In a power-assisted rack-and-pinion steering apparatus, axial movements of a helical tooth pinion resulting from a torque being applied thereto cause corresponding displacements of the movable member of a control valve of the apparatus. A piston which moves axially together with pinion defines a sealed chamber of variable volume, which is filled with an incompressible fluid, such as oil or grease. The movable member of the control valve is slidably mounted along an axis different from the pinion axis and is operated by a push-rod having one end surface arranged so as to sense the pressure within the chamber of variable volume. Means are provided for holding the movable member of the control valve in its central neutral position in case the fluid contained within the chamber is lost.

The present invention relates to a power-assisted rack-and-pinion steering 
apparatus, of the known type comprising: 
a casing, 
a rack-engaging helical tooth pinion, 
a shaft connected to said pinion, 
a bearing rotatably supporting said shaft within a cylindrical cavity of 
the casing, said bearing being slidably mounted within said cylindrical 
cavity to allow axial displacements of the pinion, 
a control valve having a movable member which is slidably mounted within an 
auxiliary cavity of the casing along an axis different from the pinion 
axis, 
means for transforming an axial displacement of the pinion into a 
displacement of said movable member of the control valve, said means 
including 
a piston operatively connected to said bearing, with respect to axial 
displacements of the latter, 
a chamber of variable volume defined within said cylindrical cavity and 
facing said piston, said chamber being filled with a deformable and 
substantially incompressible material, 
a push-rod connected to said movable member and having one end surface 
arranged so as to sense the pressure within said chamber of variable 
volume. 
An apparatus of the above type is disclosed in EP-A-O No. 078 879. In this 
known solution, a rubber element is located within the said chamber of 
variable volume, so that the rubber of this element acts as the said 
deformable and substantially incompressible material. When a torque is 
transmitted by the steering wheel to the rack-engaging pinion, the latter 
is caused to move axially, together with the associated bearing and 
piston, by the axial forces generated because of the inclination of its 
helical teeth. Depending upon such axial forces, pressure exerted by the 
rubber element onto the push-rod varies, which results in a corresponding 
variation of the position of the valve movable member. Springs acting onto 
the bearing and onto the valve control member keep the rubber element 
under pressure and the movable member in a central neutral position when 
the torque applied onto the pinion is zero. 
Studies and tests conducted by applicant have shown that in the above known 
solution, the forces absorbed by the rubber element for changing its 
shape, and also the hysteresis of the apparatus may become quite high and 
prevent regular operation of the control valve. 
The object of the present invention is to overcome the above-mentioned 
problem. 
According to the present invention there is provided a power-assisted 
rack-and-pinion steering apparatus of the above-mentioned known type, 
characterized in that said deformable and substantially incompressible 
material is a fluid, said apparatus further including sealing means for 
sealing said chamber of variable volume from the remaining part of said 
cylindrical cavity and from said auxiliary cavity. The fluid is preferably 
oil or grease. 
In the apparatus according to the invention, the above-mentioned drawback 
of the prior known solution, namely the resistance opposed by the rubber 
element to change its shape and uniformly transmit the pressure, is 
completely eliminated, while the hysteresis of the apparatus is also 
reduced.

Referring to FIG. 1, there is shown a power-assisted rack-and-pinion 
steering apparatus 1 including a steering gear casing 2 having a passage 3 
for a steering rack 4. A rack-engaging helical tooth pinion 5 forms part 
of a shaft 6 which is driven by the steering wheel via a known 
transmission (not illustrated). Shaft 6 is rotatably mounted within the 
casing 2 by means of bearings 7, 8 which both allow axial displacements of 
the pinion 5 relative to the casing. 
Bearing 8 is slidably mounted within a cylindrical cavity 9 of the casing 2 
and is urged by a spring 10 against an annular piston 11 which is arranged 
around the shaft 6. Piston 11 and the wall of the cylindrical cavity 9 
define an annular chamber 12 of variable volume, wherein a rubber annular 
element 13 is located. 
A casing 14 is associated to the steering gear casing 2, the casing 14 
forming the body of a control valve 15 of the power-assisted steering 
apparatus. Valve 15 includes a movable member 16 which is slidably mounted 
within an auxiliary cavity 17 having an axis orthogonal to the axis of the 
shaft 6. A push rod 18 is slidably mounted within casing 2 along the axis 
of the auxiliary cavity 17. Push-rod 18 is interposed between the annular 
rubber element 13 and the movable member 16, the latter being urged 
against push rod 18 by a spring 19. Spring 10 and spring 19 keep the 
annular rubber element 13 under pressure and hold the movable member 16 in 
a central neutral position (illustrated in FIG. 1) when no torque is 
applied onto the pinion 5. When a torque is applied to the latter, this is 
caused to move axially, because of its helical teeth. This results in a 
different pressure exerted by piston 11 onto the annular rubber element 
13, which causes movement of the movable member 16 of the control valve 
from its central neutral position. 
As mentioned above, such known solution has the drawback that a resistance 
is opposed by the annular rubber element 13 to change its shape and 
uniformly transmit the pressure, the hysteresis of the device being also 
quite high. 
In FIGS. 2 through 6, the parts which are common to the apparatus shown in 
FIG. 1 are indicated by the same reference numerals. 
Referring to such figures, the main difference between the various 
embodiments illustrated therein and the prior art apparatus illustrated in 
FIG. 1 lies in that the annular rubber element 13 is eliminated and the 
chamber 12 is filled with a substantially incompressible fluid such as oil 
or grease. In FIGS. 2, 4 and 5, sealing rings 20, 21 and 22 are provided 
for isolating chamber 12 from the remaining part of the cylindrical cavity 
9 and from the auxiliary cavity 17. The operation of the apparatus is 
identical to that of the apparatus of FIG. 1. When the torque applied to 
the pinion 5 is zero, the fluid pressure acting against piston 11 and push 
rod 18 is balanced by springs 10, 19 and the movable member 16 is held in 
its central neutral position. If a torque is applied to the pinion by the 
steering wheel, the pinion receives from rack 4, because of the 
inclination of its teeth, an axial thrust, so that the pressure within the 
chamber 12 varies. This results in a corresponding variation of the thrust 
of the fluid against the push-rod 18, which causes movement of the movable 
member 16 from its central neutral position. 
FIG. 3 shows a different embodiment, wherein sealing rings 20, 21 are 
replaced by a deformable sealing hollow ring 23 located within chamber 12 
and filled with oil or gease. Push rod 18 and piston 11 are in contact 
with the outer surface of the hollow sealing ring 23. By this solution, 
any possibility of leakage of fluid from chamber 12 is eliminated. 
FIG. 4 shows an embodiment which is identical to that of FIG. 2 with the 
addition of means for preventing movement of the valve movable member 16 
from its central neutral position in case of loss of the fluid contained 
within chamber 12. 
Referring to FIG. 2, if the fluid within chamber 12 is lost, spring 10 will 
urge piston 11 to its end position in which the piston is in contact with 
a shoulder 24 defined on the wall of the cylindrical cavity 9. Under these 
conditions, spring 19 pushes the movable member 16 and push-rod 18 towards 
chamber 12 thus causing an undesired activation of the power-steering 
device. This irregular operation of the apparatus is prevented in the 
embodiment of FIG. 4, in which shoulder 24 is located in such a way that 
when the piston 11 is in contact therewith, its portion of greater 
diameter partially obstructs the hole of casing 2 wherein push-rod 18 is 
slidably mounted. Push rod 18 has such a length that, in its rest position 
(corresponding to the neutral position of movable member 16), its end 
surface facing chamber 12 is adjacent to such chamber but does not protude 
into the same. Thus, if fluid pressure fails, piston 11 reaches its end 
position against shoulder 24 (see dotted line in FIG. 4) preventing the 
push rod 18 from entering into chamber 12, thus holding movable member 16 
in its central netural position. The portion of greater diameter of piston 
11 has an outer circumferential chamber 25 on its surface within chamber 
12 for forcing push-rod 18 outwardly from chamber 12 in case the end 
surface of push rod 18 protrudes within chamber 12 when the movable member 
16 in its neutral position, which may be caused, for example, by 
manufacturing errors. 
FIG. 5 shows a variant of FIG. 4, wherein the function of the outer surface 
of piston 11 is accomplished by a circumferential surface 27 intermediate 
between portions of greater and smaller diameter of the piston. Surface 27 
cooperates with an axial shank 26 provided on the end surface of push-rod 
18 in order to stop the latter out from chamber 12 when piston 11 is moved 
to its end position against shoulder 24. Chamber 25 has the same function 
as in FIG. 2. 
FIG. 6 of the annexed drawings shows a further embodiment wherein the 
sealing ring 22 is replaced by a resiliently deformable sealing diaphragm 
28, whose outer periphery is clamped between the steering gear casing 2 
and a fixed plate 29 which is interposed between casing 2 and the valve 
body 14. Plate 29 has a hole 30 wherein push rod 18 is slidably mounted. 
Hole 30 has an enlarged portion which defines a cylindrical cavity 31. 
In its central portion diaphragm 28 has a curved portion defining an 
annular ridge coaxial with push rod 18. Such annular ridge defines a 
central seat on the diaphragm 28 for the push rod 18. The opposite surface 
of idaphragm 28 is in contact with an auxiliary plunger 32 which is 
slidably mounted within a hole 33 of casing 2. Plunger 32 has an end 
surface facing chamber 12 and is provided with an axial passage 34 through 
which the fluid flows when plunger 32 and diaphragm 28 move. 
Operation of the apparatus of FIG. 6 is identical to that of the apparatus 
of FIG. 4. When the pressure whithin chamber 12 is varied, the push rod 18 
and the central movable part of diaphragm 28 move axially at the same 
time, as a unit, acting as a piston slidable within a cylindrical cavity 
whose diameter is the mean of the diameters of the push rod 18 and cavity 
31. The tapered circumferential portion 35 and the outer surface of piston 
11 acts as a safety means for preventing undesired operation of the valve 
in case of loss of pressure within chamber 12, in the same way as in FIGS. 
4 and 5. 
In all the annexed figures the axis of the valve has been supposed 
orthogonal to end in the same plane of the axis of the pinion, but it is 
evident that their related positions can be quite different without 
changing the operation of the device, on condition that the pressure in 
chamber 12 can act against push-rod 18. 
While the invention has been particularly shown and described with 
reference to some preferred embodiments thereof, it will be understood by 
those skilled in the art that various changes in form and details may be 
made therein without departing from the spirit and scope of the invention.