GB2137289A - Shock absorber seal - Google Patents

Abstract

An annular seal for use in a twin tube shock absorber comprises a radially inner face upon which primary and outer, piston rod contacting sealing lips (21, 23) are formed. A radially outer part of the seal has a depending lip (27) which extends in use generally axially of the shock absorber and which can engage a secondary sealing element (29) secured to a rod guide (11). A cavity (45) defined by the rod guide (11), piston rod (13), primary sealing lip (21), secondary sealing element (29) and depending lip (27), is filled with hydraulic fluid at least sufficiently to immerse the primary sealing lip (21) and thereby improve its life. When the fluid pressure in the cavity (45) is sufficient, the depending lip (27) is deflected to allow fluid to pass into the reservoir (9) formed between the tubes (1, 3). However, at all times the primary sealing lip (21) is advantageously immersed in hydraulic fluid. <IMAGE>

Description

SPECIFICATION Shock absorber seal The present invention relates to a seal arrangement for use in a telescopic shock absorber.

In particular the present invention relates to a seal arrangement for use in what is called a 'twin-tube' shock absorber.

Telescopic shock absorbers basically cQm- prise a piston and cylinder, a piston rod connected to the piston, passing through an aperture in one end of the cylinder. A seal secured to the cylinder engages the piston rod and allows the piston rod to move axially freely therethrough relative to the cylinder.

This seal prevents the escape of hydraulic fluid from the cylinder and the ingress of extraneous matter e.g. dirt.

'Twin-tube' telescopic shock absorbers comprise two cylinders, coaxially arranged with respect to each other, one within the other.

The piston and piston rod are movable within the inner cylinder, which is full of hydraulic fluid on both sides of the piston, and the annular cavity between the two cylinders acts as a reservoir for hydraulic fluid. The piston rod passes through an aperture in a rod guide which closes off one end of the inner cylinder, the seal previously referred to, being located beyond said rod guide and sealing between the piston rod and the outer cylinder or an end cap located on the outer2cylinder. Said reservoir serves usefully to accommodate displacements of hydraulic fluid from the inner cylinder, which occur during inward movements of the piston and piston rod owing to the necessity to displace from the inner cylinder, a volume of hydraulic fluid equivalent to the volumetric displacement of the piston rod during each inward stroke of the piston.During an outward stroke of the piston and piston rod, a volume of fluid equivalent to that displaced by the piston rod during the inward stroke, is drawn back into the inner cylinder from said reservoir via a known foot valve arrangement located in the end of the inner cylinder opposite to that in which the said rod guide is located.

Valve arrangements or orifices, or combinations of valve arrangements and orifices, are provided respectively in said piston and said foot valve. The foot valve is designed to generate resistance to movement of the piston and piston rod during inward strokes thereof.

The piston valves are designed to only restrict the flow of hydraulic fluid therethrough between opposite sides of the piston, during an outward movement of the piston and piston rod, to thereby control outward movement of the piston and piston rod.

In order for the piston rod to move freely through the aperture in the rod guide it is necessary for the bore diameter of the aperture to be slightly greater than the diameter of the piston rod. Thus a path is formed between the piston rod and aperture, through which fluid, under whatever pressure is required to generate the necessary resistance to an outward stroke of the piston rod, can escape from the inner cylinder into said reservoir, the fluid being restored back into the other end of the inner cylinder via the said known foot valve arrangement during the same outward stroke of the said piston and piston rod.

This clearance path, in addition to permitting the piston rod to slide freely within the aperture of the rod guide, also serves conveniently to permit air or gas to be easily purged from the inner cylinder and into the space above the hydraulic fluid level in said reservoir and hence serves to assist in keeping the inner cylinder charged with hydraulic fluid which is free from entrapped air or gas bubbles or pockets.

However, it will be understood that particularly during an inward stroke of the piston rod, the pressure in the upper part of the inner cylinder might be less than the pressure within the reservoir space, in which- circumstance air or gas may be caused to re-enter the upper part of the working inner cylinder and hence be disadvantageous to the efficient operation of the shock absorber.

It is known in the twin-tube shock absorber art, to provide a non-return valve arrangement to prevent such a circumstance arising, e.g.

British Patent Specification No. 709338 discloses an arrangement by which the hydraulic fluid in the reservoir space is connected directly to the inner cylinder via the aforesaid clearance path formed between the diameter of the piston rod and the slightly larger aperture of the rod guide and a tube extending downwardly from the rod guide to a point below the liquid level of the hydraulic fluid in the reservoir.

Other constructions are known, e.g. British Patent Specification No. 1,077,587, (U.S.

Patent Specification No. 4,108,287), which provide sealing arrangements comprising sealing elements arranged to cooperate with the outer surface of the piston rod and/or valve member faces, located adjacent the aperture formed in the rod guide, in which the piston rod freely slides.

As will be appreciated, the assembly of such known non-return valve arrangements is relatively complicated and owing to the number of component parts involved, may be liable to incorrect assembly.

It is known to improve the action of twintube shock absorbers by arranging for the air or gas which is contained within the reservoir space and which serves as a flexible medium to compensate for piston rod displacements, to be charged to some predetermined pressure. This causes the hydraulic fluid also to be pressurised and hence the sealing lip of the seal, which prevents oil or air or gas from leaving the shock absorber, is also subject to pressure and hence tends to tighten its grip on the piston rod.Whilst such seals must obviously be designed to best resist such pressure without permitting leakage, it is known that their performance is best met if the said sealing lip is immersed in hydraulic fluid and hence prevented from drying out and permitting friction to develop between the sealing lip and the surface of the piston rod with which it cooperates, such as might cause undesirable wearing of the sealing lip to occur. However this is not catered for in known twin-tube constructions.

The aim of the present invention is to provide an improved non-return valve assembly which comprises an improved sealing arrangement of simplified construction and hence reduces the risk of incorrect assembly and gives the possibility for reducing manufacturing costs.

According to the present invention there is provided a seal arrangement for a twin-tube telescopic shock absorber, comprising an annular seal which has a radially inner face upon which a primary sealing lip and an outer sealing lip are formed, said sealing lips, in use, contacting the piston rod of the shock absorber, a radially outer part of the annular seal having a generally axially depending lip which, in use acts as a valve member.

According to a further aspect of the present invention there is provided a twin-tube telescopic shock absorber comprising an inner and an outer tube located coaxially with respect to each other, one within the other, a piston being axially displaceable within the inner cylinder, a piston rod attached to the piston extending through a rod guide secured at one end of said inner cylinder, and an end cap closing said one end of the shock absorber, an annular seal being arranged against the end cap and having a radially inner face upon which a primary sealing lip and an outer sealing lip are formed, said sealing lips contacting the piston rod with said primary sealing lip nearest to said rod guide, a radially outer part of the integrally formed annular seal, having a generally axially depending lip which can engage a secondary sealing element secured to the rod guide, said secondary sealing element holding said annular seal against said end cap and allowing fluid to flow from the cavity defined by the rod guide, piston rod, primary sealing lip, secondary sealing element and said generally axially depending lip, when the pressure of the fluid in the cavity is sufficient to move said last mentioned lip.

Preferably, a ring-shaped reinforcement member is embedded in the seal, said secondary sealing element engaging against this reinforcement member. Further, said secondary sealing element is preferably in the form of a cylinder with axial projections at one end, said projections engaging the reinforcement member. Fluid can therefore escape from the said cavity via the spaces between the axial projections, when the pressure in the cavity is sufficient to deflect the said axially depending lip of the seal. If desired said secondary sealing element can be integrally formed with said rod guide.

Thus, with the construction of the present invention, any oil leaking past the piston and between the piston rod and rod guide will be stored in the said cavity defined by the rod guide, piston rod, secondary sealing element, and said seal. With this cavity full of oil, the pressure in the cavity will increase as fresh oil is introduced into the cavity from between the piston rod and rod guide so that the axially depending lip is deflected to allow oil to pass into the reservoir between the inner and outer cylinders. The provision of the secondary sealing element means that the primary sealing lip is always immersed in oil, this improving its life and performance.The primary sealing lip prevents oil from passing to the exterior of the shock absorber when the shock absorber is extended, and the outer sealing lip prevents the ingress of extraneous matter and air, during a compression stroke.

The seal arrangement of the present invention also assists with the recuperation of oil into the cylinder on the rebound stroke and ensures a solid column of oil. Additionally the possibility of drain back when the shock absorber is stationary is overcome, and initial priming of an installed unit is assisted by the seal arrangement of the present invention.

Further, the unit can be pressurised without affecting the required performance.

Besides the above advantages of the construction of the present invention, assembly has been simplified as compared to prior art constructions, and thus manufacturing costs can be reduced. This is in the main due to tile reduction in the number of component parts, the seal and valve member of the prior art being replaced by a single integrally moulded member and the requirement of a spring being obviated.

The present invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a cross-sectional view of a twintube telescopic shock absorber incorporating one embodiment of a seal arrangement according to the present invention; and Figure 2 is a detailed cross-sectional view of one end the shock absorber of Fig. 1; the left hand half of this figure showing a modified end cap/rod guide construction.

The twin-tube telescopic shock absorber shown in Fig. 1 of the accompanying drawings basically comprises in a known manner, an inner tube 1 and an outer tube 3, a piston 5 being axially slidable within the inner tube 1. The lower end of the inner tube 1, as illustrated in Fig. 1, is closed by a foot valve of any known configuration, the foot valve 7 being designed to allow fluid i.e. hydraulic oil, to flow relatively easily from the reservoir 9 between the tubes, into the inner tube 1 as the piston 5 moves up the inner tube 1 during an extension stroke, but to restrict flow from the inner tube during a compression stroke. The upper end of the inner tube 1, as viewed in Fig. 1, is closed by a rod guide 11 through which a piston rod 1 3 passes, the piston rod 1 3 being connected to piston 5.

The piston rod 1 3 also passes through a seal arrangement 1 5 forming the subject of the present invention and best illustrated in Fig.

2.

The seal arrangement 1 5 comprises an integrally moulded flexible seal generally designated 17, which is moulded of a suitable plastic material. The seal 1 7 is generally in the shape of a ring and has on its radially.

inner surface 19, a primary sealing lip 21 and an outer sealing lip 23, both lips pressing against the piston rod 5. A spring ring 25 presses the seal against piston rod 5 in addition to the natural resilience of the seal. The primary sealing lip 21 prevents oil from escaping from the shock absorber, and the outer sealing lip 23 prevents air and extraneous matter from entering the shock absorber during a compression stroke. A radially outer part of the annular seal 1 5 has a depending lip 27 which cooperates with a secondary sealing element 29 to form a one-way valve. The secondary sealing element 29 can be either an upstanding cylindrical projection integrally formed with rod guide 11, as illustrated, or alternatively the secondary sealing element 29 can be a separate element secured in a groove in the rod guide 11.Whichever construction is adopted, the upper end edge 31 of the secondary sealing element 29 has a number of projections 33 which engage against a reinforcement ring 35 embedded in seal 17.

The gaps between projections 33 serve as ports to allow oil to flow to the reservoir 9 when depending lip 27 has been deflected by the fluid pressure. The seal 1 7 is thus held in position by rod guide 11 and secondary sealing element 29 on the one side, and an end cap 37 on the other side. On the right hand side of Fig. 2, end cap 37 engages with its lower edge 41, against a radially outwardly extending series of projections 39 on rod guide 11, the gaps between projections 39 allowing fluid to flow to reservoir 9. On the left hand side of Fig. 2 an alternative end cap construction is shown, the main difference being that an internal shoulder 43, as opposed to end edge 41, engages against rod guide 11.

Considering the operation of seal arrangement 15, as best seen in Fig. 2, oil which escapes between piston rod 1 3 and rod guide 11 due to the pressure generated by the piston valves resisting the outward movement of the piston and piston rod, enters cavity 45 which is defined by seal 17, secondary sealing element 29, rod guide 11 and piston rod 1 3. The cavity 45 fills with oil and remains filled with oil at least up to the level of the gaps between projections 33, during normal operation life of the shock absorber. The secondary sealing element 29 thus acts as a form of weir maintaining the oil level in cavity 45 such that the primary sealing lip 21 is always immersed in oil and so the life and performance of primary sealing lip 21 is enhanced.

During operation of the shock absorber more and more leakage oil is introduced into cavity 45 and when the pressure within cavity 45 is sufficient, depending lip 27 lifts off secondary sealing element 29 to allow oil to flow into the reservoir 9 via the gaps between projections 33. With the pressure reduction in cavity 45 the depending lip 27 moves back to seal cavity 45.

With the above construction a number of advantages are obtained. The most important is the simplicity and thus relative cheapness, of manufacture due to there being only three components in the seal arrangement 1 5 i.e.

end cap, seal and rod guide. Additionally, as already mentioned, the provision of secondary sealing element 29 means that primary sealing lip 21 is always immersed in oil, so enhancing its performance.

Claims (11)

1. A seal arrangement for a twinztube telescopic shock absorber, comprising an annular seal which has a radially inner face upon which a primary sealing lip and an outer sealing lip are formed, said sealing lips, in use, contacting the piston rod of the shock absorber, a radially outer part of the annular seal having a generally axially depending lip which, in use, acts as a valve member.

2. A twin-tube telescopic shock absorber comprising an inner and an outer tube located coaxially with respect to each other, one within the other, a piston rod attached to the piston extending through a rod guide secured at one end of said inner cylinder, and an end cap closing said one end of the shock absorber, an annular seal being engaged against the end cap and having a radially inner face upon which a primary sealing lip and an outer sealing lip are formed, said sealing lips contacting the piston rod with said primary sealing lip nearest to said rod guide, a radially outer part of the integrally formed annular seal, having a generally axially depending lip which can engage a secondary sealing element secured to the rod guide, said secondary sealing element holding said annular seal against said end cap and allowing fluid to flow from the cavity defined by the rod guide, piston rod, primary sealing lip, secondary seal ing element and said generally axially depending lip, when the pressure of the fluid in the cavity is sufficient to move said last mentioned lip.

3. A shock absorber as claimed in claim 2, in which a ring-shaped reinforcement member is embedded in the seal.

4. A shock absorber as claimed in claim 3, in which the reinforcement member is embedded between said generally axially depending lip and said radially inner face.

5. A shock absorber as claimed in claim 3 or claim 4, in which said secondary sealing element engages against the reinforcement member.

6. A shock absorber as claimed in claim 5, in which the secondary sealing element is cylindrical with axial projections at one end, said projections engaging the reinforcement member.

7. A shock absorber as claimed in any one of claims 1 to 6, in which the secondary sealing element is formed with the rod guide as a unitary construction.

8. A shock absorber as claimed in any one of claims 1 to 6, in which the secondary sealing element is a separate component from the rod guide, but is secured thereto.

9. A shock absorber as claimed in any one of claims 1 to 8, in which a circular spring holds said sealing lips against the piston rod.

10. A seal arrangement for a twin-tube shock absorber, constructed and arranged substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

11. A twin-tube telescopic shock absorber constructed substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.