GB2072799A

GB2072799A - Shock absorber for MacPherson-type telescopic strut

Info

Publication number: GB2072799A
Application number: GB8109158A
Authority: GB
Original assignee: IAO Industrie Riunite SpA
Current assignee: IAO Industrie Riunite SpA
Priority date: 1980-03-26
Filing date: 1981-03-24
Publication date: 1981-10-07
Also published as: GB2072799B; DE3110677A1; IT1128244B; ES267158U; ES267158Y; FR2479388A1; DE3110677C2; IT8067464A0; FR2479388B1

Abstract

A split ring (44) of a plastics material with a low coefficient of friction is interposed between the piston (14) and cylinder (12) of a telescopic hydraulic shock absorber. The ring (44) is secured axially between an annular flange (46) forming part of the piston (14) and a bored annular resilient member (52). The annular flange (46) is situated at the end of the piston (14) opposite that from which a piston rod (18) extends. The resilient member (52) is gripped centrally between a shoulder of the piston rod (18) and an annular spacer member (24) which projects from the corresponding end face of the piston (14). Between this end face and the resilient member (44) there is defined an annular chamber (56) into which longitudinal ducts (20, 22) made in the piston (14) open. This chamber is defined peripherally by a portion (44a) of the ring (44) which projects relative to the end face of the piston (14). The pressure generated in the chamber (56) during use of the shock absorber serves to urge the portion 44a radially outwards to provide a good seal. <IMAGE>

Description

SPECIFICATION Shock absorber for MacPherson-type telescopic strut The present invention relates to telescopic hydraulic shock absorbers, particularly for incorporation in a MacPherson-type telescopic strut for a motor vehicle suspension. More particularly, the invention concerns a telescopic hydraulic shock absorber of the type comprising a cylinder in which is slidably housed an annular piston secured to a threaded stem of a piston rod by clamping between a shoulder of the rod and a nut screwed onto the stem, in which the piston has longitudinal ducts associated with disc valves, and in which a split ring of plastics material with a low coefficient of friction is interposed between the piston and the cylinder, and secured axially between an annular flange integral with the piston and an annular shoulder element mounted on the piston.

Shock absorbers of this type exhibit radial loads between the piston and cylinder which are considerable in static conditions, and which reach high levels in dynamic conditions. The friction induced by these radial loads causes a certain degree of uneveness in the behaviour of the shock absorber. In order to reduce this unevenness it is already known to provide the piston with a ring of plastics material with a low coefficient of friction (usually polytetrafluoroethylene) to improve its sliding.

An arrangement is known in which the ring is secured axially between an annular flange integral with the piston and an annular shoulder element mounted on the piston. In this known arrangement the annular flange is formed at the end of the piston fromwhich the rod extends, whilst the inserted member is rigid and is anchored against the opposite end of the piston.

This known solution presents a problem in that it is difficult to fit the ring with a minimum axial clearance between the flange and the inserted member so that, on one hand, the ring may be expanded against the wall of the cylinder by the oil pressure to provide a good seal, and, on the other hand, the ring is not subject to axial movements relative to the piston.

The machining operations necessary to achieve this optimum clearance have very close tolerances and are therefore expensive.

The known arrangement also presents a problem in that the oil pressure on the inside of the ring necessary to produce its expansion is produced only after the oil has been drawn through clearances which must, of necessity, be very restricted, and as a result of this, the expansion does not take place very efficiently in practice.

It is an object of the present invention to provide a shock absorber of the aforesaid type which avoids both the difficulties mentioned.

According to the present invention there is provided a telescopic hydraulic shock absorber comprising a cylinder in which is slidably housed an annular piston secured to a threaded stem of a piston rod by clamping between a shoulder of the rod and a nut screwed onto the stem, in which the piston has longitudinal ducts associated with disc valves, and in which a split ring of plastics material with a low coefficient of friction is interposed between the piston and the cylinder and secured axially between an annular flange integral with the piston and an annular shoulder element mounted on the piston, wherein the annular flange of the piston is arranged at the end opposite that from which the rod extends, wherein the annular shoulder element comprises a bored resilient member which is gripped centrally between the shoulder of the rod and an annular spacer member projecting from the corresponding end face of the piston, whereby to define between the end face and the resilient member an annular chamber in which are situated openings of the ducts, and wherein the chamber is defined peripherally by a portion of the ring which projects relative to the end face of the piston.

This solution ensures the absence of axial movements of the ring relative to the piston because the resilient member presses the ring axially against the flange. On the other hand, the resilient member may be designed so that the force exerted on the corresponding edge of the ring is not sufficient to prevent sliding of the latter for expansion. Furthermore, expansion of the ring is ensured in that zone where a good seal is most necessary. Indeed, it is known that, in a hydraulic shock absorber of the type being considered, the greatest oil pressure is produced in the space in which the piston rod is situated, when this chamber is reduced in length in extension strokes of the shock absorber.Since the said annular chamber communicates directly with this space through the bores of the resilient member, this high pressure acts directly and immediately against the zone of the ring which peripherally defines the annular chamber. On the other hand, the establishment of a seal in the contraction strokes of the shock absorber is not critical since, as is known, the difference in pressure between the two sides of the piston in these strokes is not particularly important.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 is a partial longitudinal section of a telescopic hydraulic shock absorber of the dualtube type and the piston thereof; Fig. 2 is an exploded perspective view of the piston and the members associated therewith, and Fig. 3 is a plan view of an alternative embodiment of a resilient member.

\with reference to Figs. 1 and 2, a hydraulic shock absorber comprises, as known per se, two co-axial cylinders, an external one 10 and an internal one 12. The internal cylinder 12 slidably housing a piston 14.

A stem 1 6 of a piston rod 1 8 extends centrally in the piston 14.

The piston 14 has a first, radially-outer ring of axial ducts 20 for transferring hydraulic fluid in the direction of the arrow A during contraction strokes of the shock absorber. There is also a second radially-inner ring of throttled axial ducts 22 made in the piston 14 for the transfer of hydraulic fluid in the opposite direction, indicated by the arrow B.

The end of the piston 14 facing the rod 1 8 has a projecting annular central boss 24 the main function of which will be described below. The ring of ducts 20 is controlled by a shutter valve, comprising a rigid annular disc 26 which has a circular series of internal radial appendages 28 which slidingly engage the boss 24. The shutter valve 26 is opposed by a conical helical spring 30 which on one hand abuts the shutter valve and on the other hand is housed in a peripheral groove 32 of the boss 24.

The radially-inner ducts 22 are controlled by a shutter valve 34 comprising a pack of elastically deformable annular discs. A strong helical spring 36 abuts the discs 34 by means of a small plate 38 which slides over a bush 40 surrounding the stem 1 6. The bush 40 is held against the piston 14 by a nut 42 screwed onto a threaded end of the stem 16. The nut 42 also serves as an abutment member for the spring 36.

A split ring 44 of polytetrafluoroethylene, or similar plastics material having a low coefficient of friction, is interposed between the piston 14 and the cylinder 12. The opposed ends of the ring 44 at the split have complementary stepped profiles to define a labyrinth intended to minimize the drawing of the hydraulic fluid between the two sides of the piston, when the ring is held between the piston 14 and the cylinder 12.

The ring 44 is slidable relative to the cylinder 12 and on one side abuts axially an annular flange 46 formed integrally with the piston 14 and situated at the end of the piston opposite that from which the rod 18 extends.

An annular shoulder 48 is formed between the rod 1 8 and its stem 1 6 because of the difference in their diameters. A spacer washer 50 is applied to the shoulder 48 and surrounds the stem 1 6 with precision. Between the washer 50 and the corresponding end face of the boss 24, there is gripped lightly an annular resilient member in the form of a disc 52 of resilient sheet steel. The clamping of the disc 52 is effected, as will be understood, by the threaded nut 42 through the bush 40 and the piston 14.

The disc 52 is bored. In Fig. 2 its perforations are in the form of a ring of holes 54, but the holes may be replaced by apertures of, for example, a sector shape.

The ring 44 has a portion 44a which projects above the end face of the piston 14 facing the piston rod 18. The periphery of the disc 52 abuts elastically the corresponding end edge of the projecting portion 44a in order to keep the opposite end edge of the ring 44 applied, without play, against the flange 46.

As may be noted, an annular chamber 56 is defined between the disc 52 and the corresponding end face of the piston. This chamber is also defined peripherally by the projecting portion 44a of the ring 44. The chamber 56 communicates, through the holes 54, with the space between the cylinder 12 and the rod 18.

In the extension stroke of the shock absorber, the high oil pressure which is produced in the space between the cylinder 1 2 and the rod 1 8 is transmitted to the inside of the chamber 56 and acts radially on the projecting portion 44a of the ring 44. In this manner the projecting portiori 44a is expanded and pressed strongly against the internal surface of the cylinder 12, improving the seal against the drawing of hydraulic fluid. The elastic force with which the member 52 presses on the ring 44 will, of course, have to be selected so as not to hamper this expansion, but eliminate constantly the axial play of the band 44.

The form of embodiment shown in Figs. 1 and 2 is not the only one possible. Thus, for example, the boss 24, which behaves as a spacer for the resilient member 52, could in fact be a spacer bush separate from the piston 14.

Thus, also, the bored disc 52 couid be replaced by a star-shaped member 1 52 (Fig. 3). The member 152 comprises a ring of sector-shaped radial spokes 58, the peripheral edges of which are intended to abut the end edge of the projecting portion 44a of the ring 44. Sectorshaped notches 60 are formed between the spokes 58 and have the same function as the holes 54.

Claims

1. A telescopic hydraulic shock absorber comprising a cylinder in which is slidably housed an annular piston secured to a threaded stem of a piston rod by clamping between a shoulder of the rod and a nut screwed onto the stem, in which the piston has longitudinal ducts associated with disc valves, and in which a split ring of plastics material with a low coefficient of friction is interposed between the piston and the cylinder and secured axially between an annular flange integral with the piston and an annular shoulder element mounted on the piston, wherein the annular flange of'the piston is arranged at the end opposite that from which the rod extends, wherein the annular shoulder element comprises a bored resilient member which is gripped centrally between the shoulder of the rod and an annular spacer member projecting from the corresponding end face of the piston, whereby to define between the end face and the resilient member an annular chamber in which are situated openings of the ducts and wherein the chamber is defined peripherally by a portion of the ring which projects relative to the end face of the piston.

2. A shock absorber according to Claim 1, wherein the resilient member comprises a disc which has a ring of holes or similar apertures.

3. A shock absorber according to Claim 1, wherein the resilient member is a star-shaped member which has on its periphery a circumferential series of radial spokes for engagement with the projecting portion of the ring, the spokes alternating with notches.

4. A telescopic hydraulic shock absorber substantially as herein described with reference to and as illustrated in Figs. 1 and 3, or Figs. 1 and 3, of the accompanying drawings.