Power steering gear ZF 8098
Function and work description
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The power steering gear is designed for a system pressure of 150bar and a maximum torque of 6726Nm from the sector shaft. The steering gear ratio is variable at 17-20:1.
The power steering gear has an integrated pressure limiting valve and a hydraulic wheel end lock.
The wheel end lock is self-adjusting.
If a fault occurs and there is no power steering, the power steering gear will operate as a mechanical steering gear. The vehicle can be steered but the steering will be much heavier.
When the steering wheel is turned, the torque is transferred to the power steering gear input shaft. A torsion bar is fixed onto the input shaft and onto the worm screw. When the torsion bar has been turned approximately 7°, a pin engages and transfers the torque mechanically to the worm screw.
The set of balls between the worm screw and piston transfers and reinforces the torque to the piston which is then displaced into the steering gear housing cylinder.
Part of the underside of the piston is shaped like a rack which engages with the sector shaft teeth. The varying contact radius between the piston rack and the sector shaft teeth along with the ball screw pitch determines the power steering gear ratio and the number of turns of the steering wheel.
The sector shaft transfers torque to the steered wheels via the droparm, draglink, guide arm and track rod.
1 Internal hexagon bolt - automatic wheel end lock
2 Valve - wheel end lock
3 Worm screw
4 Torsion bar
5 Control valve
6 Input shaft
7 Steering gear housing
8 Sector shaft
9 Set of balls
10 Servo piston
If no torque affects the torsion bar, the control valve is in neutral position. The inlet and return channels of the power steering gear are then connected with each other.
If the engine is running, oil circulates from the hydraulic pump through the control valve and back to the oil reservoir via the return line.
With power steering, only a small part of the steering work is carried out mechanically.
When the steering wheel is turned and the worm screw tries to displace the piston, the piston resists because it is engaged with the sector shaft.
The torsion bar between the input shaft and worm screw is then turned in relation to the input shaft and worm screw.
The difference between the turning angles of the input shaft and worm screw activates the control valve.
The control valve partly closes the oil flow between the inlet and return channels and opens to allow a full oil flow to the cylinder chamber at one side of the piston.
The rapidly increasing oil pressure in the cylinder chamber displaces the piston and, by meshing with the sector shaft, turns it.
Neutral position
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The oil in the chamber on the other side of the piston and the oil which is not used for steering flows out via the control valve through the return channel to the oil reservoir.
When the driver stops turning, only the oil pressure which is required to retain the wheel in the position it has reached is supplied. If the driver releases the steering wheel, the torsion bar returns the control valve to the neutral position and the wheel returns to the straight ahead position because of the front axle wheel angles and the tension from the tyres.
A high oil pressure provides steering force - a high oil flow provides a rapid steering response
The quantity of oil which passes into the cylinder chamber is determined by the speed of the steering wheel movements.
The oil pressure which is built up is determined by the resistance from the steered wheels depending on the axle weight and friction between the tyres and road surface.
The control valve controls the difference between the turning of the input shaft and worm screw so that the pressure and flow are optimally adjusted in every steering situation.
Variable steering resistance in the steering wheel gives the driver information about and control of the steering process.
If the steered wheels are suddenly exposed to a shock load, this force will be transferred to the worm screw and will cause it to turn rapidly.
When the worm screw is turned, the control valve is also turned in relation to the input shaft. The control valve then controls the full oil flow to the low pressure side of the piston where the oil pressure is rapidly increased. The oil pressure forms a counter force which dampens the shock loads before they are transferred to the steering wheel.
1 Internal hexagon bolt - automatic wheel end lock
2 Cylinder chamber
3 Valve sleeve
4 Piston pin
5 Compression spring
Full wheel lock angle is required during the adjustment, which results in the sleeve on the internal hexagon bolt being displaced.
When the piston approaches the end lock position, the piston pin is pressed into the valve sleeve and opens the valve on the low pressure side.
The function of wheel end lock is to reduce the fluid pressure in the power steering gear, and thereby also its torque when the wheels are close to their end positions.
Without this function a considerable increase in pressure and temperature would be produced in the system if the wheels were forced against their end lock positions with great force. This would also place a great load on the steering components.
The wheel end lock consists of two valves integrated into a longitudinal channel in the piston. The valves are held closed by a compression spring placed between the valves and opened by two piston pins. The ends of the piston pins protrude through valve sleeves into the ends of the piston.
When the oil pressure on the high pressure side of the piston is higher than the oil pressure between the valves, oil is allowed in through the pressure side valve.
When the piston approaches the end lock position, the relevant piston pin is pressed into the valve sleeve and opens the valve on the low pressure side. The oil pressure on the pressure side is reduced because the oil flows through the valves and piston over to the low pressure side and on to the oil reservoir via the return channel.
The closer the piston is to the end position, the more the oil pressure is reduced, resulting in ever reducing servo effect. The driver therefore cannot apply full load to the power steering gear in the end lock position.
Note: To avoid damage to the hydraulic pump, the wheel must not be pressed against the set screw for more than 10seconds.
The self-adjusting function of the wheel end lock is set at the factory after the power steering gear is fitted in the vehicle.
If the nominal wheel lock angles are subsequently increased, wheel end lock is automatically set.
If the nominal wheel lock angles are reduced, both set screws must be renewed.
It is also necessary to renew the set screws if:
• The length of the draglink, i.e. the straight ahead position, has been altered.
• The input shaft on a detached power steering gear (or on a power steering gear which has been detached from the steering linkage) is rotated more than 1.6revolutions from its centre position.
The pressure limiting valve in the power steering gear valve housing limits the maximum oil pressure in the hydraulic system.
If the oil pressure exceeds the permitted system pressure, the valve opens and oil from the pressure side is evacuated back to the oil reservoir via the return line.
The valve therefore protects the hydraulic system from damage due to excess pressure.
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Among other things, this work description covers the renewal of seals in the power steering gear.
This work requires partial dismantling of the power steering gear in order to rectify internal and external leakage.
The power steering gear function is extremely important to the vehicle's traffic safety. For this reason, any action that involves work on the power steering gear must meet high product safety requirements.
Work must only be carried out under the following conditions:
• Test the function of the hydraulic pump and power steering gear and establish the cause of the fault using the method advised by Scania.
See Workshop Manual main group 13, Steering - Checking and troubleshooting.
• Work must be carried out in accordance with this description and using the tools specified.
• No work may be carried out on the power steering gear additional to this work description.
• Cleanliness is essential when working on the power steering gear.
• Welding, grinding or other work which may generate dust must not be carried out in the vicinity while the work is carried out.
• Check function of power steering gear in vehicle as mentioned above after work is completed.
• If the power steering gear has been subjected to external impact due to a
collision or because the vehicle has been accidentally driven into a ditch, it must be renewed.
The need to keep everything clean when working on power steering gears cannot be emphasised enough.
Damage on power steering gear components and seals are mainly due to contaminants in the hydraulic system.
Pay particular attention to the following:
• Clean the power steering gear at the hose connections before these are disconnected. Remove any flaking paint from the power steering gear.
• Immediately plug the connection holes in the power steering gear.
• Always change the oil and oil filter after renewing any seals or renewing the power steering gear.
• Remove the oil reservoir when changing filter.
• Clean the outside and inside of the oil reservoir before fitting a new filter.
• Always use a clean container when filling oil.
The oil is not filtered until it returns from the power steering gear.
• Never top up with used fluid.
This work description describes the renewal of seals in the power steering gear which has been removed, and manual setting of the wheel end lock with the power steering gear in the vehicle.
Note: No work, apart from that described here, may be carried out on the power steering gear.
It is therefore essential to assess the result of the functional test on the power steering gear to decide whether it should be renewed or repaired.
The steering gear must be renewed if:
• The power steering gear has been damaged due to collision or accidental driving into a ditch.
• There is leakage through the centre of the input shaft. (Damaged seal in worm screw).
• There is unacceptably high internal leakage when the wheels are turned in either direction with centre position locked.
In this test, only the control valve is turned and there is leakage past the seals in the valve housing/control valve.
Seals should be renewed when:
• There is external leakage.
• There is excessively high internal leakage when turning the wheels in both directions.
In this case, there is probably leakage between the cylinder and the servo piston seal.
In the case of external leakage, the following work can be carried out on the power steering gear in the vehicle:
• Renewing the input shaft sealing ring.
• Renewing the side cover gasket.
• Renewing the ventilation plug in the side cover.
• Renewing the O-ring on the plug for the pressure limiting valve.
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99 237SleeveAM3
99 238Assembly sleeve with spacing ringAM3
99 281Input shaft assembly sleeveAM2
Other tools
NumberDesignationPictureToolboard
587 315Roller bearing pullerXA3
587 315Modified arms for pullerG4
587 539Puller for guide arm
587 585Circlip pliersG4
587 692Universal stand NumberDesignationPictureToolboard
1 Clean the power steering gear and place it with the sector shaft upwards in the universal stand with fixtures 87013 and 98148.
Do not stand in front of the puller. The puller comes off and can cause injury.
2 Hold the steering arm in position and remove the nut with a nut runner.
Check that the mark on the guide arm is opposite the mark on the sector shaft. If the marking is unclear, make a punch mark.
Refit the nut three turns on the sector shaft.
Pull off the guide arm using puller 587539.
3 Fit clamp sleeve 87018 with insert 87017 onto the input shaft.
Mark the clamp sleeve opposite the marking on the input shaft.
4 Put the power steering gear in the horizontal position.
5 Set the power steering gear centre position by turning the input shaft so that the markings on the input shaft and the sector shaft correspond as illustrated.
6 Check as follows, with the power steering gear in the centre position, whether there is play in the sector shaft gear mesh:
• Secure the sector shaft using a strong pipe wrench. Put a spacer between the pipe wrench and splines.
• Put a smaller pipe wrench with spacer on the input shaft.
• Check whether the input shaft has turning play.
• If it has turning play, make a mark on the pipe wrench 350mm from the centre of the input shaft.
• Ask a colleague to measure the turning play when marking.
Renew the power steering gear if the turning play exceeds 30mm.
a = Marking, input shaft
b = Marking, dust cover
c = Marking, steering gear housing
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• If only the sector shaft is to be removed to seal leaks, this can be carried out without removing the piston. If this is the case, proceed to step 14.
7 Remove the dust cover and input shaft seal.
IMPORTANT! Remove the setting screw. Do not turn the cover as this may damage the wheel end lock.
8 Remove the cover. If the cover is stuck, turn the sector shaft anti-clockwise on the low gear (L) and clockwise on the reverse gear (R).
9 Remove the needle bearing and the bearing washer underneath from the cover and inspect.
If the needle bearing is damaged or there are depressions in the bearing washer, renew the power steering gear. The steering has been subjected to external impact and there is also a risk of damage to the worm screw and to the piston ball nut.
If the needle bearing and bearing washer are not damaged, refit them in the cover.
10 Remove the piston by turning the sector shaft as mentioned above.
Do not turn the piston as there is a risk that it will be scratched. Place the piston on a clean surface.
Place your hand as illustrated to prevent the plastic cover on the underside of the piston from coming loose.
11 Check that there are no depressions or other surface damage to the piston teeth by running your fingernail over them.
Turn the worm screw and check that there is no surface damage in the ball race or on the worm screw end surface.
Check that any valve pins are straight, springloaded and move easily.
Note: If there is surface damage on the teeth or worm screw or damage on the valve pins, the power steering gear must be renewed.
12 Put your hand in the steering gear housing and remove the needle bearing and the bearing washer for the input shaft. Check the needle bearing and the bearing washer as described in step 9 above.
Remove the sealing ring using a bent marking scraper and save the sealing ring. It is used as a spacer when fitting a new sealing ring.
13 Remove the support ring and the seal on the spline side of the sector shaft. Remove the seal on the other side of the sector shaft.
14 Mark one of the sector shaft roller bearings. Make the same marking on the steering gear housing.
Note: The outer and inner roller bearing of the sector shaft must not be mixed up when fitting.
If the piston is positioned in the power steering gear, turn the input shaft 1.5 turns from the centre position without play so that there is some play in the gears when fitting the sector shaft.
Mark the changed position of the sector shaft using a dot of paint on the thread and steering gear housing.
Use protective goggles when doing this work.
15 Remove the roller bearing retaining rings using circlip pliers 587585.
16 Modify the arms of puller 587315 as illustrated.
17 Fit the puller in the roller bearing on the spline side.
Place a suitable nut between the puller bolt and the sector shaft. Since the bearings are eccentric, the centre of the bearing does not correspond to the centre of the sector shaft.
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18 Pull the bearing approximately 15mm out of the housing.
Pull the opposite bearing out approximately 15mm using support drift 99132 and remove the bearing.
19 Remove the plastic washers on both sides and lift out the sector shaft.
20 Check that there are no depressions or other damage on the sector shaft teeth or bearing surfaces by running your fingernail over them.
Note: If there is surface damage, the steering gear must be renewed.
21 Clean the cylinder in the steering gear housing. Check for scoring in the front and rear cylinder chambers.
Surface scoring does not affect the function of the power steering gear.
Deep scoring outside the working range of the piston are of no importance provided that any burring is ground off.
If there is deeper scoring in the rear cylinder chamber, within the working range of the piston seal, check whether there is any wear or damage to the piston or piston seal which can be associated with the positions of the scratches in the cylinder.
The piston seal is relatively soft and seals well even when the scoring is considerable.
Polish any burr and sharp edges using a 220-400 abrasive cloth. Thoroughly clean the cylinder after polishing.
Note: If there is deep and wide scoring within the working range of the piston seal, the power steering gear must be renewed.
22 Remove the pressure limiting valve from the steering gear housing. Clean the oilway and renew the O-ring. Oil the new O-ring and fit it from the outside of the valve plug.
Tighten the valve. Tightening torque 30Nm.
1 Check the seals of the worm screw. If they are completely undamaged, they do not need to be replaced.
If there are any scores on the seals, they must be renewed. Use assembly sleeve 99238 with spacing ring and sleeve 99237.
2 Press a marking scraper into the seals and carefully bend them out of the grooves. Take care not to damage the edges of the grooves.
3 Lubricate the assembly sleeve with power steering fluid and fit it on the worm screw without the spacing ring.
Fit the seal on the assembly sleeve and carefully push the seal into the inner groove.
Be careful not to twist or stretch it more than necessary when fitting.
Fit the spacing ring in the assembly sleeve and fit the outer seal in the same way.
4 Oil the inside of sleeve 99237 and push it carefully with the chamfered side over the seals so that they are pushed into the grooves. Leave the sleeve in place for a while so that the seals are pushed into the grooves.
5 Check the piston seal.
If it is not damaged, it is not necessary to renew it.
If it is scored, proceed as follows:
Remove the seal and the O-ring underneath using a marking scraper. Take care not to damage the edges of the grooves.
6 Lubricate a new O-ring and seal using power steering fluid and fit them in the piston groove.
Be careful not to twist or stretch them more than necessary when fitting.
Push the sealing ring into the groove by hand.
7 Renew the support washer and seal in the roller bearings. Fit the support washer in the groove with the flat side towards the bearing rollers.
Lubricate the seal well and fit it inside the support washer.
Press the seal into the groove. This is to prevent the seal from being damaged when the roller bearings are pushed onto the sector shaft.
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8 Renew the O-rings in the steering gear housing bearing seats. Lubricate the new O-rings well and push them into the grooves to prevent them from being damaged when the roller bearings are fitted in the housing.
IMPORTANT: Fit the sector shaft the right way round.
9 Place a layer of thin tape over the sector shaft splines.
Fit the sector shaft into the housing. Fit the piston in the power steering gear. Fit the sector shaft according to the marking.
Fit the plastic washers on the sector shaft with the flat sides facing outwards.
Lubricate the sector shaft bearing seats and the tape on the splines.
IMPORTANT: Do not mix up the roller bearings.
10 Fit the roller bearings in the housing with the compression of the outer races towards the recesses in the housing.
Use protective goggles when doing this work.
11 Fit drift 98332 with drift 87292 over the spline side of the sector shaft moving it towards the steering gear housing.
Fit drift 87582 moving it towards the roller bearing on the other side and push it into the housing using a screw clamp.
Only push the bearing far enough so that the retaining ring can be fitted. Fit the retaining ring with the opening opposite the recess in the housing.
Push in the bearing on the spline side of the sector shaft using drift 98743 towards the bearing and drift 98566 towards the housing. Fit the retaining ring with the opening opposite the recess in the housing.
Remove the tape from the sector shaft.
12 Fit the seal, support ring and dust cover on the spline side of the sector shaft.
13 Fit the removed input shaft sealing ring as a spacer on drift 98450 with the sprung sealing lip facing away from the drift.
Grease a new sealing ring and place it with the sprung sealing lip facing towards the drift.
Fit the sealing ring in the steering gear housing.
Fit the bearing washer and the needle bearing.
14 Lubricate assembly sleeve 99281 and fit it over the input shaft.
15 Fit the piston in the power steering gear housing as follows:
Liberally apply steering gear fluid to the cylinder and the piston.
Fit pipe wrench protection over the sector shaft splines.
Turn the sector shaft so that the first tooth on the shaft is partly visible in the cylinder.
Fit the piston in the cylinder with the gear segment upwards and push it until there is contact with the sector shaft tooth.
16 Pull the piston a little at the same time as turning the sector shaft which is in contact with the piston.
Push the piston and turn the sector shaft in the opposite direction so that the teeth on the shaft and piston engage.
Unscrew the worm screw to the end position. Turn the sector shaft and carefully pull the piston into the cylinder.
IMPORTANT! Make sure that the piston seal is not damaged by the cylinder edge.
17 Carefully screw in the worm screw. Make sure that the seals on the front part of the worm screw are guided into the housing.
18 Remove the needle bearing and the bearing washer underneath from the cover.
Put a marking scraper through the seal and remove it and the O-ring underneath. Lubricate a new O-ring and seal and fit them into the groove. Fit the bearing washer and the needle bearing in the cover. Renew the large O-ring in the cover.
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19 Fit the cover with the piston valve pins in the corresponding cover hole.
20 Fit the bolts and the cover. Tightening torque 190Nm.
21 Fit the setting screw (wheel end lock). Tightening torque 15Nm.
22 Fit a new seal and new input shaft dust cover.
Fit the dust cover marking opposite the steering gear housing marking.
Remove the assembly sleeve from the input shaft.
Set the power steering gear centre position by turning the input shaft so that the markings on the input shaft and the sector shaft correspond as illustrated.
a = Marking, input shaft
b = Marking, dust cover
c = Marking, steering gear housing
23 Place the power steering gear with the cover down.
Place a new dust cover on the sector shaft and fit the droparm as marked.
IMPORTANT! Secure the droparm with a chain in the centre position, otherwise the wheel end lock will be affected.
24 Tighten the nut and lock it in the recess of the shaft. Tightening torque 800Nm.
25 Secure the nut by staking its flange at least 2.5mm into the groove in the sector shaft.
Remove the power steering gear from the universal stand.
26 Carry out a functional test with the power steering gear in the vehicle. See Workshop Manual main group 13, Steering - Checking and troubleshooting.
General data
Left-handturnStraight ahead position Right-handturn
Steering wheel lock2.4 turns02.4 turns
Angle reading on sector shaft 47° 1) 047° 1)
Gear ratio20.0:117.0:120.0:1
Torque from sector shaft at 150bar 6726Nm5717Nm6726Nm
Weight (exc. drop arm)39kg
Oil volume3.8l
Tightening torque
Pressure limiting valve30Nm
Adjusting bolt (wheel end lock)15Nm
Bolts, cover190Nm
Nut, guide arm - power steering gear800Nm
Power steering gear TAS85 has a hydraulic wheel end lock that reduces the power steering gear oil pressure and thereby its torque when the wheels approach their end lock positions.
The wheel end lock has a factory setting that corresponds to the vehicle's power steering gear and also has set wheel lock angles.
If the input shaft on a detached power steering gear, or on a power steering gear detached from the draglink, is rotated more than 1.5turns from its centre position, the factory setting of the wheel end lock can be changed.
If this occurs, the wheel end lock is set in manual (TAS) using the special adjusting bolt and locking nuts on the power steering gear housing end plate, alternatively by using an automatic set screw (ZF).
For further information, see:
• TAS
Workshop manual main group 13, Power steering gear TAS85,-86,-87, Work description
• ZF
Workshop manual main group 13, Steering gear ZF 8098, Work description
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