05 steering hydraulic lg958l

LG958L Training Material Chapter VII Steering Hydraulic System

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Tuesday, May 19, 2015

Chapter VII Steering Hydraulic System

Overview of Steering System

CONTENTS

Load-Sensing Fully Hydraulic Steering System

Working Principle for Hydraulic Units of Steering System


Cause Analysis and Troubleshooting for Common Malfunctions of Steering System

I. Classification of steering systemThe steering system of the wheel loader is functioned to control the travelingdirection of the loader. It can stably maintain the linear traveling of theloader and can flexibly change the traveling direction as required.By steering mode, the wheel loader can be classified into deflection wheelsteering mode, skid steering mode, and articulated steering mode.For the loader with articulated steering mode, the working device is installedon the front frame. When the frame is swinging, the direction of the working

Section I Overview of Steering Hydraulic System


on the front frame. When the frame is swinging, the direction of the workingdevice is always consistent with the direction of the front frame, which canenable the working device to rapidly aim at the working face during theoperations, in order to reduce the travel and time of working cycles andimprove the working efficiency of the loader. Therefore, the articulatedsteering mode has become the mostly applied steering mode in modernloaders.

II. Requirements of steering systemThe good steering performance is one important factor to guarantee thetraveling safety of the loader, relieve the labor strength of driver, and improvethe working efficiency. The basic requirements on the steering system includelight and flexible operations, stable and reliable working, and good durability.The steering oil line requires relatively stable supply of oil flow. However, thesteering system commonly adopts fixed displacement pump. As the flow offixed displacement pump varies along with the speed, the flow of the steeringoil line is reduced when the engine runs at low speed, which will causedelayed response of steering speed and easily lead to accidents. If the highflow pump is adopted, when the engine runs at high speed, the excessive oil


flow pump is adopted, when the engine runs at high speed, the excessive oilflow will be relieved through the relief valve, which is not economical due tohigh power loss and easy heating of oil.A relatively appropriate method is to choose auxiliary pump and flow change-over valve. The pressure oil of the auxiliary pump varies along with the enginespeed under the control of the flow change-over valve to fully or partially flowinto the steering oil line, in order to guarantee the flow of steering oil line. Theremaining oil flows into the working oil line.

III. Types of steering systemThere are many types of steering system and the different types ofsteering system represent the development levels of different hydraulictechnologies respectively.At present, the types of the steering system applied in the wheel loaderare as below: Fully hydraulic steering system composed of single stabilizer valve

and open center no-reaction steering gear. Load-sensing fully hydraulic steering system composed of priority


Load-sensing fully hydraulic steering system composed of priorityvalve and loading sensing steering gear.

Load-sensing fully hydraulic steering system composed of priorityvalve and coaxial flow amplifying steering gear.

Flow amplifying steering system.

The steering system of LG958L loader is a load-sensing fully hydraulic steering systemcomposed of priority valve and coaxial flow amplifying steering gear.This system is mainly composed of steering pump, coaxial flow amplifying fully hydraulicsteering gear, steering cylinder, hydraulic oil tank, and pipelines and accessories, as shownin Figure 6-1.This steering system features the following characteristics:1. Compact structure and small size for constituent units.2. Automatic lubrication and long service life for all units.

Section II Load-Sensing Fully Hydraulic Steering System


3. Reliable steering and light and flexible operations.4. It can realize the converging with the hydraulic system of working device to reduce the

power loss and improve the system efficiency.At the time of steering, the system supplies oil preferentially to the steering hydraulicsystem and the remaining oil converges with the return oil of steering system to return tooil tank through radiator.The safety valve is set on the priority valve, with the regulated system pressure at 16MPa.


7-1 Schematic diagram of LG958L steering system 1. Priority valve 2. Steering pump 3. Oil suction filter element 4. Steering cylinder 5. Coaxial flow amplifying steering gear 6. Radiator 7. Oil return filter element

8. Hydraulic oil tank

Steering Gear BZZ6-800

I. Steering gear 1. Meaning of steering gear model

Section III Structure and Working Principle of Steering System Hydraulic Units

Model of steering gear Type of steering gear


Displacement: 800ml/r

gearBZZ1 Open center no-reaction fully hydraulic steering gear BZZ2 Open center reaction fully hydraulic steering gear BZZ3 Closed center no-reaction fully hydraulic steering gear BZZ4 Multi-functional no-reaction fully hydraulic steering gear BZZ5 Load-sensing fully hydraulic steering gearBZZ6 Load-sensing flow amplifying fully hydraulic steering gear

2. Types of fully hydraulic steering gear:

1) The BZZ fully hydraulic steering gear is mainly classified into:

Open center no-reaction type (BZZ1); Open center reaction type (BZZ2); Closed center no-reaction type (BZZ3);Load-sensing type (BZZ5);Coaxial flow amplifying type (BZZ6);


Type symbols of BZZ

Coaxial flow amplifying type (BZZ6);and other types. The functionality symbols are shown in the left figure:

2) Corresponding types of steering gears for specific loader models

NO. Applied model Name of steering gear Type of steering gear

1 952/952H/952L Steering gear BZZ1-800Open center no-reaction fullyhydraulic steering gear

2 968/969 Steering gear BZZ3-100Closed center no-reaction fullyhydraulic steering gear

3 916/918 Steering gear BZZ5-320Dynamic signal load-sensing fullyhydraulic steering gear

Steering gear Static signal load-sensing fully


4 936/938 Steering gear BZZ5-500Static signal load-sensing fullyhydraulic steering gear

5 946L Steering gear BZZ6-500Static load-sensing flow amplifyingfully hydraulic steering gear

6 933L Steering gear BZZ6-630Static load-sensing flow amplifyingfully hydraulic steering gear

7 953/956/958 Steering gear BZZ6-800Static signal flow amplifying fullyhydraulic steering gear

3. Structural composition of steering gearAs shown in Figure 5-2 (a) and (b), the structure of the BZZ6 fully hydraulic steeringgear is mainly composed of valve body, valve core, valve sleeve, linkage shaft,locating spring, shifting pin, rotor, stator, and rear cap.There are total 5 oil ports on the steering gear, namely oil inlet port P, oil return portT, left steering oil outlet port L, right steering oil outlet port R, and feedback oil portLS, which are connected with the oil outlet port CF of priority vale, oil return port ofhydraulic oil tank, rodless chamber of right steering cylinder, rodless chamber of leftsteering cylinder, and port Ls of priority valve respectively.

T: Oil returnP: Oil inlet


P: Oil inletR: Right steering L: Left steering TLPR

LS

Location of steering gear oil ports


Figure 7-2 Structure of fully hydraulic steering gear (a)

1 Connecting block 2 Front cap 3 Valve body 4 Spring plate 5 Shifting pin 6 Valve sleeve 7 Valve core

8 Linkage shaft 9 Rotor 10 Rear cap 11 Limit post 12 Spacer disc 13 Stator 14 O-ring 15 Steel ball 16 O-ring

17- X-shaped ring 18 O-ring

Spacer disc

Limit block

Oil inlet portRight steering

LeftsteeringOil return portValve body

Linkage shaft

Valve core


Figure 7-2 Structure of fully hydraulic steering gear (b)

Rear capBearing

Return spring

Valve sleeve

Shifting pin

Check valve

Stator-rotor pair

The valve body, valve core, and valve sleeve of the steering gear constitutethe follow-up swing valve of steering gear to control the oil flow direction.The valve core is within the inner chamber of valve sleeve. It is connecteddirectly with steering column via connecting block and can be driven forrotation by the steering wheel via steering column. The valve sleeve iswithin the inner chamber of valve body and is driven by the rotor vialinkage shaft and shifting pin for movement within valve body.The rotor and stator constitute the metering motor at the lower portion ofthe steering gear. The stator has 7 teeth and the rotor has 6 teeth. Thestator is fixed and the rotor rotates around the center of stator in radius of


stator is fixed and the rotor rotates around the center of stator in radius ofeccentric distance. The tooth profile for metering motor is equilong arcepicycloidal tooth, which can ensure that each point on the rotor curve canbecome the engagement point. During the rotation, seven oil orifices areconnected with the oil orifices on the valve sleeve at all times so that thevalve sleeve and valve core distribute the flow to the metering motor,namely enable the pressure to enter half of tooth chamber and drain oilfrom the other half of tooth chamber, in order to convey the pressure oil tothe steering cylinder.

The metering motor composed of rotor and stator is also referred to ascycloid gear engagement pair. During the power steering, it functions asthe metering motor to ensure that the flow into the steering cylinder isproportional to the rotation angle of the steering wheel. During themanual steering, its equivalent to a manual oil pump.The linkage shaft and shifting pin connect the rotor with valve sleeve.During the power steering, they ensure the synchronization betweenvalve sleeve and rotor (for feedback function). During manual steering,they are functioned for conveyance of torque.The spring plate is functioned to ensure the neutral position of follow-up


The spring plate is functioned to ensure the neutral position of follow-upvalve for centering purpose. Therefore, the spring plate is referred to ascentering spring.The check valve is installed between oil inlet port and oil outlet port.During the manual steering, the check valve opens so that the oil in onechamber of the steering cylinder is sucked into the oil inlet port throughoil return port and then is compressed into the other chamber of steeringcylinder by cycloid gear engagement pair, namely its functioned for oilsuction during the manual steering.

4. Working principle of steering gearThe schematic diagram for the working principle of BZZ6 fully hydraulic steering gear is shown inFigure 5-3 (a), (b), and (c). The ports A and B are connected with two chambers of steering cylinderrespectively, the port P is connected with oil outlet port of steering pump, and the port T isconnected with hydraulic oil tank.


Figure 7-3 Structure of fully hydraulic steering gear (a) 1 Connecting block 2 Front cap 3 Valve body 4 Spring plate 5 Shifting pin 6 Valve sleeve 7 Valve core 8 Linkage shaft

9 Rotor 10 Rear cap 11 Limit post 12 Spacer disc 13 Stator 14 O-ring 15 Steel ball 16 O-ring 17 X-shaped ring

18 O-ring


Figure 7-3 (b) Schematic diagram of fully hydraulic steering gear (at neutral position)


Figure 7-3 (c) Schematic diagram of fully hydraulic steering gear (at steering position)

1. No rotation of steering wheel (namely neutral position)The valve core 1 and valve sleeve 3 are at neutral positions under the action of thecentering spring plate. The hydraulic oil from the steering pump 7 enters into the valvecore through the valve core and the orifice at the end of valve sleeve and then returnsto oil tank 8 via port T. When the steering wheel is not rotated, the centering spring isfunctioning. As the oil ports A and B are blocked by the valve core 1, the oil in thesteering cylinder is restrained against input and output and the piston cant move sothat the loader is traveling towards the preset direction.

2. Leftward rotation of steering wheelThe steering wheel drives the valve core 1 for counterclockwise rotation so that thecentering spring is unilaterally compressed. As there is an up to 1030 rotationamount between the valve core 1 and valve sleeve 3, the valve core can rotate with


amount between the valve core 1 and valve sleeve 3, the valve core can rotate withrespect to the valve sleeve. In such case, the oil groove of the valve core is connectedwith oil inlet port P of valve sleeve so that the oil from the oil pump flows, through oilgrooves of valve sleeve 3 and valve core 1 and the valve sleeve, to the rotor 5 andstator 4 to drive the rotation of rotor 5 with respect to the starter 4. At the same time,the outlet oil from the rotor and stator enters into the rodless chamber of rightsteering cylinder through the valve sleeve and port A so that the cylinder piston rod isextended to drive the wheels via frame for leftward steering. The oil in the rodchamber of the cylinder flows into the valve sleeve 3 via oil port B and flows back tothe hydraulic oil tank through oil return groove of valve core 1, oil return port of valvesleeve, and the port T.

During the leftward steering, the rod chamber of left steering cylinder is connected with therodless chamber of right steering cylinder so that the rod chamber is supplied with oil to retractthe piston rod into the cylinder and drive the leftward steering of the frame. The rodlesschamber of left steering cylinder is connected with rod chamber of right steering cylinder sothat the oil from the rod chamber of right steering cylinder enters into port B via rodlesschamber of left steering cylinder and returns to the oil tank through oil return port and oil inletport of valve sleeve.When the relative rotation angle between valve core and valve sleeve is approximate 1.5, the oilline is connected so that the rotation of rotor drives the oil from steering pump to the cylinder,of which the oil supply amount is proportional to the rotation angle of the steering wheel.When the steering wheel is rotated for a certain angle and held, as the above-mentioned oil lineis connected, the oil from the steering pump drives the rightward rotation of rotor 5. When therotation angle of rotor 5 is same with the rotation angle of the steering wheel, as the valvesleeve 3 and rotor 5 are mechanically connected by linkage shaft, the rotor drives the leftward


sleeve 3 and rotor 5 are mechanically connected by linkage shaft, the rotor drives the leftwardfollow-up rotation of valve sleeve 3, till the stator spring drives the valve sleeve and valvecore to neutral position and the angle rotated is same with the rotation angle of steering wheel.In such case, the relative rotation angle between the valve sleeve 3 and valve core 1 iseliminated and the oil line to the rotor and steering cylinder is closed so that the outlet oil fromthe steering pump 7 enters into the valve sleeve via port P and returns to oil tank through oilreturn groove of valve core 1, oil return port of valve sleeve 3, and oil port T. In such case, theloader stops the steering. When the steering wheel is rotated further, the rotor and valve sleevewill follow up, till the left limit position is reached. This is the hydraulic feedback follow-upfunction.

3. Rightward steeringWhen rotating the steering wheel rightward, the steering wheel drives the valve core for clockwiserotation, of which the working principle is same with the leftward steering.For the steering at low speed (the rotation speed of the steering wheel is less than 10r/min), theeffective displacement of the steering gear is same with the metered displacement. When theinput speed of the steering wheel is increased (the rotation speed of the steering wheel is10~40r/min), the effective displacement is proportional to the rotation speed of the steeringwheel. In such case, only a part of oil from the oil inlet port P enters into the rotor-stator pair formetering and the remaining oil enters into the cylinder directly via ports A and B. Therefore, theflow amplifying function is available only at this stage. When the input speed of the steering wheelexceeds 40r/min, the effective displacement of steering gear is basically constant at its ratedequivalent displacement.4. Manual steering


4. Manual steeringIn event of sudden flameout of the engine or the malfunction of steering pump, rotate thesteering wheel with hand for static steering. While rotating the steering wheel rightward, the valvecore rotates for a 1030 angle to drive the rotation of valve sleeve, linkage shaft, and rotor viashifting pin. In such case, the rotor and stator are functioned as oil pump. The rotation of rotor 5sucks out the oil from the oil port T and inputs the oil into the oil inlet chamber of rotor pump viacheck valve, valve sleeve, and valve core. The oil pumping action during the manual steeringcompresses the hydraulic oil sucked into the steering oil so that the compressed oil enters into therodless chamber of steering cylinder to extend the piston rod and steer rightward the loader. Theoil in the rod chamber flows to the oil inlet chamber of rotor pump from oil port A through valvesleeve, valve core, valve sleeve, and check valve and continually refills into the rodless chamber, inorder to maintain the steering action.

5. Combination valve blockThe valve block is a combined hydraulic unit that is mainly composed of check valve,two-way damping valve (overload valve), and oil refilling valve. Its connectedbetween the steering pump and steering gear and is assorted to the fully hydraulicsteering gear (Generally, its directly installed on the valve body flange of thesteering gear and works along with steering gear to form an integral unit).Its functioned to guarantee the normal and stable working of steering gear andentire steering system under rated pressure on one hand and guard the steeringcylinder and connecting pipelines against damage in event of sudden overload andprotect the steering pump on the other hand.


1) Check ValveThe check valve is composed of the valve seat 1, valve core 2, and spring 3 and isinstalled within the oil inlet port of the valve block body so that the high pressure oilfrom the oil pump flows into the oil inlet port of steering gear via the check valve. It'sfunctioned to prevent the backflow of oil from automatically deflecting the steeringwheel and leading to steering failure.2) Two-way damping valveThe two-way damping valve incorporates two constant pressure direct-acting safetyvalves composed of the spring, ball valve seat, and steel ball. Its installed within thevalve port for connecting the valve body with the orifices of left and right chambers ofsteering cylinder and is connected with the oil return port, in order to protect thehydraulic steering system against impact of over-high pressure and ensure the safety of


hydraulic steering system against impact of over-high pressure and ensure the safety ofoil lines.3) Oil refilling valveThe oil refilling valve incorporates two check valves composed of steel ball and isinstalled within the valve port for connecting the valve body with the orifices of left andright 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 settingof damping valve, the damping valve relieves the load and the oil refilling valve on theother chamber of the cylinder refills the oil to prevent the formation of cavitation in thesystem.

The priority valve is mainly composed of the valve body, valve core, spring, safety valve assembly, and screw plug, as shown in Figure 7-4.

II. Priority valve


Figure 7-4 Structural diagram of priority valve 1 Safety valve assembly 2 Control spring 3 Valve core 4 Valve body 5 Screw plug

II. Priority valve There are 5 oil ports on the priority valve, namely oil inlet port P, oil outlet ports EF andCF, feedback oil port LS, and oil return port T, which are connected with oil outlet port ofsteering pump, oil inlet port of hydraulic oil radiator, oil inlet port of steering gear, portLS of steering gear, and oil return port of hydraulic tank respectively.This valve is matched with the BZZ6 steering gear to form the load-sensing steeringsystem. At the speed variation of the steering wheel, it can preferentially guarantee theflow required by the steering gear and the remaining oil flows into the working devicehydraulic system.


Port EF

Port P


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

Port CF

TPort EF

Port P


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

LsPort CF

1) Heavy steering (analysis and judgment procedure)

Section IV Cause Judgment and Troubleshooting for Common Malfunctions of Steering Hydraulic System

Cause Measure

Air content in system

Failure of manual

steering check valve

Leakage of FK

overload valve

Internal leakage of

cylinder

Damage of steering

column

Blockage of pipeline

Bleed air from system

and check oil inlet port

of oil pump for air

leakage

Check steel ball for

presence and

blockage

Replace FK

combination valve

Check cylinder for

internal leakage

Repair or replace

Clean or replace

J

u

d

g

e

a

s

p

e

r

m

a

l

f

u

n

c

t

i

o

n

s

y

m

p

t

o

m

Cylinder creepage, air bubbles in oil, and regular

sound

Heavy steering and no action of

steering cylinder

Check steering

column for flexible

rotation

Does system pressure

meet requirement?

Check feedback oil

pipe for blockage

Yes

Yes

Yes

Yes

No

No

No



Low system pressure

Breakage of priority

valve spring or

blockage of valve core

Shortage of oil


Wear or internal

leakage of steering

pump

Clean or replace

Repair or replace

Adjustment system

pressure

Add hydraulic oil

Clean or replace

Repair or replace

Is the steering heavy

at high speed and

light at low speed?

Is hydraulic oil level

too low?

meet requirement?

pipe for blockage

Adjust the system pressure.

Is there any pressure

change?

Is oil suction pipe

blocked?

Yes

Yes

Yes

Yes

No

No

No

No

No

Yes

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 thereis no feeling of end.

Malfunction cause Troubleshooting

Appropriately increase pressure of overload valve

Low pressure of overload valve


failure for steering to limit position

The steering cylinder cant be rotated to the limit position, with heavy steering

Malfunction cause Troubleshooting

Low pressure of safety valve

Appropriately increase pressure of safety valve

Due to leakage towards the cylinder port when the closedcore steering gear is at neutral position, the slight off-tracking of the closed core steering gear is normal.

Check the cylinder connecting rod for presence oflooseness.

Internal leakage of cylinder. High pressure difference between two tires. Unilateral leakage of two-way overload valve or two-wayoil refilling valve.

3) Cause for off-tracking of machine


oil refilling valve. Air content in oil.

Air content in system. Loose cylinder pin. Stagnation of priority valve or shunt

valve core. Internal leakage of cylinder. Low pump efficiency, leading to

unstable pressure. L R

4) Incorrect steering


unstable pressure.

Engine

Fixed

Pump

Reservoir

Filter

L

T

R

P

p

Serious internal leakage of two-way overload valve

5) No rotation or slow rotation of steering wheels, though the steering wheel can be rotated flexibly.


Serious leakage of cylinder piston.

Loose nuts of steering wheel.

Worn or damaged connection between steering column and steering gear.

L

T

R

PP

6) Idle travel of steering wheel


Air content in oil. Internal leakage of two-way overload valve

Internal leakage of steering cylinder

Engine

Fixed

Pump

Reservoir

Filter

p

Engine

Fixed

Pump

Incorrect assembly relationship. At thetime of reassembly after the disassemblyfor repair, its required to align thespline teeth of the linkage shaft shiftingpin slot with the corresponding inner

7) Vibration or autogiration of steering wheel


spline teeth of rotor.

When the oil pumped is connected tothe port R or L, the steering gear willrotate on its axis like a motor.

Cause:The check valve at oil inlet port ofsteering gear is damaged.Function of check valve:It prevents the backflow of oil in steeringcylinder under the action of externalforce when the pressure is higher thanthe pressure at oil inlet port. If the checkvalve is damaged, the backflow of oil will

8) Bounce of steering wheel


the pressure at oil inlet port. If the checkvalve is damaged, the backflow of oil willlead to bounce symptom of steeringwheel.

THE ENDTHE ENDTHE ENDTHE END


05 steering hydraulic lg958l

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