GB2102753A - Travel limiting device for hydraulic power steering - Google Patents

Abstract

A travel limiting device for hydraulic power steering is constituted by two valve members (31, 35, 36, 37) which are inserted in respective exhaust ducts (21, 22, 23, 24, 25) for fluid under pressure, and are housed in the wall (18) of the power steering cylinder (1) beside a sector gear (4) which drives the steering shaft. These valve members (31, 35, 36, 37) are actuated by respective operating rods (41) with free ends (44) which project into the cylinder (1) in positions such that one is engaged by a respective side (52, 53) of the sector gear (4) in each of the end of travel positions of the maximum allowed steering. <IMAGE>

Description

SPECIFICATION Travel limiting device for hydraulic power steering The present invention relates to atravel limiting device for hydraulic power steering of the type com- prising a cylinder within which is sealingly slidable a piston defining two opposed operating chambers, a distributor for supplying hydraulic fluid under pressure to one of the opposed chambers and simultaneously connecting the chamber not supplied with fluid to an exhaust, a shaft for effecting the steering of the steered wheels of a vehicle, a sector gear keyed onto the shaft and engaged with a rack on the piston to transmit rectilinear displacements of the piston to the steering shaft as corresponding angular displacements, and a screw housed axially within a cavity in the piston and engaged, by means of a reversible connection, with a nut mounted on one end of the piston for limited rotation to control the distributor, the free end of the screw projecting axially and sealingly from the cylinder for connection with a steering member.

Hydraulic power steering of the general type specified above is well known in the art. For constructional and operational details, reference may be made, for example, to GB-A-1,576,051 and GB-A-1,576,052.

In orderto allowthe proper functioning of power steering in general, and of the type specified above in particular, it is necessary to provide a device for limiting the angular movement, in both directions, of the shaft which effects the steering of the wheels.

Such a device serves primarily to avoid damage to the steering members of the vehicle in the position of maximum swivelling of the steered wheels, but also permits resting of the fluid supply pump so as to avoid excessive overheating of the fluid in the reservoirand resulting damagetothe pump.

For this purpose, several types of travel limiters have already been proposed which comprise a pair of valves mounted, for example, on the piston, and a pair of respective adjustable abutments carried by the end walls of the cylinder in alignment with the valves. One ofthevalves is opened in each maximum steering position by coming into contact with the respective abutment element, with the result that the pressure in the active chamber of the cylinder is exhausted and the piston stroke is stopped. One example of this type of travel limiter is described in DE-A-1,455,939.

Alternatively, according to the prior art, a pair of valves is mounted in a fixed position on the cylinder body, or on parts connected thereto, and respective adjustable abutments are carried by the piston and the steering shaft. Examples of this type of travel limiter are described in DE-B-1,291,644, DE-B-1,159,286 and DE-B-1,160,318.

One of the basic limitations encountered in the known hydraulically-operated travel limiting devices described above lies in the impossibility of effecting immediate adjustment during their installation in the vehicles when only the purely mechanical parts of the steering gear are present. Indeed, for such adjustment, it is neccssary to wait until the assembly of the vehicle is completed to the extent of having installed the pump, which involves the presence of its piping, the hydraulic fluid reservoir, the hydraulic fluid and its operating motor.

The object of the present invention is basically that of providing a travel limiting device for hydraulic power steering, which allows its direct adjustment during assembly of only the mechanical components of the steering gear, without waiting for the hydraulic components to be installed and become operative.

This object is achieved according to the present invention by a travel limiting device which com- prises two valve members which are inserted in respective fluid exhaust ducts and are housed in the wall of the cylinder beside the sector gear, each valve member being kept normally closed by a respective spring with a predetermined load greater than the maximum operating pressure in the cylinder, and two operating rods each connected at one end to a respective valve member and projecting at its free end into the cylinder for engagement with a respective side of the sector gear when the latter is displaced in one direction or the opposite direction to reach the respective maximum steering position, the operating rods being displaceable axially by a predetermined distance against the respective springs.

Preferably, the valve members are housed in respective tubular housings which are threaded on at least a portion of their outer walls and are engaged with complementary threading in respective holes in the cylinder wall.

In accordance with another preferred embodiment of the invention, the free end of each operating rod has a movable spherical cap, and each side of the sector gear has a respective recess with a surface which mates with that of the cap.

The invention will now be described in more detail, by way of example, with reference to the appended drawings, in which: Figure lisa longitudinal partial section of a hydraulic power steering unit provided with a travel limiting device according to the invention; Figure 2 is a section taken on the line Il-Il of Figure 1, and Figure 3 is a view on an enlarged scale of a detail ofthetravel limiting device of Figure 1 in a maximum steering position.

With reference to the drawings, the cylinder 1 of a hydraulic power steering unit houses a piston 2 with a rack 3, and a sector gear 4 which is keyed onto a steering shaft 5 and is engaged with the rack 3. In an entirely conventional manner, the piston 2 divides the cylinder 1 into two operating chambers 6,7 connected alternatively, through a distributor 8, to a source of fluid under pressure and to an exhaust (not shown). A screw 10 is movable within an axial chamber 9 in the piston 2 and is engaged with a nut The drawings originally filed were informal and the print here reproduced is taken from a later filed formal copy.

11 connected to the control rod 13 of the distributor8 through a pivoted lever 12. The end 14 of the screw 10 projects from the cylinder 1 through a removable end wall 15 thereof, and is connectible with a known steering wheel (not shown).

For further details of the construction and operation of this type of power steering unit, reference may be made, for example, to GB-A-1,576,051 and GB-A-1,576,052.

With particular reference to Figure 2, it can be seen that the piston 2 has a passage 16 for maintaining constant hydraulic communication between the chamber6 and the space 17 of the cylinder 1 housing the sector gear 4. The wall 18 ofthe cylinder 1 has two through holes 19,20 which are internally threaded and are located symmetrically with respect to the axis of the sector gear 4 and perpendicular to the axis of the cylinder 1. The holes 19,20 have respective cylindrical enlargements 21, 22 which communicate with each otherthrough a duct 23 formed in the thickness of the wall 18 ofthe cylinder 1. The duct 23 is also in communication with ducts 24,25 which are formed in the thickness of the walls 26,27 of the cylinder 1 and open into the chamber 7 opposite the chamber 6 through an aperture 28.

A respective valve member of the travel limiting device according to the invention is located in each of the threaded holes 19,20. Since the valve element inserted in the hole 19 is exactlythesame as that inserted in the hole 20 the components of each valve member are indicated by the same reference numerals.

With reference to Figures 1 and 2, a tubular housing 31 is engaged within each hole 19,20 by means of mating threads 29,30. Each housing 31 comprises a first axial hollow portion 32, a second axial hollow portion 33 communicating with the first portion 32, and a third axial hollow portion 34. The second portion 33 has a smaller diameter than the first portion 32 which defines a valve seat 35 against which bears a shutter 36 of the valve element. This shutter 36 is biassed against the seat 35, so as to be kept in a normally closed position, by a helical spring 37 in the first hollow portion 32. This first portion 32 has internal threading 38 engaged buy a nut 39 which acts as a shouider for the spring 37, allowing its load to be adjusted.

In correspondence with the valve seat 35 is a diametral passage 40 for putting the first hollow portion 32 into communication with the enlargement 21, 22 of the respective hole 19,20. The second hollow portion 33 has a diametral passage 45 which main tails the interior of this portion in constantcom- munication with the space 17 of the cylinder 1.

The second hollow portion 33 houses an operating rod 41 which is freely inserted at one end in the shutter36 and, at the other end, in the shank42 of a body 43 housed in the third hollow portion 34 with radial clearance. The body 43 is formed with a spherical cap 44 which is located in the space 17 outside the housing 31. The sides 52,53 of the sector gear4 have respective recesses 54, 55 with curved surfaces having the same radii as the spherical caps 44.

Theshank42 ofthe body43 hasaflange 47 and an annular groove 48 formed in correspondence with radial retaining pins 46. The flange 47 is engageable against the pins 46 to retain the body 43 in position, while an annular shoulder 49 of the third hollow portion 34 constitutes a striking surface for the body 43 as will be described below.

Atoroidal sealing washer 50 (Figure 3) ensures hydraulic sealing between the housing 31 and the wall 18 of the cylinder 1, while a threaded cap 51 ensures the clamping and protection of the outer end of the housing 31.

The operation ofthetravel limiting device will now be described and, for brevity, the valve members will be referred to as the right-hand and left-hand valve members, as seen in the drawings.

Assuming thatthe piston 2 is moving towards the right (as seen in Figures 1 and 2 of the drawings) and the sector gear4 is thus moving angularly in the clockwise direction, the chamber 6 and the space 17 of the cylinder 1 are under pressure. The chamber 7 however, is connected to the exhaust through the distributor 8 in the conventional manner.

During this movement and as long as the travel limit position is not reached, the right- and left-hand valve elements are both closed. In this condition, the respective shutters 36 close the valve seats 35 (Figure 2) so that the pressurised fluid is unable to pass from the space 17 to the first hollow portion 32 and, from there, through the diametral passages 40 and the enlargements 21,20, into the ducts 23,24,25 and the chamber 7 which is connected to the exhaust.

The spherical caps 44 of both valve elements are in their positions of maximum projection into the space 17, with the flanges 47 of the shanks 42 againstthe pins 46, and are thus raised by a predetermined distance S (Figure 3) from the surfaces defined by the annular shoulders 49.

As the travel limit position corresponding to the position of maximum steering of the wheels in the direction being considered is reached, the curved surface of the recess 55 on the right-hand side 53 of the sector gear 4 comes into engagement with the spherical cap 44 of the right-hand valve member (Figure 3). With a further angular displacement of the sector gear 4, the cap 44 is pressed downwarlly carrying the operating rod 41 with it. This causes the shutter 36 to move into the open position away from the seat 35 overcoming the force exerted by the spring 37, which normally exceeds the pressure of thefluid in the cylinder 1.

Upon opening of the valve seat 35, the space 17 of the cylinder 1 is immediately put into communication with the chamber 7, and hence with the exhaust, through the diametral passage 40, the enlargement 22, the ducts 24,25, and the aperture 28. The movement of the piston 2 is thus stopped automatically.

Should the sector gear4 attempt to move further in the clockwise direction as a result of manual steering action applied through the screw 10, after the cap 44 has been displaced by the entire movement allowed by the clearance S, no further movement is permitted because of the abutment of the body 43 against the annular shoulder 49. Thus, the movement of the piston 2 is also blocked as a result of mechanical conditions which ensure the stoppage in the travel limit position.

It will be appreciated, therefore, that the travel limiting device according to the invention stops the piston 2 both hydraulically and mechanically, thereby giving the maximum assurance that no damage will be caused to the steering members at the positions of maximum steering. Moreover, it renders conventional external mechanical travel limiters superfluous.

A similar operation occurs when the piston 2 is moving towards the left as seen in the drawings. In this case, the chamber7 is under pressure and the chamber 6 is in the exhaust position, the space 17 also being connected to the exhaust through the passage 6.

On reaching the travel limit position, the left-hand recess 54 of the sector gear 4 engages the cap 44 of the left-hand valve member and, in the manner described above, effects the opening of the respective valve seat 35. When the valve seat 35 is opened the fluid under pressure in the chamber 7 passes into the space 17 through the aperture 28, the ducts 25, 24,23, the enlargement 21, the diametral passage 40, the second hollow portion 33, and the further diametral passage 45, and then flows to the exhaust through the passage 16 and the chamber 6.

It will be appreciated that, as a result of the described structure and operation of the valve members, a travel limiting device according to the present invention may be adjusted mechanically before the hydraulic components of the circuit are installed on the vehicle and the pump motor is operational. Indeed, the adjustment is achieved solely by means of the correct axial positioning of the housings 31 within the holes 19,20 after the sector gear 4 has been placed in its two positions of maximum permitted mechanical displacement and the caps 44 of the two valve members have been fitted tightly into the respective recesses 55, 54 of the sector gear 4.

Claims (4)

1. Travel limiting device for hydraulic power steering of the type comprising a cylinder within which is sealingly slidable a piston defining two opposed operating chambers, a distributor for supplying hydraulic fluid under pressure to one of the chambers and simultaneously connecting the chamber not supplied with fluid to an exhaust, a shaft for effecting the steering of the steered wheels of a vehicle, a sector gear keyed onto the shaft and engaged with a rack on the piston to transmit the rectilinear displacements of the piston to the steering shaft as corresponding angular displacements, and a screw housed axially within a cavity in the piston and engaged, by means of a reversible connection, with a nut mounted on one end of the piston for limited rotation to control the distributor, the free end of the screw projecting axially and sealingly from the cylinder for connection with a steering member, in which the device comprises two valve members which are inserted in respective fluid exhaust ducts and are housed in the wall of the cylinder beside the sector gear, each valve member being kept normally closed by a respective spring with a predetermined load greater than the maximum operating pressure in the cylinder, and two operating rods each connected at one end to a respective valve member and projecting at its free end into the cylinder for engagement with a respective side of the sector gear when the latter is displaced in one direction or the opposite direction to reach the respective maximum steering position, the operating rods being displaceable axially by a predetermined distance against the respective springs.

2. A device as claimed in Claim 1, in which the valve members are housed in respective tubular housings, each housing being threaded on at least a portion of its outer wall and being engaged with complementary threading in a respective hole in the wall of the cylinder.

3. A device as claimed in Claim 1 or Claim 2, in which the free end of each operating rod has a movable spherical cap, and each side of the sector gear has a respective recess with a surface which mates with that of the cap.

4. A travel limiting device for hydraulic power steering, substantially as herein described with reference to, and as shown in, the appended drawings.