Power Steering - 2WD VEHICLES

Power Steering

COMPONENT LOCATION - 2WD VEHICLES

NOTE:
Right Hand (RH) drive shown, Left Hand (LH) drive similar









OVERVIEW
The power steering system comprises a hydraulic steering gear, a power steering pump and a reservoir.
The steering gear is a conventional end take-off, rack and pinion power assisted unit, mounted on the rear of the subframe.
A variable displacement vane pump is used on the diesel engine to give improved engine economy. The pump has a high flow rate which enhances the steering performance.

STEERING GEAR









The steering gear, manufactured by Tedrive, is located at the rear, top face, of the front subframe. It is solidly bolted to the subframe with two bolts. The bolts are passed through from the underside of the subframe, to improve service access, and are screwed into threaded bosses on the steering gear body.
The steering gear is a conventional end take-off, rack and pinion power assisted unit with lock to lock requiring 2.6 turns of the steering wheel. This gives a steering ratio (ratio of steering wheel angle to road wheel angle) of 16.7:1 which provides very quick and responsive reaction to driver inputs. The steering gear features large diameter tie-rods which optimize feedback and feel to the driver.
The steering gear comprises a steel, welded and machined, one piece housing which contains a mechanical steering rack, a valve unit and an integrated hydraulic power unit. The steering gear uses a rack with an integrated piston which is guided on plain bearings within the rack housing. The pinion, which is attached to the valve unit, runs in bearings and meshes with the rack teeth. The rack is pressed against the pinion by a spring loaded yoke which ensures that the teeth mesh without any play. The pinion is connected to the valve unit via a torsion bar. The rotary motion of the steering wheel is converted into linear movement of the rack by the pinion and is initiated by the valve unit. This movement is transferred into movement of the road wheels by adjustable tie-rods.
The piston of the hydraulic power unit is located at one end of the gear housing. Each side of the piston is connected to fluid pressure or fluid return via external metal pipes which are connected to the valve unit. Each end of the gear has a threaded hole which provides for the fitment of the tie-rod. The external ends of the steering gear are sealed with boots which prevent the ingress of dirt and moisture and allow for vertical movement of the tie-rods with the suspension in addition to linear movement when the steering wheel is turned. The boots are serviceable items and are retained on the gear housing and the tie-rod with ties.

Valve Unit

NOTE:
Typical valve unit shown









The valve unit is an integral part of the steering gear. The principle function of the valve unit is to provide power assistance (i.e. when parking) to optimize the effort required to turn the steering wheel. The pinion housing of the valve is an integral part of the main steering gear assembly.
The pinion housing has four machined ports which provide connections for pressure feed of the power steering pump, return fluid to the reservoir and pressure feeds to each side of the cylinder piston. A non-return valve and seal is fitted in the pressure feed port from the power steering pump. The valve unit comprises an outer sleeve, an input shaft, a torsion bar and a pinion shaft.
The valve unit is co-axial with the pinion shaft which is connected to the steering column via the input shaft. The valve unit components are located in the steering gear pinion housing which is sealed with a cap. The outer sleeve is located in the main bore of the pinion housing. Three annular grooves are machined on its outer diameter. PTFE rings are located between the grooves and seal against the bore of the pinion housing. Holes are drilled radially in each annular groove through the wall of the sleeve. The bore of the outer sleeve is machined to accept the input shaft. Six equally spaced slots are machined in the bore of the sleeve.
The ends of the slots are closed and do not continue to the end of the outer sleeve. The radial holes in the outer sleeve are drilled into each slot. The input shaft has two machined flats at its outer end which allow for the attachment of the steering column intermediate shaft yoke. The flats ensure that the intermediate shaft is fitted in the correct position to maintain the optimum phase angle. The inner end of the input shaft forms a dog-tooth which mates with a slot in the pinion shaft. The fit of the dog-tooth in the slot allows a small amount of relative rotation between the input shaft and the pinion shaft before the dog-tooth contacts the wall of the slot. This ensures that, if the power assistance fails, the steering can be operated manually without over stressing the torsion bar.
The central portion of the input shaft has equally spaced longitudinal slots machined in its circumference. The slots are arranged alternately around the input shaft. The torsion bar is fitted inside the input shaft and is an interference fit in the pinion shaft. The torsion bar is connected to the input shaft by a drive pin. The torsion bar is machined to a smaller diameter in its central section. The smaller diameter allows the torsion bar to twist in response to torque applied from the steering wheel in relation to the grip of the tires on the road surface. The pinion shaft has machined teeth on its central diameter which mate with teeth on the steering gear rack. A slot, machined in the upper end of the pinion shaft mates with the dog-tooth on the input shaft. The pinion shaft locates in the pinion housing and rotates on ball and roller bearings.

POWER STEERING PUMP - TD4









The pump is a variable displacement, vane type pump which supplies the required hydraulic pressure to the steering gear valve unit. The pump is located at the front of the engine and is driven by the Front Engine Auxiliary Drive (FEAD) Poly Vee belt which is directly driven from the crankshaft. The output from the pump increases proportionally with the load applied to the steering valve unit. A self-adjusting tensioner is fitted to maintain the correct tension on the belt.
The pump consists of a cartridge set which consists of 11 vanes and a rotor. These are mounted on the input shaft and are surrounded by a variable displacement cam ring. The vanes rotate within the cam ring and are driven by the shaft. As the vanes rotate, the cam ring causes the space between the vanes to increase. This causes a depression between the vanes and fluid is drawn from the reservoir via the suction hose into the space between the vanes. As the shaft rotates, the inlet port is closed to the vanes which have drawn in fluid, trapping the fluid between the vanes. The cam ring causes the space between the vanes to reduce and consequentially compresses and pressurizes the hydraulic fluid trapped between them. Further rotation of the shaft moves the vanes to the outlet port. As the vanes pass the port plate, the pressurized fluid passes from the pump outlet port into the pressure hose to the steering gear.
The cam ring can move within the valve body. By moving the cam ring it is possible to vary the eccentricity of the shaft and the vanes in relation to the cam ring. As the eccentricity is decreased, the volume of hydraulic fluid trapped between the vanes decreases, maintaining a constant fluid output. This reduces the power and torque required to turn the pump and therefore improves engine economy. The pump has an internal regulating valve which controls the eccentricity of the cam ring and therefore varies the flow rate according to demand.
At low engine speeds, the internal displacement of the variable displacement pump is at its maximum to generate the controlled fluid output. As the pump speed increases with engine speed, the increased flow inside the pump generates a back pressure within the pump. This back pressure causes an internal regulating valve to move the the cam ring and reduce the internal displacement of the pump to maintain the constant fluid flow from the pump.
A regulating, pressure relief valve within the pump limits the maximum pressure supplied to the steering gear to 115 bar (1667 lbf in2) ± 4 bar (58 lbf in2) and also limits the maximum flow to 8.3 l/min (2.2 US gal/min) ± 0.5 l/min (0.1 US gal/min) at 10 bar (145 lbf in2). The pump has a displacement of 9.6 cc/rev (0.58in3/rev).

RESERVOIR









The fluid reservoir is located on a bracket in the RH side of the engine compartment, behind the headlamp assembly. The reservoir comprises a body, cap and filter. The purpose of the reservoir is to contain a surplus of the hydraulic fluid in the system to allow for expansion and contraction of the fluid due to temperature variations.
The fluid level ensures that the supply connection on the bottom of the reservoir is covered with fluid at all operating vehicle attitudes. Any air which is present in the system is exhausted from the system in the reservoir.
The body is a plastic moulding with two ports at the bottom which provide for the connection of the suction supply and return hoses. Moulded markings on the side of the reservoir denote the upper and lower fluid levels. A non-serviceable, 100 micron nylon mesh filter is fitted in the body. The filter removes particulate matter from the fluid before it is drawn into the pump supply connection. Maximum and minimum fluid levels are moulded into the body and assist checking fluid levels when the hydraulic fluid is cold.
The cap is rotated counterclockwise to release from the body. The cap is fitted with an O-ring to prevent fluid leakage and incorporates a breather hole to allow for changes in fluid level during operation and prevent vacuum or pressurization of the reservoir.