GB2150667A

GB2150667A - Plastics shock absorber

Info

Publication number: GB2150667A
Application number: GB08426546A
Authority: GB
Inventors: Douglas Paul Taylor
Original assignee: Tayco Developments Inc
Current assignee: Tayco Developments Inc
Priority date: 1983-11-30
Filing date: 1984-10-19
Publication date: 1985-07-03
Also published as: JPS60125431A; FR2555689B1; GB8426546D0; GB2150667B; FR2555689A1

Abstract

A shock absorber has a plastics casing (11), a piston (17) in the casing, the plastics material e.g. a closed cell plastics foam (22) in the casing, the plastics casing and the plastics closed cell foam having expansion and contraction rates which are sufficiently close that the shock absorbing characteristics of the shock absorber are substantially constant throughout a relative broad temperature range. The plastics material can be a compressed polyurethane foam and the casing made of nylon. <IMAGE>

Description

SPECIFICATION Plastics shock absorber The present invention relates to a shock absorber made of plastics materials.

It is known to use plastics as a filler for shock absorbers, but the operating characteristics of known prior shock absorbers vary with temperature.

The present invention is based on the discovery that this variation in operating characteristics is due to the unequal expansion and contraction rates of the shock absorber casing and the plastics filler.

According to the invention, a shock absorber comprises a plastics casing, a piston in said casing, and plastics material within said casing, said plastics casing and said plastics material having expansion and contraction rates which are sufficiently close that the shock absorbing characteristics of said shock absorber are substantially constant throughout a broad temperature range.

The present invention thus provides a low cost plastics shock absorber which has satisfactory operating characteristics throughout a relatively broad temperature range.

In order that the present invention may be more fully understood, a preferred embodiment will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a fragmentary longitudinal sectional view of a shock absorber embodying the invention, taken along the line 1-1 of Fig. 2; Figure 2 is a cross sectional view taken substantially along the line 2-2 of Fig. 1; and Figure 3 is a graph of force versus travel of the shock absorber piston, at various temperatures.

A shock absorber 10, in the preferred embodiment of the invention, includes a plastics casing 11 which is preferably moulded and which has a cylindrical side wall 12, an integral end wall 13, and an end wall 14 which is snapped into position at the end of casing 11 remote from end wall 1 3 and retained therein by an annular protuberance 1 5 which is received in an annular groove 16. A piston 1 7 is attached to a piston rod 19 which is slidably received in a bore 20 of end wall 14.

Chamber 21 is filled with a closed cell foam 22 which is compressed as piston 17 moves to the dotted line position from right to left in Fig. 1.

The rate of contraction and expansion of the plastics material of cylinder 11 and the plastics material which comprises the closed cell foam 22 are sufficiently close that the shock absorbing characteristics of the shock absorber are substantially constant throughout a relatively broad temperature range. This is schematically shown in Fig. 2 wherein the solid lines 23 and 24 show the size of cylindrical wall 1 2 at low temperature and the broken lines 25 and 26 show the size of casing 1 2 at high temperatures.The foam 22 follows the expansion of inner wall 23 at the same rate that cylindrical wall 1 2 expands, thereby to maintain the shock absorbing characteristics of the shock absorber substantially constant throughout a relatively broad temperature range, in comparison for example with the temperature range normally encountered by a motor vehicle shock absorber. This is shown in Fig. 3 wherein the force versus travel graph of the piston of the shock absorber 10 shows the characteristics at ambient temperature, low temperature, and high temperature. The low temperature is -40"F (- 40C), the high temperature is + 160"F (71C) and the ambient temperature is approximately 70"F (21C).It will readily be appreciated that for all temperatures between -40"F and 160 (- 40C and 71 C), the characteristics of the shock absorber fall between the low temperature and the high temperature graphs.

Thus, the above-described structure is a low cost friction type shock absorber wherein the foam rubs against the inside of the casing and wherein energy is absorbed by the compression and natural hysteresis of a cellular foam, with the shock absorber providing substantially uniform operating characteristics through a wide temperature range due to the balancing of the plastics foam's thermal shrinkage and expansion and the plastics cylinder's shrinkage and expansion.

The preferred materials, which will produce the above-described results, are nylon for the cylinder 11 and closed cell polyurethane foam as the foam 22. It will be appreciated of course that other combinations of plastics may be used, provided that they have expansion and contraction rates which are sufficiently close. It will be appreciated that open cell plastics foam may also be used.

By way of example only, a model which has operated satisfactorily had a casing of nylon containing closed cell polyurethane foam. The polyurethane foam had an unstressed density of between 8 and 20 pounds per cubic foot (384 to 960 kg m-3). However, when placed in the casing, the polyurethane foam was compressed to one-third of its unstressed volume so that it had a density of between 24 and 60 pounds per cubic foot (1150 to 2880 kg m-3) when the piston was in its solid-line position, that is, when the shock absorber was not subjected to an external load. The compression of the foam to this extent was necessary to have the piston return to its full-line position of Fig. 1 when an external force thereon was removed. It will be appreciated that the foam can have any desired initial unstressed density, and also that it can be compressed to more than 60 pounds per cubic foot (2880 kg m - 3) depending on the shock absorber characteristics which are desired.

Claims

1. A shock absorber comprising a plastics casing, a piston in said casing, and plastics material within said casing, said plastics casing and said plastics material having expansion and contraction rates which are sufficiently close that the shock absorbing characteristics of said shock absorber are substantially constant throughout a broad temperature range.

2. A shock absorber as claimed in claim 1, wherein said plastics material is a plastics foam.

3. A shock absorber as claimed in claim 2, wherein said plastics foam is maintained under compression when said shock absorber is not subjected to an external load.

4. A shock absorber as claimed in claim 1, 2 or 3, wherein said plastics material is a closed cell foam.

5. A shock absorber as claimed in claim 4, wherein said closed cell foam has an unstressed density of between about 8 and 20 pounds per cubic foot (384 and 960 kg m-3) and is compressed within said casing to a density of between about 24 and 60 pounds per cubic foot (1150 and 2880 kg m-3) when the shock absorber is not subjected to an external load.

6. A shock absorber as claimed in claim 4, wherein said closed cell foam has a density in excess of 60 pounds per cubic foot (2880 kg m-3) when the shock absorber is not subjected to an external load.

7. A shock absorber as claimed in any preceding claim, wherein said casing is fabricated from nylon and wherein said plastics material is polyurethane foam.

8. A shock absorber as claimed in any preceding claim, wherein the broad temperature range is between - 40"F and + 160"F (- 40C and 71C).

9. A shock absorber substantially as described herein with reference to the accompanying drawings.