Design and function of steering gear - RA4600FBL0010X(46-0010)
- Steering gear designations
Gear
Internal designation
Designation of steering gear
The steering gear does not have a stamped model or gear designation.
The numbers stamped on the flat surface (A), e.g. D 90 1 73, identify the production number.
Casting numbers are not used as part numbers.
The "steering gear survey 46-0001" table must be used to assign a steering gear which has been removed to the respective vehicle model.
The most important distinguishing features are:
- Pitman arm designation (B),
- Color designation on steering worm, steering shaft (C),
- Rotations of steering wheel/steering worm,
- Pressure pipe joint length (D).
- Mechanical transmission section
The sliding power piston (11) is located in the steering housing (1). The pitman arm shaft (43) which is also provided with a toothed sector engages in the toothed sector of the power piston. The pitman arm shaft is pivoted on needle bearings in the steering housing (1) and in the lower housing cover.
The steering nut (25) is pivoted on thrust bearings in the power piston (11).
The steering worm (13) is held by balls inside the steering nut (25). 23 or 24 balls circulate in the ball tracks which are formed by the thread of the steering nut (25) and steering worm (13). Hence the designation as "recirculating-ball steering".
The ball circuit also forms the lower bearing of the steering worm (13). The latter's upper bearing comprises two thrust cylindrical roller bearings in the bearing cap (12).
If the steering worm (13) is turned, the steering nut (25) with the power piston (11) is moved axially in the steering housing via the ball circulation and a rotary movement is transmitted to the pitman arm shaft (43) via the toothed sectors.
The mechanical transmission section basically corresponds to a manual recirculating-ball steering system.
- Hydraulic steering section
The hydraulic control section basically comprises the pressure chambers (power chambers) for servo assistance for left (1a) and right lock (1b), the steering nut (25) with the control-bar and the spool valve (4).
A high-pressure seal (1c) is located in a groove in the steering housing (1) which seals off the moving power piston (11) from the pressure chamber (1a or 1b).
The spool valve (4) moves in a bore transverse to the power piston (11) in the steering housing and has a precise fit.
The spool valve is sealed off from the outside and secured by a screw cap (8).
The control-bar engages in a groove in the spool valve (4) and forms a unit with the steering nut (25).
The spool valve and the steering nut together with the control-bar are precision-manufactured parts and matched to each other.
Center position
The cylindrical spool valve (4) has leading edges, which are likewise located in the housing bore in order to achieve a selected oil control by displacing the servo spool valve.
When no steering forces are acting, the spool valve is held in the center position by a centering spring (12).
In this position the control cross-sections A = "from the power steering pump" and B = "to the power steering pump" are open as are the two connections to the pressure chambers (1a, 1b) so that the oil circuit is not restricted.
Left or right lock
(left lock shown)
Steering force assistance is initiated when the control-bar of the steering nut (25) moves the spool valve against the centering spring. In this way pressure oil is introduced into the pressure chamber (1a) via the leading edge (b) of the spool valve.
At the same time the circulating oil is forced back to the other side of the piston via the open leading edge (d) in the supply reservoir (B) by displacing the power piston.
Reaction process
(Example on left lock)
The reaction process is initiated by introducing pressure oil into the reaction chamber via a throttling port (arrow).
Provided the reaction piston (34a) is supported in the housing via the compression spring (35b), the pressure oil in the reaction chamber causes a restoring force to act on the spool valve (4). Within the normal driving range, i.e. with a torque at the steering wheel of ≥2 Nm, the restoring forces in the reaction chamber are proportional to the oil pressure at the power piston and thus to the steering moment at the front wheels.
Function diagram
This reaction effect causes the moment introduced at the steering wheel to correspond to the pressure assistance.
The greater the torque acting on the front wheels, the greater the torque to be applied to the steering wheel. The level of oil pressure assistance does not therefore depend on the turn angle of the steering wheel but merely on the torque acting on the front wheels.
Operating pressure in the steering gear (ΔPa ) for corresponding torque on the steering wheel (MLR ).
a=Introduction of manual effort limitation
b=Proportional increase in servo assistance
The operating range of the steering gear is shown graphically in the diagram.
The torque on the steering wheel is transmitted by the torque on the pitman arm shaft and the oil pressure.
The diagram shows how, as torque increases, the increase in servo assistance is directly proportional (up to approx. 5 Nm).
The size of the reaction surface (diameter of reaction piston) determines the level of force which is assigned to a particular oil pressure.
Manual effort limitation
The simple linear reaction with manual effort limitation limits the range of proportionality between the torque on the steering wheel and the torque on the pitman arm shaft and has a constant curve thereafter.
At the same time the reaction piston (4b) is supported from a particular oil pressure on the stop (4a) of the spool valve (e.g. circlip) so that the forces acting on the spool valve and reaction piston from the oil pressure become internal forces. Only the constant spring preload can be detected externally. The oil pressure assistance increases from this point up to maximum pump pressure of up to 125 bar, without the torque on the steering wheel exceeding 5 Nm. This avoids unacceptably high steering forces, e.g. during parking.
- Hydraulic system PARAMETER STEERING
- Hydraulic control, left lock (parking)
Hydraulic servo assistance is introduced by the displacement of the servo valve spool (4) via the control-bar engaging in the servo valve spool. In this way the pressure oil delivered from the power steering pump reaches the bearing surface area of the power piston (1a) via the open leading edges. The oil to be forced onto the other side of the power piston (1b) is led back to the oil reservoir via the leading edge of the spool valve.
At the same time the pressure oil reaches the end faces of the spool valve and the P valve (Y10) via the throttling ports in the spool valve. Since the P valve (Y10) keeps the control orifice (Y10a) closed, the same pressure occurs on both end faces of the spool valve (no differential pressure) and the servo assistance is introduced after overcoming the basic load spring (12) of the spool valve (at low torque, approx. 3 Nm).
- Hydraulic control, left lock (driving mode)
If hydraulic assistance is introduced during driving, the pressure oil is led into the return via the open control orifice (Y10a) of the parameter steering (PML) P valve (Y10) depending on vehicle speed. As a result of this process, a differential pressure is produced at the end faces of the spool valve (4) via the fixed orifice (arrow) in the governor slide valve (4a). The higher pressure on the end face of the spool valve selectively counteracts the torque on the steering wheel on left lock.
The torque to be applied at the steering wheel is adjusted between approx. 2-5 Nm - depending on speed.
- Hydraulic control, right lock (parking)
The bearing surface area of the power piston (1b) subjected to pressure on right lock can be seen in the illustration.
This pressure also occurs at the closed control orifice (Y10a) of the parameter steering (PML) P valve (Y10). The end faces of the spool valve (4) are surrounded with virtually depressurized oil (circulation pressure) determined by the fixed orifice (arrow) in the governor slide valve (4a).
This results in pressure equilibrium (no differential pressure) and servo assistance is introduced after overcoming the basic load spring (12) of the spool valve, with low torque at the steering wheel (approx. 2 Nm).
- Hydraulic control, right lock (driving mode)
If hydraulic assistance is introduced in driving mode, the control orifice (Y10a) of the parameter steering (PML) P valve (Y10) opens and the pressure is introduced into the chamber of the spool valve depending on vehicle speed. This pressure is effective on the end face of the spool valve (4) which has been introduced.
Determined by the fixed orifice (arrow) in the governor slide valve (4a), a differential pressure occurs at the end faces of the spool valve. The torque to be applied at the steering wheel is adjusted between approx. 2-5 Nm - depending on vehicle speed.
- Failure of parameter function
If there is a fault in the electrical system the parameter function is switched off. The parameter steering then functions as a conventional power steering system. The P valve (Y10) is fully opened - basic position - producing the maximum possible reaction on the spool valve (Figure Hydraulic Control, Section 2 + 4). The steering is therefore somewhat heavier to operate than in normal driving mode (>80 km/h).
- Function of manual effort limitation
This is achieved with the aid of the governor slide valve (4a).
If the pressure differential is greater than the force of the preloaded governor slide valve spring (4b), the governor slide valve closes the restrictor orifices (arrows) and thus keeps the differential pressure and the steering wheel moment (manual moment) constant.
- Hydraulic control, left lock (parking)