Power steering system for motor vehicles

A power steering system for motor vehicles, wherein a steering nut, connected to a steering worm in such a manner that the steering worm is driven thereby, is rotatably and axially non-displacably mounted in a working piston and is also provided with a control arm passing through both a radial recess in a working piston and a radial opening in a cylinder housing that receives the working piston. An end of the control arm which is radially outermost with respect to an axis of the piston actuates a spool valve for adjusting the working pressure. Two corresponding followers are provided for defining the end stroke positions of the working piston. The followers are mutually engagable to lower or shut off a working pressure in the system. One of the followers is disposed opposite the working piston relative to the piston axis and the other is located opposite the cylinder housing relative to the cylinder axis with a zero motion. The followers having a zero motion with respect to the piston axis on the control arm and followers which have a zero motion with respect to the cylinder axis on a limiting housing part are so disposed that a mutual engagement of the corresponding followers results in actuation of the spool valve.

The present invention relates to a steering system and, more particularly, 
to a power steering system for motor vehicles which includes a spool valve 
actuatable by a control arm of a steering neck for controling a working 
pressure in the power steering system, with followers being provided to 
lower the working pressure in a range of end to stroke positions of the 
working piston. 
Power steering systems for motor vehicles have been proposed wherein a 
steering neck, connected to a steering worm gear in such a manner that a 
screwing motion is permitted, is mouned in a working piston rotatably and 
axially non-displaceable and is also provided with a control arm, with the 
arm passing through both a radial recess in a working piston and a radial 
opening in a cylinder housing that accommodates the working piston. A 
radially outermost arm of the control arm, with respect to the piston 
axis, actuates the spool valve, with the spool valve being adapted to 
adjust the working pressure. Two followers are provided for defining the 
two end stroke positions of the working piston with the followers being 
mutually engageable to lower or shut-off the working pressure. One of the 
followers is disposed opposite the working piston relative to the piston 
axis and the other of the followers is located opposite the cylinder 
housing relative to the cylinder axis with a zero motion. 
A power steering system of the aforementioned type is proposed in, for 
example, ATZ 74 (1972), No. 7, pp 151-154, especially in FIGS. 17 and 18. 
In FIGS. 17 and 18, the followers, forced to move along a cylinder axis 
are formed by one flattened area on an end of the control pin, with the 
pin being disposed axially parallel to the cylinder axis in the working 
cylinder, and having a cylindrical piston cross section between its 
flattened areas. In this proposed construction, the followers, forced to 
move along the piston axis, are formed by two end openings of a bypass 
bore axially parallel to the piston axis, with the bore being traversed by 
the control pin. In a vicinity or area of each piston stroke end position, 
a bypass connection is produced between the two working pressure chambers 
whenever the opening of the bypass bore which is located opposite the 
stroke end crosses the corresponding flattened area. 
A disadvantage of the last proposed construction is that the control pin 
reduces the effective piston cross section and a sealing of the bypass 
bore of the working piston requires a considerable amount of difficult 
work to assure a fitting of such seal. 
A power steering system of another type is proposed in, for example, 
Offenlegungsschrift No. 24 08 265, wherein a steering wheel operates a 
pinion through a steering shaft, with the pinion meshing with a rack 
coupled to a pressure medium servo motor having a working pressure 
adjusted by a control valve. The control valve has a valve adjusting 
element connected to one lever arm of a two-armed control lever, with the 
other lever arm being kinematically connected to the pinion or steering 
shaft. One lever arm, at an end connected to the pinion, has an extension 
with two meshing surfaces for the stroke end positions. In a corresponding 
manner, the rack has an enaging element at each end, with the engaging 
element being adapted to actuate the control lever through engaging 
surfaces in the stroke end position so that the control valve moves into a 
neutral position which shuts off the working pressure. 
Since the followers mounted on the rack are in the form of stops, in 
Offenlegungsschrift No. 2,408,265, when the working pressure is shut off 
the control valve, is moved to a position for the opposite lock by 
additional manual mechanical steering. Because of the unavoidable 
disadvantageous tolerances in the vehicle, this proposed power steering 
system does not permit the steering to turn to the vehicle end stop, which 
corresponds to the smallest turning circle. 
Disadvantageously, however, if a limit switch is provided and constructed 
so that the smallest possible turning circle is reached, because of the 
above-noted tolerances in the prior art, the pressure drop may not be 
effective. 
In Auslegeschrift No. 24 10 077, an arrangement is proposed wherein a 
manually adjustable and lockable threaded pin is used as a stop for a 
spool valve to adjust the working pressure, with the threaded pin limiting 
the throughflow cross section and hence the working pressure. After an 
adjustment of the adjustable pin is stopped, the pin is disposed so that 
it moves together with a valve cylinder accommodating the spool valve. 
The aim underlying the present invention essentially resides in providing a 
power steering system for a motor vehicle with a shut off for a working 
pressure of the system in a vicinity of the stroke end positions of the 
working piston such that a further manual mechanical steering is possible 
until an end stop of the vehicle is reached, with the end stop determining 
the smallest possible turning circle of the motor vehicle. 
In accordance with advantageous features of the present invention, the 
followers of the control arm having no motion with respect to a center 
axis of the working piston, and the followers formed by a limiting housing 
part in the cylinder housing space traversed by the control arm and having 
no motion with respect to the longitudinal center axis of the cylinder 
housing are disposed so that a mutual engagement of the corresponding 
followers results in an acutation of a spool valve of the power steering 
system. 
Advantageously, in accordance with further features of the present 
invention, the spool valve is provided at a radially outer end with a 
control strip for engagement with a circumferential groove of the spool 
valve and a radial opening of the cylinder housing terminates in a valve 
cylinder receiving the spool valve, with the followers which have no 
motion with respect to the piston axis being disposed at axial ends of the 
control arm at a distance from the piston axis which is less than a 
distance of the control strip from the piston axis, and with the followers 
which have a zero motion with respect to the cylinder axis being disposed 
at a mutual axial distance which is less than an axial length of the 
control arm. The limiting housing part limits the radial opening 
accommodating the control arm and is disposed between the valve cylinder 
and cylinder bore accommodating the working piston. 
The followers which have no motion with respect to the cylinder axis are, 
in accordance with the present invention, formed by a housing part in 
directions of the wall surfaces delimiting the cylinder axis. 
In accordance with further features of the present invention, cams are 
integrally formed on the control arm at axial ends thereof. 
The followers having no motion with respect to the cylinder axis, have, 
according to the present invention, a mutual spacing in directions of the 
cylinder axis which is greater than a length of the control arm with 
respect to the piston axis. 
The control arm of the power steering system of the present invention is 
provided at radially outer ends thereof with a guide strip having two 
lateral sliding surfaces, with the lateral sliding surfaces being disposed 
symmetrically to a center or middle plane of the control arm which 
contains an axis of rotation of the steering nut. Each of the lateral 
sliding surfaces are adapted to be brought into sliding contact with a 
groove side wall of a circumferential groove of a control slot. The 
followers which have no motion with respect to the cylinder axis, are 
disposed at a distance from the cylinder axis generally equal to a 
distance of the guide strip from the axis of rotation of the steering nut. 
The engaging surfaces of the followers which have no motion with respect 
to the piston axis are disposed in an area of the guide strip which is 
located between the sliding surface. 
Advantageously, the followers have a distance or space with respect to the 
cylinder axis or piston axis which is greater than an effective radius of 
the working piston. 
In accordance with still further features of the present invention, at 
least one follower, which has no motion with respect to the cylinder axis, 
is adjustably mounted on the cylinder housing. 
Accordingly, it is an object of the present invention to provide a power 
steering system for motor vehicles which avoids, by simple means, the 
shortcomings and disadvantages encountered in the prior art. 
Another object of the present invention resides in providing a power 
steering system for a motor vehicle which eliminates the need for 
incurring additional expenses for sealing the power steering system. 
Yet another object of the present invention resides in providing a power 
steering system for motor vehicles which enables the power steering system 
to execute the smallest possible turning circle without the need of a 
control valve being actuated in an undesirable manner by further manual 
steering. 
A further object of the present invention resides in providing a power 
steering system for motor vehicles which functions reliably under all 
operating conditions of the motor vehicle. 
A still further object of the present invention resides in providing a 
power steering system for motor vehicles which is simple in construction 
and, therefore, relatively inexpensive to manufacture. 
Yet another object of the present invention resides in providing a power 
steering system for motor vehicles which ensures a shut off of a working 
pressure of the system in a vicinity of the stroke end positions of a 
working piston of the power steering system.

Referring now to the drawings wherein like reference numerals are used 
throughout the various views to designate like parts and, more 
particularly, to FIGS. 1-3 and 7, according to these figures, a working 
piston generally designated by the reference numeral 8 is displaceably 
received in a cylinder bore 52 of a cylinder housing generally designated 
by the reference numeral 14, with the cylinder bore being centered on a 
longitudinally extending center cylinder axis 24--24 and a piston axis 
15--15 of the piston 8 being aligned with the cylinder axis 24--24. A 
steering nut 10 is rotatably and axially non-displacably mounted in the 
working piston 10 by roller bearings generally designated by the reference 
numerals 53, 54, with an axis of rotation 44--44 of the steering nut 10 
being aligned with the cylinder axis 24--24. The steering nut 10 is 
connected to a steering worm generally designated by the reference numeral 
9 so as to permit the steering nut 10 to move by cooperating between a 
ball 55 and the steering worm 9, with the steering worm 9 passing 
outwardly through a housing cover 57 fastened to a cylinder housing 14 by 
fasteners such as, for example, bolts or screws 58. The housing cover 57 
seals off the cylinder bore 52 from the outside. Another bearing 
arrangement generally designated by the reference numeral 56 mounts the 
steering work 9 in the housing cover 57 so as to enable the steering worm 
9 to be rotatable with respect to the cylinder axis 24--24 and axially 
non-displacable with respect to the cylinder housing 14. 
The steering nut 10 includes an integrally formed control arm 11 passing 
through a radial opening 12 in the working piston 8 and through a radial 
opening 13 in the cylinder housing 14. A control strip generally 
designated by the reference numeral 41 is disposed at a radially outer end 
16 of the control arm 11, with the control strip 41 being axially parallel 
to the axis of rotation 44--44 of the steering nut 10. The control strip 
41 engages a circumferential groove generally designated by the reference 
numeral 48 of a control slide generally designated by the reference 
numeral 17, with the control slides 17 being displacably received in a 
valve cylinder generally designated by the reference numeral 15 which is 
integrated for motion with the cylinder housing 14. A valve cylinder 15 
includes a middle valve connection 59 for connecting a pressure pump (not 
shown), two adjacent valve connections 60, 61 with the valve connection 60 
being provided for a power steering booster at a left lock position and 
the valve connection 61 being provided for a power steering booster at a 
right lock position, as well as two outer valve connections 62, 63 
connected to a return line (not shown). An annular seal generally 
designated by the reference numeral 66 is disposed in a circumferential 
groove of the cylinder housing 14, with the seal 66, in cooperation with 
the working piston 8, dividing the cylinder bore 52 into a working 
pressure chamber 64 to boost the right lock position and a working 
pressure 65 to boost a left lock position. 
In FIGS. 1, 2, and 7, as well as FIGS. 4, and 5, to be described more fully 
hereinafter, the control arm 11 and hence a spool valve generally 
dsignated by the reference numeral 17 are in a middle neutral position for 
enabling a driving a vehicle in a straight line, wherein the working 
pressure is shut off, since the valve connections 60, 61 for the power 
steering boosting at the left lock position are connected by the adjacent 
valve connections 62, 63 to the return line. As the control arm 11 is 
moved in one or the other circumferential directions with respect to the 
axis of rotation 44--44 by turning the steering worm 9, one of the valve 
connections 60, 61 is connected to valve connection 59 and the return line 
is cut off from adjacent valve connections 62 or 63. 
The steering motions or circumferential displacements of the control arm 11 
are limited by the width or size of the opening 13 of the cylinder housing 
14 and, for this purpose, the opening 13 is provided with stop surfaces 
67, 68 for limiting a displacement in the circumferential direction with 
respect to the cylinder axis 24--24. If a working pressure fails or if a 
maximum pressure is reached in a vicinity of the middle stroke positions 
of the working piston 8, the control arm 11 will abut one of the stop 
surfaces 67 or 68, whereby the steering nut 10 is fixed in one 
circumferential direction with respect to the cylinder housing 14 and thus 
permits a further mechanical manual steering. 
In the power steering system of FIGS. 1-3, the followers generally 
designated by the reference numerals 20, 22 are provided on the control 
arm 11, with the followers 20, 22 being formed by cams 38 and 39 located 
at axial ends 30, 31 of the control arm 11 and form integrally with the 
control arm 11. The cams 38, 39 are located at a section of the control 
arm 11 which is located in the opening 13 of the cylinder housing 14 and 
are spaced from the axis of rotation 44--44 of the steering nut 10 at a 
smaller distance 32 than a distance 33 between the guide strip 41, located 
at the end 16 of the control arm 11, and the axis of rotation 44--44. 
Additional followers generally designated by the reference numerals 21, 23 
are provided at the cylinder housing 14, with the followers 21, 23 being 
located on a housing part 27 which has or forms an opening 13 and is 
located between the cylinder bore 52 and the valve cylinder 50. The 
followers 21, 23 are formed by wall surfaces 36, 37 which delimit or 
define the housing part 27 at a location of the opening 13 in directions 
of the cyliner axis 24--24, and are located or spaced apart at a mutual 
axial distance 34 which distance is less than an axial length 35 of the 
control arm 11. 
As shown most clearly in FIG. 2, the working piston 8 and hence the control 
arm 11 are in a middle stroke position. Thus, the control arm 11 and the 
spool valve 14 can be steered in an area of medium stroke positions 
without any impediment up to corresponding end stops 67, 68. The spool 
valve 17, in a conventional manner, then sets a working pressure which 
increases directly with steering, with the working pressure reaching a 
maximum value shortly before reaching a corresponding end stop 67 or 68. 
As shown in FIG. 3, the steering nut 10 and the steering arm 11 are in a 
position generally designated by the reference numeral 19 in which the 
working piston 8 is in an area of that end position of its stroke which 
corresponds to the left stop position. In FIG. 3, it is assumed that the 
control arm 11, prior to reaching the position 19, was actuated in a 
steering position generally designated by the reference numeral 69 in 
which the spool valve 17 sets the working pressure roughly to a maximum 
value. Upon reaching the position 19, the engaging surface 37 on the 
housing part 27 engages the cam 38 and the control arm 11, whereby the 
control arm 11 is actuated in a direction of a neutral position shown in 
FIG. 1 for travelling in a straight line. If, upon an increase in the left 
steering engagement, the cam 38 reaches the area 34 of the end stop 68, 
the control arm 11 is reset to a steering position 70 in which the spool 
valve 17 sets the working pressure to a value which is less than the 
maximum pressure. However, the cam 38 may extend in a corresponding 
circumferential direction with respect to the axis of rotation 44--44 such 
that the control arm 11, upon reaching a position 19, is reset completely 
to the neutral position illustrated in FIG. 1. When the control arm 11 is 
returning, the cam 38 may travel in an umimpeded fashion into the opening 
30 and further mechanical steering, for example, increasing the left lock 
position manually, is possible without the spool valve 17 being influenced 
by such further steering in any manner until the vehicle reaches the end 
stop position for the smallest turning circle. Correspondingly, the cam 39 
cooperates with the engaging surface 36 when the working piston 8 and, 
therefore, the steering nut 9 and the control arm 11 reach an area which 
is associated with the stroke end position corresponding to the right-hand 
stop position. 
FIGS. 4-7 provide an example of a second embodiment of the power steering 
system wherein the guide strip 41 is provided on long sides thereof with 
sliding surfaces 42, 43, respectively, through which the steering arm 11 
cooperates with the side walls 46 or 47 of a circumferential groove 
generally designated by the reference numeral 48 in an articulated and 
essentially zero-play fashion. The sliding surfaces 42, 43 are located 
symmetrically with respect to a central plane 45--45 of the steering nut 
10, with the axis of rotation 44--44 being contained in the central plane 
45--45. The followers 20, 22 of the steering arm 11 which are forced to 
move along the piston axis 15--15 in this embodiment are each formed by a 
rounded half of an end surface 71 or 72 at the ends of the guide strip 41. 
Additionally, the followers 20, 22 have a distance 33 from the axis of 
rotation 44--44 which is greater than an effective radius 51 (FIG. 7) of 
the working piston 8. 
The cylinder housing 14 is provided in directions of the cylinder axis 
24--24 on both sides of the opening 13 with one housing recess 25 or 26, 
with each recess 25, 26 opening both to the cylinder bore 52 and to the 
opening 13. 
The control arm 11 and associated guide strip enter in the housing recesses 
25, 26 during stroke movements of the working piston 8. Housing parts 28, 
29 of the cylinder housing 14 delimit or define the housing recesses 25, 
26, with a receiving bore generally designated by the references numerals 
73, or 74 being respectively provided in the housing parts 28, 29 for 
enabling an insertion of a thread-adjusting pin 75 or 76. At an end of the 
thread adjustment pins 75, 76 facing the interior of the housing 14, each 
of the adjustment pins 75, 76 have an end pin 77 or 78, with an end of the 
end pins 77, 78 being formed to serve as an engaging surface 79 or 80 for 
engaging corresponding engaging surface 71 or 72 of the guide strip 41. 
The engaging surfaces 79, 80 are displacable in the directions of the 
corresponding longitudinal center pin axes 83, 84 of the adjustment pins 
75, 76 by corresponding threaded connections 88 or 89 of the thread 
adjustment pin 75 or 76 with the housing part 28 or 29, whereby a 
corresponding distance 85 or 86 of the engaging surface 79 and 80 in the 
directions of the cylinder axis 24--24 may be changed to a stroke center 
position 87--87 shown in FIG. 7. 
The thread adjustment pins 75, 76 are lockable on the cylinder housing 14 
in a conventional manner through the use of appropriate lock nuts 81, 82. 
However, despite the possible change in the distances 85 and 86, the 
arrangement of the thread adjustment pins 75 and 76 on the cylinder 
housing 14 is accomplished in such a manner that the mutual spacing 40 the 
engaging surfaces 79, 80 which are non-movable relative to the housing 14 
is always greater than an axial length 35 of the control arm 11. In this 
manner, the control arm 11, as shown most clearly in FIG. 5, may be 
steered or rotated in an unimpeded fashion up to end stops 67, 68 in a 
vicinity of the middle stroke positions of the working piston 8, in which 
the corresponding engaging surfaces 71, 79 and 72, 80 are disengaged. 
FIG. 6 provides an illustration of a status of the power steering system of 
the present invention when the working piston 8 and, therefore the 
steering or control arm 11 are in an area of the stroke end position 18 
which corresponds to the right-hand stop position. In this area, the 
control arm 11 is re-set to the neutral position shown in FIG. 4 by a 
mutual engagement of the corresponding engaging surfaces 71, 79 relative 
to the end stop areas 67. Consequently, the spool valve 17 has set the 
working pressure to the value which is less than the maximum value. In 
this construction also, when the right-hand lock position is manually 
increased, there is no undesired influence on the spool valve 17 because 
the sliding surface 42 of the control arm 11 can slide in an unimpeded 
manner along the engaging surface 79 of the end pin 77. Additionally, in 
this construction, the thread adjustment pin 75 may be disposed so that, 
in the stroke end position 18, the control arm 11 is reset completely to a 
neutral position to shut off the working pressure. In a vicinity of the 
stroke end position of the working piston 8, at the left stop position, 
the corresponding engaging surface 72, 80 cooperate in corresponding 
fashion to reset the spool valve 17. 
In order to be able to shut off or lower the working pressure in a vicinity 
of the end stroke positions of the working piston 8 according to the 
present invention, in both of the above described constructions, the 
followers 20, 22 of the control arm 11 and the followers 21, 23 of the 
cylinder housing 14, respectively, cooperate in such a manner that the 
spool valve 17 is actuated in the corresponding stroke end position in a 
direction of its neutral position for enabling the vehicle to travel in a 
straight line and, therefore, in a direction for shutting off or lowering 
the working pressure. The two constructions described hereinabove differ 
only in the spatial arrangement of the followers 20, 22, 21, 23 with 
respect to the control arm 11 or control housing 14 as well as the 
structural configuration of the respective followers. 
While we have shown or described several embodiments in accordance with the 
present invention, it is understood that the same is not limited thereto 
but is susceptible of numerous changes and modifications as known to one 
having ordinary skill in the art and we therefore do not wish to be 
limited to the details shown and described herein, but intend to cover all 
such modifications as are encompassed by the scope of the appended claims.