GB2111636A - >Hydropneumatic shock absorber with gas return valve - Google Patents

Abstract

A two-tube pressurized shock absorber has, at the upper end through which the piston rod 11 exits, a seal cavity 38 containing a resilient washer 41 which surrounds and grips the piston rod 11. The seal cavity is in communication through a passage 39 with the reservoir and through the piston rod clearance with the working chamber 2. The lower surface 28 of the seal cavity is conical. On a compression stroke, friction between the piston rod and the washer 41 causes the washer to be drawn down tightly against the conical surface to prevent flow between the washer and the conical surface, the distortion of the washer into a conical shape pressing the upper edge of the bore of the washer more tightly against the piston rod (Figure 3); flow from the working chamber to the seal cavity is thus prevented. On a rebound stroke, the piston rod draws at least the central portion of the washer away from the surface (Figure 4); flow can therefore take place from the working chamber to the seal cavity and hence to the reservoir. Any gas which has collected at the top of the working chamber is therefore returned to the reservoir on each rebound stroke, but there is no flow in the opposite direction. <IMAGE>

Description

SPECIFICATION Shock absorbers This invention relates to two-tube gas pressurised hydraulic shock absorbers of the kind in which: the working cylinder, i.e. the cylinder which is bounded by the pressure tube and in which the piston is reciprocated by a piston rod which passes through a closed end (the upper end when the shock absorber is in use) providing sealing and providing guidance for the piston rod, is bounded by an annular reservoir which is in communication (possibly through a valve or valves) with the other end of the working cylinder and contains a reserve of fluid and most of the pressurised gas; in which at least a part of the compression damping resistance is provided by the resistance to flow from below to above the piston afforded by means in or mounted upon the piston; piton; in which at least a part of the reaction to such piston flow is afforded by transmission of the gas pressure through the fluid (as defined below); and in which the gas and fluid are in direct contact. Such shock absorbers will be referred to herein as shock absorbers of the kind specified.

The term "fluid" is used herein to refer to the damping fluid of the shock absorber, which is a liquid that may contain some absorbed gas.

In shock absorbers of the kind specified, as also in unpressurised two - tube shock absorbers, gas may enter the working cylinder during manufacture, storage or fitting to the vehicle of the shock absorber or whilst the shock absorber is at rest or in operation on the vehicle. Such of the gas as is not absorbed into the fluid, or is later released by the fluid, either forms bubbles within the fluid or collects upon the fluid within the working cylinder and affects the damping operation of the shock absorber. It is desirable therefore to control, and in some circumst ances to eliminate as far as is practical, the amount of gas in the working cylinder.

The devices and processes for expelling gas from the working cylinder of unpressurised two - tube shock absorbers are well known. Some gas in the lower part of the working cylinder will, on operation of the shock absorber, be transferred to the lower part of the reservoir but the invention is not con cerned with this transfer. The invention is concerned with gas which rises, because of gravity, to the top of the working cylinder, which is generally vertical when the shock absorber is installed in the vehicle; this gas may initially be above the piston or may initially be below the piston and pass through the piston as a result of operation of the shock absorber.

The rising gas collects at the top of the working cylinder and is pressurized to a higher level than that in the reservoir during extension of the shock absorber. A connection, a so - called priming passage, is provided between the upper end of the working cylinder and the reservoir and gas is expelled by operation of the shock absorber which is then said to be "primed". A few "priming" cycles only are normally required and may be carried out by band or constitute the first few cycles of operation on the vehicle. The cross section of the priming passage is made very small so that, although gas will pass readily under the rebound damping pressure there will be substantial restriction to fluid flow; otherwise desired damping characteristics would not be achievable.Customarily the priming passage is provided as small notches in the region of the junction between the pressure tube and the piston rod guide or is constituted by a small clearance between the piston rod and rod guide that connects the working cylinder to a piston rod seal cavity which is, in turn, connected to the reservoir through somewhat larger notches or holes.

These well known provisions for unpressurised two - tube shock absorbers also work well for pressurised units but have the characteristic that the priming passage through which gas passes from the working cylinder to the reservoir allows gas to pass also in the reverse direction because when a shock absorber is set to damping characteristics that are customary for vehicle suspension, that is to say for this type of unit there will be at least some passageway between the lower end of the working cylinder and the reservoir that is permanently open, than at least on some occasions during compression of the shock absorber the pressure drop across the piston from below to above will be greater than that from below the piston to the reservoir and so the gas pressure in the reservoir will be greater than the fluid pressure in the working cylinder above the piston.

Those occasions will be during movements that are towards the lower end of the compression velocity range but may extend over the whole range.

The quantity of gas that passes from the reservoir into the working cylinder depends not only upon the size of the priming passage and the duration and, perhaps, the velocity of movement but also upon the gas pressure and the hydraulic resistance means in or on the piston and between the lower end of the pressure tube and the reservoir. It may amount to a quantity that is not expelled to a sufficient extent by the priming action associated with on-vehicle operation of the shock absorber, and so impairs its operation. This situation calls for ways of limiting, of stopping completely, gas flow from the reservoir.

Such ways are already and generally comprise obstructing the priming passage with a valve, or with a seal operating as a valve, that allows flow from the working cylinder to the reservoir but not in the reverse direction. The valve/seal may act upon passageways connecting directly from the working cylinder to the reservoir or may be associated with passageways via the rod seal cavity acting either upon the piston rod itself or upon components on the same side of the rod guide as the rod seal.

Examples are either the subject of, or illustrated in, Patent Specifications GB 1 077 587, 1 227 949, 1 290 948 and 2 045892.

It has now been appreciated that these known provisions impose a common penalty on the benefit The drawing(s) originally filed was/were informal and the print here reproduced is taken from a later filed formal copy.

of preventing gas flow from the reservoir; this is because the opening of the valve/seal for flow from the working cylinder to the reservoir is effected solely by the pressure difference across the valve and a substantial pressure difference is needed to open the valve/seal. Expulsion of gas from the working cylinder can only occur when the pressure differential between the working cylinder and reservoir is above this level, which is not always the case during extension of the shock absorber.As there is a part of the velocity end of the extension velocity range, of magnitude depending upon the hydraulic settings of the shock absorber, within which the pressure differential does not rise above that necessark to open the valve/seal and, as some extension movement of the shock absorber is required to compress the gas in the working cylinder to the valvelseal opening pressure, expulsion of any gas contained in the upper end of the working cylinder does not accompany all extension movements and is, on occasions therefore, incomplete. The residual gas quantity may as much as can impairthe damping operation of the shock absorber. It is therefore an object of the present invention to reduce or avoid this impairment.

According to one aspect of the present invention, in a shock absorber of the kind specified, the said closed end comprises a seal cavity which is in communication with the reservoir and the working cylinder and contains a slide element which is in frictional engagement with the piston rod and is arranged to be moved by virtue of this frictional engagement on movement of the piston rod in the rebound direction from a first position to a second position to facilitate flow from the working cylinder through the seal cavity to the reservoir. Thus the relative movement between the piston rod and the remainder of the shock absorber is utilized to facilitate flow in total or partial substitution for the pressure difference which has hitherto been required. The relative movement can be utilized in various ways.For example, it would be possible to provide a passage through the piston rod guide closable by a rigid closure member constituting the slide element. Alternatively, the slide element could be rigid and engage in its first position a flexible sealing member to hold the sealing member on to a seat, disengagement of the slide element from the sealing rnember occuring on movement of the slide element to its second position to allow flexing of the sealing element by a small pressure difference across it. Preferably, however, the slide element comprises a flexible annular washer surrounding the piston rod and having one generally axially - facing surface which is engageable in its first position with a surface of the seal cavity to prevent flow in the direction from the reservoir to the working cylinder.

According to a second aspect of the present invention, in a shock absorber of the kind specified, the said closed end includes gas transfer means comprising: a seal cavity extending around the piston rod, a first passage connecting the seal cavity to the working cylinder, a second passage connecting the seal cavity to the reservoir, and a flexible annular seal member within the seal cavity, the seal member engaging frictionally around the piston rod so that the part of the seal member engaging the piston rod is moved within the seal cavity axially of the shock absorber by movements of the piston rod between rod and between a first position produced by a compression movement in which the seal member provides a seal preventing flow from the reservoir to the working cylinder and a second position produced by an extension movement in which the seal member permits flow from the working cylinder to the reservoir.Preferably, the seal member has an annular surface which faces generally axially towards the working cylinder and the seal cavity has an annular surface which faces generally axially away from the working cylinder, the two said surfaces being in face to face contact when the seal member is in the first position while in the second position of the seal member only a part or none of the said surfaces are in contact. In this arrangement preferably, with the seal member in the second position, the two said surfaces have different conicities so arranged that the inner annular portion of the surface on the seal member is spaced from the surface on the seal cavity and the seal member is supported in the seal cavity by means such that the outer annular portion of the seal member is free to flex away from the surface on the seal cavity.

It will be understood that the invention provides a shock absorber in which movement of the piston rod is used to operate (or to modify the operation of) a valve which prevents flow from the reservoir back to the working cylinder on the compression stroke. As has been mentioned, in previous constructions the valveforthis purpose has been opened by the pressure differential developed during the rebound stroke. By means of the invention, the pressure differential required to open the valve can be reduced or, if required, be eliminated entirely. It will be realised that by appropriate design the degree of reduction in the required pressure differential produced by movement of the piston rod can be selected as required.

The invention may be carried into practice in various ways but one shock absorber embodying the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure lisa longitudinal section through the shock absorber and Figures 2,3 and 4 are enlarged details of the region within the ring A in Figure 1 with the parts shown respectively in the at rest condition, the condition during compression and the condition during rebound.

Referring first to Figure 1 the shock absorber comprises a pressure tube 1 bounding a working cylinder 2 which is divided into two regions by a piston 3 containing passages which are closed by a compression valve 4 and a rebound valve 5 respectively. Bleed passages may be provided bypassing one or other or both of the valves 4 and 5; such passages may for example be formed in the valve seats. The pressure tube 1 is surrounded by an outer tube 6 to form an annular reservoir 7 which communicates with the lower part of the working cylinder 2 through a base valve 8 which opens against spring resistance on recuperation and the seat of which includes notches 9 to permit restricted flow from the working cylinder to the reservoir on compression. The piston 3 is attached to one end of a piston rod 11 which passes through a piston rod guide 12 and an external seal 13.The working cylinder 2 and the reservoir up to a frothy transition region 14 are filled with a hydraulic fluid comprising oil in which some gas may be absorbed and also possibly including bubbles of gas. The upper part of the reservoir contains only gas. The construction and operation of the shock absorber as so far described will be well understood and will not therefore be discussed in greater detail.

Figure 2 shows to a larger scale the region of the shock absorber shown in the circle A in Figure 1 and it will be seen that the upper end of the shock absorber is closed by a top cap 21 which is welded to the outer tube 6 and an outer guide 22 to provide with the piston rod guide 12 a seal chamber 23 containing the outer seal 13 which has a lip 24 held against the piston rod 11 by a garter spring 26. This seal holds the pressure within the shock absorber and also provides a means for pressurising the shock absorber by displacement of the lip 25 to permit gas to flow from outside the shock absorber between the lip and the piston rod into the interior of the shock absorber. The upper part of the piston rod guide 12 contains a recess 27 with a lower conical wall 28 and a side wall 29, the recess thus being surrounded by a cylindricai wall 31.Trapped between the upper edge of the wall 31 and the underside of the seal 13 is a pair of annular discs 32 and 33, the former disc having its central portion deformed into a double conical section as can be seen from Figure 2 so that in section the disc comprises an outer flat portion 32 whose upper and lower surfaces are radial of the shock absorbed, a central portion 35 whose upper and lower surfaces are conical and dished downwardly and a small central portion 36 whose upper and lower surfaces are conical and dished upwardly. At intervals around the circumference the disc 32 is formed with notches 37 that provide communication between the interior of the seal cavity at 38 and the outside of the disc and hence through notches 39 (which are also clearly visible in Figure 1) in the outside surface of the piston rod guide 12 with the upper end of the reservoir 7.By contrast with the complex cross-section of the disc 32, the disc 33 is flat.

Located between the disc 32 and the bottom wall 28 of the recess 27 is an annular sealing disc 41 whose depth is substantially less than that of the depth of the side wall 29 of the recess and whose depth is equal to the axial distance between the iowest part of the disc 32 and the point 42 on the lower wall 28 of the recess 27 at which this lower wall intersects with the outer circumference of the sealing disc 41. Alternatively, the depth of the sealing disc could be slightly smaller or slightly greater than the said axial distance.

As stated above, Figure 2 shows the as installed condition with no pressure difference across the sealing disc 41. The dimensions of the hole through the sealing disc are such there is a light grip on the piston rod. During compression of the shock absorber by the arrow 51 in Figure 3, when the piston rod moves downwardly the friction between the piston rod and the seal ring 41 draws the seal ring downwardly and increase the amount of the undersurface of the sealing ring 41 which is in contact with the conical lower surface 28 of the cavity 27; in the event of a long or fast compression stroke pressure will build up to such an extent that the whole of this lower surface of the sealing ring is in contact with the wall of the cavity as is indicated in Figure 3.A concomitant effect which can be seen in Figure 3 is that the distortion of the seal disc 41 to a conical shape presses the upper inner edge of the sealing ring into tighter contact with the piston rod. Thus during compression any differential pressure between the seal cavity 23 and the working cylinder 2 is sealed and gas is not able to flow in this direction, the greater the pressure the greater the sealing effect.

During extension of the shock absorber, i.e. during upward movement of the piston rod as indicated by the arrow 52 in Figure 4, pressure in the upper end of the working cylinder is as least equal to that in the seal cavity 23 and so there is no pressure urging the seal downwardly into the recess 27. Further the friction generated by the grip of the seal ring 41 on the piston rod 11 assisted by the elastic tendency of the seal ring 41 to return to its own flat state withdraws the seal ring 41 from the conical surface 28 of the recess 27 in the piston rod guide 12 and the seal ring 41 returns to the unstressed condition shown in Figure 2.Any fluid and/or gas passing through the working clearance between the piston rod 11 and the piston rod guide 12 caused by an excess of pressure in the upper end of the working cylinder over that in the seal cavity 23 can now escape into the cavity by deflecting the unclamped edge 53 of the seal ring 41 away from the conical wall 28 is allowed by the conical form of the washer 32 and thus fluid and/or gas can pass through the notches 37 and the notches 39 to the reservoir 7. It should be noted that the gap between the edge 53 and the conical surface 28 as shown in Figure 4 has been somewhat exaggerated for clarity.

It will be apparent that various modifications can be made to the construction shown in Figures 1 to 4 and in particular it will be realised that other configurations of the seal ring 41, the recess 27 and the disc 32 can be employed. For example, the upper and lower faces of the seal disc 41 may be upwardly conical with the lower face 28 of the recess 27 being flat, i.e. in a plane perpendicular to the axis of the piston rod 11.

Claims (10)

1. A shock absorber of the kind specified in which the said closed end comprises a seal cavity which is in communication with the reservoir and the working cylinder and contains a slide element which is in frictional engagement with the piston rod and is arranged to be moved by virtue of this frictional engagement on movement of the piston rod in the rebound direction from a first position to a second position to facilitate flow from the working cylinder through the seal cavity to the reservoir.

2. A shock absorber as claimed in Claim 1 in which the slide element comprises a flexible annular washer surrounding the piston rod and having one generally axially - facing surface which is engageable in its first position with a surface of the seal cavity to prevent flow in the direction from the reservoir to the working cylinder.

3. A shock absorber as claimed in Claim 2 in which, with the washer in an unstressed condition, the two said surfaces have different cone angles.

4. A shock absorber as claimed in Claim 3 in which, with the washer in an unstressed condition, the cone angle of the said surface on the washer is 1800 and that of the said surface of the seal cavity is less than 1800 with the point of the cone directed towards the working cylinder.

5. A shock absorber as claimed in Claim 4 in which the seal cavity includes a second surface which faces the first with the washer between them, the second surface being generally conical with the point of the cone directed towards the working cylinder.

6. Ashock absorberofthe kind specified in which the said closed end includes gas transfer means comprising: a seal cavity extending around the piston rod, a first passage connecting the seal cavity to the working cylinder, a second passage connecting the seal cavity to the reservoir, and a flexible annular seal member within the seal cavity, the seal member engaging frictionally around the piston rod so that the part of the seal member engaging the piston rod is moved within the seal cavity axially of the shock absorber by movements of the piston rod between a first position produced by a compression movement in which the seal member provides a seal preventing flow from the reservoir to the working cylinder and a second position produced by an extension movement in which the seal member permits flow from the working cylinder to the reservoir.

7. A shock absorber as claimed in Claim 6 in which the seal member has an annular surface which faces generally axially towards the working cylinder and the seal cavity has an annular surface which faces generally axially away from the working cylinder, the two said surfaces being in face to face contact when the seal member is in the first position while in the second position of the seal member only a part or none of the said surfaces are in contact.

8. A shock absorber as claimed in Claim 7 in which, with the seal member in the second position, the two said surfaces have different conicities so arranged that the inner annular portion of the surface on the seal member is spaced from the surface on the seal cavity and the seal member is supported in the seal cavity by means such that the outer annular portion of the seal member is free to flex away from the surface on the seal cavity.

9. A shock absorber as claimed in Claim 6 or Claim 7 or Claim 8 in which the first passage is constituted by the clearance between the piston rod and a piston rod guide.

10. A shock absorber constructed and arranged to operate substantially as described herein with reference to the accompanying drawings.