GB2164415A

GB2164415A - Tension-compression fluid spring

Info

Publication number: GB2164415A
Application number: GB08520297A
Authority: GB
Inventors: Douglas Paul Taylor; David A Lee
Original assignee: Tayco Developments Inc
Current assignee: Tayco Developments Inc
Priority date: 1984-09-10
Filing date: 1985-08-13
Publication date: 1986-03-19
Also published as: FR2570152B1; DE3530429C2; DE3530429A1; FR2570152A1; GB8520297D0; GB2164415B

Abstract

A tension-compression fluid spring comprises a casing (11) and end walls (14-17) defining first and second spaced chambers (12, 13). An axially slidable elongate shaft (35) extends through seals in the end walls, and has surfaces (36,37; 39,37) constituting pistons (faces 52, 51) in each chamber. A compressible fluid maintained under pressure in each chamber acts on the pistons to resist tension or compression forces applied between the piston shaft (via 57) and the casing (via 47). The fluid also acts on the pistons, with zero applied force, to centre the piston shaft at a predetermined position. The spring has zero force centring, i.e. it is capable of responding to minimal applied forces when at its centred position. The fluid can be a silicone or butane liquid or a high pressure gas. A damping head (53) can be mounted on one of the pistons. <IMAGE>

Description

SPECIFICATION Tension-compression fluid spring The present invention relates to a fluid spring operable in both tension and compression.

It is known to provide fluid springs which are operable in both tension and compression, for example as described in U.S. Patent No.

2,842,356. However, liquid springs of this type do not have zero force centring because they have an inherent preload which has to be overcome before the spring is operative. Zero force centring is the quality of being movable from a neutral position in response to any minimal force (or additional force) applied thereto. In addition, tension-compression fluid springs of the foregoing type are affected by temperature changes which vary the parameters of the spring.

The invention provides a fluid spring comprising casing means defining first and second chambers containing first and second bodies of pressurized, compressible fluid respectively, and piston means comprising first and second piston means in said first and second chambers respectively and coupled by coupling means, the arrangement being such that the spring is operable to resist both tension and compression forces applied between said piston means and said casing, and the pressurized fluid acts on said first and second piston means to maintain said piston means in a predetermined centred position relative to said casing with a predetermined applied force.

The predetermined applied force may be zero or non-zero; in either case, small differences in the applied force will cause corresponding movements of the piston, thus providing zero force centring.

Such a fluid spring is responsive to applied forces of small magnitudes, because the spring is preloaded by opposing fluid forces to a centred position, that is, it has zero force centring. The zero force centring characteristic operates both to attenuate high frequency vibrations as well as shock pulses.

The fluid spring is also capable of maintaining its preset centre position when subjected to temperature changes, by arranging the two fluid chambers and piston means to have similar characteristics.

In order that the invention may be better understood, a preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, wherein: Figure 1 is a fragmentary cross-sectional view of a tension-compression fluid spring embodying the present invention; Figure 2 is a fragmentary enlarged view of a portion of the spring of Figure 1; and Figure 3 is a force-displacement diagram illustrating the manner in which the fluid spring of Figure 1 operates.

A tension-compression fluid spring 10 has its piston centred relative to its casing in such a manner that it does not have a net preload force, and this is known as zero force centring. Therefore, forces of any small magnitude will cause the spring to operate, and the spring will therefore attenuate high frequency vibrations as well as shock pulses. The zero force centring of the piston is achieved by balanced fluid pressures acting thereon, and they are affected equally by changes in temperature so that such changes will not alter the centring of the spring.

The tension-compression fluid spring 10 of Figure 1 includes a cylindrical casing 11 divided into fluid chambers 12 and 13. Annular end walls 14 and 15 define chamber 12, and annular end walls 16 and 17 define chamber 13. Annular end walls 15 and 16 are mirror images of each other and annular end walls 14 and 17 are mirror images of each other.

Annular end walls 15 and 16 are mounted in casing 11 in the following manner: Threaded ends 19 and 20 of end walls 15 and 16, respectively, are threaded into tapped portion 21 of cylinder 11. There is a close sliding fit between the outer cylindrical surfaces 15" and 16" of end walls 15 and 16, respectively, and internal surfaces 24 and 28, respectively, of cylinder 11. O-rings 22 and 23 provide a seal between end walls 15 and 16, respectively, and surfaces 24 and 28 respectively. Outer cylindrical surfaces 14' and 17" of annular end walls 14 and 17 have a close sliding fit with the inner surfaces 24 and 28, respectively, of casing 11. O-rings 25 and 26 provide a seal between inner surfaces 24 and 28 and end walls 14 and 17, respectively.

Each of the end walls 14, 15, 16 and 17 have PTFE or TEFLON (Registered Trade Mark) seals 27, 29, 30 and 31, respectively, which are identical to each other. Therefore, only seal 27 will be described in detail. Seal 27 includes an annular outer portion 32 and an annular lip 33 spaced therefrom with an annular chamber 34 therebetween for receiving pressurized fluid within chamber 12. Portions of seal 29 which correspond to portions 32, 33 and 34 of seal 27 are designated 32a, 33a and 34a, respectively; and corresponding portions of seal 30 are designated 32b, 33b and 34b, respectively; and corresponding portions of seal 31 are designated 32c, 33c and 34c, respectively.

A piston 35 has a portion of a first diameter extending through seal 27, a portion 37 of a second and larger diameter extending through seals 29 and 30, and a portion 39, which is of smaller diameter than portion 37, extending through seal 31. When the chambers 12 and 13 have pressurized fluid therein, the existence of pressurized fluid in the annular spaces 34, 34a, 34b and 34c, will cause annular lips 33, 33a, 33b and 33c, respec tively, to provide sealing engagement with the portions of piston 35 which are received therein, and will also cause annular seal portions 32, 32a, 33b and 33c, respectively, to provide sealing with the internal cylindrical surfaces 14', 15', 16' and 17', respectively, of end walls 14, 15, 16 and 17, respectively.

Guide rings 38 of hard plastics material are located behind the seals to prevent extrusion thereof. Seals, like 27, 29, 30 and 31, are fully disclosed in U.S. Patent No. 3,256,005.

In order to assemble the tension-compression fluid spring of Figure 1, end walls 15 and 16 are threaded into position. Annular shoulders 39 and 40 of end walls 15 and 16, respectively, will abut annular shoulders 41 and 42, respectively, of casing 11. Piston 35 is then inserted through seals 29 and 30 to substantially the position shown in Figure 1 with seals 29 and 30 engaging the central portion 37 of shaft 35. Thereafter, the space to the left of end wall 15 is filled with a suitable compressible fluid and end wall 14 is slid into casing 11 with seal 27 encircling piston portion 36. Thereafter, annular ring 43 is threaded into the left end of casing 11 and its annular surfaces 44 will bear against annular shoulder 45 of end wall 14 and thus move it to the right and thereby compress the fluid in chamber 12. End wail 17 is instailed in a like manner.In this respect, the space to the right of end wall 16 is filled with compressible fluid, and end wall 17 is slid into casing 11 with seal 31 encircling piston portion 39.

Thereafter, the annular end 46 of bracket 47 is threaded into the right end of casing 11 so that annular end 49 thereof bears against annular shoulder 50 of end wall 17 to thereby move it to the right to compress the fluid in chamber 13. Annular ring portions 43 and 46 are adjusted until the pressures in chambers 12 and 13 are substantially equal.

The pressurized fluid in chamber 13 will exert a force on piston 35 biasing it to the left in Figure 1. In this respect, an annular shoulder 51 is located between piston portions 37 and 39, and the fluid pressure in chamber 13 is applied against annular shoulder 51 which is the effective piston area. There is also an annular shoulder 52 on piston 35 in chamber 12. Shoulder 52 is the same size as shoulder 51. Damping head 53 has a side 54 and an opposite side 55. Side 54 abuts shoulder 52.

The difference in area between sides 55 and 54 is equal to the size of annular shoulder 52 which is equal to annular shoulder 51. Thus, there will be a fluid pressure exerted on damping head 53 biasing piston 35 to the right. Since the pressures in chambers 12 and 13 are equal and since the effective areas of piston 35 in chambers 12 and 13 are equal, piston 35 will be maintained in a neutral centred position.

As can be seen from Figure 1, bracket 47 is attached to casing 11 and bracket 57 is attached to piston 35. The forces which are applied to spring 10 are applied to brackets 47 and 57. Compressive forces are resisted by spring 10 in the following manner. The compressive forces will be applied in the-direction of arrows 59. This will cause piston 35 to move to the right in Figure 1 relative to casing 11. Therefore, more of the larger diameter central portion 37 of piston 35 will enter chamber 13 and simultaneously leave chamber 12. The pressure in chamber 13 will thus increase and the pressure in chamber 12 will decrease. This will create a spring force biasing piston 35 to the left. When the compressive forces 59 are removed, the biasing force in chamber 13 will return piston 35 to the neutral position shown in the drawings.

Damping head 53 will merely dampen movement of piston 35 to reduce oscillation thereof. When tension forces in the direction of arrow 60 are applied to brackets 47 and 57, the larger central portion 37 of piston 35 will enter chamber 12 and simultaneously leave chamber 13. This will cause an increase in pressure in chamber 12 and a decrease in pressure in chamber 13. Therefore, the increased pressure in chamber 12 will bias piston 35 to the right in Figure 1. After the tension forces 60 are removed, the piston 35 will move to the right to a netural centred position shown in Figure 1. In the foregoing manner, tension-compression spring 10 resists compressive forces 59 and tension forces 60.

Representative compressible fluids which may be used in chambers 12 and 13 are silicone liquid which is 9.6% compressible at 20,000 psi (1.4X10B N/m2), or butane liquid which is 20-25% compressible at 20,000 psi (1.4X108 N/m2), or any other suitable compressible liquid which provides significant compressibility at high pressures. In addition, the compressible fluids may be suitable gases at high pressures.

The ends 61 and 62 (Fig.2) of end walls 15 and 16, respectively, are spaced from each other and define an annular space 63 around piston 35. Annular space 63 is in communication with a bore 64 in casing 11. Thus, the facing ends 61 and 62 of end walls 15 and 16, respectively, are vented to the atmosphere, this being necessary for proper operation of seals 29 and 30.

The diagram of Figure 3 shows how the tension-compression spring of Figure 1 responds to changes in temperature. Piston 35 will remain centred during temperature fluctuations because the pressure in each of chambers 12 and 13 will vary equally with changes in temperature. Curve 66 shows the piston displacement at a given temperature as a function of the forces which are applied both in tension and compression. If the temperature should increase so that there is greater pressure in both chambers 12 and 13, the forcedisplacement characteristic of spring 10 is shown by the curve 69. Thus, while the parameters of the spring change, it remains centred in the same position. Curve 70, which is superimposed on curve 66, shows the characteristic of the curve as affected by the damping head 53.

In Figure 1 the tension-compression spring 10 is shown as being centred to a neutral position when no external forces are applied thereto, and any minimal tension or compression forces applied thereto will move it from its neutral position. This is known as zero force centring. However, under certain circumstances it may be desirable to have the spring 10 have zero force centring after an external force has been applied thereto.Therefore, if a force, such as 65, is applied to piston 35, and it is desired that piston 35 have a zero force centring in the same neutral position, bracket 47 can be rotated to move end wall 17 into chamber 13 a greater amount so as to increase the pressure in chamber 13 over that which exists in chamber 12 so that the total of force 65 plus the force exerted by the fluid pressure on piston 35 in chamber 12 would equal the force exerted by the pressure of the fluid on piston 35 in chamber 13, and thus the piston will be returned to its neutral centred position shown in Figure 1. Thereafter, minimal forces applied in either tension or compression will move the spring from its neutral position.Thus, in certain situations where spring 10 is to be subjected to an unbalancing force, the spring can be adjusted to counterbalance the unbalancing force, so that it will have zero force centring from which it will resist external forces. In the latter condition, any minimal forces in either direction will move the spring from its neutral position without the requirement of having to overcome the external force 65.

It will also be appreciated that variations in the parameters of the spring 10 may be made to meet various conditions by either varying the effective areas of the pistons in chambers 12 and 13 and/or by varying the pressure of the fluids therein, or by having fluids with different characteristics in each of the chambers.

By way of example, and not of limitation, chambers 12 and 13 may contain silicone fluid at a pressure of 15,000 psi (1.0X103 N/m2), or any of the other above mentioned fluids at proper pressure to provide the desired spring parameters.

Claims

1. A fluid spring comprising casing means defining first and second chambers containing first and second bodies of pressurized, compressible fluid respectively, and piston means comprising first and second piston means in said first and second chambers respectively and coupled by coupling means, the arrangement being such that the spring is operable to resist both tension and compression forces applied between said piston means and said casing, and the pressurized fluid acts on said first and second piston means to maintain said piston means in a predetermined centred position relative to said casing with a predetermined applied force.

2. A fluid spring according to claim 1, comprising first force receiving means coupled to said piston means, and second force receiving means coupled to said casing means, the coupling means coupling said first and second piston means for correlated movement relative to said first and second chambers, respectively, to cause increases of pressure produced in either of said chambers by either of said first and second piston means to be accompanied by decreases in pressure in the other of said chambers by the other of said first and second piston means, whereby the movement of said first and second piston means relative to said casing means in a first direction under a tension force applied to said first and second force receiving means produces a spring force in said fluid spring opposing said tension force, and whereby the movement of said first and second piston means relative to said casing means in a second direction under a compression force applied to said first and second force receiving means produces a spring force in said fluid spring opposing said compression force.

3. A fluid spring according to claim 2, wherein said casing means comprises an elongate cylinder, and wherein said first and second chambers are axially spaced in said cylinder, and wherein said first and second piston means comprise an elongate shaft having first and second portions of different diameters in each of said chambers.

4. A fluid spring according to claim 3, comprising first end wall means in said cylinder defining said first chamber, first seal means in said first end wall means, second end wall means in said cylinder defining said second chamber, second seal means in said second end wall means, and wherein said elongate shaft extends through said first and second seal means.

5. A fluid spring according to claim 4, wherein said first end wall means comprises a pair of first end walls spacedly mounted in said cylinder, and wherein said second end wall means comprises a pair of second end walls spacedly mounted in said cylinder.

6. A fluid spring according to claim 5, comprising means for moving at least one of said first and second end wall means to vary the volume of its associated chamber.

7. A fluid spring according to claim 5 or 6, wherein one of said first end wall means is proximate one of said second end wall means, and vent means are provided in said cylinder between said one of said first end wall means and said one of said second end wall means.

8. A fluid spring according to claim 3, com prising seal means in each of said first and second chambers, and wherein said shaft exends through said seal means.

9. A fluid spring according to any of claims 1 to 5, including means for adjusting the pressure of the fluid in at least one of said first and second chambers.

10. A fluid spring according to any preceding claim, including damping means for damping any movement of said piston means.

11. A fluid spring according to claim 10, wherein said damping means comprises a piston head mounted on at least one of said first and second piston means.

12. A fluid spring according to claim 1, wherein the coupling means and the pressurized fluid in said first and second chambers acting on said first and second piston means co-operate, with zero applied force on the spring, to maintain said piston means in the said predetermined centred position relative to said casing.

13. A fluid spring according to claim 12, including means for varying the pressure of said compressible fluid in at least one of said first and second chambers, thereby to vary the centred position of said piston means under zero applied force.

14. A fluid spring, substantially as described herein with reference to the accompanying drawings.