Fluid handling – Processes – Cleaning – repairing – or assembling
Reexamination Certificate
1999-07-06
2001-04-17
Rivell, John (Department: 3753)
Fluid handling
Processes
Cleaning, repairing, or assembling
C091S37500R, C137S625230
Reexamination Certificate
active
06216718
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to control valves for motor vehicle hydraulically-assisted steering. More specifically, this invention relates to a method which makes it possible, in the context of the industrial-scale production of such control valves, to reduce the spread on the torque of the “valve law”.
DESCRIPTION OF THE PRIOR ART
FIG. 1
of the appended diagrammatic drawing depicts, in longitudinal section, a conventional control valve (such as the one disclosed, for example, in French patent application No. 2752809 or its European equivalent No. 0827892 in the name of the Applicant Company), as a reminder of the structure and operating principle, and to explain the problem underlying the present invention.
The control valve comprises, arranged along its longitudinal axis
1
, a pinion
2
which is designed to mesh permanently with a rack (not depicted) of the power-assisted steering. A torsion bar
3
, arranged along the longitudinal axis
1
, is connected to the pinion
2
by a rigid connection
4
, at one end of this torsion bar
3
. A spool
5
, of tubular overall shape, mounted around the torsion bar
3
, is centered by one of its ends in the pinion
2
, via a bearing
6
which allows it a certain degree of angular travel under no stress, but which travel is limited by a positive stop (not depicted) provided between this spool
5
and the pinion
2
. The spool
5
is centered, at its other end, on the torsion bar
3
and is rigidly connected thereto, near this end, by a connecting element, particularly a pin
7
.
A sleeve
8
partially surrounding the spool
5
, is centered on this spool
5
and angularly connected to the pinion
2
by another pin
9
. Complementary hydraulic fluid control grooves
10
,
11
are formed in the sleeve
8
and in that region of the spool
5
which is covered by the sleeve
8
, so as to allow the hydraulic fluid to pass to one or other of the chambers of the hydraulically-assisted steering ram. In operation, the relative angular positions of the grooves
10
,
11
define variable passage cross sections for the hydraulic fluid.
More specifically, when the control valve is in operation, a resistive torque (resulting from the adhesion of the wheels of the vehicle and transmitted through the rack) is exerted on the pinion
2
and an input torque originating from the vehicle steering wheel is exerted on the spool
5
. These two torques are in opposite direction, and they place load on the torsion bar
3
, leading to an angular offset between the spool
5
on the one hand, and the sleeve
8
(angularly connected to the pinion
2
) on the other hand. This angular offset opens the hydraulic passage to one chamber of the ram in proportion to the resultant torque, at the same time as the opposite chamber is placed in communication with the hydraulic return circuit. The pressure of the hydraulic fluid conveyed to one of the ram chambers, and known as the “assistance pressure” increases as the angular offset becomes greater, and is therefore an increasing function of the torque applied to the control valve.
The way the control valve operates is characterized by a “control valve law” which is the law P=f(C) of the variation in assistance pressure as a function of torque, represented by the curve in FIG.
2
.
To determine the control valve law, the pinion
2
is prevented from rotating by a rigid device, and a torque is applied to the spool
5
in the clockwise direction and then in the counterclockwise direction, this respectively corresponding to the vehicle turning to the right and to the vehicle turning to the left. While torque is being applied to the spool
5
, the pressure in the corresponding hydraulic circuit increases. It is this hydraulic pressure which is measured, as a function of the torque applied to the spool
5
, the measurement points thus determined defining the curve and therefore the control valve law.
For a control valve of a given type, this law is determined, in particular, by the characteristics of the torsion bar
3
which acts as a spring and allows the torque needed to angularly offset the spool
5
with respect to the sleeve
8
to be adapted in order to meet the motor manufacturers' specifications. This characteristic of the torsion bar
3
is known as the angular stiffness; it depends on the geometry of the said torsion bar and therefore on the various diameters and lengths which geometrically define this bar.
The torsion length of the torsion bar
3
, that is to say the axial distance between the rigid connection
4
with the pinion
2
and the pinned connection (by pin
7
) with the spool
5
also affect the ultimate stiffness of the torsion bar
3
. This length is a predefined dimension D which depends, in particular, on the axial position of the pin
7
on the spool
5
and the torsion bar
3
. In the current state of the art, the pin
7
, for a given control valve, has a single axial position and therefore an unvarying position. The machining needed (simultaneous diametral drilling of the torsion bar
3
and of the spool
5
) to push the pin
7
in at its predefined position is achieved, when the control valve is being assembled, after a hydraulic regulation has been performed in order to obtain a valve law curve which is identical (symmetric) for turns to the right and turns to the left.
However, as illustrated by the dotted lines in
FIG. 2
, there is, for a set of theoretically identical control valves, a spread on the values of torque for a given pressure. Put another way, for the same pressure P, the applied torque C may be higher or lower, because of machining inaccuracies and other variations in geometric or assembly parameters that affect the hydraulic passage cross sections and the stiffness of the torsion bar
3
. In particular, the inaccuracies in the machining of the hydraulic fluid. control grooves
10
,
11
, which are the result of the limitations of the industrial machining facilities used, have a direct influence on the spread of the valve law. The geometry of the torsion bar
3
and the torsion length D of this bar also exhibit variations which influence the stiffness of the said torsion bar and therefore the spread in the valve law. A combination of these causes leads to a not insignificant amount of spread, as illustrated by FIG.
2
.
SUMMARY OF THE INVENTION
The present invention sets out to avoid the drawbacks of such a spread on the control valve law, and its purpose is therefore to greatly reduce the spread on torque to obtain practically the same assistance pressure on a significant number of valves produced on an industrial scale.
To this end, the subject of the invention is a method for reducing the spread on the torque of a control valve for motor vehicle hydraulically-assisted steering, the method consisting, for each control valve produced, in adapting the stiffness of the torsion bar and/or of the connection between this torsion bar and the spool of the control valve according to the difference between the valve law or power-assistance law measured for this valve and the theoretical law for the type of control valve concerned.
Thus, the inventive step consists in adapting or modifying particularly the stiffness of the torsion bar according to the observed difference between the measured valve law and the theoretical law, so as to “recenter” the actual valve law as best possible with respect to the desired theoretical law. As this operation is repeated individually for each control valve produced, this solution makes it possible to reduce the spread on the valve law across an entire industrial-scale production run of some arbitrary size.
In the context of the present invention, adaptation may be achieved in various ways, possibly combined with each other.
In a first embodiment of the method that is the subject of the invention, the stiffness of the torsion bar is adapted by selecting the axial position of the pin that connects this torsion bar with the spool of the control valve, so as to alter the torsion length of said torsion bar. In the contex
Dubno Herbert
Rivell John
Schoenfeld Meredith H
Societe de Mecanique d'Irigny
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