GB1576053A

GB1576053A - Hydraulic servo steering unit for a motor vehicle

Info

Publication number: GB1576053A
Application number: GB1301977A
Authority: GB
Original assignee: Riva Calzoni SpA
Current assignee: Riva Calzoni SpA
Priority date: 1976-03-29
Filing date: 1977-03-28
Publication date: 1980-10-01
Also published as: HU175026B; FR2346198A1; CS212789B2; SU689608A3; FR2346198B1; IT1063862B; ES457135A1; DE2630616A1; DE2630616B2; DE2630616C3

Description

(54) A HYDRAULIC SERVO STEERING UNIT FOR A MOTOR VEHICLE (71) We, RIVA CALZONI S.p.A., an Italian joint stock company of 34 via Stendhal, Milan, Italy, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a hydraulic servo steering unit in which angular movement of a steering shaft in one sense or another can be achieved by a hydraulic mechanism connected with a supply duct, for connection with a source of fluid under pressure and connected with a discharge duct for connection to an unpressurised reservoir, the steering unit incorporating a limiting device for angular movement of the steering shaft.

Such a limiting device may, for example, be incorporated in a hydraulic servo steering unit of the kind having a cylinder receiving slidably, in sealing-tight relationship, a piston dividing the cylinder into two chambers, a distributor for directing fluid under pressure to one or other of these chambers while at the same time connecting that chamber to which pressure medium is not supplied to the discharge duct, the steering shaft for control ling the steering of wheels on the vehicle having fixed thereto a toothed segment which is keyed on to the steering shaft and which engages with a rack formed by a toothed portion of the body of the piston whereby rectilinear movements of the piston are converted into corresponding angular movements of the steering shaft in one direction or the other.

Such a construction of a hydraulic servo steering unit is known.

The limiting device in a servo steering unit of the last-mentioned kind is preferably arranged so as to relieve the hydraulic forces acting on the piston at the limits of movement thereof which correspond to the desired limits of angular displacement of the steered wheels of the vehicle, so as to prevent an excessive stroke of the piston from damaging the steering mechanism.

A number of mechanical and hydraulic devices have already been suggested for this purpose. It has, for example, been proposed to provide two stops on the end of the steering shaft, the stops being apart from each other and co-operating with respective valves, each said valve being disposed in a respective duct connected to a respective one of the two chambers of the servo steering unit cylinder, each said valve including a movable part which can be actuated to open the valve by the respective stop in the respective angular end position which must not be exceeded by the steering shaft. The particular cylinder chamber which is pressurized during movement of the steering shaft in one direction is therefore immediately connected to the discharge when the respective limiting position is reached so that the piston with its connected mechanisms stops.

However, this prior art solution to the problem of limiting the movement of the steering shaft has numerous disadvantages, including the necessity of using two separate valves, one for each of the cylinder chambers of the servo steering unit, so that the device becomes complicated and uncertainty arises as regards reliable operation, more particularly bearing in mind that, as in all hydraulic devices, the cylinder of the servo steering unit also has a main valve for the relief of excess pressure, and so that the total number of valves is increased to three.

It is an object of the invention to provide a hydraulic servo steering unit incorporating an improved limiting device which is simple in construction and reliable in operation as compared with known devices.

According to the invention there is provided a hydraulic servo steering unit in which pivotal movement of a steering shaft in one sense or the other can be achieved by a hydraulic mechanism connected with a supply duct for connection with a source of fluid under pressure and connected with a discharge duct for connection to an unpressurized reservoir, the hydraulic servo steering unit including a limiting device including a discharge valve having a valve member biased by a spring towards a closed position, the discharge valve when open establishing a connection between said supply duct and said discharge duct to relieve the supply duct, and thus the hydraulic mechanism, of fluid pressure, the device including a mechanism arranged to lift said valve member off its seat, to open the valve, in each of two angular positions of said steering shaft about its axis, said discharge valve also being arranged to be lifted from its seat by a fluid pressure exceeding a predetermined value in said supply duct, whereby said discharge valve also serves as a pressure relief valve.

Thus the stroke limiting device serves to limit the angular movement of the steering shaft in both directions, using a single valve which also acts as an excess pressure discharge valve.

preferably the valve member of said discharge valve includes an enlarged closure member for engagement with said valve seat, an elongate stem projecting beyond said closure member from the side thereof which ligages said valve seat, said valve including a body part providing said valve seat and also providing an annular chamber through which said elongate stem extends, said chamber being connected with said supply duct, the free end of said elongate stem bearing against two independent rocker arms, two stops being provided on the periphery of the steering shaft, spaced angularly about the axis of said shaft, and each stop acting on a projection of a respective said rocker arm to determine a respective limit of the angular movement of said shaft, each said rocker arm, when acted on by the respective said projection, abutting the free end of the elongate stem of the valve closure member, and therefore opening the valve against the bias of the spring to prevent further angular movement of said shaft by the hydraulic mechanism.

The invention therefore affords a very considerable advantage, namely that the use of a single discharge valve gives security against undesirable excess pressures, while at the same time limiting the angular movement of the steering shaft in both directions.

According to a very advantageous preferred feature of the invention, each of the rocker arms is arcuate and engages around the outer periphery of the end portion of the steering shaft and each is articulated to an end of a respective arcuate bearing member which alsp partially extends around the end portion -of the steering shaft, each bearing member having a serrated portion engaging with an adjusting screw or rack which can be operated from outside the servo steering unit.

This feature of the invention also affords the advantage that each rocker arm can be moved pivotally about the axis of the steering shaft in relation to the respective stop on the steering shaft periphery, so that the angular movement which the steering shaft must perform before the discharge valve comes into operation, to stop said angular movement, can be altered.

An embodiment of the invention is described below with reference to the accompanying drawings, wherein: Fig. 1 is a simplified partial view in longitudinal section of a servo steering unit embodying the invention; Fig. 2 is a view of a stroke limiting device, incorporated in the steering unit, in section along the line II---II in Fig. 3 and Fig. 3 is a sectional view taken along the line Ill-Ill in Fig. 2.

Fig. 1 shows a cylinder 1 of a hydraulic servo steering unit which has a steering screw 2 extending axially in the cylinder 1 and connected to the steering wheel (not shown).

The unit also has a steering nut 3 on the steering screw 2, the nut 3 being located in an axial cavity 5 in a piston 4 sealingly slidable in the cylinder 1. The piston 4 subdivides the cylinder 1 into two chambers 6, 7 to which the pressure medium can be supplied as directed by a hydraulic distributor 8 which is disposed in the piston 4 in a cavity 9 and actuated by a rod 10 which can be moved axially by a lever 11 (shown only in part) pivotally mounted in the piston 4 and connected to the steering nut 3. The piston 4 is constructed with a toothed portion 12 a rack engaging with a toothed segment 12a fixed on to a steering shaft 13, which controls the vehicle steered wheels (not shown). The shaft 13 is not shown in Fig. 1 but its longitudinal axis extends perpendicular to the plane of Figure 1, through the pivotal axis of the toothed segment 12. The shaft 13 extends within a tubular casing part which branches off from and is integral with the part of the cylinder 1 providing the bore in which the piston 4 slides. Figure 3 is a view in section along the axis of shaft 13 and shows an end part of shaft 13 accommodated in an end part of said tubular casing part remote from the part providing the bore in which the piston 4 slides. A cover 1b is bolted to the free end of said tubular casing part as shown in Fig. 3.

As shown in Figs. 2 and 3, the cover 1b has a bore lathe axis of which is perpendicular to and intersects with the axis of the shaft 13. Within bore la is mounted a discharge valve 14 which has a tubular sleeve 15 forming a valve seat 16 against which the frustoconical underside of a closure member 17 of a valve member bears. The valve member has an actuating stem 18 extending along the axis of the bore la through the axial passage of sleeve 15, the stem 18 projecting from the side of the closure member which engages the valve seat. The stem 18 is formed, in the region inside the sleeve 15, with a circumfer- ential groove 19 co-operating with the inner wall of the sleeve 15 to form an annular chamber 20. The chamber 20 is permanently connected via radial ducts 21 and a circumferential groove 22 on the exterior of the sleeve 15, to the main supply duct 23 for the pressure medium.

The duct 23 is the means by which fluid under pressure is fed from the source of fluid under pressure, e.g. a fluid pump, to the distributor 8 to be directed to one or other of the chambers 6, 7. The closure member 17 is urged into engagement with the seat 16 by a spring 24 which is prestressed and disposed in a hollow closure screw 25 closing the bore la and bounding a cavity 26 (Fig. 2) on the side of the closure member remote from seat 16. The cavity 26 is permanently connected to the unpressurized discharge reservoir (not shown) via ducts 27. The free end 1 8a of the stem 18 of the valve member extends out of the fixed sleeve 15 of the valve into a cavity 28 in which an end portion 1 3a of the steering shaft 13 is disposed, said end portion protecting from a bearing member 29 accommodating a roller bearing 30 for the shaft 13 and tubular casing part.

A bolt 31 has an enlarged head which is held captive in an axial bore in the end portion 1 3a and a shank which extends axially from the last mentioned bore through a bore in the cover ib and is held in place by a nut on the outside of cover ib. The bolt 31 serves to prevent, or at least limit, axial movement of the shaft 13.

Encircling the extreme free end part of portion 13a of shaft 13 is an assembly of parts which in its entirety has the general form of annulus received in an annular groove defined between an abutment provided by an end part 39 of bearing member 29 and an opposing abutment provided by a part 38 of the cover lb.

The said annulus comprises arcuate bearing members 35 and 37 which occupy respective positions along the axis of the shaft 13 and lie in the part of the annular groove referred to furthest from the valve 14. The said annulus further comprises an arcuate rocker arm 32 which is pivotally connected at one end by a pin 34 to the bearing member 35 and extends around the part of the circumference of portion 13a nearer valve 14 and passes stem 18. The said annulus still further comprises an arcuate rocker arm 33 which is pivotally connected at one end by a pin 36 to the arcuate bearing member 35 and extends around the part of the circumference of the portion 13a of the shaft 13 nearest the valve 14 and passes the stem 18. The rocker arms 32, 33 overlap axially and each is aligned with a respective half of the end face of part 18a.

The major portions of the bearing members 35, 37 have only a slight radial clearance with the portion 13a but the arms 33, 32 and the adjoining parts of the corresponding bearing members 35, 37 have a substantial radial clearance 40, 41 with respect to the outer surface of the portion 13a. The free ends 32,a 33a of the arms 32, 33 bear against the aforementioned surface via respective projections 42, 43 on the arms. Spaced angularly from one another on the outer surface of portion 13a, and disposed at the axial position of the arms 32, 33 are stops 44, 45 which are fixed to portion 13a. The arcuate bearing members 35, 37 also have toothed portions 46, 47 engaging with racks 48, 49 of screws 50, 51 which can be adjusted from the outside of the cover lb and have lock nuts 52, 53. The racks 48, 49 may comprise a series of circumferential grooves on each screw 50, 51 or simply screw threads.

Operation of the device described is as follows:-- Assuming the valve 14 to be closed initially and that the servo steering unit is operating at normal pressure, a movement of the piston 4 in one direction produces a rotation of the shaft 13, for instance, clockwise, as viewed in Fig. 2. After a predetermined angular movement of shaft 13 as a result of a corresponding movement of the piston 4, the stop 44 moves under the projection 43 of the rocker arm 33. This results in the adjacent part 33a of the arm being lifted the arm pivoting outwardly about the pin 36, so that the end 33a co-operates with the end 18a of the stem 18 of the closure member 17, and the latter is lifted, the force of the spring 24 being overcome. This lifting of member 17 results in the opening of the valve 14 and the discharge of the pressure medium from the annular chamber 20 to the cavity 26 of the discharge duct 27. The hydraulic pressure under the action of which the piston 4 and thus the shaft 13 was moving is therefore relieved and the piston 4 stopped in the stroke end position reached.

With a movement in the direction opposite to the previously described one, during which as a result of the axial movement of the piston 4, an angular movement of the shaft 13 takes place anti-clockwise, the stop 45 moves under the projection 42 and lifts the arm 32. In this case also, the closure member 17 is lifted, the force of the spring 24 being overcome, and the valve 14 connects the main supply duct with the discharge reservoir, again relieving the pressure of the fluid in the cylinder 1.

In this case also the piston 4 is retained in the stroke end position reached. If the stroke end positions of the piston 4 are to be adjusted the screws 50 and/or 51 are adjusted, and by their rotation and consequent axial movement impart angular movement to the arcuate bearing members 35, 37 about the axis of shaft 13 and as a result make the projections 42, 42 approach or move away from the fixed stops 44, 45.

If excess pressure occurs in the hydraulic circuit, the valve 14 opens automatically, the closure force of the spring 24 being overcome.

The device described therefore affords the advantage that the stroke ends for the piston 4 can be determined and also security obtained against excess pressures by means of a single discharge valve in addition to the advantage that the length of the stroke in one direction or other of the servo control system piston can be controlled, in dependence on necessities, in relation to the maximum steering angles to be achieved.

Other features of the hydraulic servo steering unit as a whole, and to which the accompanying drawings relate, are described in greater detail in our co-pending applications No. 13017/77 Serial No. 1576051 and No. 13018/77 Serial No. 1576052 to which reference should be had.

WHAT WE CLAIM IS: 1. A hydraulic servo steering unit in which pivotal movement of a steering shaft in one sense or the other can be achieved by a hydraulic mechanism connected with a supply duct for connection with a source of fluid under pressure and connected with a discharge duct for connection to an unpressurized reservoir, the hydraulic servo steering unit including a limiting device including a discharge valve having a valve member biased by a spring towards a closed position, the discharge valve when open establishing a connection between said supply duct and said discharge duct to relieve the supply duct, and thus the hydraulic mechanism, of fluid pressure, the device including a mechanism arranged to lift said valve member off its seat, to open the valve, in each of two angular positions of said steering shaft about its axis, said discharge valve also being arranged to be lifted from its seat by a fluid pressure exceeding a predetermined value in said supply duct, whereby said discharge valve also serves as a pressure relief valve.

2. A hydraulic servo steering unit as claimed in Claim 1 wherein the valve member of said discharge valve includes an enlarged closure member for engagement with said valve seat, an elongate stem projecting beyond said closure member from the side thereof which engages said valve seat, said valve including a body part providing said valve seat and also providing an annular chamber through which said elongate stem extends, said chamber being connected with said supply duct, the free end of said elongate stem bearing against two independent rocker arms, two stops being provided on the periphery of the steering shaft, spaced angularly from one another about the axis of said shaft and each stop acting on a projection of a respective said rocker arm to determine a respective limit of the angular movement of said shaft, each said rocker arm, when acted on by the respective said projection, abutting the free end of the elongate stem of the valve member, and therefore opening the valve against the bias of the spring to prevent further angular movement of said shaft by the hydraulic mechanism.

3. A hydraulic servo steering unit as claimed in Claim 2, in which each of the rocker arms is arcuate and engages around the outer periphery of an end part of the steering shaft and is articulated to the end of a respective arcuate bearing member which also partially extends around the pin, each said bearing member having a serrated portion engaging with an adjusting screw or rack which can be operated from outside the servo steering unit.

4. A hydraulic servo steering unit as set forth in claim 3 in which the rocker arms and the adjoining parts of the bearing members are spaced radially outwardly from the outer surface of the end part of the steering shaft, the stops projecting correspondingly from the outer surface of the end part of the steering shaft, the free end of each rocker arm bearing via a respective projection against the said outer surface.

5. A hydraulic servo steering unit as set forth in claim 3 or claim 4 in which the rockers arms and the corresponding arcuate bearing members are guided in an annular groove which is defined between a stop provided by a bearing member for the steering shaft and an opposing stop provided by a cover of the servo steering unit.

6. A hydraulic servo steering unit, substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. advantage that the stroke ends for the piston 4 can be determined and also security obtained against excess pressures by means of a single discharge valve in addition to the advantage that the length of the stroke in one direction or other of the servo control system piston can be controlled, in dependence on necessities, in relation to the maximum steering angles to be achieved. Other features of the hydraulic servo steering unit as a whole, and to which the accompanying drawings relate, are described in greater detail in our co-pending applications No. 13017/77 Serial No. 1576051 and No. 13018/77 Serial No. 1576052 to which reference should be had. WHAT WE CLAIM IS:

1. A hydraulic servo steering unit in which pivotal movement of a steering shaft in one sense or the other can be achieved by a hydraulic mechanism connected with a supply duct for connection with a source of fluid under pressure and connected with a discharge duct for connection to an unpressurized reservoir, the hydraulic servo steering unit including a limiting device including a discharge valve having a valve member biased by a spring towards a closed position, the discharge valve when open establishing a connection between said supply duct and said discharge duct to relieve the supply duct, and thus the hydraulic mechanism, of fluid pressure, the device including a mechanism arranged to lift said valve member off its seat, to open the valve, in each of two angular positions of said steering shaft about its axis, said discharge valve also being arranged to be lifted from its seat by a fluid pressure exceeding a predetermined value in said supply duct, whereby said discharge valve also serves as a pressure relief valve.