SERV2631- Steering & Brakes - Text - 992G.pdf

Service TrainingSERV2631-01March 2002

TEXT REFERENCE

PART OF TECHNICAL INSTRUCTION MODULE SERV2631-01

992G WHEEL LOADER

STEERING AND BRAKE SYSTEMS

SERV2631-01 - 2 -3/02

TABLE OF CONTENTS

INTRODUCTION ..................................................................................................................3

STEERING SYSTEM ............................................................................................................4Steering System Block Diagram.......................................................................................5Steering Pilot System Components...................................................................................7Main Steering System Components................................................................................12Main System Pump Schematics......................................................................................16Steering System Schematics ...........................................................................................19

SECONDARY STEERING SYSTEM .................................................................................24Secondary Steering System Schematic...........................................................................25

BRAKE SYSTEM ................................................................................................................27Brake System Components ............................................................................................28Brake Pump Schematics ................................................................................................37Brake System Schematics ...............................................................................................42

AXLE COOLING SYSTEMS..............................................................................................45

CONCLUSION.....................................................................................................................48

VISUAL LIST ......................................................................................................................49

SERV2631-01 - 3 -3/02

Objective 1Visuals 1 - 19


• Introduction

Distribute Lab A and BWorksheets

1

INTRODUCTION

This presentation discusses the major features, component locations, andsystems operation for the steering and brake systems on the 992G WheelLoader.

When discussing systems operation, both sectional view and graphicsymbol schematics are provided. Systems are explained by tracing oilflow from the tank, through the system and back to the tank.

The color codes for hydraulic oil used throughout this presentation are asfollows:

Red - System or high pressure

Red and White Stripes - Reduced pressures

Orange - Pilot pressure

Green - Tank or return oil

Blue - Blocked oil

992G WHEEL LOADER992G WHEEL LOADERSTEERING AND BRAKE SYSTEMSSTEERING AND BRAKE SYSTEMS

© 2002 Caterpillar Inc.© 2002 Caterpillar Inc.

2

SERV2631-01 - 4 -3/02

Identify components:

- Pilot system

- Main system

- Common to bothsystems

STEERING PILOTCONTROL VALVE

NEUTRALIZERVALVES

STEERINGCYLINDERS

DUAL PILOTCHECK VALVE

STEERINGPUMP

PILOT RELIEFVALVE

PILOTPUMP

STEERINGCONTROL

VALVE

PILOTFILTER

STEERING AND BRAKE TANK

PILOT SYSTEM MAIN STEERING SYSTEM COMMON TO MAIN AND PILOT SYSTEMS

STEERING SYSTEMCOMPONENT IDENTIFICATION

STEERING SYSTEM

Shown are the components of the steering system on the 992G WheelLoader. Components of the pilot system are shown in orange,components of the main steering system are shown in red, andcomponents that are common to the the pilot system and main steeringsystem are shown in green.

3

SERV2631-01 - 5 -3/02

Explain schematic

STEERING HYDRAULIC SYSTEMHOLD

STEERING PUMPCOOLER

STEERINGCYLINDERS

NEUTRALIZERVALVE

NEUTRALIZERVALVEFILTER

PILOTRELIEFVALVE

STEERLEVER

TANK

PILOTPUMP

STEERINGCONTROL

VALVE

QUADCHECKVALVE

CHECKVALVE

CHECKVALVE

PILOT CONTROLVALVE

Steering System Block Diagram

This block diagram of the 992G steering system shows the componentsand oil flow.

The steering pilot circuit consists of the pilot pump, the pilot filter, thepilot relief valve, two check valves, the steer lever, the pilot control valve,two neutralizer valves, and the quad check valve. When the steer lever ismoved to the left or right, pilot oil is sent through the respectiveneutralizer valve to the steering control valve. The pilot oil moves thesteering control valve and allows steering pump oil to flow to the steeringcylinders.

The main steering circuit consists of the steering pump, the steeringcontrol valve, and the steering cylinders. The steering control valvehouses the back-up relief valve, the pressure reducing valve, the valvespool, the cylinder crossover relief valves, the makeup ball check valves,and the shuttle valve.

SERV2631-01 - 6 -3/02

The steering pump draws oil (green) from the tank and sends flow (red) tothe steering control valve. The steering control valve directs oil to thesteering cylinders. The steering control valve also supplies oil (orange) tothe steering pilot circuit if a shortage of pilot system oil occurs. Thesteering cylinders extend and retract to steer the machine.

SERV2631-01 - 7 -3/02


1. Steering pilotpump

2. Main steeringpump

3. Steering pilotsystem pressuretap

4. Implement fixeddisplacementpump

4

Steering Pilot System Components

The steering pilot pump (1) is located on the rear of the main steeringpump (2). The steering pilot pump pulls oil from the tank and sends pilotoil to the pilot filter.

Also shown are the steering pilot system pressure tap (3) and theimplement fixed displacement pump (4).

1

2

4

3

SERV2631-01 - 8 -3/02


1. Pilot filter

2. Left frame

3. Pilot relief valve

4. Pressure tap

5. Steering controlvalve

5

The steering system pilot filter (1) is located on the inside of the leftframe (2) below the deck access panel (behind the operator's station).From the filter, pilot oil flows to the pilot relief valve (3) and to the pilotcontrol valve (not shown).

The pilot relief valve maintains the pilot pressure at 2400 ± 100 kPa (350 ± 15 psi) with the engine at LOW IDLE. Pilot pressure is tested atthe pressure tap (4).

Also shown is the steering control valve (5).

2

3

4

5

1

SERV2631-01 - 9 -3/02


1. Control lever

2. Transmissiondirection control

3. Up and down shiftswitches

4. Pilot control valvecover

5. STIC lock lever

6

The control lever (1) is located at the left front of the operator's seat. Thecontrol lever is part of the Steering/Transmission Integrated Control(STIC). Other components of the STIC are the transmission directionalcontrol (2) and the transmission upshift and downshift switches (3).

The pilot control valve is located below the control lever and behind thecover (4). When the control lever is moved left or right, the control levermoves the respective valve spool in the pilot control valve. The pilot oilflows past the pilot control valve spools to the steering control valve.

Also visible is the STIC lock lever (5).

1

2

3

45

SERV2631-01 - 10 -3/02


1. Neutralizer valves

2. Adjustable strikers

3. Parking brake valve

7

The two neutralizer valves (1) are mounted on the rear frame. Eachneutralizer valve has an adjustable striker (2) mounted on the front frame.When the machine makes a maximum turn (either left or right), therespective striker will contact the neutralizer valve at the end of the turn.The striker moves the neutralizer valve and blocks oil flow to thedirectional control valve. With the oil flow stopped, the directionalcontrol valve centering spring centers the valve and stops the machinefrom turning.

Also shown is the parking brake valve (3).

1

2

3

SERV2631-01 - 11 -3/02

• Quad check valve(arrow)

8

The quad check valve (arrow) is located on the rear frame to the rear ofthe left neutralizer valve. The check valves in the steering pilot lines havedifferent cracking pressures. The supply side to the control valve opens at15 psi while the return side opens at 145 psi. The supply side checkvalves prevent pilot oil from flowing backwards when the STIC is movedto the hold position. This ensures the main control valve spool shifts tothe hold position in a smooth, relatively quick, predictable manner.

If the operator wants the machine to behave more aggressively, he canmove the STIC from steering in one direction, through the hold position,and steer in the opposite direction. The 145 psi check valve on the returnside will then open and the main control valve spool shifts moreaggressively.

The orifice to tank from the pilot lines should handle the flow of oilexiting the pilot cavity on the control valve during normal spoolmovement. If the flow is greater than can pass through the orifice duringrapid spool movement and the pressure increases to greater than 145 psi,the check valve will open and allow the pilot oil to return to tank throughthe pilot control valve.

SERV2631-01 - 12 -3/02


1. Main steering pump

2. Steering pilot pump

3. Steering pilot systempressure tap

4. Implement fixeddisplacement pump

9

Main Steering System Components

The main steering pump (1) is located on the left front of the pump drive.The variable displacement piston pump senses both pressure and flowrequirements for the system. The steering system pump draws oil fromthe hydraulic tank and sends flow to the steering control valve.

Also shown are the steering pilot pump (2), the steering pilot systempressure tap (3), and the implement fixed displacement pump (4).

1

2

3

4

SERV2631-01 - 13 -3/02

Case drain filter (arrow)

10

The case drain filter (arrow) for the main steering pump is located in thepump bay, just above and forward of the main steering pump. The filterremoves contaminants from the case drain oil before the oil enters thesteering and brake tank.

SERV2631-01 - 14 -3/02


1. Steering controlvalve

2. Selector valve

3. Crossover relief valve

4. Secondary steeringrelief valve

11

The steering control valve (1) is located behind the cab on the inside ofthe right frame below the deck access panel. The control valve sendssystem oil to the steering cylinders and sends signal oil to the marginspool spring chamber in the compensator valve group on the pump.

Also shown are the selector valve (2), the crossover relief valve (3), andthe secondary steering relief valve (4).

1 2

3

4

SERV2631-01 - 15 -3/02


1. Right steeringcylinder

2. Steering cylinderhead end

3. Steering cylinderrod end

12

Shown is the right steering cylinder (1). The steering cylinder head end(2) is pinned to the front frame and the rod end (3) is pinned to the rearframe.

123

13

SERV2631-01 - 16 -3/02

• Objective 2• Visuals 13 - 19

Explain schematic

SWASHPLATE

LARGEACTUATOR

PISTON

STEERING PUMP ANDCOMPENSATOR VALVE

ENGINE OFF

FROM STEERINGCONTROL VALVE

FLOWCOMPENSATOR

SPOOL

PRESSURECOMPENSATOR

SPOOLCOMPENSATOR

VALVE

SMALLACTUATOR PISTON

MINIMUM ANGLESTOPMAXIMUM ANGLE

STOP

CHARGE PUMP

Main System Pump Schematics

Shown is a sectional view of the steering pump and compensator valvegroup.

The pump has two control pistons which work together to continuallyadjust the angle of the swashplate. The small actuator piston, assisted bythe small actuator spring, is used to upstroke the pump. The large actuatorpiston is used to destroke the pump.

The compensator valve group consists of a flow compensator (margin)spool and a pressure compensator (cutoff) spool. The valve group keepsthe pump flow and pressure at a level needed to fulfill the demands of thesteering system.

SERV2631-01 - 17 -3/02

The margin compensator spring maintains the pump supply pressure a t2100 ± 105 kPa (305 ± 15 psi) above the signal pressure. The pressurecompensator spring limits the system pressure to 31000 kPa (4500 psi).

When the engine is OFF, the small actuator spring moves the swashplateto maximum angle.

14

SERV2631-01 - 18 -3/02

Objective 7Visual 14

Explain schematic

SWASHPLATE

LARGEACTUATOR

PISTON

STEERING PUMP ANDCOMPENSATOR VALVE

LOW PRESSURE STANDBY

FROM STEERINGCONTROL VALVE

FLOWCOMPENSATOR

SPOOLPRESSURE

COMPENSATORSPOOL

COMPENSATORVALVE

SMALLACTUATOR PISTON

MINIMUM ANGLESTOPMAXIMUM ANGLE

STOP

CHARGE PUMP

At machine start-up, the small actuator spring holds the swashplate atmaximum angle. When the steering control valve is in the HOLDposition, pump flow is blocked at the steering control valve and no signalpressure is generated. As the pump produces flow, the system pressurebegins to increase. This pressure is felt at the lower end of both the flowcompensator spool and the pressure compensator spool. The flowcompensator spool moves up against the low spring force and permitssystem oil to go to the large actuator piston. The oil pressure at the largeactuator piston overcomes the combined force of the small actuator springand system oil pressure at the small actuator piston.

The large actuator piston moves the swashplate (destroke) to the LOWPRESSURE STANDBY position. In LOW PRESSURE STANDBY, thepump produces enough flow to compensate for system leakage atsufficient pressure to provide instantaneous response when the steeringcontrol valve is moved.

15

SERV2631-01 - 19 -3/02

Explain schematic

HOLD

COMPENSATORVALVE

FLUIDSAMPLING

PORT

PILOTRELIEFVALVE

COOLER FILTER

PILOTPUMPTANK

STEERINGCYLINDERS

STEERLEVER

RIGHTNEUTRALIZER

SELECTORVALVE

STEERING SYSTEM

LEFTNEUTRALIZER

QUADCHECKVALVE

STEERING CONTROL VALVE

SHUTTLEVALVE

LINERELIEFVALVE BACK-UP

RELIEFVALVE

PRESSUREREDUCING

VALVE

CONTROLSPOOL

SECONDARYPUMP

STEERINGPUMP

DIVERTERVALVE

SECONDARYRELIEFVALVE

PILOTCONTROL

VALVE BYPASSSWITCH BYPASS

SWITCH

Steering System Schematics

When the engine is running and the steering system is in HOLD, pilotpump oil is blocked at the pilot control valve. Oil from the main steeringpump flows to the steering control valve. The steering control valve spoolblocks oil flow to the steering cylinders and no signal pressure is generated.

System pressure is sensed at the pump margin spool and pressurecompensator spool, and at the small actuator piston (rod end symbol). Assystem pressure increases, the margin spool moves against the spring forceand opens a passage for pump oil to flow to the large actuator piston (head end symbol).

SERV2631-01 - 20 -3/02

The pressure in the large actuator piston overcomes the combined force ofthe actuator spring and the pressure in the small piston and moves theswashplate to the LOW PRESSURE STANDBY position.

In LOW PRESSURE STANDBY, the pump produces enough flow tocompensate for system leakage and sufficient pressure to provide forinstantaneous response when the steering control valve is moved.

INSTRUCTOR NOTE: System pressure specifications are found inthe 992G Wheel Loader Steering Testing and Adjusting Module(Form SENR1373).

16

SERV2631-01 - 21 -3/02

Explain schematic

GRADUAL RIGHT TURN

FLUIDSAMPLING

PORT

PILOTRELIEFVALVE

COOLER FILTER

PILOTPUMPTANK

STEERINGCYLINDERS

STEERLEVER

RIGHTNEUTRALIZER

SELECTORVALVE

STEERING SYSTEM

LEFTNEUTRALIZER

QUADCHECKVALVE


SHUTTLEVALVE


RELIEFVALVE

PRESSUREREDUCING

VALVE

CONTROLSPOOL

SECONDARYPUMP

DIVERTERVALVE


PILOTCONTROL


SWITCH

COMPENSATORVALVE

STEERINGPUMP

When the operator moves the steering lever to the right, pilot oil flowsthrough the pilot control valve and the right neutralizer valve to the leftend of the steering control spool. Pilot oil pressure moves the steeringcontrol spool to the right. System oil from the steering pump flowsthrough the check valve and the control spool to the steering cylinders.As pressure increases in the steering cylinders, the pressure (signalpressure) is sensed in the flow compensator valve spring chamber at thepump. The signal pressure combines with the force of the flowcompensator spool spring and moves the flow compensator spool down.The flow compensator spool restricts the flow of oil to the large actuatorpiston (head end). The spring and pressure in the small actuator pistonovercome the pressure in the large piston to move the swashplate towardmaximum angle. The increase in swashplate angle increases pump oilflow.

SERV2631-01 - 22 -3/02

The increase in oil flow through the control spool increases the systempressure. The system pressure is sensed at the flow compensator spool.The increased pressure moves the flow compensator spool against thecombined forces of the spring and signal pressure and sends oil to thelarge actuator piston. The actuator piston moves the swashplate to areduced angle that produces flow relative to the position of the controlspool.

17

SERV2631-01 - 23 -3/02

Explain schematic

SHARP RIGHT TURN

FLUIDSAMPLING

PORT

PILOTRELIEFVALVE

COOLER FILTER

PILOTPUMPTANK

STEERINGCYLINDERS

STEERLEVER

RIGHTNEUTRALIZER

SELECTORVALVE

STEERING SYSTEM

LEFTNEUTRALIZER

QUADCHECKVALVE


SHUTTLEVALVE


RELIEFVALVE

PRESSUREREDUCING

VALVE

CONTROLSPOOL

SECONDARYPUMP

DIVERTERVALVE


PILOTCONTROL


SWITCH

COMPENSATORVALVE

STEERINGPUMP

When making a SHARP RIGHT TURN, the right striker (not shown)contacts the right neutralizer valve. Oil flow from the pilot control valveto the steering control valve is blocked by the movement of the neutralizervalve. The steering control spool returns to the center position. Flow tothe steering cylinders is blocked and the machine stops turning. Thesteering pump returns to the LOW PRESSURE STANDBY condition.

The neutralizer valves prevent the machine front frame from contactingthe machine rear frame when making a SHARP RIGHT or SHARP LEFTturn.

INSTRUCTOR NOTE: Check the 992G Wheel Loader SteeringSystem Testing and Adjusting Module (Form SENR1373) for thecorrect adjustment procedures.

SERV2631-01 - 24 -3/02


1. Secondary pump

2. Transmissionoutput transfer gear

3. Supply hose

4. Case drain hose

18

SECONDARY STEERING SYSTEM

The secondary pump (1) is splined to the transmission output transfer gear(2). The pump is ground driven and turns when the machine is moving.

When the machine is moving, the pump pulls oil from the tank through asuction hose (not shown). Pump oil flows through the supply hose (3) tothe diverter valve (not shown) in the steering control valve.

When the engine is running, the diverter valve sends the secondary pumpoil to the tank. When the engine is not running, the diverter valve sendssecondary pump oil to the steering control valve.

Also shown is the case drain hose (4).

1

2 3

4

19

SERV2631-01 - 25 -3/02

Explain schematic

GRADUAL RIGHT TURN

FLUIDSAMPLING

PORT

PILOTRELIEFVALVE

COOLER FILTER

PILOTPUMPTANK

STEERINGCYLINDERS

STEERLEVER

RIGHTNEUTRALIZER

SELECTORVALVE

SECONDARY STEERING SYSTEM

LEFTNEUTRALIZER

QUADCHECKVALVE


SHUTTLEVALVE


RELIEFVALVE

PRESSUREREDUCING

VALVE

CONTROLSPOOL

SECONDARYPUMP

DIVERTERVALVE


PILOTCONTROL


SWITCH

COMPENSATORVALVE

STEERINGPUMP

Secondary Steering System Schematic

Shown is a schematic of the steering system with the engine off and themachine moving.

When the engine is off, no oil flows from the pilot pump and the steeringpump. The steering system oil comes from the secondary steering system.The secondary steering system consists of the secondary pump, secondaryrelief valve, diverter valve, and check valve.

When the machine is moving, the output transfer gear turns the secondarypump. Secondary pump oil flows through the diverter valve and thecheck valve to the steering control valve.

SERV2631-01 - 26 -3/02

Secondary pump oil also flows through the pressure reducing valve andthe check valve to the pilot control valve.

When the operator moves the steer lever, secondary pump oil is used tosteer the machine.

INSTRUCTOR NOTE: At this time, introduce and perform "Lab A:Steering System Component Identification Quiz" and "Lab B:Steering System Tests" individually. These labs will reinforce thematerial discussed in the preceding presentation.

20

SERV2631-01 - 27 -3/02


Distribute Lab CWorksheet

PARKING BRAKELEVER

FRONT SERVICEBRAKES

SERVICE BRAKEPEDALS

BRAKEPUMP

MANIFOLDPARKINGBRAKE

BRAKEACCUMULATORS

PARKING BRAKEVALVE

REAR SERVICEBRAKES

SERVICE BRAKEVALVE

STEERING AND BRAKETANK

SERVICE BRAKECOMPONENTS

PARKING BRAKECOMPONENTS

SERVICE AND PARKING SYSTEMSSHARED COMPONENTS

BRAKE SYSTEM COMPONENTS

BRAKE SYSTEM

Shown are the components of the brake system on the 992G WheelLoader. Components of the service brake system are shown in orange,components of the parking brake system are shown in brown, andcomponents that are common to the service brake system and the parkingbrake system are shown in red.

21

SERV2631-01 - 28 -3/02

Identify componentsand explain function:

- Brake pump

- Check valves

- Brake accumulators

- Manifold

- Tandem brake valve

- Service brake

- Parking brake valve

- Parking brake

- Pressure switch

BRAKEPUMP

CHECKVALVE

MANIFOLD

REAR BRAKEACCUMULATOR

FRONT BRAKEACCUMULATOR

MANIFOLD

TANDEMBRAKEVALVE

FRONTBRAKE

REARBRAKEPARKING

BRAKE

PARKINGBRAKEVALVE

CHECKVALVE

TANK

BRAKE HYDRAULIC SYSTEM

PRESSURESWITCH

Brake System Components

Shown is a simplified schematic of the brake system. The brake systemcomponent functions are:

Brake pump: Pulls oil from the tank and sends supply oil to theaccumulator charging valves.

Check valves: Allows oil flow in one direction.

Brake accumulators: When the engine is running, the front and rearbrake accumulators supply oil within a controlled pressure range to thetandem brake valve and to the parking brake valve. If the engine stopsrunning, the accumulators provide an emergency oil supply to providebraking.

SERV2631-01 - 29 -3/02

Manifold: Serves as a junction block for oil lines, pressure taps, sensorsand other monitoring components.

Tandem brake valve: Controls brake oil flow and pressure to the frontand rear service brakes.

Service brakes: Slow or stop the machine during operation.

Parking brake valve: Controls oil pressure to the parking brake.

Parking brake: Prevents the machine from moving when parked.

Pressure switch: Sends a signal to VIMS when the case drain filterbegins to bypass.

SERV2631-01 - 30 -3/02


1. Brake pump

2. Pump drive

3. Pressurecompensator valve

4. Pump supply port

22

The brake pump (1) is located on the left rear of the pump drive (2).The brake pump supplies oil flow to charge the two accumulators.

Also shown are the pressure compensator valve (3) and the pump supply port (4).

1

2

3

4

SERV2631-01 - 31 -8/97

Case drain filter (arrow)

23

The case drain filter (arrow) for the brake pump is located on the left sideof the pump bay, just above the pump drive. The filter removescontaminants from the case drain oil before the oil enters the steering andbrake tank.

SERV2631-01 - 32 -3/02


1. Accumulators

2. Deck frame

3. Deck plate

24

The accumulators (1) are bolted to the deck frame (2) below the deckplate (3) on the left side of the cab.

The accumulators supply oil at a constant pressure range to the brakesystem. The accumulators also supply pressure oil to the brake system tostop the machine if the engine stops running while the machine is moving.

1

2

3

SERV2631-01 - 33 -3/02


1. Manifold

2. Service brake valve

3. Parking brake valve

25

Oil from the rear brake accumulator flows through the manifold (1) to thetop section of the service brake valve (2) and to the parking brake valve(3). Oil from the front brake accumulator flows through the manifold tothe bottom section of the service brake valve.

When the parking brake valve is in the BRAKE RELEASED position,accumulator oil pressure is sent through the parking brake valve to thebrake chamber. Oil pressure compresses the spring in the brake chamberand releases the parking brake. When the parking brake valve is movedto the BRAKE ENGAGED position, accumulator oil pressure is blockedat the parking brake valve. Oil in the parking brake chamber flowsthrough the parking brake valve to drain. The spring in the brakechamber engages the brake.

1

2

3

SERV2631-01 - 34 -3/02


1. Oil activated discbrake

2. Service brakesupply line

3. Brake cooling oilline

4. Final drive housing

26

The oil activated disc brake (1) is the same at all four wheels.

Also shown are the service brake supply line (2), the brake cooling supplyline (3), and the final drive housing (4).

1

2

3

4

SERV2631-01 - 35 -3/02

• Parking brake controllever (arrow)

27

The parking brake control lever (arrow) controls the valve spool in theparking brake valve.

SERV2631-01 - 36 -3/02


1. Parking brake

2. Secondary steeringpump

28

The parking brake (1) is located on the output transfer gear case above thesecondary steering pump (2). The parking brake is spring engaged andhydraulically released.

INSTRUCTOR NOTE: At this time, introduce and perform "Lab C:Brake System Component Identification Quiz." This lab willreinforce the material discussed in the preceding presentation.

1

2

29

SERV2631-01 - 37 -3/02

Distribute Lab DWorksheet

Identify components andexplain function:

- Pressurecompensator valve

- Control piston

- Spring

- Swashplate

- Piston assembly

- Barrel

- Drive shaft

PRESSURECOMPENSATOR VALVE

CONTROL PISTON

CONTROL PISTONPASSAGE

SPRING

SWASHPLATEPISTON ASSEMBLY

DRIVE SHAFT

BARREL

OUTLET PASSAGE

INLET PASSAGE

BRAKE PUMPLOW PRESSURE

Brake Pump Schematics

Shown in this illustration are the main components of the brake pump.The components are:

- Pressure compensator valve

- Control piston

- Spring

- Swashplate

- Piston assembly

- Barrel

- Drive shaft

SERV2631-01 - 38 -3/02

When pressure in the brake system is less than 16000 ± 345 kPa (2300 ± 50 psi), the spring keeps the swashplate at maximum angle. Thepump piston stroke is longest and pump displacement is maximum. Asmall amount of pressure oil from the outlet passage flows to the pressurecompensator. A spool in the pressure compensator blocks the flow of oilto the control piston passage.

30

SERV2631-01 - 39 -3/02

Identify componentsand explain function:

- Adjustment bolt

- Locknut

- Spring

- Spool

ADJUSTMENTBOLT

LOCKNUT PRESSURECOMPENSATOR SPOOL

FROM PUMPOUTLET

TO CONTROLPISTON

ADJUSTMENTBOLT

LOCKNUT PRESSURECOMPENSATOR SPOOL

FROM PUMPOUTLET

TO CONTROLPISTON

LOW BRAKE SYSTEM PRESSURE HIGH BRAKE SYSTEM PRESSURE

PUMP COMPENSATOR VALVE

SPRINGSPRING

This illustration shows the main components and the operation of thepressure compensator valve. The components are:

- Adjustment bolt

- Locknut

- Spring

- Pressure compensator pool

SERV2631-01 - 40 -3/02

The left illustration shows the operation of the pressure compensator valvewhen the brake system pressure is less than 16000 ± 345 kPa (2300 ± 50psi). Pump output oil flows around the right land of the pressurecompensator spool and into the chamber at the right end of the spool.

When the brake system pressure increases to 16000 ± 345 kPa(2300 ± 50 psi), the pressure of the oil in the chamber is high enough tomove the spool against the spring. Movement of the spool permits oil toflow past the spool to the control piston in the pump.

INSTRUCTOR NOTE: Check the 992G Wheel Loader Brake SystemTesting and Adjusting Module (Form SENR1353) for the correctpressure compensator spool adjustment procedure.

31

SERV2631-01 - 41 -3/02

Explain schematic

PRESSURE COMPENSATOR VALVE

CONTROL PISTON

CONTROL PISTONPASSAGE

SPRING

SWASHPLATEPISTON ASSEMBLY

DRIVE SHAFT

BARREL

OUTLET PASSAGE

INLET PASSAGE

BRAKE PUMPHIGH PRESSURE

When the brake system pressure reaches 16000 ± 345 kPa (2300 ± 50 psi),oil from the pressure compensator valve fills the chamber in the controlpiston. As the brake system pressure increases above 16000 ± 345 kPa(2300 ± 50 psi), the oil pressure from the pressure compensator valvemoves the control piston against the control spring. This movementdecreases the angle of the swashplate, the stroke of the pistons, and thedisplacement of the pump. The amount of oil per pump revolution isdecreased to the amount needed to maintain the system pressure at 16000 ±345 kPa (2300 ± 50 psi).

32

SERV2631-01 - 42 -3/02


Explain schematic

MANIFOLD



MANIFOLD

TANDEMBRAKEVALVE

FRONTBRAKE

REARBRAKE

PARKINGBRAKE

PARKINGBRAKEVALVE

CHECKVALVE

PUMP

CHECKVALVE

TANK

BRAKE HYDRAULIC SYSTEMSERVICE BRAKES OFF / PARKING BRAKE OFF

PRESSURESWITCH

Brake System Schematics

The brake pump draws oil from the tank and directs the flow through thecheck valves to the accumulators. When the engine is running, the brakepump maintains the oil pressure in the front and rear brake accumulatorsat 16000 ± 345 kPa (2300 ± 50 psi).

The tandem brake valve controls brake oil flow and pressure to the frontand rear service brakes. When the brakes are released, oil from theaccumulators is blocked at the tandem brake valve. Oil in the servicebrakes drains through the tandem brake valve to the tank.

33

SERV2631-01 - 43 -3/02

Explain schematic

MANIFOLD



MANIFOLD

TANDEMBRAKEVALVE

FRONTBRAKE

REARBRAKEPARKING

BRAKE

PARKINGBRAKEVALVE

CHECKVALVE

PUMP

CHECKVALVE

TANK

BRAKE HYDRAULIC SYSTEMSERVICE BRAKE ON / PARKING BRAKE OFF

When the operator moves the parking brake control knob to the BRAKEOFF position, a cable moves the spool in the parking brake valve.Pressure oil from the rear brake accumulator flows around the valve spooland RELEASES the parking brake.

When the operator depresses either brake pedal, the tandem brake valvespools move down against the spring at the bottom of the brake valve.The tandem brake valve spools close the passages to the tank and open thepassages to the front and rear brake accumulators. The rear brakeaccumulator oil flows through the spool for the rear brakes in the tandembrake valve and ENGAGES the rear brakes. The rear brake accumulatoroil also fills the chamber between the tandem brake valve spools. Thefront brake accumulator oil flows through the spool for the front brakes inthe tandem brake valve and ENGAGES the front brakes.

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The front brake accumulator oil also fills the chamber at the bottom of thevalve spool for the front brakes in the tandem brake valve.

The combined force of the oil pressure and the spring at the bottom of thevalve spool for the front brakes moves the valve spool for the front brakesup against the oil pressure in the chamber between the front and rearbrake spools in the tandem brake valve. This movement increases the oilpressure in the chamber. The increased oil pressure moves the valvespool for the rear brakes up against the force on the brake pedal.

The force at the bottom of each brake spool in the tandem brake valvebalances the brake spool against the force at the top of the brake spool.Each brake spool acts as a pressure reducing valve to limit the pressure inthe brakes proportionally to the force at the top of the respective brakespool.

When the brake pedal is RELEASED, the oil passages from theaccumulators to the brakes are closed and the passages from the brakes tothe tank are opened. Oil in the rear and front service brakes flowsthrough the tandem brake valve to the tank. The force at the bottom ofeach brake spool in the tandem brake valve moves the respective brakespool up.

INSTRUCTOR NOTE: At this time, introduce and perform "Lab D:Brake System Tests." This lab will reinforce the material discussed inthe proceeding presentation.

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Objective 5Visual 34

Explain schematic

FRONTAXLE

REARAXLE

REARPUMP

FRONTPUMP

RELIEFVALVE

RELIEFVALVE

COOLER

COOLER

AXLEMANIFOLD

AXLEMANIFOLD

BRAKEBRAKE

BRAKE BRAKE

AXLE COOLING SYSTEMS

AXLE COOLING SYSTEMS

Shown is a schematic of the front and rear axle cooling systems. Eachaxle cooling system cools the oil in the respective axle. The operation ofboth systems is the same.

When the operator applies the service brakes, friction generates heat in thebrake plates and discs. Also, heavy loads generate heat in the differential.The front and rear pumps pull the high temperature oil from thedifferential cases and send the flow to the oil cooler. The oil coolerremoves the heat and returns the oil to the axle manifold. The cooler oilflows from the manifold and enters the axles at each brake chamber and atthe differential housing.

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When the brakes are ENGAGED or RELEASED, the cooler oil passesbetween the brake discs and plates. The cooler oil flows through groovesin the face of the brake discs and absorbs the heat from the brake discsand plates. The high temperature oil collects in the bottom of the brakechamber and returns internally to the bottom of the differential housing.

In the differential, the cooler oil flows over the gears and removes theheat. The high temperature oil flows to the bottom of the differentialhousing and joins with the oil from the brakes. The oil is again pulledinto the pump and the cycle continues.

When the oil is cold, the high resistance to flow through the cooler causesthe pump pressure to increase. When the pump pressure increases to345 kPa (50 psi), the relief valve opens allowing cold oil to flow to thepump inlet.

INSTRUCTOR NOTE: At this time, introduce and perform "Lab E:Axle Cooling System Component Identification Quiz." This lab willreinforce the material discussed in the preceding presentation.

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1. Rear axle coolingpump

2. Front axle coolingpump

3. Relief valve for therear axle coolingpump

4. Relief valve for thefront axle coolingpump

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The front and rear axle cooling pumps (1) and (2) are located on the frontaccessory drive on the front of the engine. The axle cooling pumps drawoil from the differential cases and pump the oil through the axle oil cooler.The oil then returns to the brake and differential housings.

The relief valves (3) and (4) are located on the axle cooling pumps. Therelief valves prevent cold oil from flowing to the axle oil cooler and limitthe maximum pressure in the axle cooling system.

12

4 3

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CONCLUSION

This concludes this presentation on the 992G Wheel Loader Steering andBrake Systems.

Always check the Service Manual for the latest service information andspecifications when servicing, testing and adjusting, and/or makingrepairs.

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VISUAL LIST

1. Title 2. Steering component location graphic3. Steering system block diagram - HOLD4. Steering pilot pump5. Steering pilot filter6. Steering transmission integrated control

(STIC)7. Neutralizer valves8. Dual check valve9. Main steering pump

10. Case drain filter - main steering pump11. Steering control valve12. Steering cylinder13. Main steering pump schematic -

ENGINE OFF14. Main steering pump schematic - LOW

PRESSURE STANDBY15. Steering system schematic - HOLD16. Steering system schematic - GRADUAL

RIGHT TURN17. Steering system schematic - SHARP

RIGHT TURN18. Secondary steering pump

19. Secondary steering system schematic -GRADUAL RIGHT TURN

20. Brake component location graphic21. Brake hydraulic system schematic22. Brake pump23. Case drain filter - brake pump24. Brake accumulators25. Service brake valve26. Service brake27. Parking brake control lever28. Parking brake29. Brake pump graphic - LOW PRESSURE30. Brake pump servo valve31. Brake pump graphic - HIGH

PRESSURE32. Brake hydraulic schematic - SERVICE

BRAKE OFF, PARKING BRAKE ON33. Brake hydraulic schematic - SERVICE

BRAKE ON, PARKING BRAKE OFF34. Axle cooling system schematic35. Axle cooling pumps36. Model view

SERV2631- Steering & Brakes - Text - 992G.pdf

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Transcript of SERV2631- Steering & Brakes - Text - 992G.pdf