GB2207730A

GB2207730A - Elastomeric springs

Info

Publication number: GB2207730A
Application number: GB08816606A
Authority: GB
Inventors: Andrew Stevenson; John Harris
Original assignee: MATERIALS ENG RES
Current assignee: MATERIALS ENG RES
Priority date: 1987-07-15
Filing date: 1988-07-13
Publication date: 1989-02-08
Anticipated expiration: 2008-07-13
Also published as: GB2207730B; GB8716711D0; GB8816606D0

Abstract

A compression spring useful in vehicle suspension applications has an extended plateau region in its force/deflection characteristics, such extended plateau being obtained by virtue of the formation of the spring as a tubular elastomer body 1 of progressively increasing cross-section from one end to the other provided with longitudinally spaced-apart reinforcements 2 defining bulging instability sites 3 therebetween, the spring when under compression undergoing bulging sequentially at said sites. The reinforcements may be formed as ribs 2 or can be rings 5 of metal, plastics or filamentary material, and may be helical rather than annular. The elastomeric material can include magnetic material to enable magnetic damping. The internal void may be filled with soft elastomer or plastics material. <IMAGE>

Description

IMPROVEMENTS RELATING TO ELASTOMERIC SPRINGS This invention concerns improvements in and relating to elastomeric springs and particularly, though not exclusively, is concerned with the application of such springs to the field of vehicular suspension.

There have been many proposals in the prior art to utilize the properties of elastomeric materials in the design of shock absorption devices for vehicle suspensions, the earliest of such proposals concentrating upon utilization of natural rubber elastomers and more recent proposals making use of the advantageous characteristics of modern synthetic elastomeric materials. The prior art proposals have also embraced the use of elastomeric materials in conjunction with metal and other spring devices in socalled composite springs and, as will be explained hereinafter, the present invention also embraces this possibility.

Described in European Patent Application No.

85115879 (Serial No. EP-A2-0184848) is a composite spring comprising a tubular elastomeric body, preferably of rubber, having a coil spring embedded in and bonded to it; the coil spring is said to control the occurrence of symmetric bulging instability in the elastomeric body under axial load conditions so that this bulging instability occurs sequentially along the length of the body between adjacent coils of the coil spring until it assumes the form of a continuous coil of elastomer.By virtue of this arrangement, the force/deflection characteristic of the composite spring is said to be controllable to provide selected stiff and soft regions in accordance with specific requirements, particularly the provision of a soft load bearing characteristic over an extended deflection range at a predetermined load, with the soft region corresponding to the symmetric bulging of the elastomer body. The application of such a composite spring device to automotive suspensions is further discussed in rubber Chemistry and Technology, Vol 59, No.5, November-December 1986, pp. 740 to 764, in the article entitled "On the role of nonlinearity in the dynamic behaviour of rubber components" by J.

Harris and A. Stevenson.

Whilst the force/deflection characteristic described in EP-A2-0184848 abovementioned has attractions for automotive suspension applications, it is our belief that some of the explanations provided in EP-A2-0184848 as to how such characteristics might be obtained may be flawed and that a composite spring device as described in EP-A2-0184848 may not be capable of achieving characteristics significantly different from those obtained by much earlier and more recent prior art devices. In this connection, reference may be made to US Patent Specification No.

1032454 issued in 1912 which discloses a rubber body within which there is embedded a helical spring.

Other disclosures of composite springs comprising generally cylindrical elastomer bodies having helical springs embedded therein are shown in French Patent Specification No. 34669 (patent of addition to FR-A559350) issued in 1929, US Patent Specification No.

2156580 issued in 1939, US Patent Specification No.

2605099 issued in 1952, US Patent Specification No.

2822165 issued in 1958, and European Patent Specification No. EP-B1-0045497. A composite spring comprising a cylinder of elastomeric material having metal annuli embedded therein at spaced apart locations is described in European Patent Specification No. EP-A1-0155209.

The present invention resides in the realization that in order to achieve a force/deflection characteristic of the kind described in EP-A2-0184848 it is necessary to positively constrain the elastomer body to buckle progressively in a predetermined manner by appropriately configuring the body and/or any composite materials associated therewith; the right cylindrical elastomer body described in EP-A2-0184848 with a helical spring embedded therein will not, it is believed, exhibit the sequentially occurring bulging instabilities required to produce the required force/deflection characteristics in a predetermined manner, as neither will any of the spring devices disclosed in any of the other prior art documents disclosed herein.

In accordance with the present invention therefore, an elastomeric body providing advantageous force/deflection characteristics has a generally frusto-conical cylindrical shape such that the crosssection of the body changes progressively throughout its length whereby sequential bulging of the body is assuredly a structural feature of -the shape of the body.In one exemplary embodiment, a spring made entirely of elastomeric material has periodic annular reinforcements formed integrally in its wall so as further to predetermine the force/deflection characteristic of the spring by achievement of progressive bulging instabilities, and in another arrangement equivalent reinforcements are provided by means of reinforcing annuli spaced axially from one another, advantageously at progressively changing distances, the annuli being formed of metal or of any other suitable material capable of reinforcing the elastomer.

By virtue of the generally conical shape of the elastomer body, and it is to be noted that the present invention is not to be regarded as limited to strictly conical shapes nor to shapes of circular cross-section, a spring device is obtained which can be made to be globally intrinsically stable at its design loading and yet demonstrates local buckling instabilities which come into effect progressively and in a predetermined manner so as to contribute to a non-linear static force/deflection behaviour of specific type.In application of the spring device of the invention to a vehicle suspension, the non-linear spring function can advantageously be predetermined so that the normal static weight of the vehicle is supported in the plateau region of the non-linear force/deflection characteristic of the device, and the elastomer functions dynamically to provide substantial attenuation of vibration passing into the vehicle body whilst the vehicle is in motion.

Further features of the invention are set forth in the appended claims, and in order that the invention might be more clearly understood, several exemplary embodiments thereof will hereinafter be described with reference to the accompanying drawings wherein: Fig.l is a cross-sectional view of a first embodiment of the invention shown in unloaded condition; Fig.2 is a view of the embodiment of Fig.l as axially compressed under the action of a force F; Fig.3 shows a cross-sectional view of a second embodiment of the invention shown in unloaded condition; Fig.4 shows the embodiment of Fig.3 as axially compressed under the action of a force F; Fig.5 shows the static load/deflection characteristic obtainable by virtue of the present invention; ; Fig.6 is a showing of the dynamic stiffness/frequency characteristic of a device in accordance with the present invention.

Fig.7 illustrates the variation of the phase change of a dynamic input transmitted through an elastomeric spring as a function of dynamic frequency; Fig.8 illustrates the transmissibility of dynamic inputs through an elastomeric spring in accordance with the invention; and Fig.9 illustrates the dynamic behaviour of a spring in accordance with the invention under varying load conditions.

Referring first to Figs.l and 2, shown therein is an integrally-formed elastomeric spring device in accordance with the present invention comprising a moulded hollow frusto-conical elastomer body 1 having annular reinforcing ribs 2 provided on its outer surface and defining therebetween sites 3 for the occurrence of bulging instabilities as the device is axially compressed.As shown in Fig.l, the ribs 2 are equi-axially spaced apart from each other and are all of the same size and configuration, but they could alternatively be differently spaced from each other and/or they could be of different sizes so as to have different reinforcing effects upon the tendency of the body 1 to buckle and bulge outwardly under axial compression, the essence of the present invention being that bulging instabilities should be constrained by the structure of the device to occur in the elastomer body in a predetermined progressive manner.

Fig.2 shows the compression of the body 1 under axial loading and indicates how controlled sequential bulging will occur on account of the different diameters of the elastomer at the different sites 3.

Whereas the device of Figs.l and 2 is integrally formed (though it need not be, the reinforcing rings 2 could be assembled to the device separately), the device of Figs.3 and 4 comprises an elastomer body 4 of hollow frusto-conical shape provided at axially spaced-apart locations with separately formed reinforcing rings 5 which might be formed of metal for example, or could be formed of an appropriate rigid or semi-rigid plastics material or could be formed by winding filamentary material onto the external -surface of the elastomer body 5. As with the device of Figs.l and 2, in the device of Figs.3 and 4 the rings need not be the same and could be formed in different shapes and sizes, and/or of different materials, and/or differently spaced apart from each other.

Many other variations of the devices of Figs.l to 4 are possible within the ambit of the present invention. For example, the wall thickness of the conical elastomer body could be lesser at one axial end of the body than at the other so as to assist the formation of progressive bulging instabilities in the device under compression. The cone angle as a design parameter can also be varied not only to ensure controlled sequential bulging, but also to adjust the stages in the force/deflection characteristic of the device at which bulging occurs. Helical, rather than annular, reinforcing ribs could be utilized, though annular ribs are presently preferred. Furthermore, whilst in the devices of Figs.1 to 4 the conical structure is such that buckling occurs outwardly, an equivalent device could be configured so that buckling occurred inwardly.

The elastomeric material of the elastomer body might be of natural or synthetic rubber for example or might be formed of a composite elastomer material exhibiting multi-phased behaviour under compressive load conditions, and could possibly incorporate magnetic material so as to enable controlled magnetic or electromagnetic damping of dynamic vibrations to be achieved. The internal void of the hollow elastomer body might further incorporate dynamic damping means, which might comprise a soft elastomeric filling material such as of vulcanized or unvulcanized polysulphide or a foamed plastics material, and/or might incorporate active or passive fluid damping components enabling the intrinsic transmissibility performance characteristics of the device to be adapted to the specific application.

The essence of the embodiments of the present invention as hereinbefore described is that the reinforcements, be they integrally or separately formed, are arranged so as to constrain the elastomer body to deform by means of periodic bulging deformations which contribute together to a non-linear static force/deflection behaviour of specific type as shown schematically in Fig.5. The local bulging between reinforcements comprises a local buckling instability, with the overall generally conical design ensuring that the device overall is intrinsically stable at its design load F.

The device of the present invention is particulary well suited to such applications as (a) primary or secondary suspensions for road vehicles, (b) mounting of sensitive electronic equipment in vehicles such as aircraft and space vehicles, and (c) other general suspension applications. In application to vehicle suspensions, for example, the present invention provides a device having a non-linear spring function such that the normal weight of the vehicle is supported in the plateau region of the non-linear force deflection curve, shown in Fig.5. It is furthermore essential to the practical functioning of the device in vehicle suspension applications that the design and materials employed incorporate sufficient damping to prevent unacceptable resonances at input frequencies likely to occur while the vehicle is in motion.The device of the invention provides attenuation of vibrations occurring from various sources passing into the vehicle body while it is in motion, and it is essential to this function that the dynamic properties of the device be well defined.

Dynamic properties are normally defined by means of dynamic stiffness (see Figure 5) (the real part of the complex modulus) and phase angle (Figure 7) (describing phase lag between in phase and out of phase components). This behaviour can be usefully summarised by means of transmissibility (Figure 8).

Each of these parameters is defined as a function of frequency. The frequency of input vibrations can vary in practice over the range 0.1 to 500 Hz. The amplitude of the vibrational input will be smaller at higher frequencies. The device thus also needs to be characterised as a function of amplitude since dynamic stiffness and phase angle can vary with amplitude, even at the same frequency. Dynamic behaviour can also be represented as hysterisis loops. Figure 9 illustrates the desired behaviour at different static preload levels of the device.

It is an advantageous feature of the invention that the device enables the following features to be provided in one unit: 1. static force deflection behaviour to enable a vehicle body to be supported at the correct height; 2. intrinsic global stability at the design load; 3. non-linear force/deflection behaviour to provide improved ride characteristics, namely low stiffness under normal steady conditions and increasing stiffness with increasing load under extreme conditions such as fast cornering or hard braking of the vehicle; 4. correct dynamic stiffness at the appropriate static preload and service frequency and amplitude to give the desired natural frequency and hence desired vibration isolation characteristics; and 5. sufficient damping from the elastomeric phase to avoid the need for external hydraulic or other damping systems, though damping may be enhanced by such means despite the fact that the composite design will provide a good initial level of damping.

Claims

CLAIMS:

1. A compression spring comprising an elongate tubular elastomer body provided with longitudinally spaced-apart reinforcements defining therebetween sites for local bulging instabilities, and wherein the spring is constrained to develop such local bulging instabilities sequentially throughout said sites when under compressive load by virtue of the spring having a non-uniform cross section progressing from a lesser to a greater end of the spring.

2. A spring as claimed in claim 1 wherein the elastomer body has a generally frusto-conical shape.

3. A spring as claimed in claim 1 or 2 wherein the wall thickness of the elastomer body is generally constant throughout the length of the body.

4. A spring as claimed in claim 1 or 2 wherein the wall thickness of the elastomer body varies from one end thereof to the other.

5. A spring as claimed in any of the preceding claims wherein said reinforcements comprise reinforcing ribs of elastomeric material.

6. A spring as claimed in claim 5 wherein said reinforcing ribs are formed integrally with the elastomer body and of the same elastomeric material.

7. A spring as claimed in claim 5 wherein said reinforcing ribs are formed non-integrally with the elastomer body.

8. A spring as claimed in claim 5 or 6 or 7 wherein said reinforcing ribs comprise a plurality of spacedapart annular reinforcements of the elastomer body.

9. A spring as claimed in claim 5 or 6 or 7 wherein said reinforcing ribs comprise a helical reinforcement of the elastomer body.

10. A spring as claimed in any of claims 5 to 9 wherein said reinforcing ribs are on the exterior of said elastomer body.

11. A spring as claimed in any of claims 5 to 9 wherein said reinforcing ribs are within said tubular elastomer body.

12. A spring as claimed in any of claims 1 to 4 wherein said reinforcements comprise reinforcing elements of metal.

13. A spring as claimed in any of claims 1 to 4 wherein said reinforcements comprise reinforcing elements of rigid or semi-rigid plastics material.

14. A spring as claimed in claim 12 or 13 wherein said reinforcing elements comprise a plurality of spaced-apart annuli.

15. A spring as claimed in claim 12 or 13 wherein said reinforcing elements comprise a helical reinforcement of the elastomer body.

16. A spring as claimed in any of claims 12 to 15 wherein said reinforcements are on the exterior of said elastomer body.

17. A spring as claimed in any of claims 12 to 15 wherein said reinforcements are on the interior of said tubular elastomer body.

18. A spring as claimed in any of claims 12 to 15 wherein said reinforcements are embedded in said elastomer body.

19. A spring as claimed in any of the preceding claims wherein the spacing of said longitudinally spaced-apart reinforcements varies from one end of the spring to the other.

20. A spring as claimed in any of the preceding claims wherein the size of said reinforcements varies from one end of the spring to the other.

21. A spring as claimed in any of the preceding claims wherein said elastomer body is formed of a composite elastomer material exhibiting multi-phased behaviour under comprehensive load conditions.

22. A spring as claimed in any of the preceding claims wherein the elastomeric material of the spring incorporates magnetic material for enabling magnetic or electromagnetic damping of dynamic vibrations.

23. A spring as claimed in any of the preceding claims wherein dynamic damping means are incorporated within the interior of the elastomer body.

24. A spring as claimed in claim 23 wherein said dynamic damping means comprises a soft elastomeric material.

25. A spring as claimed in claim 23 or 24 wherein said dynamic damping means comprises fluid damping components.

26. A spring as claimed in any of the preceding claims wherein the sequential bulging instabilities experienced under compressive load of the spring are such as to provide the spring with an extended plateau region in its non-linear force: deflection characteristic.

27. A compression spring as claimed in claim 1 and substantially as hereinbefore described with reference to Figs. 1 and 2 of the accompanying drawings.

28. A compression spring as claimed in claim 1 and substantially as hereinbefore described with reference to Figs. 3 and 4 of the accompanying drawings.

29. The use of a compression spring as claimed in claim 26 in an application wherein the static loading of the spring occurs in the plateau region of the force: deflection characteristic of the spring.

30. The use claimed in claim 29 wherein said application is to a vehicle suspension.

31. The use claimed in claim 30 wherein the vehicle is a road vehicle and the compression spring provides primary or secondary suspension for the vehicle.

32. The use claimed in claim 29 wherein said application is to the mounting of instrumentation in vehicles.

33. The use claimed in claim 32 wherein said instrumentation comprises sensitive electronic or other equipment.

34. The use claimed in claim 32 or 33 wherein said vehicle comprises an aircraft.

35. The use claimed in claim 32 or 33 wherein said vehicle comprises a space vehicle.

36. The use of a compression spring as claimed in any of claims 1 to 28 in an application comprising (a) primary or secondary suspensions for road vehicles, or (b) mounting sensitive equipment in aircraft or space vehicles, or (c) any other application.