GB2276930A - Shock absorber with seal protection - Google Patents

Abstract

A piston rod 32 for a shock absorbing damper comprises an upper portion 47 welded to a cylindrical lower portion 44 which is to reciprocate through a seal 38 inside one end of a hydraulic cylinder 31. The upper portion 47 includes an enlarged abutment structure 58 at the junction between the portions 44, 47. A separate collar 59, e.g. of plastics material, is provided on the lower portion 44 so as to serve as a spacer between the abutment structure 58 and the end of the hydraulic cylinder 31, thereby preventing contact between the seal 36 and any region of poor surface finish on the lower portion 44 near the weld zone. <IMAGE>

Description

Shock absorber with seal protection This invention relates to a piston rod for a shock absorbing damper.

A shock absorber for a vehicle, for example, includes a shock absorbing damper which usually comprises, as shown in Figure 1 of the accompanying drawings, a hydraulic cylinder 1, one end of which has a pivot connection (not shown) for mounting it on a suspension structure and the other end of which has an end plate 2 for cooperating with a resiliently compressible bumper, and a piston rod 3, one end of which carries a piston 4 which reciprocates in the hydraulic cylinder 1 and the other end of which has a screwthreaded connection 6 for mounting it on a suspension structure. The rod 3 has an enlarged abutment structure 7 for retaining the resiliently compressible bumper.

The known piston rod 3 shown in Figure 1 is fabricated from high carbon steel rod of high tensile strength, which is machined to form a partly-screwthreaded reduced-diameter bottom portion for connection to the piston 4 and to form the screwthreaded connection 6 and a reduced-diameter upper portion 8.

The main cylindrical rod portion 9, which is to reciprocate in the hydraulic cylinder 1 and which passes freely through a central hole 11 in the bumper plate 2, is plated with chromium (or is hardened by nitriding) and is ground and honed to achieve a polished surface which resists corrosion and which provides a smooth low-friction surface cooperating with a seal 12 in the upper end of the hydraulic cylinder 1, the seal including a lower lip 13 (primarily for retaining the hydraulic fluid) and an upper lip 14 (primarily for preventing ingress of dust). The abutment structure 7 is constituted by a top-hat section steel ring 16 which is press-fitted on the reduced-diameter portion 8 and abuts against the upper end of the chromium-plated portion 9.

It will be appreciated that the machining of the parts 6 and 8 is a costly procedure. It would be desirable to be able to produce the piston rod by forging an upper portion including the screwthreaded connection 6 and welding the upper portion to a polished cylindrical rod. Figure 2 shows what has been found to happen when a cold-forged steel upper portion 17, including an enlarged abutment portion 18, is connected to a chromium-plated steel cylindrical portion 19 by electrical resistance welding. A steel ring 21 is fitted on the upper end of the cylindrical portion 19 to prevent weld spatter contaminating the smooth surface.

A welding electrode is connected to the upper portion 17 and an earthing (grounding) electrode is connected to the cylindrical portion 19 as near as practicable to the ring 21. The resulting weld 22 secures the ring 21 to the resulting welded rod 23.

A problem which has been found to result from the welding technique is that the current passing through the thin chromium plating tends to burn it away and produce a relatively rough region 24 which may extend to a distance beyond the ring 21 which is greater than the distance, d, between the upper surface of the hydraulic cylinder end plate 2 and the upper lip 13 of the seal 12. To avoid burning, the chromium plating 26 may be stopped short of the upper end of the cylindrical portion 19, leaving a region 27 for engagement by the earthing electrode close to the anti-spatter ring 21, which in this instance has a peripheral flange 28 encircling the weld zone. However, the unplated region 27 is of relatively poor surface finish, is delimited by an uneven edge 29 of the plating, and extends beyond the ring 21 which is greater than the distance d.

Consequently, the rough region 24 or 27 could come into contact with the seal 12, possibly causing damage which would not be visible from the outside and could result in premature failure of the damper. Such contact would be most likely to occur during manufacture of the damper and during storage or transport of the damper before it is incorporated into a shock absorber, but if a highly compressible bumper is used there is also a risk that such contact could occur if the shock-absorber was fully compressed.

What is desired is a design of piston rod which is less costly to manufacture than the known one-piece machined rod but which allows one to avoid the problems found to be associated with a two-piece rod.

It would also be desirable to provide a method of making a shock absorbing damper which could guarantee avoidance of contact between a seal and a region of poor surface finish.

The present invention provides a piston rod for a shock absorbing damper including a hydraulic cylinder, the piston rod comprising a first, cylindrical, longitudinal portion which is to reciprocate through a seal inside one end of the hydraulic cylinder and a second longitudinal portion welded to the first portion, the second portion including an enlarged abutment structure at the junction between the said portions, a separate collar being provided on the first portion so as to serve as a spacer between the abutment structure and the end of the hydraulic cylinder.

Thus, the shock absorbing damper may be made by a method including inserting the first portion of the piston rod through an end plate, which is to constitute the end of the hydraulic cylinder, and through the seal, the collar remaining on the first portion in order to prevent contact between the seal and the region of the rod surface near the abutment structure, which region may be of poor surface quality.

The invention will be described further, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is an axial section through part of a shock absorbing damper, including a known form of one-piece piston rod; Figure 2 is an axial section through part of a damper, including a two-piece welded piston rod; Figure 3 is a side view of part of another two-piece welded piston rod; Figure 4 is an axial section through part of a shock absorbing damper, in accordance with the present invention; Figure 5 is an enlarged detail of Figure 4; Figure 6 is an axial section similar to Figure 4, showing the fully compressed state of the damper; Figure 7 is an axial section through part of a shock absorber including a damper according to Figures 4 to 6; and Figures 8 and 9 is an enlarged perspective view of two alternative embodiments of a collar.

The shock absorbing damper shown in Figure 4 comprises a hydraulic cylinder 31 and a piston rod 32.

The hydraulic cylinder 31 has an end plate or bumper plate 33 welded to a cap 34 fitted on the upper end of an outer tubular casing 36. The lower end of the cylinder 31 has a pivotal connection (not shown) for mounting on a vehicle suspension structure. A bearing assembly 37 inside the upper end of the casing 36 carries a seal 38 having an upper lip 39 and a lower lip 41. A tubular inner sleeve 42 has its upper end fixed to the bearing assembly 37. A piston 43, including valves for controlling the flow of hydraulic fluid during compression and extension of the damper, runs in the inner sleeve 42.

The piston rod 32 has a first or lower portion 44 which reciprocates through the seal 38 and which is integral with a reduced-diameter stepped portion 46 carrying the piston 43. The piston rod 32 also has a second or upper portion 47 which includes a squared end part 48, a screwthreaded part 49, a cylindrical part 50, and an enlarged abutment part 51 (including a flange 52). The parts 48-51 constitute a single piece produced by cold-forging a mild-steel workpiece.

The lower portion 44 of the piston rod 32 is made from a length of high carbon steel rod which is machined to form the stepped portion 46. The cylindrical surface of the lower portion 44 is then chromium plated and is subsequently ground and honed to achieve a polished surface. The upper end of the lower portion 44 is then welded to the upper portion 47 by electrical resistance welding in the following way.

First, an anti-spatter ring 53 (made of mild steel and having a peripheral flange 54) is provided as a friction fit on the lower portion 44, with the flange 54 substantially flush with the upper end of the lower portion 44. Then a welding electrode is connected to the upper portion 47 and an earthing electrode is connected to the lower portion 44 just beneath the anti-spatter ring 53, the two portions 44,47 are urged axially towards each other, and welding current is supplied to the welding electrode. This results in a weld zone 56 at the junction between the two portions 44,47 and weld pool metal 57 which also welds the ring 53 to the abutment part 51 so that the ring 53 and the part 51 become integral parts of an enlarged abutment structure 58.

The resulting two-piece welded piston rod is then provided with a separate collar 59 of plastics material, which is passed over the stepped portion 46 and slid along the cylindrical lower portion 44 as a friction fit until it is adjacent the abutment structure 58. The plastics material of the collar 59 is selected in order to avoid scoring or damaging the chromium plating, and to withstand the high and low temperatures at which the damper will operate, without cracking, falling off, or becoming loose. Suitable materials for this purpose are polypropylene and nitrile rubber, for example.

The collar 59 has an outer diameter approximately equal to that of the ring 54 and greater than the diameter of the hole 61 in the end plate 33 through which the lower portion 44 passes freely. The axial length of the collar 59 is greater than its radial thickness, and the sum of this length and the distance between the outer surface of the end plate 33 and the upper lip 39 of the seal 38 is sufficient to prevent contact between the seal 38 and any region of poor surface finish which may result from the practical requirements by the electrical resistance welding process. Preferably, the collar 59 is sufficiently long to cover any such region of poor surface finish (e.g.

absent or damaged chromium plating).

To assemble the damper, the end plate 33, cap 34, and casing 36 are fitted on the lower portion 44 of the piston rod 32, and the upper surface of the end plate 33 abuts against the collar 59. Subsequently, the bearing assembly 37, with the seal 38, is slid up the lower rod portion 44, the piston 43 is fixed to the stepped portion 46, and the inner sleeve 42 is fitted. Assembly is completed with the outer casing 36 deformed over and retaining the seal 38 and then the damper is fully compressed, in which state it is stored and then transported to the vehicle assembly plant for incorporation into a shock absorber as shown in Figure 7.

To assemble the shock absorber, a bumper 62 comprising a rubber base 63 and a resiliently compressible expanded plastics body 64 with a downwardly widening axial bore 66 (whose minimum diameter is at least equal to the outer diameter of the collar 59) is fitted over the upper portion 47 of the piston rod 32 and engaged with the abutment structure 58. A cap 67 carrying a dust cover 68 is then fitted and abuts against the abutment structure 58. The upper portion 47 is then passed through a suspension structure 69, comprising a body member 71 sandwiched between two rubber elements 72, 73, and is secured by a nut 74 threaded on the part 49. The collar 59 remains within the bumper 62, even when the bumper is compressed to its maximum extent.

It will be appreciated that the collar 59 prevents any region of poor surface finish near the junction of the upper and lower rod portions 47,44 from coming into contact with the seal 38 at any time during the assembly of the damper, storage and transport of the damper, and assembly and use of the shock absorber. Once the collar is fixed and the assembly of the damper is completed, the presence of the collar in the damper and in the shock absorber serves as a guarantee that such undesirable contact has been prevented at all times.

Various modifications may be made within the scope of the invention. For example, the collar may be freely slidable along the piston rod during assembly of the damper and may be held adjacent to the abutment structure during assembly of the shock absorber, e.g. by bonding the collar to the lower rod portion and/or to the abutment structure or by retaining the collar by means of engagement with the bumper. Alternatively, the collar may be bonded to the lower portion and/or to the abutment structure before assembly of the damper is begun. The collar may be moulded onto the lower rod portion. The collar may be of a material other than plastics material, e.g. it may be a metal collar.

Figures 8 and 9 show collars 76,77 which are not circumferentially continuous and which can be fitted from the side, without having to slide the collar along the lower rod portion. This facilitates fitting of the collar. The collar 76 is in the form of a split ring of resiliently flexible plastics material. The collar 77 is of flexible plastics material and has an integral pin 78 which is a snap fit in a hole 79 in order to retain the collar 77 in frictional engagement around the lower rod portion.

Claims (25)

Claims: -

1. A piston rod for a shock absorbing damper including a hydraulic cylinder, the piston rod comprising a first, cylindrical, longitudinal portion which is to reciprocate through a seal inside one end of the hydraulic cylinder and a second longitudinal portion welded to the first portion, the second portion including an enlarged abutment structure at the junction between the said portions, a separate collar being provided on the first portion so as to serve as a spacer between the abutment structure and the end of the hydraulic cylinder.

2. A piston rod as claimed in claim 1, in which the collar is in contact with the abutment structure.

3. A piston rod as claimed in claim 2, in which the collar is held in contact with the abutment structure by frictional engagement with the first portion.

4. A piston rod as claimed in any of claims 1 to 3, in which the collar is made of plastics material.

5. A piston rod as claimed in any of claims 1 to 4, in which the collar is circumferentially continuous.

6. A piston rod as claimed in any of claims 1 to 5, in which the collar has an axial length greater than its radial thickness.

7. A piston rod as claimed in any of claims 1 to 6, in which the abutment structure extends radially beyond the collar.

8. A piston rod as claimed in any of claims 1 to 7, in which the first portion has a region of relatively poor surface finish near the abutment structure, the collar being sufficiently long to cover the said region.

9. A piston rod as claimed in any of claims 1 to 8, in which the abutment structure comprises an enlarged part of the second portion.

10. A piston rod as claimed in claim 9, in which the abutment structure further comprises a ring surrounding the end of the first portion adjacent the second portion, the weld zone between the first and second portions also securing the ring to the first and second portions.

11. A piston rod as claimed in claim 10, in which the ring has a circumferential flange spaced from the first portion and in contact with the enlarged part of the second portion.

12. A piston rod as claimed in any of claims 1 to 11, in which the first portion is chromium plated.

13. A shock absorbing damper comprising a hydraulic cylinder, a seal inside one end of the hydraulic cylinder, and a piston rod extending through the end of the hydraulic cylinder and through the seal, the piston rod being in accordance with any of claims 1 to 12.

14. A shock absorbing damper as claimed in claim 13, in which the end of the hydraulic cylinder is constituted by an end plate having a hole through which the first portion of the piston rod passes freely, the collar having an outer diameter greater than the diameter of the hole.

15. A shock absorber comprising a shock absorbing damper in accordance with claim 13 or 14.

16. A shock absorber as claimed in claim 15, in which the shock absorber includes a resiliently compressible bumper, the collar being adjacent the abutment structure, the bumper surrounding the abutment structure and the collar, extending towards the hydraulic cylinder, and being compressible from an uncompressed length to a minimum compressed length between the abutment structure and the end of the hydraulic cylinder.

17. A shock absorber as claimed in claim 16, in which the minimum compressed length is greater than the axial length of the collar.

18. A shock absorber as claimed in claim 16 or 17, in which the bumper has a bore which widens towards the hydraulic cylinder.

19. A shock absorber as claimed in claim 18, in which the minimum diameter of the bore is at least equal to the outer diameter of the collar.

20. A method of making a shock absorbing damper, in accordance with claims 13 or 14, comprising the sequential steps of (a) providing a piston rod in accordance with any of claims 1 to 12, and (b) inserting the first portion of the piston rod through an end plate, which is to constitute the end of the hydraulic cylinder and through the seal, the collar remaining on the first portion.

21. A method as claimed in claim 10, in which step (a) includes sliding the collar as a friction fit along the first portion until the collar is adjacent the abutment structure.

22. A piston rod substantially as described with reference to, and as shown in, Figures 4 and 5 of the accompanying drawings.

23. A shock absorbing damper substantially as described with reference to, and as shown in, Figures 4 to 6 of the accompanying drawings.

24. A shock absorber substantially as described with reference to, and as shown in, Figure 7 of the accompanying drawings.

25. A method of making a shock absorbing damper, substantially as described with reference to Figures 4 to 6 of the accompanying drawings.