specifies the volume of space in which a risk is likely to be present and the nature of the risk. With this information the necessary design decisions can be made. Postal Address: FEED FORWARD PUBLICATIONS, PO Box 578, BENTLEY, West Australia, 6102. E-mail Address: [email protected] Because the authors and publisher do not know the context in which the information presented in the flyer is to beused they accept no responsibility for the consequences of using the information contained or implied in any articles

HAZARDOUS ZONES The designation (naming) of the zones reflect whether the hazard is a flammable gas or an explosive dust and the likelihood that a hazard will be present. For a gas/vapour the zones and their definition are listed below. • Zone 0 – a volume of space an explosive gas atmosphere is continuously present. An example is the vapour space in a fuel storage tank. • Zone 1 – a volume of space an explosive gas atmosphere occurs periodically in normal operation. An example is while filling the fuel tank of a car. • Zone 2 – a volume of space an explosive gas atmosphere is not normally expected and if it does occur, it will only be present for a short period of time. An example is a spill from overfilling a car fuel tank. R3.0 3.0

VENT

BUND WALL

BUND WALL 4.0

4.0

GROUND

1.0

15.0

15.0

* ALL DIMENSIONS IN METERS SOURCE OF RELEASE

ZONE 0

ZONE 1

ZONE 2

ABOVE GROUND FIXED ROOF VENTED STORAGE TANKS, ADEQUATELY VENTILATED

Figure No. 1 Zone classification for a flammables tank. For dusts the zone designations are noted below and reflect the probability of the occurrence of an explosive mixture. • Zone 20 – a volume of space where a combustible dust cloud is present for lengthy periods during normal operation or layers of combustible dust will form. An example is inside a dust collector. • Zone 21 – a volume of space where a combustible dust cloud is likely to occur during normal operation or layers of combustible dust will gather during operation. An example is beside a 25-kg bag filling-head. • Zone 22 – a volume of space where due to abnormal conditions a combustible dust cloud may occur infrequently and for short periods of time or layers of combustible will gather over an extended period of time. An example is inside a grain milling room that gradually accumulates dust over years of operation. Once the zone is designated the appropriate hazard protection measures suited to the zone must be applied. MINIMISING SURFACE TEMPERATURES To prevent hot surfaces from causing gases and dusts to ignite, their temperatures must be kept below the ignition temperature. Electrical apparatus, like motors, build up heat in operation. Their surface temperature rises and unless they are properly selected for the hazardous area they may introduce an explosion risk. Electrical equipment can be designed and built to a specific temperature class that limits the maximum surface temperature. There are two designations within the surface temperature classification system. Group I apparatus are used in the mining industry and Group II equipment are used everywhere else. Within Group II there is a second rating system known as the maximum surface temperature designation and is shown in Table No. 1. T1 T2 T3 T4 T5 T6 450oC 300oC 200oC 135oC 100oC 85oC Table No. 1. Maximum Surface Temperature EXPLOSION PROTECTION TECHNIQUES As already noted the three requirements that must coincide for an explosion to occur are - the presence of a fuel, at the right concentration to burn, when ignition is present. One explosion control principle is to purposely introduce a non-flammable atmosphere into the process. An example is the use of inert nitrogen or carbon dioxide gas to replace air inside reactors mixing flammable chemicals or inside mills grinding explosive powders.

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The other explosion principle used in hazardous areas is to select electrical equipment and designs that remove the ignition source. Table No. 2 lists the explosion techniques available. They must be selectively used as they can only be applied in the appropriate hazard zone. Method Exclusion – sealing ignition source away from gas or dust inside an enclosure

Explosion Containment – if explosion does occur it remains within the enclosure Energy limitation – the energy available is below the minimum ignition requirement of a gas/dust. Dilution – the flammable atmosphere is kept below the LEL. Avoid ignition source – use of no spark creating equipment Special design – not of the above techniques but can be experimentally proven as suitable.

Symbol DIP Ex m Ex n Ex o Ex p Ex q Ex d Ex n

Type of Protection Dust ingress protection Encapsulation Non-sparking (Permanently sealed devices, restricted breathing enclosures) Oil-immersion Pressurised enclosure Powder or sand filled Flameproof enclosure Non-sparking (enclosed-break devices)

Ex i Ex n

Intrinsically safe Non-sparking (non-incendive components)

Ex v

Ventilation

Ex e Ex n Ex s

Increased safety Non-sparking (inherently the operating temperatures are low) Special protection (designed for a purpose)

Table No. 2. Explosion Protection Techniques An alternative that should be considered to using hazardous area equipment is whether the electrical equipment can be located outside of the hazardous area. Hazardous area equipment is more costly and deliveries are longer because of the precision and quality requirements. By using standard off-the-shelf equipment located outside the hazardous area the cost and time for the work can be greatly reduced. EQUIPMENT APPROVAL AND CERTIFICATION Electrical equipment for hazardous areas is marked with symbols that indicate their certification and classification. They can only be used if the correct markings are in place to verify they are suited to the particular hazardous area concerned. An example is –

Ex II 1 G EEx ia IIC T6. The markings Ex II 1 G EEx show it is approved and certified to the specifications of relevant international bodies, while ia indicates the type of protection rating, IIC indicates the explosion group and T6 the temperature class. The markings are permanently on the equipment and recorded on the documentation accompanying the apparatus. INSTALLATION AND MAINTENANCE To insure hazardous area equipment retains its effectiveness it must be installed and maintained so that the protection it provides is continuously available. Only competent and qualified persons can design, install and work with hazardous area apparatus. Cables, glands, sockets, plugs, enclosures, etc must all meet the hazardous area designation. Special cable and gland installation methods and sealing techniques are required to prevent combustible atmospheres and flames from being transferred to connected, neighbouring equipment. Unless the exact requirements are followed the explosion protection is voided. Always replace equipment in hazardous areas with a certified, exact duplicate. If it is necessary to use an alternative because the exact duplicate is not available a qualified and competent person must check the rating and approve the alternative. The change approval process must be documented. Once hazardous area equipment is installed there are ongoing inspection and upkeep requirements. The integrity of the apparatus must be examined periodically. There have been numerous occasions where bolts have been left out of rated enclosures resulting in the loss of explosion protection capabilities. Inspection frequencies need to be determined and set to suit the operating requirements and risks in the plant. The use and keeping of documented maintenance records is necessary on hazardous area plant and equipment. Mike Sondalini – Maintenance Engineer

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Electric Motor Current Protection Saves Your Plant. ABSTRACT Electric motor current protection saves your plant. A lot of equipment failure results from just simple, plain mistakes, sometimes from unintentional forgetfulness, sometimes from ‘short-cut’ taking and sometimes because of ignorance of the consequences. The result is plant destroyed at great cost and inconvenience. But for plant being driven by an electric motor use of under and over current protection is something that can be done very cheaply to protect it from being run outside of its design 'envelope'. A current transformer is placed around the electrical cables leading to the motor. The transformer is connected to a monitoring device that alarms when the current is outside preset limits. Keywords: motor load, overload, motor characteristic.

ELECTRIC MOTOR CURRENT DRAW The power required to operate a 3-phase induction electric motor depends on the torque load on the motor. Low load means a low power draw and causes a low current draw, high load leads to a high power draw and a high current draw. Low load means the motor is turning near full speed and doing little work while drawing little power, high load means the motor is turning at lesser speed and working hard while causing a greater power draw. Overload means the motor has too much load and cannot turn at all. Figure No. 1 shows a 3-phase induction motor performance characteristics.

Figure No 1. Electric motor characteristics The stator current draw characteristic is a good variable to monitor and use for a decision to stop the motor before it gets damaged. Anything attached to the motor will also stop. In this fashion the equipment is protected from any condition that produces a low power draw or any condition that causes a high power draw.

DETECTING CHANGING ELECTRIC CURRENT When an electric current flows through a wire it produces a magnetic field around the wire. The greater the current, the stronger the magnetic field. The magnetic field will induce electrical fields and cause current to flow in neighbouring wires. This phenomenon causes problems for process logic computer (PLC) equipment and field equipment communications because the electrical fields can interrupt signals sent between equipment and computer. In this case communication cabling is specially shielded away from power cabling. However the phenomenon is useful as a means of monitoring electric power draw. By installing a current transformer onto a power cable the transformer develops its own current which is proportional to the current in the cable being monitored. Using a current transformer means there no wires to cut, the transformer is low cost, readily available and installation is quick. When coupled with a metering relay and a timer it is possible to turn off the power to a motor when the current goes outside set limits for a given period of time. If the limits are set to the current draw at maximum and minimum working situations and the motor is turned off when the limits are passed then the motor is protected from abnormal load conditions. The use of a current transformer is but one way to detect the presence of an electrical current. Other methods are also available to detect electrical current and involve installing the monitoring device into the electrical circuit. Postal Address: FEED FORWARD PUBLICATIONS, PO Box 578, BENTLEY, West Australia, 6102. E-mail Address: [email protected] Because the authors and publisher do not know the context in which the information presented in the flyer is to beused they accept no responsibility for the consequences of using the information contained or implied in any articles

UNDER-CURRENT SITUATIONS When a motor is freewheeling, or it is very lightly loaded, the current required to just turn the motor is only a small portion of the full load current. If such a situation develops when a motor in operation it is probably because something abnormal has occurred. A typical low-load situation arises when centrifugal pumps are deadheaded against a closed discharge valve or if a downstream suction valve is shut and the pump is cavitating because it is starved of liquid. In this case undercurrent protection would detect the low load on the motor and turn it off. If the pump had a mechanical seal it would be protected from damage before loss of lubricating fluid across the seal faces destroys the seal. Another situation where undercurrent protection would be useful is in the detection of unloaded conveyors or bucket elevators. They could be turned off automatically after a period of time. Anytime the components in a drive train fail, for example the shaft coupling breaks, or a drive shaft breaks or drive belts come off or snap, the motor load would suddenly drop and the fall in load current could be used to trigger a

shutdown and/or an alarm. OVER-CURRENT AND OVERLOAD SITUATIONS As the load on an electric motor increases, the spinning rotor starts to slip more and slow down. The electric current draw rises as the motor tries to maintain speed. The higher current flow causes more heat to develop inside the motor. The heat builds up and can destroy the motor’s internals. The motor fails due to being overloaded. Here again current detection can be used to shut the motor down and protect it and/or raise an alarm. Overloads are likely in bulk materials handling situations such as bucket elevators and screw feeders. Where equipment has to combat a dragging, digging or scraping action as part of the process, sudden overloads should be expected. Over-load protection should be incorporated into the original design. ALLOW FOR THE OPERATING REQUIREMENTS There are times when motors are required to free wheel for a short period or there will be a short, temporary overload situation. An example is when a pump sending liquid into one tank is required to send liquid into an alternate tank. For a short period both tank’s valves may be closed. It is often better to keep the pump running for a few seconds against a deadhead rather than to put it off and restart it. In this case the motor current will intentionally drop low but do not want the undercurrent protection to stop the motor. The timer is used to prevent the motor shutting down unnecessarily. Observations are made of normal operating duty current draws and delays to permit normal operation, such as high current draw at start-up, are set into the timer. The current protection only activates after the timer counts out in the presence abnormal loads. There is one issue to be weary of when using current protection. It is possible to have a ‘false’ load on the motor. As long as a motor experiences ‘normal’ loads the current stays within the permitted operating band. Should this load be the result of a part failing, e.g. a collapsed bearing or a slipping vee-belt, then the current could still be in its working band and the protection will not operate. Current protection is a cheap, simple way to protect your equipment against those unexpected and unforeseeable errors that happen. HIGH GEARING RATIOS When electrical current detection is used on highly geared drives (above about 50:1) it tends to become insensitive to sudden changes in load. By the time the unexpected load condition is transferred through the drive to the motor, the torque involved is very much reduced. There will be an eventual effect on the motor current but it may not last long enough to trigger the protective systems. The problem can be overcome if torque detection is also incorporated into the load protection methodology. With torque, electric current, electric voltage and shaft speed detection and control all incorporated into a variable speed drive (VSD) it is possible to control the loads on high geared mechanical drives. They can be slowed, sped-up or stopped electronically if unplanned load conditions occur. Mike Sondalini – Equipment Longevity Engineer References: A. Mychael, Electric Circuits and Machines Edition 2, McGraw-Hill Book Company

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Dust control concepts ABSTRACT Dust control concepts. Industrial dust is the result of material escaping from the confines of a process or a storage location. The size of the problem depends on the dust’s characteristics and the means of its distribution. The best policy to control dust is to not let it escape from where it ought to belong. There are simple techniques that can be used to manage dust and prevent it from becoming a major health and environmental problem once the mechanism of creation and distribution are under stood. Keywords: bulk material handling, dust collector, stock piles, transfer points

By Evan Platts, Mechanical Engineer. Overview The following briefly describes some of the basic mechanisms that are involved in the creation of dust and also describe some associated concepts. It does not cover the technical and chemical side of things. It is essentially common sense things listed out. Essentially for the creation of dust there is a requirement for a particle and mechanism to induce movement in the particle to make in airborne. The volume of particles made airborne determines the volume of dust. Once a particle is airborne then other factors influence the density and spread of the dust. Particle Size and Density The particle size and density is a significant influence on dust creation. • •

The smaller the dust particle then the easier it is to make airborne and the longer that the particle will remain suspended once airborne. The less dense the dust particle then the easier it is to make airborne and the longer that the particle will remain suspended once airborne.

So, smaller lighter particles are more prone to creating dust, and once airborne are less likely to disperse. Within a dust collection system this means that they are easier to capture and transport. Volume and Scatter of Particles The greater the volume of particles then the more opportunity there is to create dust. The exposed surface area or scatter of the particles also influences the opportunity to create dust. A bucket of particles compactly piled will generally require more effort to make airborne than the same volume spread widely as it presents more particles. The greater the scatter then generally the greater the area affected by dust. Wind or Air Flow The volume and velocity of wind can result in both a positive and negative effect on dust. If dust is being created by other source then wind or airflow will disperse the dust. This may be beneficial if small amounts of dust are being created, as it will prevent the build up of a high dust concentration. If the amount of dust being created by other source is high then wind or airflow will spread the dust over a large area. Whilst this may reduce the dust concentration at the source area it may result in a larger dust problem to both people and environmental emissions. Wind is the cause of lift off or pick up from stockpiles and the ground. Essentially the stronger the wind the larger the particles able to be lifted off and the greater the volume of particles lifted off. Within a plant area tunneling of the wind often occurs where by the velocity of wind increases when it is forced through a constricted area. This often occurs between buildings. The stack or chimney effect is also caused by wind. This is the principle behind the natural draft stacks prior to draft fans. Essentially the wind blows across the top of an exposed opening creating a low pressure point that draws dust and fumes out. This is generally noticeable from open hoppers or tanks by small eddies of wind disturbed dust. Wind velocity increases with distance from the ground. Higher points are more susceptible to the effects of wind i.e. buildings, stockpiles, transfer points. Air flow is induced by equipment and product movement. Air can be entrained in the product, induced by the boundary layer effect of movement or captured by equipment movement. When air is drawn into a transfer or a piece of equipment, this air Postal Address: FEED FORWARD PUBLICATIONS, PO Box 578, BENTLEY, West Australia, 6102. E-mail Address: [email protected] Because the authors and publisher do not know the context in which the information presented in the flyer is to beused they accept no responsibility for the consequences of using the information contained or implied in any articles

either needs to be controlled (direction, velocity, volume) or removed/expelled. Note the difference between air and dust. The dust can be filtered or removed from the air. Moisture or Wetting Agents This describes the use of water as a wetting agent however it also applies to other liquids that are used to control dust such as oil. Water can be used to two main ways to control dust as a prevention measure and then as a capture method. By having a moist bulk product, the particles adhere together and are less likely to disperse and scatter. The particles adhere to the water and put simply, they make the particle larger. This principle is often applied to stockpiles to prevent lift off or pick up. If particles become airborne then water sprays can be used to capture the particles by having the dust particle adhere to the water droplet. This principle is often applied but as particle sizes become smaller then it becomes less effective. Vehicles Whilst vehicles are generally not responsible for being the initial creator of dust they often result in being a significant dust source. When a vehicle travels over particles on a roadway several things may occur. • • •

The particles may be pulverised resulting in smaller particles that are more easily able to become airborne. The particles are picked up by tyres and scattered over a larger area and may become airborne by lifting into the air. The particles may become airborne from the airflow induced by the vehicle movement.

The amount of airflow is directly related to the speed of the vehicle. The degree of scatter and airborne particles are inversely related with moisture content. The greater the moisture the greater amount of pick up by tyres and the less airborne dust is produced. Build up of particles on vehicles and machinery such as front-end loaders (FEL) and bobcats result in a significant amount particle scatter and particles becoming airborne through induced wind lift off. Evan’s Rabbit vs Sheep Theory of Dust Control Often people try the rabbit theory on dust. Firstly they say it is just a couple of rabbits and nothing to worry about and certainly not worth spending money on. As the problems gets larger very quickly the method used to control the problem is shoot all rabbits on sight. This ends up being very time consuming, labour intensive and ends up costing lots of money in bullets. This is often occurs in dust collection by putting lots of dust take-offs on dust collectors. There ends up being too many take-offs that don’t work efficiently because the dust collector has a limited useful capacity, and then there is a call for a bigger dust collector. More dust take-offs and a bigger dust collector means collecting more dust, which must mean the dust problems, are being fixed. Wrong! The dust problem is what is on the outside of the dust collector. Once you collect more dust you then have the problem of what to do with it. Once you have more dust collectors the capital cost goes up, the maintenance cost goes up and the operating cost goes up. You need to minimise dust collection points to those that are most effective. The sheep theory to dust control is containment, control and collection. Like sheep, dust is dumb and tends to have a mind of it’s own wandering around aimlessly following each other. Sheep are kept in fenced paddocks and herded all together in a flock to a collection point. This is what to do with dust. If there is a hole in the fence and sheep are getting out you fix it. If a small flock of sheep runs off bring them back to the main flock rather than try herding two flocks. Don’t try to move them too quickly or they will break up. Have somebody watching the flock for stray sheep that run off at all times, get a dog. If a sheep falls on the ground pick it up put it in the ute and deliver with the remainder of the flock.

Postal Address: FEED FORWARD PUBLICATIONS, PO Box 578, BENTLEY, West Australia, 6102. E-mail Address: [email protected] Because the authors and publisher do not know the context in which the information presented in the flyer is to beused they accept no responsibility for the consequences of using the information contained or implied in any articles

Belt conveyor tuning. ABSTRACT Belt conveyor tuning. A belt conveyor is a moving surface used to transport product from one end to the other. In its basic form it consists of a driving head pulley, a tail pulley, the moving belt, support rollers, cleaning devices, tensioning mechanisms and a structural frame. Though simple in concept its many components need to work together as a system to get the best performance and operating life. Critical to that is an understanding of how to care for a belt conveyor and tune it for successful operation. Keywords: materials handling, bulk material transport

Belt conveyors are used to transport anything from matches to bulk material such as iron ore and quarried stone. The belt can be made of natural fibres, rubber, plastic or metal. Regardless of its construction and purpose there are basic requirements to its successful operation that must be met. How a Belt Conveyor Works Figure No 1 is a simple sketch of a belt conveyor. An electric motor and gearbox turn the head drum (or head pulley). The belt is pulled tight to produce friction between it and the head drum. The friction overcomes the load and drag forces and the belt moves around the circuit from the head pulley to the tail pulley and back to the head pulley.

Figure No. 1. A Basic Belt Conveyor Only friction is used to drive the belt. If the friction falls the belt will slip or stop moving even though the head pulley keeps turning. The friction between the belt and pulley depends on the friction properties of the surfaces in contact, the amount of surface in contact (the arc and width in contact) and the tension in the two lengths of the belt. The loaded side of the belt is the tight side and the return side is the loose side. The tight side needs to carry as much tension as possible to minimise the load on the drums, the shafts and the bearings. Getting the maximum friction possible between belt and head pulley does this. Often a head pulley will be herringbone grooved or coated in rubber (or other such treatment) to increase the friction. Another option is to increase the arc of contact. A jockey (snubber) pulley is placed under the slack side close to the drum. By lifting the return belt higher so it comes off the head pulley further around the circumference the contact area and hence the friction is increased. Tensioning the belt also increases friction. This can be by jacking the head and tail pulleys further apart and forcing the belt harder against the drums or by making the slack (loose) side tighter. Tightening the slack side goes against the ideal of keeping the slack side tension low and the tight side tension high. If the loose side is used for tensioning, the load carrying components are made larger to take greater forces. Maximising Belt Conveyor Operating Life. Once a belt conveyor is designed and installed it is there for years to come! The very best practice to adopt to promote long, trouble-free life is to be sure that the designer has designed it with quality components that can handle the entire range of forces generated in its use. One way to insure that is to engineer every part taking a load and then review the design calculations and the component selection using independent, experienced equipment users and maintainers querying the designer for the assumptions, reasons and proof behind each design selection. The list below highlights some of the issues and problems with belt conveyor installations. Once you are aware of them you can be on the watch-out and get to them fast. Postal Address: FEED FORWARD PUBLICATIONS, PO Box 578, BENTLEY, West Australia, 6102. E-mail Address: [email protected] Because the authors and publisher do not know the context in which the information presented in the flyer is to beused they accept no responsibility for the consequences of using the information contained or implied in any articles

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Belt wear and gouging from product impact at loading zones, from belt drag across solid surfaces, from scrapper rubbing, from the belt touching caught product, from product hardening on scrappers, from drive pulleys slipping during start-up or during a belt jam, from product build-up on trough and return rollers. Frayed belt at edges due to rubbing against structures, due to rubbing against product caught in structures, from rubbing against seized rollers. Belt Stretch from excessive belt tensioning, from belt aging, from high product impact, from overloading with product, from running the belt beyond belt design speed, from too much stop-start inertia forces. Belt distortion due to out-of-square joining, due to a join being too thick, due to ripping and buckling within the belt material as internal fibres tear because of overloads, due to loading one side of the belt. Bad tracking due to head pulley misalignment, due to no head pulley crowning, due to trough and return roller misalignment, due to roller seizure producing more drag. Also can be due to the last upper roller being too close to the head pulley and lifting the belt so it makes first contact far around on the head pulley circumference. Gearbox/drive mounting deflection causing shaft misalignment by support frame and load carrying members being undersized and insufficiently braced for the operating forces and inertia force changes. Cut belts from impact of product, from dragging across jammed sharp objects, from tearing due to sudden overloads, from bolts and foreign metal objects gouging. Scrapper failure from product build-up on the scrapping edge, from jammed scrapper parts, from wrong set-up. Slipping belt due to product jam, due to loss of belt tension, due to dust/dirt/moisture under head drum.

Proper Belt Conveyor Set-Up and Use Become familiarised with the manufacturers recommended operating practice and then get operators and maintainers together to discuss how to achieve them in-house. In the simplest of ways decide how to: ƒ Make sure the belt is tracking properly and to detect when it is going off-track. Crown the pulleys (1% to 1.5% of the pulley width), align guide and troughing rollers square to the belt, on short conveyors insure the head and tail drum centers are aligned to within 0.25 millimeter. Keep guide rollers and pulleys clean of product build-up. ƒ Make sure the scrapper is working well. If necessary change designs. Maybe water jet spray instead of a solid edge scrapper. (Be wary of brush scrappers for powdered and damp products. Fine particles build up in the bristles and clog the entire brush making the bristles rigid and stiff, which then scratches the belt.) ƒ Prevent overloading product by slowing loading rates to below removal rates. Install deflection plates in loading chutes to take the momentum of falling product and stop it from pounding into the belt. Install more rollers in the loading section to distribute the pounding forces. ƒ Prevent product jams. ƒ Keep friction low by detecting and replacing seized rollers. Reduce transfer station skirt-to-belt contact. If dust is a problem keep skirt contact area and pressures low enough to minimise the amount of dust escape. Higher pressures force the skirt hard into the belt and both parts wear. When you sort the issues out write down how it was done and make them standard operating procedure so the discoveries are not lost. Mike Sondalini - Equipment Longevity Engineer References: Applied Mechanics, A K Hosking, M R Harris, H & H Publishing.

Postal Address: FEED FORWARD PUBLICATIONS, PO Box 578, BENTLEY, West Australia, 6102. E-mail Address: [email protected] Because the authors and publisher do not know the context in which the information presented in the flyer is to beused they accept no responsibility for the consequences of using the information contained or implied in any articles

Tracking belts on elevators and conveyors. ABSTRACT Tracking belts on bucket elevators and short belt conveyors. Flexible flat belts are used on bucket elevators and belt conveyors to carry loose, bulk product. The belt is stretched tight from head drum to tail drum and the friction generated on the turning head drum is used to drive the belt and carry the product. The belt must run true on the end pulleys (drums) and stay within the sides of the equipment structure. If the belt runs off the drums, buckets will be destroyed and belt edges frayed. In the worst cases the belt runs off the pulleys resulting in a breakdown repair. Proper and long-lived tracking of belts is critical for trouble-free operation. Keywords: belt alignment, Head and Tail Pulley Designs The head and tail drums on belt bucket elevators and short conveyors are usually steel cylinders. A shaft is mounted thorough them at the center and extending outward long enough to mount bearings on both sides. Often the shell is steel pressure pipe and the ends are cut from flat plate and welded in place. The shaft can be welded into the end plates, but for easy repair, it is best mounted in taper lock hubs that are welded to the end plates and the shaft clamped into them. The shell length is usually 25 – 40 mm (1” – 1-1/2”) longer than the belt width Where additional friction is required the head pulley can be lagged in rubber or grooved with a 45o herring bone pattern. On belt bucket elevators carrying powdered or dusty product the shell on the bottom pulley can be replaced with round bars spaced around the end plates so that the gaps between them and the edge of the belt allow the product to fall out and the pulley self-cleans. Drum Adjustment and Alignment To insure the belt runs central the two pulleys must be aligned center to center. This is done with a string line stretched from the mid-point on both drums. The distance from a straight edge datum on the support structure is measured and the position of the shaft bearings adjusted until the string lines up with the straight datum edge. The head and tail pulley axes must also initially be aligned parallel to each other and in the same plane. Once the drum centers are inline the centerline distance on both sides from shaft to shaft is measured from a datum line on the support structure. Bring the drums as close together as possible so that the belt can be later stretched. Once the drums are aligned both horizontally and axially the belt is then fitted on the pulleys. Belt Joining Belts for short conveyors are usually purchased with the ends joined and are slipped into place over the pulleys. A splice at 45o to the belt running direction and not square is preferred. This allows the edge to lead into the scrapper and lessens the chance of the join catching and ripped. The belt for belt bucket elevators can be supplied either with buckets already mounted or without. Once the buckets are mounted it must be fed into the structure from the top and the ends of the belt joined later through a hatch in the wall of the structure. There are three common methods to join the ends of bucket belts - overlap, butt and strap and turn-up clamp. Figure No. 1 shows the joining methods.

Figure No 1. Joining Bucket Elevator Belts. Postal Address: FEED FORWARD PUBLICATIONS, PO Box 578, BENTLEY, West Australia, 6102. E-mail Address: [email protected] Because the authors and publisher do not know the context in which the information presented in the flyer is to beused they accept no responsibility for the consequences of using the information contained or implied in any articles

Insure joins are square to the running direction of the belt so that the belt runs as straight and true as possible. Drum Crowning Belt tracking is greatly improved when one of the drums is crowned. The drive pulley is the one usually crowned though the bottom drum can also be used. Only one of the drums is crowned and not both. Crowning allows the natural elasticity of the belt to act like a spring and pull the belt back down the rise of the crown if it starts to wander to one side. The height of the crown is 1.5% to 2% of the pulley length. Tracking Belts in Place Once the belt is installed and pulled tight, the distance between drum shafts is measured on both sides and positioned the same distance apart. The belt is run unloaded and if it wanders the tail pulley is adjusted and made tighter on the side the belt moves toward. The basic rule is that the belt moves toward the slack side and away from the tight side (this is also why crowning one pulley works). Another way to look at it is that the belt goes from the high-energy side (tight) to the low-energy side (loose). Once the unloaded belt is running true product is then introduced onto it. If the belt wanders under load the tail pulley is further adjusted as was done when set-up unloaded. Only tension the belt enough to insure it does not slip under full load. Over-tensioning the belt rips the internal fibers and the belt stretches and creeps continuously. All snub rollers, carrying rollers and return rollers must be square to the centerline of the belt and parallel to each other. Check this by measuring diagonals, which should be equal. Some times the belt can take on a ‘banana’ shape or ‘crescent’ camber along its length. Usually this is a sign of uneven internal fibre lengths. In this case the belt wanders from side to side during a rotation and little can be done to control it except to get it running as near to center as possible. Depending on the severity of the ‘banana’ it may even require that one of the drums be offset from the center-to-center alignment so that the belt stays on the pulleys. If the camber exceeds 1% of the belt length replace it. It can also be useful to install ‘digging’ buckets every tenth bucket. These buckets are slightly oversize in length and depth to the rest of the buckets and act as a scrapper to clear away any product build-up on the sides of the structure. Make them of metal or a harder material than the regular buckets. Mike Sondalini – Equipment Longevity Engineer

Postal Address: FEED FORWARD PUBLICATIONS, PO Box 578, BENTLEY, West Australia, 6102. E-mail Address: [email protected] Because the authors and publisher do not know the context in which the information presented in the flyer is to beused they accept no responsibility for the consequences of using the information contained or implied in any articles