04 05 Hydraulic System LG956L

LG56L Training Material Chapter VI Hydraulic System

CHAPTER VI Hydraulic System

SECTION I WORKING

SECTION II STEERING

HYDRAULIC SYSTEM

HYDRAULIC SYSTEM

I. Overview

I. Overview

II. Working pump

II. Loader sensor hydraulic steering system

III. Control valve

1. Overview

1. Main relief valve

2. Steering gear

2. Overload valve of big chamber

3. Priority valve

3.Over load valve of small chamber

III. Common faults and repair of steering

4. Check valve

hydraulic system

IV . Pilot valve V. Pressure selector valve VI. Cylinder VII. Trouble shooting for hydraulic system

RELIABILITY IN ACTION

SECTION I WORKING HYDRAULIC SYSTEM I OVERVIEW LG956L applied the pilot control system, to control the high main oil flow which is high pressured and mass flowed. The working hydraulic system is mainly composed by the working pump, pressure selector valve, control valve, lift arm cylinders, bucket cylinders, oil tank and oil tubes. Compared with the machinery control hydraulic system, this system has the features as below: ① It is portable, flexible and efficient, which can be controlled even by a finger. ② By using proportional pilot valve control, it can greatly reduce the commutation control force. ③The relief valve, overflow valve, oil refill valve and check valve applied cartridge structure, thus they have good commonality, and they are easy to maintenance.

RELIABILITY IN ACTION 2

SECTION I WORKING HYDRAULIC SYSTEM ④ The pilot valve is monolithic construction, which makes it small in seize, and easy to place. ⑤ When the engine stops, the lift arm can be lower and the bucket can be unload by the effect of pressure selector valve and the pilot valve. ⑥ There are electromagnets set in the lift arm lifting position and bucket retraction position, it can realize the limitation of the lift arm’s height and the balance of the bucket at any position. This can simplify the control process, reduce the labor intensity and avoid the energy loss and pressure impact due to the frequent action of the relief valve.

RELIABILITY IN ACTION 3

SECTION I WORKING HYDRAULIC SYSTEM II WORKING PUMP The working pump is a gear pump. The liquid is transmitted due to the change of the volume formed by the gears and the pump body. And the mechanical energy is transferred into hydraulic energy. ①Main parts： 1-Pump body 2-Bearing 3-Washer 4-Driven gear 5-Side plate 6-Sealing 7-Bearing 8-Drive gear

1

2

3

4

5

6

7

8

RELIABILITY IN ACTION 4

SECTION I WORKING HYDRAULIC SYSTEM ②Working principle When the working pump runs, many volumes formed by the pump body, the end cover and the gears. When the gears rotates in the direction of the picture shown right, the volume of the oil suck chamber increases gradually due to the meshing tooth release little by little. The oil sucked in will full-fill the volume between the gears and the pump body. The oil go with the gears to the oil outlet chamber. The volume changes to be smaller because the gears run into meshing, and the oil will be pushed out. The oil suck chamber and the oil outlet chamber are separated by the gears. As the volume changes, the oil sucked in and squeeze out constantly.

RELIABILITY IN ACTION 5

SECTION I WORKING HYDRAULIC SYSTEM III .CONTROL VALVE

1

①Compose The control valve is mainly composed of valve body, lift arm valve kit, bucket tilting valve kit, auxiliary valve kit, over flow valve kit for big chamber of lift arm cylinder, over flow valve kit for small chamber of bucket cylinder, logic valve kit, check valve etc. 1-Relief valve 2-The third function 3-Bucket tilting valve 4-Lift arm valve 5-Relief valve for small chamber of bucket tilting cylinders 6-Relief valve for big chamber of bucket tilting cylinders 7-Valve body 8-End cover 9-Spring 10-Spring seat 11-Spool

8

2

3

4

5

6

9

7

RELIABILITY IN ACTION 6

10

11

SECTION I WORKING HYDRAULIC SYSTEM

11 1




Oil ports set

1-To small chamber of bucket cylinders（B1） 2-To big chamber of bucket cylinders （A1） 3-To small chamber of lift arm cylinders（B2） 4-To big chamber of lift arm cylinders （A2） 5-Oil return port（T） 6-Oil inlet port（P） 7&8-To the third function ports 9-Logic valve 10-Check valve 11-To pilot oil

2 8 3 7 4 6

9

5 11

10

RELIABILITY IN ACTION 7

SECTION I WORKING HYDRAULIC SYSTEM ②Main function To control the movement of the lift arm cylinders and bucket cylinders or to keep the bucket and lift arm stay at a position through controlling the position of the spools, which determines the direction, the flow and the pressure of the oil flow. And the movement of the spools are based on the pilot oil. Neutral position When the pilot lever at the neutral position, the spools of bucket kit and lift arm kit stay at the neutral position. The A1, A2, B1 and B2 chambers are locked. The oil inlet port is connected to the oil return port. The oil from working pump go straight back to the oil tank through the control valve.


RELIABILITY IN ACTION 8

SECTION I WORKING HYDRAULIC SYSTEM


Bucket backward tilting

When the pilot lever at the position of bucket backward tilting, the pilot oil come from a1 channel push the spool to the b1 side. Then the channel between the P chamber and the T chamber is cut off. The P chamber is connected with the A1 chamber and the T chamber connected with the B1 chamber. The high pressure oil from P chamber goes into the big chamber of bucket cylinders through A1 chamber. The oil from the small chamber of the bucket cylinders goes back to the oil tank through the B1 chamber. Thus the bucket tilts backward.

a1

b1

RELIABILITY IN ACTION 9

SECTION I WORKING HYDRAULIC SYSTEM


Bucket forward tilting (unload)

When the lever is placed at the bucket forward tilting position, the oil from channel b1 pushes the spool to the a1direction, and the spool cut the channel between P chamber and T chamber off, and connects the P chamber and the B1 chamber, and also connects T chamber and A1 chamber. The pressured oil flows to the small chamber B1 of the bucket cylinder. The oil in the big chamber A1 of the bucket cylinder returns to the tank through the T chamber. And the bucket forward tilting (unload) action can be achieved.

a1

b1

RELIABILITY IN ACTION 10

SECTION I WORKING HYDRAULIC SYSTEM


Lift arm lifting

When the lever is placed at the lift arm lifting position, the oil from a2 channel pushes the spool to the b2 direction. The spool cuts the channel between the P chamber and T chamber off, and connects the P chamber and A2 chamber, and also connects T chamber and B2 chamber. The pressured oil flows to the big chamber of the lift arm cylinder through P chamber. The oil in the small chamber B2 of the lift arm cylinder flows to the tank through T chamber. And the lift arm lifting action can be achieved.

a2

b2

RELIABILITY IN ACTION 11

SECTION I WORKING HYDRAULIC SYSTEM


Lift arm down

When the lever is placed at the position of lift arm down, the oil from b2 channel pushes the spool to a2 direction. The spool cut the channel between P chamber and T chamber off, connect the P chamber and the B2 chamber, and also connects the T chamber and A2 chamber. The pressured oil flows to the small chamber B2 of the lift arm cylinder through the P chamber, and the oil in the big chamber A2 of the lift arm cylinder flows to oil tank through T chamber. And the lift arm down action achieved.


RELIABILITY IN ACTION 12

SECTION I WORKING HYDRAULIC SYSTEM


Lift arm float

When the lever is placed at the lift arm float position, the logic valve opens, the B2 chamber is connected with the T chamber. The ports P, A2, B2 and T are connected, the lift arm cylinders float under the effect of the external force.

T

B2

A2

P

逻辑阀

K

RELIABILITY IN ACTION 13

SECTION I WORKING HYDRAULIC SYSTEM 1 1.Main relief valve 2 The main relief valve is used to control the pressure of the whole hydraulic system. The pressure set is 17MPa. ①Compose 1-Spring seat 2-Hexagon nut 3-Spring 4-Lock nut 5-Support sleeve 6-Poppet 7-Poppet vale seat 8-Spring 9-Main spool 10-Valve seat 11-Valve body

3

4

5

6

7

8

9

10

11

RELIABILITY IN ACTION 14

SECTION I WORKING HYDRAULIC SYSTEM ②Working principle Chamber C and chamber D are separated by the control vale body and the main relief valve body. Chamber C is connected with oil returning port, and chamber B is connected with the oil inlet port. The orifice 1 in chamber B is connected with the orifice 2 in chamber C. When the pressure of the oil higher than the set pressure, the oil flows to chamber A through orifice 3 and pushes the poppet to move upward. The pressured oil flows to chamber B, at this time the poppet valve seat moves upward by the force of the spring. Because of the pressure difference between the two ends of the orifice, the main spool is forced to move upward. And thus chamber C and chamber D are connected by orifice 4, parts of the oil return to the oil tank. With the increase of the oil pressure, the distance the main spool travels become longer. The size chamber C to chamber D become larger, the ability for oil relief is larger. When the pressure is below the set pressure, the main spool return by the force of the spring.

1

2

B

A C 3

4

D

RELIABILITY IN ACTION 15

SECTION I WORKING HYDRAULIC SYSTEM 2.Relief valve for big chamber 1 ①Compose 2 1-Valve seat 3-Spring 5-Spool

2-Hexagon nut 4-Valve seat 3

②Working principle Chamber C is connected with oil return port, chamber A is connected with big chamber of bucket cylinder, chamber B is connected with chamber C through orifice 7, chamber D is connected with chamber C through the orifice in the spool. When the pressure of the oil in chamber A higher than the set pressure, the high pressure pushes the spool move upward, and chamber A is connected with chamber C to drain the oil. When the pressure decreases below the set pressure, the spring returns by the force of the spring.

4

D 6 5

B

7

C A

RELIABILITY IN ACTION 16

SECTION I WORKING HYDRAULIC SYSTEM 1 3.Relief valve for small chamber 2 ①Compose 3 1-Spring seat 2-Hexagon nut 3-Screw plunger 4-Valve body 5-Spring 6-Spool 4 ②Working principle Chamber A is connected with small chamber of the bucket cylinders, and chamber B is connected with oil return port. When the pressure of the oil in the small chamber is higher than the set pressure(12MPa),the oil pushes the spool to move upward. And chamber A is connected with chamber B, the oil draining started. When the oil decreases to below the set pressure, the spool returns by the force of the spring.

5

6

B

7

A

RELIABILITY IN ACTION 17

SECTION I WORKING HYDRAULIC SYSTEM The function of relief valves 1.When the bucket tilting spool at the neutral position, both the chambers of the bucket cylinder are closed. If some external impact works on the bucket, the relief valve can prevent the pressure from been increased sharply. 2.It can drain the oil automatically at the process of lift arm up and down. For example, When the lift arm reached to a given position, the piston rod of the bucket cylinder will be forced out. Thus the pressure of the oil in the small chamber will rise sharply. The relief valve can makes the oil return the oil tank through the relief valve. And this can prevent the cylinder and hoses from been broken by the high pressure.

RELIABILITY IN ACTION 18

SECTION I WORKING HYDRAULIC SYSTEM 4.Check vale There are check valves in every spool unite of the control valve. There is a oil refill valve between the small chamber B1 of the bucket cylinder and chamber T. This oil refill valve is a check valve. The main function is to refill the small chamber of the bucket cylinder. For example, when the loader unloading, the bucket forward tilting fast. When the barycenter of the bucket passes the below hinge joint, the bucket accelerate tilts by the effect of the gravity. The travelling speed is limited by the oil volume pumped by the oil pump. This check valve can refill the small chamber of the bucket cylinder, and makes the bucket tilt fast, strike onto the stop block realistic.

RELIABILITY IN ACTION 19

SECTION I WORKING HYDRAULIC SYSTEM IV.PILOT VAVLE ①Compose 1 The pilot valve is mainly composed of valve body, electromagnet kit, metering valve kit, logic valve etc. The details of the parts in next page.

2

1-Lever 2-Dust cover 3-Ledge plate 4-Valve body 5-Metering valve 6-Logic valve 3

4

5

6

RELIABILITY IN ACTION 20

Seat Dust cover Ledge plate Plate Electromagnet valve Ledge plate

Spine Ledge plate

Valve body Push rod Seal Spring seat Metering spring (internal) Oil return port

Spring Spring Oil return chamber 2C Logic valve

Oil inlet port

Metering valve seat

Logic valve seat Metering valve spool

RELIABILITY IN ACTION 21

SECTION I WORKING HYDRAULIC SYSTEM ②Working positions The lever is fixed on the pilot valve. There are 3 positions for the bucket, like forward tilt position, neutral position and backward tilt position. There are 4 positions for the lift arm, like lifting position, neutral position, down position and float position. There are electromagnets at the lifting, float and backward tilt position. When the lever is placed at the maximum position of lift arm lifting position or backward tilt position, the lever is locked until the lift arm or the bucket moved to the specific position. The approach switch works, and the electromagnet lost its magnetic force. The lever is forced back to the neutral position by the force of the returning spring.

RELIABILITY IN ACTION 22

SECTION I WORKING HYDRAULIC SYSTEM ③The hose connection 收斗 P-Oil inlet port T-Oil return port

举升

Chamber 2C is connected with K port in logic valve.

T P

2C

下降 卸料

RELIABILITY IN ACTION 23

SECTION I WORKING HYDRAULIC SYSTEM ④Working principle

3

When the lever is placed at the bucket backward tilting position, the push rod pushes the spring seat, and the spring is compressed, then the spool is pushed downward by the inferior spring seat. The oil inlet chamber is connected with the pilot oil chamber through the orifice 1 and orifice 2 in the spool. The oil flows to chamber a1 and chamber b1, and pushes the metering spool upward. The pilot oil chamber is connected with the oil return chamber. And the spool in the control valve moves.

4 5 6 7 8

T 1

P

3-Spine 4-Push rod 5-Spring seat 6-Metering spring 7-inferior spring seat 8-metering valve spool

2

控制腔

RELIABILITY IN ACTION 24

SECTION I WORKING HYDRAULIC SYSTEM Principle of direct ratio control When the press of the pilot control chamber equals with the force of the metering spring(or a little bit bigger), the oil pushes the spool upward, and the inferior valve seat pushes the metering spring. The channel between oil inlet port and control oil chamber is cut off, at the same time, the spool in control valve stop moving. So the angle the lever placed determines the movement of the spool in control valve. And we can control the working speed by controlling the angle of the lever placed.

RELIABILITY IN ACTION 25

SECTION I WORKING HYDRAULIC SYSTEM When the lever is placed at the float position, the control chamber is connected with the oil inlet chamber, the pressured oil flows into the control chamber of the logic valve, and pushes the spool upward. because of the difference of the two ends of the spool in logic valve, chamber 2C is connected with the oil return chamber. The logic valve works and thus the float action achieved.

T

P

RELIABILITY IN ACTION 26

V .PRESSURE SELECTOR VALVE ①Function The function of the pressure selector valve is to supply specific pressured oil to the pilot valve, and guarantee the lift arm down when the diesel is off. ②Compose 1-Valve body

2-Check valve

SECTION I WORKING HYDRAULIC SYSTEM ③Port sets 1-P1 port to pilot pump 2-P2 port to pilot valve 3-Pr port to big chamber of the lift arm cylinder 4-L port to the tank

1

2 4

3

RELIABILITY IN ACTION 28

SECTION I WORKING HYDRAULIC SYSTEM ④Working principle

Pr

When the valve is working , the oil from pilot pump flows to pilot valve through P1 port, the orifice in the spool and P2 port.

L 接先导阀

P1

RELIABILITY IN ACTION 29

SECTION I WORKING HYDRAULIC SYSTEM When the pressure of P2 port is higher than 1.5MPa, the spool moves to the left, and the channel connecting P1 port and Pr port is cut off. The check valve in Pr port can prevent the oil in the big chamber of the lift arm cylinder from flowing back.

Pr

L 接先导阀

P1

RELIABILITY IN ACTION 30

SECTION I WORKING HYDRAULIC SYSTEM When the engine is off, there is no oil in P1 port, and the spool moves back to the position when the P1 port and Pr port are connected. If the working device is lifted high, and the lever is placed at the neutral position, the oil in the big chamber of the lift arm cylinder is sealed in the chamber. At this time, just put the lever at the lift arm down position, and the oil in the big chamber of the lift arm cylinder will flow into pressure selector valve. The high pressured oil flows to the pilot valve through P2 port and pushes the spool in the control valve to the lift arm down position. And then the lift arm can down to the ground. In this process, the pressure of the oil to the pilot valve is controlled by the spool to 1.5 MPa. If the pressure is higher than 1.5 MPa, the spool moves to left, and the flow will be decreased, vice versa. And thus the pressure of the oil can be controlled.

RELIABILITY IN ACTION 31

SECTION I WORKING HYDRAULIC SYSTEM VI.CYLINDER The cylinder is used to transmit pressure energy into mechanical energy. And to drive the working device to move in line or swing back and forth. The cylinders can be divided into 2 types based on the different working type. There is 1 port in the single acting cylinder, and the oil can push the piston out, while it depends on the force of the spring or the gravity. There are 2 ports in the double acting cylinder the movement of the piston is determined by the oil pressure. For this training material, we just talk about double acting cylinder. 1 1-Cylinder body

2

2-Piston rod

RELIABILITY IN ACTION 32

SECTION I WORKING HYDRAULIC SYSTEM

①Compose 1-Bottom 2-Spring ring 3-Ring 4-Clamp ring 5-Piston 6-O-ring 7-Support ring 8-Ring 9-Yx seal 10-Cylinder body 11-Joint 12-Guide sleeve 13-Cylinder cover 14-Dust cover 15-Piston rod 16-pisitioning screw 17-Ring 18-Buffer plunger ②Working principle The cylinder is mainly composed of the cylinder body 10, piston 5, piston rod 15, guide sleeve 12 etc. To simplify, we divide the cylinder into the small chamber and the big chamber. The high pressured oil flows into the cylinder through port A and pushes the piston to the right. The oil in the right chamber flows to the tank through port B.

RELIABILITY IN ACTION 33

VII.TROUBLE SHOOTING FOR HYDRAULIC SYSTEM 1.Why the gear pump fails? How to prevent? Reason : (1)There are scrap iron, gravel or something like that, and this will cause the scratch, wearing to the parts that moves. (2)The clearance between the shaft of the pump and the drive shaft is too large to drive the pump. (3)The friction between the side plate and the gears is too serious. (4)The oil suck hose jams or leakage, the oil intake shortage and this result in the failure of the pump. (5)The tightening torque of the bolts used fixe the flange is not equal. Thus cause the failure of the pump. Prevention measures : A. Check the drive shaft B. Check the bear of the drive shaft, there should not be axial float. C. Check the tightening torque of the bolts used to fix the cover to the pump body. The tightening torque should be equal. D. Check the oil sucking hose, prevent the air from flowing into the system. E. Keep the oil clear.

RELIABILITY IN ACTION

2. Why the bucket weakness, the bucket tilt forward or backward automatically? (1)The sealing rings in the piston fails, and it cause internal leakage. (2)The scratch or wearing appeared in the spool and the valve body, and it cause the oil leakage in the control valve. (3)There are some damages in the conical valve spool or the conical surface of the relief valve, and thus makes the sealing failure. The leakage in the relief valve appeared.

3. The working pump is easy to burst, and the leakage often appears at the end cover of the pump, why? The main reason is the spool of the main relief valve jams, the system stay at the closed station. The pressure in the system increase rapidly. And this lead to the burst of the pump. The reasons below can account for the oil leakage: The long-term bearing of high pressure result in the elongation of the bolts used to fix the cover to the pump body. And this cause the flexible of the cover. The radial scratch in the cover or the valve body is another reason that can account for the leakage.

RELIABILITY IN ACTION

4.What is “pulling cylinder”? What is the hazard? How to prevent? (1)Pulling cylinder: the part protruding outward of bucket cylinder piston rod is shortest when the bucket is at limit position of downward tilting. When the lift arm is lifted to a certain height, the limit point of the contacts the bottom of the bucket and the bucket can not tilt downward with the lifting of the lift arm. If the lift arm is lifted continuously, however, relative position of supporting point on two sides of bucket cylinder can change with continuous lifting of the lift arm, which forces the bucket cylinder piston rod to extend outward. For small and big chambers of the bucket cylinder are closed when the boom is lifted, outward extension of the piston rod can lead to rapid pressure increase in the small chamber of the cylinder. Such phenomenon that outward extension of bucket cylinder piston rod is caused by lifting of the lift arm when the bucket is dumped fully is called” pulling cylinder”. (2) Hazard: bending and deformation of bucket cylinder piston rod; damage of knuckle for bucket cylinder pine shaft; teeth falling of lock nut for bucket cylinder piston. (3)solution: retract the bucket and then lift the lift arm after the bucket is dumped.

RELIABILITY IN ACTION

5.Why does“ “nodding” ”(falling first and then rising)occur sometimes when the lift arm is lifted? Floating status that oil inlet and outlet are connected with working port will appear at a certain time of reversing for negative closing is adopted by lift arm control valve of distribution valve. Lower the lift arm first and then lift it after the negative closing and the floating status of oil inlet and outlet and the working port end due to the gravity of the lift arm and bucket.

6.Why the bucket is lifted and tilted slowly and powerlessly? The seals of lifting and tilting bucket cylinders are damaged, resulting in inner leakage of the hydraulic cylinder, which causes powerless action. Symptom : oil leakage occurs at surface junction, the pressure drops faster and abnormal sound of oil leakage appears during operation.

RELIABILITY IN ACTION

7.Failures of control valve. (1)Dirt in control valve blocks the spool or the pipeline to cause unsmooth oil flow. Symptom : the resistance when operating control valve is larger, the spool fails to move smoothly and can’t be reset normally. (2)Excessive wear of control valve and too large fit clearance between the spool and the valve body result in inner leakage thus to cause insufficient working flow. (3)The relief valve fails. The relief vale opening pressure is too low. At this time, do not fasten pressureregulating screw of relief valve blindly. It is required to remove and check the relief valve to see if the relief valve spring is ruptured, guide valve sealing is favorable, the spool is seized and damping hole of spool is blocked. The opening pressure of the relief valve shall be adjusted if none of the above problems occurs. (4)Inner leakage of the working pump is large, pump outlet flow is insufficient, loop pressure decreases and lifting and tilting buckets act powerlessly. Symptom : The hydraulic pump produces large noise when working and abnormal sound of leakage in the pump can be heard; engine speed raises and the hydraulic pump produces larger noise; a large amount of copper scales exist in the oil filter and tiny copper scales also exist in the oil. (5)The oil suction pipe and the oil filter are blocked or the oil suction pipe is aged, twisted or sealed improperly, which can result in insufficient flow.

RELIABILITY IN ACTION

8.Why the machine shivers in the raising or tilting process? The specific cause and countermeasure for the failure is as below: (1)The oil is insufficient to cause unstable working pressure. It is necessary to fill up hydraulic oil. (2)The oil suction rubber pipe nozzle is improperly sealed and air enters in the system to cause unstable working pressure. It is necessary to check sealing performance. (3)The oil is invaded by air and the invaded oil is forced to become compressible object due to lots of tiny bubbles in the oil. It is necessary to eliminate the position which is sealed improperly in the lowpressure oil pipeline and then expel air in the oil. (4)Hydraulic cylinder piston is loosened to cause runout of the piston rod in the hydraulic cylinder. It is necessary to dismantle the hydraulic cylinder and trace the cause. (5)The opening pressure of relief valve is unstable to cause pressure change of high pressure oil thus to bring about shivering. It is necessary to check pressure-regulating spring of the relief valve and adjust opening pressure. (6)The unequal leakage of the two lift arm cylinders result in the flow fluctuation, which brings about shivering. It is necessary to check the cylinders respectively and repair the broken cylinder.

RELIABILITY IN ACTION

9.Why the bucket tilts slowly and powerlessly with the lift arm working normally? (1)The pressure of two relief valves of the bucket cylinder is abnormal. It is necessary to check the pressure and adjust it to normal value. (2)There are particles in the oil and seized the relief valve spool, and the relief valve keep open. It is necessary to remove sundries and check if the spring is broken or disabled, the sealing ring is aged and if the fit clearance between the spool and the valve body is suitable(normal fit clearance is 0.0060.012mm). (3)There is inner leakage in the bucket cylinder. It is necessary to make a check.

10.Why the oil temperature of hydraulic system is too high? Too high oil temperature is mainly caused by the following cause except for the system: (1)Wear of pump gear pair, side plate, pump body and seals in the pump result in the reduced efficiency of pump capacity, which causes too high oil temperature. (2)Oil brand used fails to meet the requirements, which causes poor clearance of mixed oil deterioration etc.

RELIABILITY IN ACTION

Section Ⅱ Steering Hydraulic System

Ⅰ.Overview Steer Mode
The steering system’s function of the wheel loader is to control the drive direction of the wheel loader, keep the wheel loader straight drive stably and change the drive direction flexible according to the need. In terms of steering mode, the wheel loader can fall into deflection wheel steering, articulated steering, skid steering. The working equipment of the articulated steering wheel loader is installed on the front frame. When the frame deflected relatively, the direction of the work equipment is same with the direction of the front frame. This type can makes the work equipment aim at the work plane quickly, and reduces the distance and time of operation circulate, which improves the operate efficiency of the wheel loader. So the articulated steering becomes the most popular steer mode.

Articulated wheel loader




Skid steering wheel loader

Deflection wheel loader

RELIABILITY IN ACTION 41

Section Ⅱ Steering Hydraulic System

Ⅱ.Load sensing full hydraulic system 1.Over view
The steering system of the LG956L wheel loader mainly consists of priority valve and coaxial flow amplifying steering gear. ①Composition of the system This system mainly consists of steering pump, coaxial flow amplifying steering gear, steering cylinder, hydraulic tank, pipelines accessory and so on.


1.Priority valve 2.Steer pump 3. Oil suction filter 4.Steer Cylinder 5.Steer Gear 6.Hydraulic oil radiator 7.Oil return filter 8.Hydraulic oil tank

RELIABILITY IN ACTION 42

Section Ⅱ Steering Hydraulic System

②Features: The steering system have the following features: a. The components structure is compact, and the size is small. b. All the components has the function of self-lubrication, which achieve long service life. c. The steer is reliable, and the control is easy and flexible. d. Achieve the interflow with working equipment hydraulic system, reduce the loss of power, improve the system efficiency


As steering, the system supplies oil to the steering hydraulic system. And the remain oil, interflowed with the steering system , return to the hydraulic oil tank through the radiator.




The safety valve is installed on the priority valve. The set pressure of the system is 16Mpa.




RELIABILITY IN ACTION 43

Section Ⅱ Steering Hydraulic System

2、Steer Gear ①The meaning of the steer gear type

Steer Gear BZZ6BZZ6-800

Steer gear model

Steer gear type

BZZ6

Load sensing flow amplifying full hydraulic steer gear

Displacement 800ml/r

RELIABILITY IN ACTION 44

Section Ⅱ Steering Hydraulic System

②Coaxial flow amplifying symbol Coaxial flow amplifying type（BZZ6） symbol is labeled at the right picture.

RELIABILITY IN ACTION 45

Section Ⅱ Steering Hydraulic System

③The port location


There are 5 ports on the steer gear,

that are inlet port P, return port T, left turn exhaust oil port L, right turn exhaust oil port R, feed back port LS, which are corresponding connected with exhaust oil port CF of the priority valve, return oil port of the hydraulic oil tank, right turn cylinder big cavity,

T：Exhaust oil P：Inlet oil R：Right turn L：Left Turn

left turn cylinder big cavity and the LS port of the priority.

LS

T L P R

RELIABILITY IN ACTION 46

Section Ⅱ Steering Hydraulic System

④The composition of the steer gear


The structure of BZZ6 type full

hydraulic steer gear is shown in the picture. It is mainly consists of valve body, valve core, valve pocket, universal driving shaft, guide spring, pin, rotor, stator, rear cover, etc.

1.Connect body 2.Front cover 3.Valve body

4.Spring

5.Pin

6.Valve cover

7.Spool

8.Connected Shaft

9.Rotor

10.End cover

11.Limit Rod

12.Separation disc

13.Stator

14.O-ring

15.Steel ball

16.O-ing

17.X-ring

18.O-ring

RELIABILITY IN ACTION 47

Section Ⅱ Steering Hydraulic System




The steering gear following rotary

valve, which mainly consists of valve body, valve core and the valve pocket, controls the direction of the oil flow.




The valve core located inside the

valve pocket is connected with steering rod directly through the connect block, and rotated with the steering wheel by the drive of steer rod. The valve pocket, located in the valve body, moves in the valve body by the drive of connected shaft and the pin.

RELIABILITY IN ACTION 48

Section Ⅱ Steering Hydraulic System The metering motor, which consists of rotor and stator, is located at the lower end of the steering gear. The stator has 7 teeth, rotor six teeth. And the stator is stationary, the rotor rotates around the center of the stator in the radius of throw of eccentric.




The shape of metering motor teeth is Equidistant circular arc epicycloids tooth. The teeth shape make sure that every point of the motor curve is contact point. When rotating, there are always 7 holes connected with the valve pocket. The valve pocket and valve core supply oil to the metering motor. Then the pressed oil enters one half teeth cavity, and the other teeth cavity exhaust oil, sending the pressure to the steering cylinder.

1




1-rotor

2

2-stator

RELIABILITY IN ACTION 49

Section Ⅱ Steering Hydraulic System The metering motor, which consists of rotor and stator, is also called cyclonical meshing pair. When power steering, it plays the function of metering motor to ensure the oil mass is in proportion to angel of the steering wheel. When manual steering, it plays the roles of manipulating fuel pump.




The connect rod and pin are connected with the rotor and the valve pocket. When power steering, it can ensure synchrony between valve pocket and rotor. When manual steering, it plays the role of torque transmission.

1




1-Connect rod

2

3

2-Rotor

3-stator

RELIABILITY IN ACTION 50

Section Ⅱ Steering Hydraulic System

The function of the spring is to ensure the following rotary remain the centering position. So the spring is called centering spring. 1 1.Valve core 2.Spring 3.Valve pocket

2

3

RELIABILITY IN ACTION 51

Section Ⅱ Steering Hydraulic System




The one-way valve is installed

between the inlet valve and exhaust valve. When manual steering, the one-way valve opens, and the oil in one of the steering cylinder cavity return to the inlet port through the return port. Then the oil passes into the other cavity of the steering cylinder by the negative pressure of 1 cyclical meshing pair. Achieve the function of oil sorting as manual steering.

1-One-way valve 2-P port(oil inlet port) 3-T port(oil return port)

2

3

RELIABILITY IN ACTION 52

Section Ⅱ Steering Hydraulic System

⑤Operation Principle The Work functional diagram of the BZZ6 type full hydraulic steering gear is shown in the picture. The port A and port B are connected with the two cavities of the steering cylinder. The port P is connected with the exhaust port of the steering pump, and the port T is connected with the hydraulic oil tank.




1-connect block

2-front cover

3.valve body

4.spring 5.pin

6.Valve pocket

7.valve core

8.Linkage shaft

9.rotor 17.X-ring

10.End cover

11. limited post

12. separation disc 13.sator 14.16.18.O-ring

15.Steel ball

RELIABILITY IN ACTION 53

Section Ⅱ Steering Hydraulic System

A. The steering wheel doesn’t turn. The valve core 1 and valve pocket 3 stay center position at the function of the centering spring. The hydraulic oil from the steering pump 7 enters inside the valve core through the small hole located at the end of valve core and the valve pocket, and return to the hydraulic oil tank 8 through the port T.




When the steering wheel doesn’t turn, the centering spring plays role. As the port A and port B are blocked by the valve core 1, the oil in the steering cylinder can’t flow in nor out. The piston can’t move, so the wheel loader runs at the former direction.


RELIABILITY IN ACTION 54

Section Ⅱ Steering Hydraulic System

B. The steering wheel turns left. The steer wheel drives the valve core 1 rotate counterclockwise. The centering spring are pressed. As the biggest rotatio between the valve core 1 and valve pocket 3 is 10°30, the valve core can turn around the valve pocket.




The oil groove of the valve core is connected with the inlet port P. The oil from pump flows through the oil groove of valve pocket 3, valve core 1,and reflow to stator 4 and rotor 5 driving rotation of the rotor 5 around stator 4.


RELIABILITY IN ACTION 55

Section Ⅱ Steering Hydraulic System

B. The steering wheel turns left. At the same time, the exhaust oil of stator and rotor passes into the big cavity of right steering cylinder through the port A, making the piston rod to reach out and making the frame to drive the wheel turn left.




The oil in the small cavity of the cylinder passes into he valve pocket 3 through the port B, and return to the oil return groove through the valve core 1. In the end, the oil return to the hydraulic tank through hole of valve pocket.


During the motion of turning left, the small cavity of the left steering oil cylinder (with rod cavity) is connected with the big cavity of the right steering oil cylinder. The inlet oil of the small cavity makes the piston rod to shrink into the cylinder, driving the frame turn left.




RELIABILITY IN ACTION 56

Section Ⅱ Steering Hydraulic System B. The steering wheel turns left. The big cavity of left steering oil cylinder is connected with the small cavity of the right steering cylinder. The oil from the small cavity of right steering oil cylinder enters into the port B through the big cavity of the left steering oil cylinder, and returns to the oil tank from the oil return hole of the valve pocket.




When the rotary angle between valve core and the valve pocket is about 1.5°, the oil channel begins to combine. The rotation of rotor makes that the oil of steer pump is connected with oil cylinder. The mass of supply oil is in direct proportion to the angle of the steering wheel.


RELIABILITY IN ACTION 57

Section Ⅱ Steering Hydraulic System

B. The steering wheel turns left. When the turn angle of the steering wheel remains some, the oil from the steer pump drives the rotor 5 rotate right because the oil channel above is open. When the rotary angle of rotor 5 is same with that of steering wheel.




Because the valve pocket 3 is mechanical jointed with rotor 5 through the linkage shaft , the rotor drives the valve pocket 3 turn left at the same time until the centering spring makes the valve pocket, valve core to return the center position. The angle between the stator and the rotor is same with the rotary angle of the steering wheel.


RELIABILITY IN ACTION 58

Section Ⅱ Steering Hydraulic System

B. The steering wheel turns left. At this time, the valve pocket 3 and the valve core 1 stays the position where they don’t have relative rotary angle. The oil channel, which goes through rotor and steering cylinder, is closed. The exhaust oil of the steer pump enters into the valve pocket through the port P, and then return to the hydraulic tank through the return hole 3 of valve pocket 3 after entering into the oil return groove of valve core 1. Thus the wheel loader stops steering.


If the steering wheel continues rotating, the rotor and the valve pocket follows to rotate with the steering wheel until the left turning position is limited. This is called the hydraulic feed back function.




RELIABILITY IN ACTION 59

Section Ⅱ Steering Hydraulic System B. The steering wheel turns left. When the steering speed is low(the rotary speed of the steering wheel is below 10r/min), the effective displacement of the steering gear is same with the metering displacement. When the input rotary speed of the steering wheel increases, the effective displacement is in direct proportion to the speed of the steering wheel.




Part of the oil from oil inlet port P enters into the stator, rotor parts to meter, and the others passes into the oil cylinder through the port A and port B. So it plays the role of flow amplifying.




When the input speed of the steering wheel is above 40 r/min, the effective displacement of the steering gear is constant at the rating displacement.




RELIABILITY IN ACTION 60

Section Ⅱ Steering Hydraulic System

C. The steering wheel turns right


When the steering wheel turns right,

the steering wheel drive the valve core rotate clockwise. The operation principle is same with that of turning left.

RELIABILITY IN ACTION 61

SECTION 2 STEERING HYDRAULIC SYSTEM d.manual steering In event of sudden flameout of the engine or the malfunction of steering pump, rotate the steering wheel with hand for static steering. While rotating the steering wheel rightward, the valve core rotates for a 10º30’ angle to drive the rotation of valve sleeve, linkage shaft, and rotor via shifting pin. In such case, the rotor and stator are functioned as oil pump. The rotation of rotor 5 sucks out the oil from the oil port T and inputs the oil into the oil inlet chamber of rotor pump via check valve, valve sleeve, and valve core. The oil pumping action during the manual steering compresses the hydraulic oil sucked into the steering oil so that the compressed oil enters into the rodless chamber of steering cylinder to extend the piston rod and steer rightward the loader.


Valve body

Oil return port

Right Oil inlet Left Spacer disc steering steering port Limit block

Linkage shaft Valve core

Bearing

Check valve Valve sleeve Return spring Shifting pin Stator-rotor pair

Rear cap

RELIABILITY IN ACTION 62

SECTION 2 STEERING HYDRAULIC SYSTEM d.manual steering The oil in the rod chamber flows to the oil inlet chamber of rotor pump from oil port A through valve sleeve, valve core, valve sleeve, and check valve and continually refills into the rodless chamber, in order to maintain the steering action.


RELIABILITY IN ACTION 63

SECTION 2 STEERING HYDRAULIC SYSTEM ⑥VALVE BLOCK Valve block is a combined hydraulic components ,mainly consists of Check valve, bi-directional buffer valve (overload valve) and oil refill valve , etc . It connects between the steering pump and steering gear, completes with full hydraulic steering gear(Usually fixed on the flange of the steering gear,with the steering gear make up a whole)。 It’s use:On the one hand keeps the steering gear and entire steering system work normal under the pressure rated; On the other hand, can guarantee the steering cylinder and connecting pipe without damage in a sudden overload, and can protect the steering pump.

RELIABILITY IN ACTION 64

SECTION 2 STEERING HYDRAULIC SYSTEM a.CHECK VALVE Check valve consists of valve seat 1、valve core 2 and spring 3,etc.It is installed in the oil inlet port of the valve body.The high pressure oil from the pump flows into the oil inlet port of the steering gear through the check valve. Its role is to prevent the backflow of oi,steering wheel selfdeflection,steering failure.

1-Valve core 2-Spring 3-Valve seat 1

2

3

RELIABILITY IN ACTION 65

SECTION 2 STEERING HYDRAULIC SYSTEM b. TWO-WAY DAMPING VALVE The two-way damping valve incorporates two constant pressure direct-acting safety valves composed of the spring, ball valve seat, and steel ball. It’s installed within the valve port for connecting the valve body with the orifices of left and right chambers of steering cylinder and is connected with the oil return port, in order to protect the hydraulic steering system against impact of over-high pressure and ensure the safety of oil lines.

L

R

T

P 1

2

3

4

RELIABILITY IN ACTION 66

SECTION 2 STEERING HYDRAULIC SYSTEM C.REFILL VALVE The oil refilling valve incorporates two check valves composed of steel ball and is installed within the valve port for connecting the valve body with the orifices of left and right chambers of steering cylinder and is connected with the two-way damping valve. When the pressure within one chamber of cylinder is higher than the pressure setting of damping valve, the damping valve relieves the load and the oil refilling valve on the other chamber of the cylinder refills the oil to prevent the formation of cavitation in the system. 1-Valve seat 2-Steel ball 3-Spring

L

R

T

P

4-Valve core

1

2

3

4

RELIABILITY IN ACTION 67

SECTION 2 STEERING HYDRAULIC SYSTEM

3. Priority valve ①FUCTION The load sensor steering system is made up of the priority valve and the BZZ5 steering gear. When you turning the steering wheel, the priority valve can ensure the oil flow demanded by the steering gear and the rest will flow to the working hydraulic system.

RELIABILITY IN ACTION 68

SECTION 2 STEERING HYDRAULIC SYSTEM

②COMPOSITION The priority valve is mainly composed of valve body, valve core, a spring, the relief valve assembly and parts such as screw. Upper hydraulic principle diagram for it. 1- The group of relief valve 2-Spring 3-Valve core 4-Valve body 5-Plug screw

RELIABILITY IN ACTION 69

SECTION 2 STEERING HYDRAULIC SYSTEM ③OIL PORT ARRANGEMENT Priority valve has five oil port, Oil inlet P, oil outlet EF、CF, feedback oil port LS and oil return port T, respectively. The five oil ports respectively connect with the oil outlet port of steering gear, the inlet port of hydraulic oil radiator, the inlet port of steering gear, the LS port of steering gear and oil return port of hydraulic oil tank.
1.P port for oil inlet, connected with steering pump
2.T port for oil drain, connect with hydraulic tank
3.EF port connected with the working equipment(or connected with the hydraulic oil tank)
4.CF port connected with the inlet port of steering gear
5.LS port connected with the LS orifice of the steering gear.

P

CF

EF

LS

T

RELIABILITY IN ACTION 70

SECTION 2 STEERING HYDRAULIC SYSTEM

④working principle a. When the steering wheel is stationary, the pressure oil from the steering pump flows to port EF from port P through valve core and enters into the working device hydraulic system or returns to oil tank directly.

RELIABILITY IN ACTION 71

SECTION 2 STEERING HYDRAULIC SYSTEM b. When the steering wheel is rotating, under the joint action of the spring force and LS pressure, the valve core moves rightward to connect the port P with port CF so that the pressure oil enters into the steering gear and drives the cylinder for steering of loader and the excessive oil diverges into the working device hydraulic system or oil tank through port EF.

RELIABILITY IN ACTION 72

Section III Cause Judgment and Troubleshooting for Common Malfunctions of Steering Hydraulic System ① Heavy steering (analysis and judgment procedure) Cause s y m p to m

J u d g e a s p e r m a lf u n c ti o n

Yes

Cylinder wriggle, air bubbles in oil, and regular sound

Check steering column for flexible rotation No

Is the steering heavy at high speed and light at low speed?

air from system Air content in system Bleed and check oil inlet port of oil pump for air leakage

Yes

Heavy steering and no action of steering cylinder

Measure

Yes No Does system pressure meet requirement?

No

Yes

Check feedback oil pipe for blockage

Check steel ball for presence and blockage
Replace FK combination valve
Check cylinder for internal leakage




Failure of manual steering check valve
Leakage of FK overload valve
Internal leakage of cylinder




Damage of steering column

Repair or replace

Blockage of pipeline

Clean or replace

No

Yes

Adjust the system pressure. Is there any pressure change?

Yes

Low system pressure Repair or replace

No Breakage of priority Adjustment system pressure valve spring or blockage of valve core

Yes

Yes

Is hydraulic oil level too low?

Shortage of oil

Add hydraulic oil

Blockage of pipeline

Clean or replace

Wear or internal leakage of steering pump

Repair or replace

No Yes

Is oil suction pipe blocked?

No

RELIABILITY IN ACTION

② No steering end or failure for steering to limit position (analysis and judgment procedure)

No end After the steering cylinder rotates to limit position, when the steering wheel is rotated with high force, the steering wheel can rotate lightly, namely there is no feeling of end.

Malfunction cause Low pressure of overload valve

Troubleshooting Appropriately increase pressure of overload valve

failure for steering to limit position

The steering cylinder can’t be rotated to the limit position, with heavy steering

Malfunction cause

Troubleshooting

Low pressure of safety valve

Appropriately increase pressure of safety valve

RELIABILITY IN ACTION

SECTION 2 STEERING HYDRAULIC SYSTEM ③ CAUSE FOR OFF-TRACKING OF MACHINE a. Due to leakage towards the cylinder port when the closed core steering gear is at neutral position, the slight off-tracking of the closed core steering gear is normal. b. Check the cylinder connecting rod for presence of looseness. c. Internal leakage of cylinder. d. High pressure difference between two tires. e. Unilateral leakage of two-way overload valve or two-way oil refilling valve. f. Air content in oil.

RELIABILITY IN ACTION

SECTION 2 STEERING HYDRAULIC SYSTEM

④ INCORRECT STEERING a. Air content in system. b. Loose cylinder pin. c. Stagnation of priority valve or shunt valve core. d. Internal leakage of cylinder. e. Low pump efficiency, leading to unstable pressure.

L

R

T

P

p Fixed Pump

Engine

Filter Reservoir

RELIABILITY IN ACTION

SECTION 2 STEERING HYDRAULIC SYSTEM ⑤No rotation or slow rotation of steering wheels, though the steering wheel can be rotated flexibly. a. Serious internal leakage of twoway overload valve . b. Serious leakage of cylinder piston.

RELIABILITY IN ACTION 77

SECTION 2 STEERING HYDRAULIC SYSTEM

⑥ IDLE TRAVEL OF STEERING WHEEL a. Worn or damaged connection between steering column and steering gear. b. Loose nuts of steering wheel c. Air content in oil. d. Internal leakage of two-way overload valve e. Internal leakage of steering cylinder

L

R

T

P

p Fixed Pump

Engine

Filter Reservoir

RELIABILITY IN ACTION 78

SECTION 2 STEERING HYDRAULIC SYSTEM ⑦ VIBRATION OR AUTOGIRATION OF STEERING WHEEL a. Incorrect assembly relationship. At the time of reassembly after the disassembly for repair, it’s required to align the spline teeth of the linkage shaft shifting pin slot with the corresponding inner spline teeth of rotor. b. When the oil pumped is connected to the port R or L, the steering gear will rotate on its axis like a motor.

RELIABILITY IN ACTION 79

SECTION 2 STEERING HYDRAULIC SYSTEM

⑧ Bounce of steering wheel. Cause: The check valve at oil inlet port of steering gear is damaged. Function of check valve: It prevents the backflow of oil in steering cylinder under the action of external force when the pressure is higher than the pressure at oil inlet port. If the check valve is damaged, the backflow of oil will lead to bounce symptom of steering wheel.

RELIABILITY IN ACTION 80

•THE END

RELIABILITY IN ACTION 81