Hydraulic assisted steering device for a motor vehicle

A hydraulic assisted steering device for a motor vehicle, of the type having a rotary distributor, comprising a distribution sleeve which extends a steering control member, a rotor of generally tubular shape, which is disposed within the sleeve, a torsion bar which is housed within the rotor and which has one end fixed to the rotor and the other end fixed to the steering control member, and a hydraulic circuit for generating a control value in response to an angular movement imparted to the rotor. According to the invention, the torsion bar comprises an active portion of constant section of high-resiliency steel, and two end members and ordinary steel, which are respectively engaged on the ends of the active portion and are fixed to said ends, the end members being respectively fixed with respect to the rotor and the steering control member.

The present invention concerns a hydraulic assisted steering device for a 
motor vehicle of the type having a rotary distributor comprising a 
distribution sleeve which extends a steering control member, a rotor of 
generally tubular shape, which is disposed within the sleeve, a torsion 
bar which is housed within the rotor and which has one end fixed to the 
rotor and the other end fixed to the steering control member, and a 
hydraulic circuit for generating a control value in response to an angular 
movement imparted to the rotor. 
A device of this type is described for example in French Pat. No. 2 178 
379. In the known device, the torsion bar which forms the resilient 
connection between the rotor and the steering control member is made of a 
high-resiliency steel and has a first end portion which is fixed to the 
rotor, whose diameter is equal to the inside diameter of the rotor, a 
central portion of smaller diameter, and a second end portion which is 
fixed to the steering control member and which is received in a bore 
formed in the steering control member, the diameter of the second portion 
being substantially the same as that of the first portion. The connection 
between the central portion and the end portions is made progressively, 
according to a curvated correcting outline. 
Disadvantageous stress concentrations occur in those connecting regions of 
variable section, and such stress concentrations increase the fatigue of 
the torsion bar. The present invention aims to overcome this disadvantage 
and for this purpose, in the assisted steering device according to the 
invention, the torsion bar comprises an active portion of constant section 
made of high-resiliency steel and two end members made of ordinary steel, 
which are respectively engaged on the ends of the active portion and which 
are fixed thereto, the end members being respectively fixed with respect 
to the rotor and the steering control member. 
The invention makes it possible to avoid the stress concentrations since 
the active portion is of constant section and the end members are of a 
material which is much more malleable than the material of the active 
portion. 
Machining is markedly simplified since the active portion, being of 
constant section, can be produced by drawing. 
In addition, as only the active portion is of high-resiliency material, it 
is possible to use a steel which has a much higher level of strength than 
in the prior art, in which the torsion bar is in one piece and in which, 
in consequence, the end portions are also of high-resiliency steel. This 
makes it possible to reduce the length of the torsion bar and therefore 
the space occupied by the device, this advantage being important as far as 
vehicle components are concerned. 
The end members are advantageously fixed to the active portion by a force 
fit process or by crimping of the end members which for that purpose are 
provided with blind bores of a diameter adapted to the type of operation 
envisaged.

The distributor for a hydraulic assisted steering device as shown in FIG. 1 
comprises a body 1 provided with bore 2. Mounted in the bore 2 is a 
distribution sleeve 3 which is fixed in position by means of a pin 4 to a 
steering control member 5 which in the present embodiment comprises the 
pinion of a rack and pinion steering assembly. 
Mounted in the bore 6 of the distribution sleeve 3 is a tubular rotor 7, 
the rotor being the component to which the steering forces produced by the 
driver are applied. 
The rotor 7 is resiliently connected to the pinion 5 by a torsion bar 
which, in accordance with the invention, comprises a central portion 10, 
an end member 11 which is fixed to the rotor by means of a pin 12 and a 
second end member 13 which is fixed to the pinion 5 by a pin 14, the two 
end members being rigidly connected to the central portion 10. 
The central portion 10 alone is of high-quality steel, for example spring 
steel of type 50 CV4 with a strength R.sub.N =150 kg/mm.sup.2. By way of 
contrast, the end members are of ordinary steel, for example of type XC 
35, with a strength R.sub.N =45 kg/mm.sup.2. 
The central portion 10 is of constant section, advantageously being a 
prismatic section and in particular hexagonal, as in the illustrated 
embodiment. 
The method of connecting the end members to the central portion must ensure 
a rigid connection. FIGS. 2a and 2b show a method of assembly by force 
fitting the end of the central portion in a blind bore 15 formed in the 
end member, the diameter of the bore being less than the circle 
circumscribed about the section of the portion 10. 
FIGS. 3a and 3b show another method of assembly wherein the part 16 of the 
end member 11 or 13 into which the end of the central portion 10 engages 
is crimped radially onto the end of the central portion. In this case, the 
blind bore 17 formed in each end member is of a diameter such that it can 
receive the end of the central portion 10, with clearance. 
As for the rest, the device illustrated is conventional, in particular as 
regards the hydraulic elements. It will be pointed out only that the space 
between the distribution sleeve 3 and the body 1 is sealingly partitioned 
by seals as illustrated at 20, that the chambers so defined are 
respectively connected by ports as indicated at 21 to a pressure system 
and to a discharge means, and to the two compartments of an assistance 
jack (not shown), and that conduits as indicated at 22 and 23 are 
provided, the arrangement being such that when the rotor 7 turns as a 
consequence of a steering movement, the relative rotation occurring 
between the rotor and the sleeve results in an imbalance between the 
pressures of the assistance jack, which depends on the steering angle. All 
this is known and there would be no need for detailed explanation in this 
respect. 
Furthermore, the active or control portion 10 at least may be 
advantageously submitted to a "shot-peening" treatment in order to 
eliminate superficial miscroscopic irregularities and cavities at the 
rough surface of the bar forming said active portion which constitute 
incipient features.