CHE10209 Couplings Seals Bearings

Engineering Encyclopedia Saudi Aramco DeskTop Standards

Miscellaneous Mechanical Components

Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.

Chapter : Process File Reference: CHE10209

For additional information on this subject, contact R. A. Husseni on 874-2792

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Process Miscellaneous Mechanical Components

CONTENTS

PAGES

INFORMATION COUPLINGS

1

Flexible Disc Coupling

3

Flexible Diaphragm Coupling

5

Lubricated Gear Coupling

6

Coupling Life

7

GEARS

8

Speed Increasing Gears

8

Speed-Decreasing Gears

8

Gear Design

9

Lubrication of Gears

9

Power Consumption in Gears

9

VARIABLE SPEED COUPLINGS

10

BEARINGS

11

Journal Bearings

11

Thrust Bearings

13

Ball Bearings

14

BEARING LUBRICATION

15

Oil Ring Lubrication for Small Power Loads

15

Lubrication by Forced Circulation

16

PUMP SEALS

17

Packing

17

Mechanical Seals

19

Tandem Mechanical Seals

21

Double Mechanical Seals

22

Barrier Fluid System

23

COMPRESSOR SEALS

24

Labyrinth Seals

24

Oil Seals

25

VIBRATION MONITORING TECHNIQUES

26

VIBRATION PROBE TYPES

28

Non-Contacting Eddy Current Probe

28

Velocity or Seismic Sensors

29

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Thrust Bearing Monitoring GLOSSARY

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COUPLINGS The function of a coupling is to transmit power from one component of a machinery train to another and allow a certain amount of lateral and axial misalignment between the connected equipment. Some examples are: • • • •

A motor to a pump. A motor to a gear. A gear to a compressor casing. One compressor casing to another compressor casing.

Figure 1 shows some typical coupling locations.

Couplings must be designed to absorb movements due to slight misalignment between the shafts. Before startup, mechanics align machinery components; however, some small misalignment always remains. The two types of misalignment are parallel and angular. They are shown in Figure 2.

Coupling

Motor

Gear

Compressor Casing 1

Compressor Casing 2

FIGURE 1. LOCATIONS OF COUPLINGS

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COUPLINGS (CONT’D) Parallel Misalignment

Angular Misalignment

FIGURE 2. FUNCTIONS OF COUPLINGS -- (PARALLEL AND ANGULAR MISALIGNMENT)

Couplings perform one other function that is shown in Figure 3. They permit axial movement of the two shafts relative to each other, which is called free end float. The source of this movement is a change in shaft axial position resulting from a change in temperature during operation.

FIGURE 3. FUNCTIONS OF COUPLINGS -- (FREE-END FLOAT)

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Flexible Disc Coupling Figure 4 shows the most common type of coupling, the flexible disc. This type of coupling is used on most centrifugal pumps and on some compressors. It transmits power from one shaft to the other through sets of thin, flexible metallic discs. Flexing of the discs permits the movements necessary during each rotation. The flexible discs also permit a limited amount of free end float, as shown in Figure 5. An advantage of the flexible disc type coupling is that it does not require lubrication.

FIGURE 4. COUPLING TYPES - FLEXIBLE DISC, NONLUBRICATED

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Flexible Disc Coupling (Cont’d)

Free End Float

FIGURE 5. COUPLING TYPES - FREE END FLOAT

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Flexible Diaphragm Coupling Another type of nonlubricated coupling is the flexible diaphragm. This coupling is commonly used on large loads with high speeds such as centrifugal compressors. Power is transmitted from one shaft to the other through relatively thin flexible diaphragms that permit movement. Figure 6 illustrates the flexible diaphragm coupling.

FIGURE 6. COUPLING TYPES - FLEXIBLE DIAPHRAGM - NONLUBRICATED

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Lubricated Gear Coupling An older type of coupling is the gear coupling shown in Figure 7. In this coupling, power is transmitted through gear teeth. All of the gears are concentric and they rotate at the same speed. However, the elements are free to slide axially within each other, and this provides the required flexibility. A disadvantage of this gear is that it must be lubricated. Because poor lubrication can result in frequent coupling failures, this type of coupling is not commonly used today.

With Permission from Zurn Industries (Amerigear)

FIGURE 7. COUPLING TYPES - GEARS, LUBRICATED

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Coupling Life A coupling that is properly designed and installed should not experience failures during normal operation of machinery trains. Factors that can decrease coupling life are: • • • • •

A high torque stress on the coupling, greater than the design value. Large misalignments between the two shafts. Operation at relatively high speeds. Corrosion or mechanical scratches on thin discs and diaphragms. Dirt or sludge in a lubricated coupling.

A coupling can often tolerate any one of these contributing factors, but two or more in combination increase the chances for a coupling failure.

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GEARS Gears are sometimes needed to change the speed of rotation from one component to the next. Some examples are: Speed Increasing Gears Motors typically run at a maximum speed of 3600 rpm. Centrifugal compressors operate at 5000 to 13,000 rpm. A speed-increasing gear is used to run a compressor at the higher speed. Speed-Decreasing Gears Steam turbine drivers run at high speeds, 5000 to 10,000 rpm. If the driven equipment is a pump or an electric generator, the speed required is typically 3600 rpm. A speed-decreasing gear is therefore required. Gas turbines are also high speed machines running at 5000 to 10,000 rpm. Speed-decreasing gears are required for pumps and electric generators driven by gas turbines. The second stage of a centrifugal compressor may run at a higher speed than the first stage. Reciprocating compressors are low speed machines, typically running at 500 to 1000 rpm. A speed-decreasing gear is necessary with most drivers, including 2-pole and 4-pole electric motors. Gears are usually not needed for the following applications because the driver and the driven equipment can operate at the same speed. •

An electric motor connected to a centrifugal pump.

•

A steam turbine connected to a centrifugal compressor.

•

A gas turbine connected to a centrifugal compressor.

•

A slow-speed synchronous electric motor connected to a reciprocating compressor.

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Gear Design The three basic types of gears--spur, helical, and double helical--are shown in Figure 8.

Spur

Helical

Double Helical

FIGURE 8. TYPES OF GEARS

The spur gear is the simplest type of gear. The teeth are parallel to the axis of the shaft. This type of gear is not used for large, high-speed loads. In the helical gear, the teeth are not parallel to the shaft. Instead, they follow the shape of a helix, or spiral, on the surface of the gear. This results in gradual loading of each tooth as the gear rotates. Vibration and noise levels are much lower for helical gears than for spur gears. A double helical gear has two sets of teeth included at opposite angles. This arrangement neutralizes the large thrust, or axial load, on the shaft, which is a disadvantage of the single helical gear. Double helical gears are common for large, high-speed loads. Lubrication of Gears Gears are lubricated by a spray of oil onto the teeth at the point of contact. The oil is circulated by a pump. The circulation system may be for the gear alone, or it may be combined with a system that lubricates all the major bearings in the train. Power Consumption in Gears Gears are not 100% efficient in transmitting power. A power loss of 2 to 3% is normal.

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VARIABLE SPEED COUPLINGS Variable speed couplings permit the speed ratio to be changed during operation. The different types include hydraulic couplings, magnetic eddy current couplings, and v-belt couplings. These couplings are not commonly used in Aramco. In fact, their use is declining in industry in general. It is now more common to use variable speed electric motors. Therefore, these couplings are not covered in detail in this course.

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BEARINGS Two types of bearings are used in rotating equipment. The first is the journal bearing, which supports the shaft and absorbs all radial forces. Radial forces are perpendicular to the shaft. The second type is the thrust bearing, which absorbs axial forces on the shaft. Axial forces are parallel to the shaft. Journal Bearings Journal bearings are usually sleeve bearings or tilting pad bearings. Typical journal bearing types are shown in Figure 9. The shaft rotates within a simple sleeve. A film of oil is maintained between the bearing and the rotating shaft. The shaft does not touch the bearing but is supported by the dynamic forces of the oil. If contact occurs, the sliding friction will damage the bearings in a short time.

FIGURE 9A. JOURNAL OR SLEEVE BEARINGS

FIGURE 9B. JOURNAL OR SLEEVE BEARINGS - TILT PAD

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The rotation of the shaft causes the oil in the annular space to rotate. Since the annular space is smaller at the bottom of the shaft, the oil is forced into a narrow wedge at this point. This causes the oil pressure to be higher at the bottom of the shaft. The higher pressure supports the weight of the shaft and prevents contact between the shaft and the bearing. This type of bearing is the one most commonly used on large machinery such as centrifugal compressors, steam turbines, and gas turbines. At high speeds, the simple sleeve bearing may cause unacceptable vibration, because the single oil wedge may rotate, or orbit, around the shaft. Specially designed bearings with lobes or tilting pads (See Figure 9B) produce multiple oil wedges and greater stability at high speeds.

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Thrust Bearings Hydraulic, aerodynamic, and magnetic forces can cause axial forces on a shaft. These forces are absorbed by bearings called thrust bearings. Each compressor casing, turbine, and electric motor has at least one thrust bearing. A rotating collar on the shaft presses against stationary pads mounted to the casing. A rotating oil film between the collar and the pads supports the load without metal-to-metal contact in the same manner as a journal bearing. See Figure 10.

Soft Metal Surfaces Shoe Pads Mounted to Casing

Pivots or Levelling Plates

Thrust

Shaft Oil In

Oil Out Collar FIGURE 10. THRUST BEARINGS

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Ball Bearings Another type of bearing is the ball bearing or rolling contact bearing. Figure 11 is a diagram of a ball bearing. Ball bearings are commonly used in pumps. They absorb both the radial and the axial thrust forces on the shaft. Metal-to-metal contact does occur, but the contact is at small points that are constantly moving. Also, rolling friction occurs, not sliding friction. Therefore, only small amounts of heat are generated by the friction.

Outer Sleeve Balls Inner Sleeve

Ball Guide or Cage

FIGURE 11. BALL BEARING OR ROLLING CONTACT BEARING

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BEARING LUBRICATION Oil Ring Lubrication for Small Power Loads Small pieces of rotating equipment such as pumps usually have oil-ring lubrication. See Figure 12. An oil reservoir is located below the shaft. An eccentric ring suspended on the shaft dips into the oil level. As the shaft rotates, this ring also rotates but at a slower speed. As the ring rotates through the oil, it picks up oil and carries it up to the shaft. The oil then flows sideways into the bearing. A level indicator on the side of the equipment shows the level of oil in the reservoir. Operators check the level periodically and refill with oil when necessary. Figure 12 also shows a constant level oiler, which keeps the level in the bearing housing constant until the glass bottle is empty.

FIGURE 12. BEARING LUBRICATION Note that the glass bottle is not vented and does not show the oil level in the bearing housing. Level is checked in the sight glass.

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Lubrication by Forced Circulation Large pieces of equipment such as compressors, turbines, and large electric motors have forced lubrication systems. Oil is circulated through the bearings and through a cooler and filter to remove the heat generated within the bearings and dirt. The major components are as follows: •

A reservoir to hold a supply of oil.

•

Circulating pumps. Normally two pumps are provided, one driven by an electric motor, one driven by a small steam turbine. Two pumps insure that circulation of the lubricating oil is never lost, even during an electric power failure. Loss of circulation would cause significant damage to the bearings.

•

Piping carries the lubricating oil to and from each bearing. Usually small indicator glasses are located in these lines so that an operator can visually check that the oil is flowing.

•

A cooler to remove heat generated by friction. The oil cooler may be a water cooler or an air cooler.

•

A dual filter to remove solid particles greater than 10 microns from the oil.

•

Control valves to control oil flow and pressure.

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PUMP SEALS Seals are required between the rotating shaft and the stationary casing of a pump to prevent the leakage of liquid. Seals are mounted in the section of the casing called the stuffing box. Packing The original type of seal on pumps was packing. Packing consists of rings of flexible material in the annular space between the shaft and the casing. See Figure 13. The rings are compressed by a device called the gland follower. Compression of the packing rings causes a tight fit between the packing and the shaft and between the packing and the casing. Lubrication of the surface between the packing and the shaft may be required. If so, a lubricant is injected into the space called the lantern ring. This fluid can also act as a barrier fluid if it is maintained at a pressure higher than the pressure inside the pump.

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The disadvantage of packing as a seal is that tightening is required during operation. A small but finite amount of leakage occurs with most packing services. Therefore, packing is not used very often today. The mechanical seal has become the standard sealing device.

FIGURE 13. PACKING TYPE - PUMP SEAL

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Mechanical Seals Most pumps today are equipped with mechanical seals rather than packing. Mechanical seals do not require tightening during operation. Leakage rates are extremely small. Figure 14 is a diagram of a mechanical seal. The principal parts of a mechanical seal are two seal rings. One ring is mounted on the shaft and rotates with the shaft; the other ring is stationary and is attached to the casing. Where the two rings touch each other, the surfaces are extremely flat and smooth. Therefore, contact between the two rings is so perfect that liquid cannot pass between them. A spring pushes the one ring against the other to maintain contact. O-ring seals prevent flow of liquid through any other path.

FIGURE 14. MECHANICAL SEAL

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If the pumping service is hot, or if the fluid viscosity is high, heat must be removed from the rotating seal. A flushing liquid is circulated through the stuffing box to remove the heat. The flushing liquid also provides lubrication between the two seal surfaces. The flushing liquid can be a liquid circulated from the discharge of the pump if this liquid is clean. At other times, the flushing liquid may be a separate fluid. The flushing liquid also prevents the accumulation of solids in the stuffing box. Figure 15 shows several methods for providing flushing liquid to a pump seal.

FIGURE 15. SOME TYPICAL SEAL FLUSHING ARRANGEMENTS

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Tandem Mechanical Seals For most pumping services, a single mechanical seal is adequate. However, if the pump fluid is hazardous or toxic, more than one seal may be required to make sure leakage does not occur. Two seals may be arranged in tandem or as double seals. If one seal fails and leaks, the other seal serves as a backup and prevents leakage to atmosphere. The tandem arrangement is shown on Figure 16. In this case, the inner seal contains the pressure. The pressure in the buffer zone between the two seals will be near atmospheric pressure. If the inner seal fails and leakage occurs, liquid will accumulate in the buffer zone but will be contained by the outer seal. A level instrument or a pressure instrument in the buffer zone will sound an alarm to notify operators that leakage of the inner seal is occurring. If the outer seal fails first, only barrier fluid leaks to the atmosphere, not pumped fluid. Again an alarm indicates the malfunction. The barrier fluid is nonhazardous.

FIGURE 16. TANDEM MECHANICAL SEALS

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Double Mechanical Seals Double mechanical sealing is an alternative arrangement for two seals on a single shaft. See Figure 17. In this case, the pressure between the seals is higher than either the fluid pressure inside the pump or atmospheric pressure. Thus, if any leakage occurs, it is barrier fluid which leaks. Barrier fluid can leak into the product or out to the atmosphere. However, the product and the atmosphere are isolated from each other at all times. This arrangement is used for materials which are very hazardous, toxic, or corrosive. With this arrangement, the corrosive liquid does not contact the sealing surfaces.

FIGURE 17. DOUBLE MECHANICAL SEALS

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Barrier Fluid System Figure 18 shows a typical barrier fluid system. A reservoir is provided at an elevation approximately 6 ft above the pump seal. This reservoir is filled with a barrier fluid. The fluid circulates by thermal convection or by a pumping ring located on the rotating seal component to the seal and back to the container. A nitrogen connection maintains pressure on the container. If both seals are functioning normally, the level in the reservoir and the pressure will remain constant and the respective instruments will function normally. If one of the seals leaks, an instrument will sound an alarm, notifying the operator of leakage.

FIGURE 18. TYPICAL BARRIER FLUID SYSTEM

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COMPRESSOR SEALS There are two major types of seals on compressor casings. Labyrinth seals minimize internal leakage, for example, backflow from one impeller to the preceding impeller. Oil seals are located at the ends of the casing to prevent leakage of gas to the atmosphere. Labyrinth Seals In a compressor, leakage between the rotating impeller and stationary casing must be minimized in order to maintain efficiency. Backflow of gas is minimized by means of labyrinth seals. See Figure 19. A labyrinth seal contains multiple teeth located very close to the rotating shaft. This results in a breakdown of pressure across the seal with very little flow. Labyrinth seals are usually made of a soft metal so that they will not damage the shaft, but as a result, they are very easily damaged by excessive vibration of the shaft.

FIGURE 19. LABYRINTH SEAL

Labyrinth seals can also be used as external seals on air compressors and gas turbines. Some leakage of the internal gas to the atmosphere can be tolerated in these places.

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Oil Seals Most centrifugal compressors have oil seals at each end of the casing. See Figure 20. A bushing type oil seal shown has two rings mounted on the casing. These seal rings fit very closely to the shaft but with a finite clearance. Seal oil is pumped into the space between the two seals and circulates through the annular spaces between the shaft and the seal rings. The pressure of the seal oil is controlled to a level 5 to 10 psi higher than the gas pressure on the other side of the ring. Seal oil pressure is also higher than atmospheric pressure. This means that oil will flow outward through the seals, but gas and atmospheric air cannot flow through the seal. Seal oil is circulated by a pump. Seal oil from the outer seal ring can be returned directly to the reservoir and the pump. Oil that passes through the inner seal contains dissolved gas and must be treated differently. If the gas is clean, the oil passes through a low-pressure vessel to vent the gas. The oil is then returned to the system. If the gas is dirty or corrosive, the oil from the inner seal is usually discarded. The small pressure difference between the seal oil and the gas minimizes the leakage to the inner seal. Typical rates are 5 to 10 gallons of oil leakage per day.

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VIBRATION MONITORING TECHNIQUES Compressors, large turbines, and electric motors are expensive pieces of equipment. They are also usually essential to the operation of a process. One way to prevent mechanical failures is to continuously monitor equipment vibration levels. Very often, a gradual increase in vibration level precedes a significant failure. If increased vibration can be detected, the machine can be shut down and repaired before the damage becomes significant. Hand-held devices are used for routine monitoring of vibration in pumps and small motors. Large equipment is fitted with permanently mounted vibration probes. When the vibration reaches a specified level, these monitors sound an alarm. They may also automatically shut down the machine.

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FIGURE 20. OIL SEALS - CENTRIFUGAL COMPRESSORS

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VIBRATION PROBE TYPES Non-Contacting Eddy Current Probe This type of probe is usually used for compressors, motors, and turbines. See Figure 21. A probe is installed in the bearing housing very near to the rotating shaft. A d.c. voltage signal is fed to the probe. Any metal mass near the tip of the probe will cause a loss of signal strength as the signal produces eddy currents in the metal. The losses are inversely proportional to the width of the gap between the probe and the shaft. An electronic instrument connected to the probe translates the signals into amplitude of vibration and frequency of vibration. These devices can measure up to 5000 Hz. The eddy current probe measures the distance between the shaft and the probe. Therefore, it can be used to detect radial vibrations and position of the shaft. Two radial probes are installed, at right angles, in each major journal bearing.

FIGURE 21. VIBRATION PROBE TYPES - EDDY CURRENT

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Velocity or Seismic Sensors The velocity sensor is usually used in equipment with ball bearings. A moving coil is attached to the bearing housing. See Figure 22. A stationary magnet surrounds the coil. The movement of the coil generates a signal that is monitored by an external electronics package. This type of monitor can measure much higher frequencies than the eddy current monitor so it is used on gear housings. The vibration in gears can be much higher in frequency than in other types of equipment, because vibrations are generated by the teeth.

FIGURE 22. VIBRATION DETECTOR - VELOCITY (SEISMIC) SENSOR

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Thrust Bearing Monitoring Thrust bearing failure due to excessive wear is an important cause of machinery shutdowns. Monitors are installed to give early warning of a problem. Two variables are normally monitored on a thrust bearing, the temperature of the bearing and the position of the shaft. Temperature sensors are installed in the shoes of the thrust bearing. A high temperature at this position indicates excessive load on the bearing, insufficient lubrication, or both. The shaft position indicator is an eddy current sensor similar to the axial vibration monitor. The instrument measures the distance between the tip of the probe and the thrust collar. As the thrust pads gradually wear down, the collar moves toward the probe. A disadvantage of velocity sensors is large size. Recently, smaller sensors called accelerometers have been applied to measure vibrations other than shaft vibration. They operate by the piezoelectric principle and can measure high frequencies.

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GLOSSARY

Accelerometer

A device used to measure high frequency vibration of a gear housing or bearing housing.

Alignment

The relative orientation of two shafts with respect to each other. For good alignment, the shafts should be parallel and concentric.

Ball Bearing

A bearing that has rotating spheres between two sleeves.

Barrier Fluid

An intermediate fluid that prevents contact between a process fluid and the atmosphere.

Buffer Zone

An area that contains a barrier fluid.

Coupling

A device that connects the shafts of two machines.

Double Helical Gear

A gear with two rows of teeth that are inclined to the shaft at opposite angles.

Double Mechanical Seals

Two seals installed back-to-back, with a high pressure barrier fluid between them.

Eddy Current Probe

A type of vibration monitor that detects distance between the probe and a shaft.

Flexible Diaphragm

A type of coupling that uses flexible diaphragms to transmit power, used for large equipment such as gas compressors.

Flexible Disc

The most common type of coupling, usually used for pumps.

Flushing

A liquid injected into a stuffing box to cool, lubricate, and clean a seal.

Forced Lubrication

A system of bearing lubrication where oil is pumped into the space between the bearings and the shafts.

Free-End Float

Axial movement of the shaft.

Gear

A device that transmits power and changes speed of rotation. Output speed is in a fixed ratio to input speed.

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Gear Coupling

A coupling that transmits power by means of gear-type elements. All elements are concentric and rotate at the same speed, but axial motion is possible.

Helical Gear

A power transmission gear with teeth that are not parallel to the shaft. Each tooth follows the path of a helix on the surface of the gear.

Journal Bearing

A bearing that absorbs the radial forces on a shaft.

Labyrinth Seal

A seal made of a series of sharp-edged rings. The rings are very close to a shaft, without any sealing liquid.

Mechanical Seal

A seal that prevents leakage by means of two rings whose faces fit together very closely. One ring is stationary, the other rotates.

Oil Ring

A device used to lubricate bearings without forced lubrication. As it rotates, it dips into an oil reservoir and carries oil up to a shaft.

O-Ring

A ring of flexible material used to seal an annular space in mechanical seals where there is no relative rotation between the two surfaces to be sealed.

Packing

A method of sealing a rotating shaft, using rings of flexible material around the shaft.

Piezoelectric

Producing an electric voltage by stressing a crystalline material, such as quartz.

Seal

A device that prevents or minimizes leakage of fluid between a stationary component such as a casing and a rotating component such as a shaft or impeller.

Sleeve Bearing

A cylinder-shaped bearing that fits around a shaft like a sleeve.

Spur Gear

A gear with teeth that are parallel to the axis of the shaft.

Stuffing Box

The part of a pump body that contains the mechanical seal or the packing.

Tandem Mechanical Seal

Two mechanical seals, in series, on the same shaft.

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Train

A series of machinery components connected together; for example, driver, gear, and compressor.

Thrust Bearing

A bearing that absorbs the axial force, or thrust, on a shaft.

Variable Speed Coupling

A coupling that can continuously change the ratio of output speed to input speed.

Velocity Sensor

A vibration probe that has a moving coil attached to a machine part. The coil moves inside a fixed magnet to generate a signal.

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