EXTENDED TRAVEL DAMPER

Abstract: A damper comprises a cylinder (4) having an outer surface (15); a piston rod (2) slidably received in the cylinder (4); and a strut case (14) attached to the piston rod (2) and acting on the outer surface (15) of the cylinder to maintain the alignment of the piston rod (2) with the cylinder (4). An intermediate tube (20) is located between the cylinder (4) and the case (14) such that an inner surface of the intermediate tube (20) engages the outer surface (15) of the cylinder (4) and an inner surface of the case (14) engages an outer surface (22) of the intermediate tube (20). The intermediate tube (20) allows the damper to be extended in a telescopic manner such that the maximum travel distance of the damper is increased.

Description: TITLE Extended travel damper DESCRIPTION Field of the invention The invention relates to the field of dampers using a fluid as a damping medium. It has particular application to a damper used as a strut for a motor vehicle. Background of the invention Figure 1 illustrates a known damper including a piston rod 2 slidably received in a cylinder 4 containing a fluid damping medium 5. A piston (not shown) is mounted on the piston rod 2 within the cylinder 4 so that movement of the piston along the cylinder 4 is resisted by the damping fluid 5. Thus forces applied along the piston rod 2 are damped by the dissipation of energy as heat in the damping fluid 5. The cylinder 4 may be connected via a hose 6 to a reservoir (not shown) containing damping fluid and a pressurized gas compartment separated from the fluid by a floating piston. Such a reservoir allows for the displacement of fluid 5 from the cylinder 4 as the piston rod 2 is inserted and increases the volume of damping fluid available for the dissipation of energy. Alternatively there may be a pressurized gas compartment separated by a floating piston within the cylinder 4. The illustrated damper is in the form of a MacPherson strut, which is a load-bearing member associated with the front or rear wheel of a vehicle. The strut may include means 8 for mounting a suspension spring, means 10 for attaching the strut to the body of the vehicle and means (not shown) for attaching the strut to the vehicle wheel, hub assembly or upright . To allow the strut to withstand bending loads, a strut case 14 attached to the piston rod 2 surrounds and supports the cylinder 4. As the piston rod 2 slides into and out of the cylinder 4, the strut case 14 slides over an outer surface 15 of the cylinder 4 on two or more bearings 16. Between the bearings 16 is a space 18 which may be filled by a lubricant, typically grease. Generally both bearings 16 are formed on an inner surface of the strut case 14 so that the spacing between them is fixed. As an alternative, one bearing may be formed on the outer surface of the cylinder 4, in which case the spacing between the bearings 16 will vary as the piston moves. The arrangement in which the vehicle body attachment 10 is on the damper cylinder 4, the vehicle wheel attachment is on the strut case 14 and the working piston acts directly on the bore of the cylinder 4 is known as an inverted monotube acPherson strut . An alternative arrangement of damper, illustrated for example in patent application EP-A-0607545 , is a non- inverted monotube. It comprises an inner cylinder in which a piston rod is slidably received and an outer cylinder surrounding the inner cylinder to form an annular chamber. A further, annular piston is coupled to the piston rod and slidably received in the annular chamber. The overall length of the strut in the rest position is determined by the vehicle design. Within that constraint, it is desirable to achieve the maximum travel distance of the damper so that the damper can accommodate large displacements of the piston rod 2 away from its rest position. In the known damper of Figure 1, the maximum extension of the damper is limited by the spacing of the bearings 16. The two bearings 16 must be at least a minimum distance apart for two reasons. The first reason is to maintain the rigidity of the strut under bending loads at maximum extension by providing sufficient overlap between the strut case 14 and the outer surface 15 of the cylinder 4. The second reason is to allow a sufficient volume of lubricant in the space 18 between the bearings 16. It is an aim of the invention to increase the maximum travel distance of the damper without the aforementioned disadvantages of reducing the spacing between the bearings 16. Summary of the invention The invention provides a damper comprising: a cylinder having an outer surface,- a piston rod slidably received in the cylinder,- a case attached to the piston rod and cooperating with the outer surface of the cylinder to maintain the alignment of the piston rod with the cylinder,- CHARACTERIZED IN THAT: an intermediate tube is located between the cylinder and the case such that an inner surface of the intermediate tube engages the outer surface of the cylinder and an inner surface of the case engages an outer surface of the intermediate tube. The intermediate tube allows the damper to be extended in a telescopic manner such that there is very little overlap between the case and the cylinder but there is sufficient overlap between the case and the intermediate tube and between the intermediate tube and the cylinder for the damper to remain sufficiently rigid. Thus the maximum travel distance of the damper according to the invention is greater than in the prior art. Preferably the inner surface of the intermediate tube includes a first pair of bearings for engagement with the outer surface of the cylinder, wherein one bearing of the first pair of bearings is formed at an end of the inner surface of the intermediate tube remote from the piston rod and the other bearing of the first pair of bearings is formed at a fixed distance away along the inner surface of the intermediate tube,- and the inner surface of the case includes a second pair of bearings for engagement with the outer surface of the intermediate tube, wherein one bearing of the second pair of bearings is formed at an end of the inner surface of the case remote from the point of attachment between the case and the piston rod and the other bearing of the second pair of bearings is formed at a fixed distance away along the inner surface of the case. This arrangement allows each pair of bearings to have a spacing at least as great as in the prior art, while still allowing the maximum travel distance of the damper to be greater than in the prior art, as previously described. The invention may be adapted for application to a monotube or a twin tube damper, inverted or non- inverted, with or without a separate fluid/gas reservoir. It can also be applied to the aforementioned arrangement in which an outer cylinder defines an annular chamber containing an annular piston coupled to the piston rod. In that arrangement according to the invention, the intermediate tube is located concentrically between the annular piston and the inner cylinder or between the annular piston and the outer cylinder so as to be capable of sliding motion relative to the annular piston and relative to the inner or outer cylinder. The invention is also applicable to arrangements wherein one or more of the bearings is formed on the outer surface of the cylinder or of the intermediate tube. The principle of the invention may be further extended to a damper comprising a plurality of concentric intermediate tubes between the cylinder and the case . Brief description of the drawings Figure 1 illustrates a damper according to the prior art, partially cut away. Figure 2 illustrates a damper according to the invention, partially cut away. Description of a preferred embodiment In Figure 2 is shown a damper according to the present invention. Corresponding parts have been given the same reference numerals as in Figure 1 and the explanation of those parts will not be repeated. The damper according to the invention differs from the prior art in having an intermediate tube 20 located concentrically between the cylinder 4 and the strut case 14. A first pair of bearings 16a is located on an inner surface of the intermediate tube 20 in contact with the outer surface 15 of the cylinder 4, one of the bearings 16a being located near one end of the inner surface of the intermediate tube 20 and the other of the bearings 16a being located a fixed distance away along the inner surface of the intermediate tube 20. A second pair of bearings 16b is located on an inner surface of the strut case 14 in contact with an outer surface 22 of the intermediate tube 20, one of the bearings 16b being located near the end of the inner surface of the strut case 14 remote from its attachment 12 to the piston rod 2 and the other bearing 16b being located a fixed distance away along the inner surface of the strut case 14. A first lubricant space 18a is formed between the first pair of bearings 16a and a second lubricant space 18b is formed between the second pair of bearings 16b. As the piston rod 2 moves into and out of the cylinder 4, the intermediate tube 20 slides over the outer surface 15 of the cylinder 4 on the first pair of bearings 16a and the strut case 14 slides over the outer surface 22 of the intermediate tube 20 on the second pair of bearings 16b. The intermediate tube 20 slides freely between the cylinder 4 and the strut case 14, its position being constrained only by an outer lip 24 on the intermediate tube 20 that prevents the end of the tube 20 that carries the lip 24 from sliding past the second pair of bearings 16b. A similar outer lip 26 is formed on the cylinder 4 to prevent the end of the cylinder 4 that carries the lip 26 from sliding past the first pair of bearings 16a. Figure 2 shows how the presence of the intermediate tube 20 enables the damper to achieve increased maximum extension, represented by additional travel distance d, when compared with the prior art . The intermediate tube 20 allows the first and second pairs of bearings 16a, b to slide almost past each other so that there is very little overlap between the cylinder 4 and the strut case 14. Meanwhile, the intermediate tube 20 maintains the rigidity of the strut under bending loads . In the dampers of Figures 1 and 2, the spacing between the bearings 16 of each pair is the same. However, the tolerances associated with each pair of bearings 16 result in a small reduction in rigidity when two pairs of bearings 16a, b are used in the present invention. Accordingly, in an alternative embodiment of the invention each pair of bearings 16a, b may be more widely spaced, which restores the desired rigidity and allows a greater lubrication volume 18a, b of each bearing, while the intermediate tube 20 still allows a maximum travel as great as in the prior art. A combination of the two effects is preferred, whereby both the maximum travel and the bearing spacing are increased. The invention provides a further advantage over the prior art, namely that during reciprocation of the piston rod 2, the speed of relative movement between the strut case 14 and the cylinder 4 is shared between two pairs of bearings 16a, b. Thus the average sliding speed at each bearing surface 15,22 is approximately half that in the prior art, given the same total extension, with a consequent reduction in the heat generated through friction at the bearing surfaces. This improves the reliability and operational life of the bearings, and also the efficiency of the damper because it operates through the dissipation of heat .

Claims: CLAIMS 1 . A damper comprising : a cylinder (4) having an outer surface (15) ; a piston rod (2) slidably received in the cylinder (4) ,- and a case (14) attached to the piston rod (2) and cooperating with the outer surface (15) of the cylinder to maintain the alignment of the piston rod (2) with the cylinder (4) ,- CHARACTERIZED IN THAT: an intermediate tube (20) is located between the cylinder (4) and the case (14) such that an inner surface of the intermediate tube (20) engages the outer surface (15) of the cylinder (4) and an inner surface of the case (14) engages an outer surface (22) of the intermediate tube (20) . 2. A damper according to claim 1, wherein the inner surface of the intermediate tube (20) includes a first pair of bearings (16a) for engagement with the outer surface (15) of the cylinder, the first pair of bearings (16a) defining a first space for lubricant (18a) therebetween. 3. A damper according to claim 2, wherein one bearing of the first pair of bearings (16a) is formed at an end of the inner surface of the intermediate tube (20) remote from the piston rod (2) and the other bearing of the first pair of bearings (16a) is formed at a fixed distance away along the inner surface of the intermediate tube (20) . 4. A damper according to claim 2 or claim 3, further including a lip (26) at an end of the outer surface (15) of the cylinder for retaining the first pair of bearings (16a) in engagement with the outer surface (15) . 5. A damper according to any of claims 1 to 4, wherein the inner surface of the case (14) includes a second pair of bearings (16b) for engagement with the outer surface (22) of the intermediate tube, the second pair of bearings (16b) defining a second space for lubricant (18b) therebetween. 6. A damper according to claim 5, wherein one bearing of the second pair of bearings (16b) is formed at an end of the inner surface of the case (14) remote from the point of attachment (12) between the case (14) and the piston rod (2) and the other bearing of the second pair of bearings (16b) is formed at a fixed distance away along the inner surface of the case (14) 7. A damper according to claim 5 or claim 6, further including a lip (24) at an end of the outer surface (22) of the intermediate tube for retaining the second pair of bearings (16b) in engagement with the outer surface (22). 8 A damper according to any preceding claim, wherein the damper is an inverted monotube 9. A damper according to any preceding claim, wherein the damper is a strut 10. A damper according to claim 9, wherein the damper is a MacPherson strut . 11. A damper comprising: an inner cylinder,- a piston rod slidably received in the inner cylinder,- an outer cylinder surrounding the inner cylinder to form an annular chamber therebetween; and an annular piston slidably received in the annular chamber, the annular piston being coupled to the piston rod; CHARACTERIZED IN THAT: an intermediate tube is located concentrically between the annular piston and the inner cylinder or between the annular piston and the outer cylinder so as to be capable of sliding motion relative to the annular piston and relative to the inner or outer cylinder. 12. A damper comprising: a cylinder (4) having an outer surface,- a piston rod (2) slidably received in the cylinder (4) ,- a case (14) attached to the piston rod (2) and cooperating with the outer surface (15) of the cylinder (4) to maintain the alignment of the piston rod (2) with the cylinder (4) ,- CHARACTERIZED IN THAT: a plurality of intermediate tubes (20) is arranged concentrically between the outer surface (15) of the cylinder (4) and an inner surface of the case (14) .

Inventors: ANDERSON RICHARD FRANKLIN ANDE, ANDERSON, RICHARD, FRANKLIN, ANDERSON
Applicants: ANDERSON RICHARD FRANKLIN ANDE, PILOT PRECISION DAMPERS LIMITE, ANDERSON, RICHARD, FRANKLIN, ANDERSON, PILOT PRECISION DAMPERS LIMITED
Application year: 1996
Main classification: B60G3
IPC codes: B60G3, B60G13, F16F9