Principle of Power Steering

Basic PrincipleBasic Principle

Hydraulic Power Steering

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Content

> FUNCTIONING OF STEERING> RACK & PINION TYPE STEERING> POWER STEERING

- WORKING PRINCIPLE - LINKAGES - PINION VALVE ASSEMBLY

> ON LINE TESTING> KEY POINTS FOR POWER STEERING> WORKING OF PUMP

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OVERALL VIEW OF STEERING SYSTEM

Rack & Pinion Type Steering system

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HOW STEERING SYSTEM WORKS.

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Turning the Car> For a car to turn smoothly, each wheel must follow a different

circle. Since the inside wheel is following a circle with a smaller radius, it is actually making a tighter turn than the outside wheel. If you draw a line perpendicular to each wheel, the lines will intersect at the center point of the turn. The geometry of the steering linkage makes the inside wheel turn more than the outside wheel.

> .

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DIFFERENT TYPES OF STEERING SYSTEMSDIFFERENT TYPES OF STEERING SYSTEMS

• Rack and Pinion Type• Re-circulating Ball Screw Type

MANUAL STEERING POWER ASSISTED

Hydraulic Power Electrically Assisted

Column Type EPS (CEPS)

Pinion Type EPS (PEPS)

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The pinion gear is attached to the steering shaft. When you turn the steering wheel, the gear spins, moving the rack. The tie rod at each end of the rack connects to the steering arm on the spindle (see diagram below).

WHAT IS RACK &PINION STEERING SYSTEM & HOW IT WORKS

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The rack-and-pinion gear set does two things: •It converts the rotational motion of the steering wheel into the linear motion needed to turn the wheels. •It provides a gear reduction, making it easier to turn the wheels.

On most cars, it takes three to four complete revolutions of thesteering wheel to make the wheels turn from lock to lock (from far left to far right).

RACK & PINION TYPE STEERING (RPS)

Some cars have variable-ratio steering, which uses a rack-and-pinion gear-set that has a different tooth pitch (number of teeth per inch) in the center than it has on the outside. This makes the car respond quickly when starting a turn (the rack is near the center), and also reduces effort near the wheel's turning limits.

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CONSTRUCTIONAL DETAIL OF POWER CONSTRUCTIONAL DETAIL OF POWER STEERINGSTEERING

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Power SteeringThere are a couple of key components in power steering in addition to the rack-and-pinion or re-circulating-ball mechanism.

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In

Out

Left Turn

Right Turn

Right Turn Oil Flow

Left TurnOil Flow

Power Steering System (Rack & Pinion type)

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WORKING PRINCIPLE•The engine driven hydraulic pump supplies oil at a pressure to a control valve (Pinion Valve Assembly) situated in housing that supports the pinion shaft.•The movement of steering wheel imparts movement to the control valve through the steering column. •The torsion bar moves the control valve, which in turn directs the oil to one side of the piston inside the steering housing( Cylinder Tube), depending on the direction of rotation of steering wheel. •The control valve is a rotary spool type valve controlled by the torsion bar impregnated in the steering case.

•The spool valve is a shaft with six flutes and a sleeve, which has six internal axial grooves. Radial ports in the sleeve and shaft pass the oil from the supply to the lines connected to the piston chamber/rack housing.

•The amount of twist of the torsion bar an the movement of the spool valve is proportional to the effort applied by the driver on steering wheel in any direction. •Initial power steering assist is obtained at about 0.5 degree deflection of the torsion bar and the assist rises progressively as the bar deflection rises to about 4 degrees. •The maximum deflection allowed in any direction is limited to 7 degree with the help of a series of the splines between the shaft and the sleeve.

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POWER STEERING WORKING PRINCIPLE (Continue)

Part of the rack contains a cylinder with a piston in the middle. The piston is connected to the rack. There are two fluid ports, one on either side of the piston. Supplying higher-pressure fluid to one side of the piston forces the piston to move, which in turn moves the rack, providing the power assist. We'll check out the components that provide the high-pressure fluid, as well as decide which side of the rack to supply it to, later in the article. First, let's take a look at another type of steering.

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Power Rack-and-pinion

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Power steering gear and linkage

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The power steering system consists of:

- Rack and pinion steering gear box

- Power steering oil pump

- Oil reservoir

- Tubes

The power steering system uses a hydraulic pressure which is generated by the power steering pump to reduce the effort required to turn the steering wheel. The power steering pump is mounted on the front of the engine. The pump has a vane-type design, and is driven by the crankshaft through a drive belt.

The power steering fluid is drawn into the pump from the reservoir when the engine is running. The fluid is pressurized by a power steering switch and a control valve located in the power steering pump.

System construction

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Steering gear box assembly

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1. Tie rod end2. Lock nut3. Boot clip4. Boot5. Boot wire6. Inner ball joint7. Rack bushing8. O-ring9. Seal10. Lock nut11. Yoke plug12. Spring13. Support yoke

14. Snap ring15. Seal16. Bearing17. Pinion shaft assembly18. Bolt19. Valve housing20. O-ring21. Steering gear housing22. Mounting rubber23. Mounting bracket24. Oil pipe25. Rack26. Inner guide

Steering gear box assembly

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Construction of the power steering gear box

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Operation of the pinion and valve assembly (at the center position)

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Operation of the pinion and valve assembly (at the center position)

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Operation of the pinion and valve assembly (turning left)

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Operation of the pinion and valve assembly (turning left)

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Operation of the pinion and valve assembly (turning right)

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Operation of the pinion and valve assembly (turning right)

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Rotary Valve

A power steering system should assist the driver only when he isexerting force on the steering wheel (such as when starting a turn). When the driver is not exerting force (such as when driving in a straight line), the system shouldn't provide any assist. The device that senses the force on the steering wheel is called therotary valve. The key to the rotary valve is a torsion bar. The torsion bar is a thin rod of metal that twists when torque is applied to it. The top of the bar is connected to the steering wheel, and the bottom ofthe bar is connected to the pinion or worm gear (which turns thewheels), so the amount of torque in the torsion bar is equal to the amount of torque the driver is using to turn the wheels. The more torque the driver uses to turn the wheels, the more the bar twists.

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The input from the steering shaft forms the inner part of a spool-valve assembly. It also connects to the top end of the torsion bar. The bottom of the torsion bar connects to the outer part of the spool valve. The torsion bar also turns the output of the steering gear, connecting to either the pinion gear or the worm gear depending on which type of steering the car has.

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CONTINUE

As the bar twists, it rotates the inside of the spool valve relative to the outside. Since the inner part of the spool valve is also connected to the steering shaft (and therefore to the steering wheel), the amount of rotation between the inner and outer parts of the spool valve depends on how much torque the driver applies to the steering wheel. When the steering wheel is not being turned, both hydraulic lines provide the same amount of pressure to the steering gear. But if the spool valve is turned one way or the other, ports open up to provide high-pressure fluid to the appropriate line.

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PART II

IMPORTANT ON LINE TESTS

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1. AIR LEAK TEST

• Low Pressure Test ( 1 bar)

• High Pressure Test ( 7 bar)

Purpose :

• To check external leakage from

- Tube fittings

- Feed tubes

- Oil seals

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2. High Pressure Endurance Test

• Test Pressure 120 bar (Hydraulic)

• Cyclic test

Purpose :

• To check internal leakage from

- Piston ring

- PV Assembly seals

Note : In case of external leakage ,oil leakage can be checked visually from outside portion of cylinder tube and Pinion Housing

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IMPORTANT ON LINE TESTS3. Rotational Torque

• Test done in Power On condition

• Hydraulic oil flow 6 LPM

Purpose :

• To check that the assembly meet the specified rotational torque criteria

Typically

- Maximum torque

- Torque variationNote : Rotational torque value is adjusted by adjusting rack twist (yoke clearance amount)

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IMPORTANT ON LINE TESTS4. Reverse sliding force



Purpose :

• To check that the assembly meet the specified sliding force criterion

Typically

- Maximum sliding force

- Sliding force variationNote : This is important to get good returnability of steering

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5. Performance Characteristics



Purpose :

• This is the test of valve characteristics in term of Hydraulic pressure vs torque characteristics

• Evaluation is done at low pressure and high pressure

Note : This characteristics determines the operation of power steering function

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5. Hydraulic Nose Test


• Test is performed along with Performance Characteristic test

Purpose :

To detect noise due to oil flow in the hydraulic power steering system

KEY TO QUALITY OF KEY TO QUALITY OF POWER STEERINGPOWER STEERING

PARTPART--IIIIII

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IMPORTANT FACTORS

• CLEANINESS -THE ASSEMBLY ROOM- WASHING LIQUID- PARTS

- Rack Bar- PV Assembly- Pinion Housing- Valve Housing- Oil seals- Bearing- Feed tube

- ASSEMBLY FIXTURES & TOOLS

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IMPORTANT FACTORS

• CLEANINESS -THE ASSEMBLY ROOM- WASHING LIQUID- PARTS

- Rack Bar- PV Assembly- Pinion Housing- Valve Housing- Oil seals- Bearing- Feed tube

- ASSEMBLY FIXTURES & TOOLS- PART HANDLING BINS

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IMPORTANT FACTORS

• NO DENTS & SCRATCHE-Rack Bar- Cylinder Tube- Oil seals- PV Assembly- Valve housing

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DAILY CHECK ITEMS

- Cleanliness- Contamination- Machine check sheet- Pokayoke check sheet- Torque wrench calibration- Rocket condition- Fixtures and locators condition- Leak master check- Torque and performance heck master- Daily analysis of rework / rejection

POWER STEERING POWER STEERING PUMPPUMP

PARTPART--IVIV

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PUMPThe hydraulic power for the steering is provided by a rotary-vane pump(see diagram below). This pump is driven by the car's engine via a belt and pulley. It contains a set of retractable vanes that spin inside an oval chamber.

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1. Pulley2. Shaft3. Pressure switch4. P/S pump body5. Flow control

valve ass’y (built in relief valve)

6. Cam ring7. Rotor8. Vane9. Cover

Oil pump construction

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Oil pump construction

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Intake side

Vane

Rotor

Cam ring

DischargeDischarge

DischargeDischarge

SuctionSuction

SuctionSuction

Rotating

direction

Operation of the oil pump

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As the discharge rate of the power steering pump increases in proportioning to the pump revolution speed, a flow control valve is added to control it so that the optimum amount of fluid for steering operation is supplied according to the engine speed (driving condition).

Described below is its operation at different engine speeds.

Flow control valve

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Flow control valve (when idling)

1. To gear case2. From pump3. To pump4. Rod5. Orifice A1

The fluid discharged from the pump is supplied through the clearance around the rod in orifice A1 to the gear box.

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As the engine speed rises, the pump discharge rate increases and causes a pressure difference to occur between both ends of the orifice (P1 – P2). Thus the pressure exceeding the flow control spring force pushes the flow valve to the right in figure, making the opening in the orifice narrower through which only a necessary amount of fluid is fed to the gear box and the excess fluid is returned to the pump.

1. Flow control valve2. Flow control spring

Flow control valve (when running at Low Speed)

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The relief valve located in the flow control valve controls the maximum hydraulic pressure.

The steel ball in the relief valve is under the hydraulic pressure in the circuit coming through orifice A2. When the steering wheel is turned and the hydraulic pressure increases higher than 75-82kg/cm2 (1060-1160 psi), it compresses the relief spring to push the steel ball which then allows the fluid to flow to the power steering pump.

1. Orifice A2

2. Relief valve spring

3. Steel ball4. Chamber A5. Chamber B

Relief Valve

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As the engine speed rises higher, opening in the orifice is madenarrower and fluid flow to the gear box reduces. As a result, hydraulic pressure application is slow at the start of the steering wheel turn. This provides straight-ahead stability to suit the driving condition with the steering wheel operated near its neutral position.

Flow control valve (when running at High Speed)

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Such relief valve operation causes a pressure difference to occur between chamber A and B.

Then the flow valve moves to the right to make opening in orifice A1, maintaining the hydraulic pressure constant.

Relief Valve

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CONTINUE

As the vanes spin, they pull hydraulic fluid from the return line at low pressure and force it into the outlet at high pressure. The amount of flow provided by the pump depends on the car's engine speed. The pump must be designed to provide adequate flow when the engine is idling. As a result, the pump moves much more fluid than necessary when the engine is running at faster speeds. The pump contains a pressure-relief valve to make sure that the pressure does not get too high, especially at high engine speedswhen so much fluid is being pumped.

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BLEEDINGBLEEDINGBleeding is required when we are doing fluid Bleeding is required when we are doing fluid replacementreplacement1. Jack up the Front Wheels and Support them by using a

rigid rack.

2. Fill the Oil reservoir with the specified fluid(Mitsubishi ATF-11).up to the lower Position of the filter.

3. Manually turn the Oil pump pulley a few minute.

4. Turn the Steering wheel all the way to the left &to the right five or six times.

5. Disconnect the High tension Cable .

6. While operating the starting motor intermittently,turn the steering wheel all the way to the left &right five or six times.(15to20seconds).

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7. Connect the high 7. Connect the high --tension cable.tension cable.

8.Turn the steering wheel to the left and right until there are 8.Turn the steering wheel to the left and right until there are no no air bubbles in the Oil reservoir.air bubbles in the Oil reservoir.

9.Confirm that the fluid is not milky,and then level is up to th9.Confirm that the fluid is not milky,and then level is up to the e specified position on the level gauge.specified position on the level gauge.

10. Confirm that there is very little change in the fluid level 10. Confirm that there is very little change in the fluid level when the steering wheel is turned left&right. when the steering wheel is turned left&right.

Fluid level changeFluid level changewithin 5mmwithin 5mm

11. Check whether or not the 11. Check whether or not the change in the fluid level is change in the fluid level is within 5mm when the engine is within 5mm when the engine is stopped &when it is running.if stopped &when it is running.if the change of the fluid level is the change of the fluid level is 5mm or more,the air has not 5mm or more,the air has not been completely bleed from the been completely bleed from the system,and thus must be bleed system,and thus must be bleed completely.completely.

While engine

running

While engine

stopped

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1. During Air bleeding,replenish the fluid supply so that the level never fails below the lower position of the filter.

2. If Air bleeding is done while engine is running,the air will be broken up and absorbed into the fluid;be sure to do the bleeding only while cranking.

3.If the fluid level rises suddenly after the engine is stopped the air has not been completely bled.

4. If the bleeding is not complete, there will be abnormal noises from the pump and the flow control valve, and this condition could cause a lessening of the life of the pump.

The following points should be taken care:The following points should be taken care:--

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•Steering Gear Type Rack &Pinion, End take off, Integral Power Assisted.

•Rack Travel (Steering Gear) LH-75mm & RH-75mm,Maximum

•Overall Steering Ratio 20:1

•Total Turns Available on 3.75

Input shaft of the steering gear

•No. of steering Wheel rotation 3.6

(lock to lock)

•Torque required on input shaft 1.5 Nm

to move the rack (preload)

•Normal operating Pressure 85 bar

•Steering wheel diameter 395mm/365mm

•Power steering pump Sliding vane type-Positive displacement.

Specifications

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•Pump make Koyo

•Direction of pump rotation Clockwise when viewed from shaft end

•Pump flow 8.5LPM @ 1000rpm

•Pump-Pressure Relief 75 kg/cm² or 75 bar

•Pump Drive Gear driven

•Wide Operating speed-pump 600 rpm-6500 rpm

•Wide Operating Temp.Pump -40° C to +120° C

Gear.

•Flow control cum pressure relief In built system

Valve in pump 7.5 +0.5/-0Mpa

8.5 ± 0.7 lit/min @ 1500 rpm

•Oil Capacity of reservoir 0.8 approx

•Oil capacity of system 0.8 approx

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OIL PUMP PRESSURE TESTOIL PUMP PRESSURE TEST

1.Disconnect the pressure hose 1.Disconnect the pressure hose from the Oil pump,and then from the Oil pump,and then connect the special tools.connect the special tools.

2. Bleed the air,and then turn the 2. Bleed the air,and then turn the steering Wheel several times while steering Wheel several times while the vehicle is not moving so that the vehicle is not moving so that the Temp. of the fluid rises to the Temp. of the fluid rises to Approx. 50Approx. 50--6060ººC.C.

3.Start the engine and idle at 3.Start the engine and idle at 1,0001,000±±100r/min.100r/min.

4.Fully close the shut off valve of 4.Fully close the shut off valve of the pressure gauge and measure the pressure gauge and measure the oil pump relief pressure to the oil pump relief pressure to confirm that it is within the confirm that it is within the standard value rangestandard value range.(Standard .(Standard valuevalue--8.8mpa).8.8mpa).

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5. If it is not within the standard value,replace the Oil pump.5. If it is not within the standard value,replace the Oil pump.

6. Check whether or not the hydraulic pressure is the standard v6. Check whether or not the hydraulic pressure is the standard value alue when nowhen no-- load condition are created by fully opening the shutload condition are created by fully opening the shut--off valve off valve of the pressure gaugeof the pressure gauge.(Standard value.(Standard value-- 0.20.2--0.5Mpa)0.5Mpa)

7.If it is not within the standard value,the probable cause is a7.If it is not within the standard value,the probable cause is amalfunction of the Oil line or steering gear box,so check these malfunction of the Oil line or steering gear box,so check these parts parts and repair as necessary.and repair as necessary.

8. Fully open the shut off valve of the pressure gauge.8. Fully open the shut off valve of the pressure gauge.

9. Turn the steering wheel all the way to the left or right;then9. Turn the steering wheel all the way to the left or right;then check check whether or not the retention hydraulic pressure is the standard whether or not the retention hydraulic pressure is the standard value.(value.(Standard valueStandard value-- 8.8Mpa)8.8Mpa)..

10.When not within the standard value,replace the Power steering When not within the standard value,replace the Power steering gear box .gear box .

11.Remove the special tool,and then tighten the pressure 11.Remove the special tool,and then tighten the pressure hose.(hose.(Tightening torque:17 Nm).Tightening torque:17 Nm). And bleed the system. And bleed the system.

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POWER STEERING OIL PRESSURE SWICTH POWER STEERING OIL PRESSURE SWICTH CHECKCHECK

1.Disconnect the pressure 1.Disconnect the pressure hose from the Oil pump,and hose from the Oil pump,and then connect the special tool.then connect the special tool.

2. Bleed the air,and then turn 2. Bleed the air,and then turn the steering wheel the several the steering wheel the several times while the vehicle is not times while the vehicle is not moving so that the temp. of moving so that the temp. of the fluid rises to Approx. 50the fluid rises to Approx. 50--6060ººC.C.

3. The engine should be idling.3. The engine should be idling.

4. Disconnect the connection 4. Disconnect the connection of the connector for the Oil of the connector for the Oil pressure switch,and place an pressure switch,and place an ohmmeter in position.ohmmeter in position.

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5. Gradually close the shut5. Gradually close the shut--off valve of the pressure gauge and off valve of the pressure gauge and increase the hydraulic pressure ,then check whether or not the increase the hydraulic pressure ,then check whether or not the hydraulic pressure that activates the switch is the Standard hydraulic pressure that activates the switch is the Standard value(value(Standard valueStandard value--1.51.5--2.0Mpa).2.0Mpa).

6. Gradually open the shut6. Gradually open the shut--off valve and reduce the hydraulic off valve and reduce the hydraulic pressure;then check whether or not the hydraulic pressure pressure;then check whether or not the hydraulic pressure that deactivates the switch is the standard value(that deactivates the switch is the standard value(Standard Standard value:0.7value:0.7--2.0Mpa)2.0Mpa)

7.Remove the special tool,and then tighten the pressure hose 7.Remove the special tool,and then tighten the pressure hose to the specified torque.(to the specified torque.(Tightening torque:17 Nm)Tightening torque:17 Nm)..

8. Bleed the system.8. Bleed the system.

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Principle of Power Steering