AU3942000A - Suspension apparatus - Google Patents

Description

AUSTRALIA

Patents Act COMPLETE SPECIFICATION

(ORIGINAL)

Class Int. Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority Related Art: Name of Applicant: Liebherr America, Inc.

Actual Inventor(s): ALAN B CORAM, ALAN W DICKERSON Address for Service: PHILLIPS ORMONDE FITZPATRICK Patent and Trade Mark Attorneys 367 Collins Street Melbourne 3000 AUSTRALIA Invention Title: SUSPENSION APPARATUS Our Ref: 618579 POF Code: 862/452863 The following statement is a full description of this invention, including the best method of performing it known to applicant(s): -1- SUSPENSION APPARATUS FIELD OF THE INVENTION The present invention relates generally to oleo-pneumatic shock absorbing systems for vehicles. More specifically, the present invention is directed to oleo-pneumatic suspension springs or devices for mining equipment, especially those vehicles that carry large payloads.

BACKGROUND OF THE INVENTION Oleo-pneumatic suspension springs (or devices) have become the preferred suspension apparatuses for certain vehicles such as large mining trucks and aircraft. Typical configurations of oleo-pneumatic suspension springs OP, of the type used in large mining trucks, are shown in Figs. 1, 3, and 4.

Fig. 1 illustrates the general components in prior art suspension springs of this type.

Piston rod 10, which is disposed at one end of the suspension spring OP, is attached indirectly o to one of the wheels of the-vehicle (see, Figs. 2 and At the opposite end of the suspension spring OP, cylinder housing 12 is attached to the main frame or chassis of the truck (see, Figs. 2 and Typically, oleo-pneumatic suspension springs OP are installed on mining trucks in the locations shown in Figs 2 and 5. In Fig. 2, for example, oleo-pneumatic suspension unit OP is disposed between frame F of the truck and the elements that connect to the truck's tire T.

In Fig. 5, oleo-pneumatic suspension device OP is mounted between truck frame F and suspension arm SA on which the wheel and tire T is mounted.

Within the oleo-pneumatic suspension spring OP, head 14 defines an internal volume that can be divided into three parts 16, 18, 20. As will become apparent in the discussion that follows, the internal volume changes as piston rod 10 moves in relation to cylinder housing 12.

Portions 18, 20 of the internal volume are filled with oil 0, which lubricates the sliding bearings and seals between piston rod 10 and cylinder housing 12. The remaining portion 16 is filled with pressurized gas G. Typically, nitrogen is selected as the pressurized gas G both because it is inert and because it may be purchased at a moderate cost. In operation, gas G transfers its pressure to oil 0 in portions 18, 20 which, in turn, generates a force on piston rod 10 that matches the force required to support the vehicle's sprung weight.

The pressure of gas G in portion 16 and the pressure of oil O in volume portions 18, depends on the effective area of piston rod 10, the geometry of the vehicle suspension system, and the mass supported by the individual suspension unit OP. Since the effective area usually is fixed and since the geometry of the vehicle's suspension system is nearly constant, the pressure is almost directly proportional to the portion of the vehicle's total sprung weight supported by spring unit OP.

In spring unit OP, annular oil chamber 20 functions primarily to dampen the travel of piston rod 10 as oil O flows into and out of annular chamber 20 through orifices 22 and 24.

Ball check valve BCV in orifice 22 restricts the oil flow and, therefore, increases the damping effect in one direction of travel of piston rod 10 over the other direction of travel.

Oil O in internal volume portions 18, 20 inside cylinder housing 12 of oleo-pneumatic suspension spring OP is nearly incompressible. Therefore, oil O retains a nearly constant volume regardless of the pressure applied to oil O by gas G, which is compressible. The pressure versus volume characteristic of gas G can be determined by physical laws that are ooo.

15 well known in the art.

Fig. 6 illustrates the typical relationship between a load on suspended wheel T and the o travel (deflection) of the piston relative to the housing of an oleo-pneumatic suspension unit known in the art (such as that shown in Fig. In Fig. 6, the typical operating ranges for both the empty vehicle condition and the loaded vehicle condition are shown.

*o It is widely known, however, that the relationship illustrated in Fig. 6 provides less than a satisfactory balance between the load and deflection characteristics for both loaded and empty vehicles. When the vehicle is subjected to a minimal load, the curve shows a satisfactory deflection response in the increasing load direction. In particular, the deflection response between the truck when it is empty (empty weight) and when it is subjected to a load that is twice the empty weight (2 x empty weight) is about 4 inches.

However, the deflection response of the suspension spring OP varies greatly depending upon the load applied. As the curve in Fig. 6 illustrates, when the vehicle is fully loaded, the deflection response is much smaller than the deflection response when the vehicle is nearly empty. In particular, the suspension spring OP experiences a difference in deflection of only 0.5 inch (13 mm) between the fully loaded weight and a force equivalent to twice the fully loaded weight. (Refer to the deflection distance between the "Loaded Weight condition and the "2 x Loaded Weight" condition in Fig. The preferred situation would be S S that the suspension spring provides a significantly larger deflection when the vehicle has been loaded to capacity.

When the suspension's travel is restricted to only a small deflection distance (in this case, 0.5 inch for a 100% increase of suspension load), the suspension spring is unable to absorb the large deflections generated as the truck traverses uneven ground. As a result, the deflections are absorbed by the truck's frame, which can be bent, twisted, or otherwise damaged by the associated forces. In addition, when the suspension deflection distance is small, there are two further disadvantages: the tires may be overloaded (which can result in damage to the tires, among other things), and for the driver, the vehicle rides poorly (a poor quality ride). On the other hand, if the suspension spring provides a larger deflection, the suspension spring can more easily absorb variations in the terrain without detrimentally transmitting high forces to the vehicle's frame and can provide a ride with improved quality.

Many solutions have been proposed to overcome the disadvantages inherent in the prior art suspension spring. One suggestion has been to replace the oiLand gas within the "15 suspension spring with an elastomeric liquid that is compressible at high pressures and displays a more linear deflection-versus-load curve than a gas. Another option has been to replace the oil with a compressible elastomeric liquid while retaining a volume of gas acting S on the top of the liquid. Yet another option has been to use two discrete volumes (masses) of gas, as described in U.S. Patent No. 4,798,398, to alter the deflection versus load response of the suspension spring.

None of these options, however, has proven commercially viable or successful. The first two options, which teach the use of compressible elastomeric liquids, have not been widely accepted because the liquids compress only at very high pressures (typically operating pressures exceeding 10,000 pounds per square inch or 70 MPa) and, when they compress, 9 they exhibit only relatively small volume changes. Moreover, at such pressures, the liquids are prone to leak from the suspension springs, making it difficult to reduce leakage to acceptably small rates so that the suspension springs can operate for long service periods in harsh environmental conditions like those encountered in mining operations. With these types of suspension systems, the sealing designs are very costly and the life between recharging or overhaul of the suspension cylinder frequently much shorter than desired.

The third alternative mentioned, the system described in U.S. Patent No. 4,798,398 (which is illustrated in Fig. 1 of that patent), displays a deflection response that is illustrated in Fig. 2 of that patent. The deflection curve shows that the compliance for the loaded truck condition can be improved by more than a factor of 2.5. Potentially, a system such as this could provide significant benefits such as improved ride quality for the truck driver, significantly reduced twisting of the truck's main frame, reduced tire damage, and an improved ability to operate the truck at higher speeds over uneven ground surfaces.

Despite all of these advantages, the system described in U.S. Patent 4,798,398 is not commercially available. Probably, this is due to the mechanical complexity of the system described. System complexity translates into a significant manufacturing cost as well as a much higher cost (and greater difficulty) for system maintenance. Also, the system requires accumulators 12, 16 external to the suspension apparatus. It is not easy to find a suitable place to mount either the external accumulators or the hydraulic connections between the main suspension cylinder and the external accumulators. In addition, external accumulators 12, 16 and hydraulics 66, 68, 80 typically are additional sources of fluid leakage (with a consequent loss of correct suspension operating effect). All of these factors mean that the system described in U.S. Patent No. 4,798,398 probably fails to offer the touted advantages :15 for a reasonable cost (both for the system and for maintenance).

Finally, each of the systems described above is known as a passive system. In other words, there is no external power or control input to the suspension system during operation of the vehicle. Various non-passive (or active) suspension systems have also been developed.

However, all active suspension systems are much more complex than the passive systems 20 described. To date, no active suspension system has become widely accepted in the mining truck field.

SUMMARY OF THE INVENTION An object of the present invention is to provide a suspension apparatus that offers the 25 advantages described in U.S. Patent No. 4,978,398 without the disadvantages previously described with respect to that system.

Another object of the invention is to provide a passive suspension apparatus with an improved deflection response.

To do this, the present invention provides a suspension apparatus including a housing, a piston rod slidably disposed within the housing, wherein the housing and the piston rod together define a variable sealed internal volume. A liquid is disposed within the variable sealed internal volume that occupies a volume that is less than the variable sealed internal volume. A first volume of gas is disposed within the variable sealed internal volume that occupies a volume less than the variable sealed internal volume and is always subject to the same pressure as the liquid disposed within the variable sealed internal volume. A second volume of gas is disposed within the variable sealed internal volume, which occupies a volume less than the variable sealed internal volume and, being separated from the liquid and the first volume of gas by means that includes a piston which slides inside a cylindrical wall, expansion of the second gas volume is limited by a stop which limits the travel of the piston inside the cylindrical wall.

The present invention also provides for a suspension apparatus where the pressure of the gas within the second gas volume is greater than the pressure of the gas in the first gas volume for light suspension loads.

The present invention also provides that the gases in both the first and the second gas volumes may be nitrogen.

Low friction bearings may be disposed in the piston which separates the second gas volume from the liquid and the first gas volume to minimize friction between the piston and 15 the cylinder wall in which it slides.

In another embodiment, the piston which separates the second gas volume from the liquid and the first gas volume may be replaced with a rubber bladder or a rubber diaphragm S of the type used in accumulators for fluid power systems.

In still another embodiment, the suspension apparatus may include a third gas volume disposed inside the variable sealed internal volume and separated from the liquid. The first gas volume and the second gas volume are disposed within the variable sealed internal volume, with the third gas volume being separated from the liquid, the first gas volume, and the second gas volume by means that includes a second piston which slides inside a cylindrical wall. Expansion of the third gas volume is limited by a stop which limits the travel of the second piston inside the cylindrical wall.

In this embodiment, the limit to expansion of the third gas volume causes the pressure of the gas in the third gas volume to be greater then the pressure of the gas in the second gas volume for light and medium suspension loads.

As with the first embodiment, the present invention contemplates that the first, second, and third gases may be nitrogen, or any other appropriate compressible fluid.

In a further embodiment, the second piston which separates the third gas volume from the liquid, the first gas volume and the second gas volume may be replaced with a rubber bladder or a rubber diaphragm of the type used in accumulators for fluid power systems, 0 The embodiments of the present invention that are discussed above will be described below with reference to the accompanying figures. Where appropriate throughout the specification, the same reference numerals are used to indicate the same structures.

BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments of the invention are schematically illustrated in the accompanying drawings, in which: Fig. 1 illustrates a typical oleo-pneumatic suspension spring known in the art for mining vehicles; Fig. 2 shows a typical placement of an oleo-pneumatic suspension spring on the front suspension strut of a truck; Fig. 3 depicts a cross-sectional view of the oleo-pneumatic suspension spring illustrated in Fig. 2, which is another typical example of conventional oleo-pneumatic suspension springs; oo.o Fig. 4 illustrates the details of yet another typical example of oleo-pneumatic 1.: suspension springs available in the prior art; Fig. 5 shows the placement of the oleo-pneumatic suspension spring from Fig. 4 Sbetween the frame and suspension arm of a truck; Fig. 6 depicts the typical deflection versus load characteristics for a single stage oleopneumatic suspension spring of the type found in the prior art; Fig. 7 illustrates an oleo-pneumatic suspension spring incorporating one embodiment of the present invention, shown under a light load; Fig. 8 shows a detailed enlargement of the contact region between the housing, piston rod, and first piston illustrated in Fig. 7; Fig. 9 shows the oleo-pneumatic suspension device of Fig. 7 under a heavy load; Fig. 10 illustrates an alternative embodiment of the present invention where there are three separate gas volumes, shown under a light load; Fig. 11 shows the oleo-pneumatic suspension device of Fig. 10 under a medium load; Fig. 12 shows the oleo-pneumatic suspension device of Fig. 10 under a heavy load; Fig. 13 graphically illustrates the predicted deflection versus load curve for an application of the present invention, which incorporates one additional internal gas volume (this mirrors the curve shown in Fig. 2 of U.S. Patent No. 4,798,398); and Fig. 14 graphically illustrates a deflection versus load curve for an application of the present invention, which incorporates two additional internal gas volumes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Oleo-pneumatic suspension springs of the first embodiments of the present invention are generally designated 100 and are illustrated in Figs. 7-9. Suspension spring 100 includes piston rod 102 that is slidable within housing 104. Piston rod 102 is sealed at its exposed end 106 by closure 108. Similarly, housing 104 is sealed at its exposed end 110 by closure 112.

Both piston 102 and housing 104 are cylindrical in the example illustrated and, for a suspension spring used in a large mining truck, the diameter of piston rod 102 ranges from about 8 to about 14 inches. Piston rod 102 and housing 104 are preferably made from steel.

However, it should be noted that piston rod 102 and housing 104 can be manufactured so that they do not have a circular cross-section. Moreover, piston rod 102 and housing 104 may be fashioned from any suitable material other than steel, so long as the material can withstand •o 15 the forces typically encountered by a truck in a mining environment.

o* Piston rod 102 is hollow. So is housing 104. When piston rod 102 is slidably inserted into housing 104, the two components define an internal volume 114 for suspension spring 100. Seal 116, which is disposed between piston rod 102 and housing 104, seals internal volume 114 from the external environment, permitting internal volume 114 to be pressurized.

Seal 116 is designed to withstand the high pressures to which it will be subjected during operation of suspension spring 100. Seal 116 may be any type known to those skilled in the art.

In the preferred embodiment of the present invention, a substantially incompressible liquid, such as oil 118, partially fills internal volume 114, occupying part of the interior of both piston rod 102 and housing 104. Any oil may be used in suspension spring 100, so long as it does not impair the operation of the spring.

Ball check valve 111 may be disposed through the wall of piston rod 102 so that main volume 113 may communicate with annular volume 115 to permit oil 118 to move from one volume to the other as piston rod 102 moves within housing 104. Ball check valve 111 controls the flow of oil from main volume 113 to annular volume 115 to control the damping effect of suspension spring 100. In addition to ball check valve 111, other apertures (not shown) also may be disposed through the wall of piston rod 102 to facilitate flow of oil 118 from main volume 113 to annular volume 115.

The remaining portion of internal volume 114 in suspension spring 100 is occupied by gas. A first gas 120 fills a first volume 122 within housing 104. First gas 120 is preferably nitrogen because it is inert and readily available. Naturally, first volume 122 is less than internal volume 114.

A first valve 124 is disposed through housing 104 that communicates with first volume 122. First valve 124 permits first gas 120 to be introduced into first volume 122 from the exterior of suspension spring 100. Also, if the pressure of first gas 120 must be changed during operation of suspension spring 100, first gas 120 may be easily added (or removed) through first valve 124. In addition, if the pressure of first gas 120 is to be checked, a pressure gage (not shown) can be fitted on first valve 124.

A second gas 126 occupies a second volume 128 within piston rod 102. Like first gas 122, second gas 126 is preferably nitrogen. Like first volume 122, second volume 128 is less than internal volume 114.

Like first valve 124 through housing 104, a second valve 130 is disposed through piston rod 102, which communicates with second volume 128. Second valve 130 permits second gas 126 to be introduced into second volume 128 from the exterior of suspension spring 100. Also, if the pressure of second gas 126 must be changed during operation of S suspension spring 100, second gas 126 may be easily added (or removed) through second valve 130. If the pressure of second gas 126 needs to be checked, a pressure gage (not •0°0 shown) can be attached to second valve 130.

In piston rod 102, a piston 132 is disposed between second gas 126 and oil 118.

Piston 132 contains second gas 126 within second volume 128 to prevent second gas 126 from escaping into oil 118 and mingling with first gas 120 in first volume 122. Piston 132 is preferably made from a metal such as steel or an aluminum alloy.

*o Within suspension spring 100, second gas 126 is initially charged to a higher pressure than first gas 120 (with the suspension spring in the extended (or lightly loaded) condition).

Therefore, to prevent piston 132 from travelling past a predetermined point within piston rod 102 (and, therefore, equalizing the pressure between second gas 126 and first gas 120), piston stop 134 is attached to the interior surface of piston rod 102. Piston stop 134 limits how far piston 132 can travel within piston rod 102 toward first volume 122.

Two mechanisms are provided to prevent leakage of second gas 126 around piston 132. When suspension spring 100 is under a small (light) load, piston 132 rests against piston stop 134. In this condition, the oil pressure and the pressure of first gas 120 may be significantly lower than the pressure of second gas 126 in second volume 128. To prevent leakage of second gas 126 past piston 132 under these conditions, piston 132 includes an elastomeric seal 136 that engages with a precisely machined surface on the interior of piston stop 134 to provide a pressure resistant seal when piston 132 is against the piston stop 134.

When the pressure of oil 118 within suspension spring 100 exceeds the initial charging pressure of second gas 126, causing second gas 126 to compress (as shown in Fig. piston 132 moves from piston stop 134. In that condition, elastomeric seal 136 does not prevent the leakage of second gas 126 from second volume 128 because elastomeric seal 136 no longer engages piston stop 134. Piston 132, however, includes a second seal 138 to prevent the leakage of second gas 126 when piston 132 has moved free from piston stop 134.

To minimize friction between piston 132 and the interior wall of piston rod 102, piston 132 also includes low friction bearings 140. Low friction bearings 140 assure that piston 132 will move freely to nearly equalize the pressure of second gas 126 with the pressure of oil 118 (and first gas 120) whenever the pressure in oil 118 (and first gas 120) is .0 :15 above the "initial charging pressure" for second gas 126. To minimize the mass inertia of piston 132, the wall thickness is kept to a minimum. If necessary to further reduce its inertia, piston 132 may be fashioned from a low-density material such as an aluminum alloy.

S. The operation of the present invention will now be described with reference to Figs. 7 and 9. In suspension spring 100, first gas 120 acts as the primary gas volume that provides the deflection characteristics graphically illustrated in Fig. 13 for an empty truck. While second gas 126 is also included in internal volume 114, second gas 126 cannot contract unless the pressure within suspension spring 100 exceeds a lower pressure limit (the "initial charging pressure" for second gas 126). This lower pressure limit is higher than the pressure of oil 118 (and of first gas 120) when the truck is in a normally empty (or lightly loaded) condition.

25 When the load on suspension spring 100 rises sufficiently so that the pressure on oil 118 exceeds the lower pressure limit, second gas 126 begins to compress at a rate consistent with that for the combined volume of first gas 120 and second gas 126. Since the combined gas quantity is higher than for just the first volume 122, the rate at which the pressure increases (with reduction of internal volume 114 movement of piston rod 102 into housing 104)) is lower than it would be if second volume 128 were not included in suspension spring 100. This improves considerably the deflection versus load characteristics of suspension spring 100.

0 In order to move piston 132 clear of piston stop 134, the pressure of first gas 120 (and oil 118) must be just above the "initial charging pressure" for second gas 126. When the pressure of first gas 120 is above the "initial charging pressure" for second gas 126, any difference in the pressure between first gas 120 and second gas 126 is due to friction between piston 132 and the inside wall of piston rod 102 and/or the effect of piston 132's inertia when the pressure of first gas 120 changes rapidly (due to rapid movement of piston rod 102 relative to housing 104).

As discussed, first gas 120 may be charged into first volume 122 through first valve 124 at the top of suspension spring 100. So that a proper amount of first gas 120 is charged into first volume 122, suspension spring 100 is generally lightly loaded or not loaded at all for this operation. Similarly, second gas 126 can be supplied to second volume 128 through second valve 130 disposed at the bottom of suspension spring 100. So that the required amount of gas 126 is charged into second volume 128, the suspension spring must be either lightly loaded or not loaded at all. Because first valve 124 and second valve 130 are readily accessible on the exterior of suspension spring 100, and because charging of both gas volumes can be done with suspension spring 100 unloaded, charging suspension spring 100 requires little effort more than that currently required to charge a standard (single-stage) oleo- S pneumatic suspension spring (as shown in Figs. 1, 3, and 4).

Figs. 10-12 illustrate a second embodiment of the oleo-pneumatic suspension spring 200 present invention where the variable sealed internal volume is further divided to add a third gas 202, contained within a third volume 204, to further modify the deflection versus load characteristics of spring 200 (as shown in Fig. 14). To create third volume 204, an internal housing 206 is attached to the internal surface of closure 113 of housing 104.

Internal housing 206 may be made from steel or any other suitable material that is capable of 25 handling the pressures to which it will be subjected during normal operation of suspension spring 200. Internal housing 206 may be attached to closure 113 by welding or any alternative attachment method that seals third gas 202 within third volume 204 so that it cannot escape from internal housing 206 to mix with first gas 120 in first volume 122.

Internal housing 206 necessarily has a smaller cross-section than piston 102 so that internal housing 206 does not interfere with the movement of piston rod 102 in housing 104.

In addition, the length of internal housing 206 is such that it does not interfere with the movement of piston 132 when suspension spring 200 is under a heavy load (as shown in Fig.

12).

Like first valve 124, third valve 208 is disposed through closure 113. Third valve communicates with third volume 204 to permit third gas 202 to be pressurized to a predetermined magnitude when suspension spring 200 is either lightly loaded or not loaded at all. If the pressure of third gas 202 must be changed during operation of suspension spring 200, third gas 202 may be easily added (or removed) through third valve 208. Also, if the pressure of third gas 202 needs to be checked, a pressure gage (not shown) can be attached to third valve 208.

In internal housing 206, second piston 210 is disposed between third gas 202 and oil 118. Second piston 210 contains third gas 202 within third volume 204 to prevent third gas 202 from escaping into oil 118 and mingling with first gas 120 in first volume 122. Like piston 132, second piston 210 is preferably made from a metal such as steel or aluminum alloy.

Within suspension spring 200, third gas 202 is initially charged to a higher pressure than second gas 126 (with the suspension spring in the extended (or lightly loaded) condition). Therefore, to prevent second piston 210 from travelling past the end of internal housing 206 (and thereby equalizing the pressure between third gas 202 and first gas 120), os..

second piston stop 212 is attached to the end of internal housing 206 that is not attached to 0 closure 112.

As with piston 132, two mechanisms prevent leakage of third gas 202 around second o o piston 210 because second piston 210 may have the same construction as piston 132.

Specifically, when suspension spring 200 is under a light load, second piston 210 rests against second piston stop 212. In this condition, the oil pressure and the pressure of first gas 120 may be significantly lower than that of third gas 202. To prevent leakage of third gas 202 past second piston 210 under these conditions, second piston 210 includes an elastomeric seal 214 that engages with a precisely machined surface on the interior of second piston stop 212.

When the pressure of oil 118 exceeds the initial charging pressure of third gas 202, causing third gas 202 to compress (as shown in Fig. 12), second piston 210 moves from second piston stop 212. In that condition, elastomeric seal 214 does not prevent leakage of third gas 202 from third volume 204 because elastomeric seal 214 no longer engages second piston stop 212. Second piston 210, however, includes a second seal 216 to prevent leakage of third gas 202 when second piston 210 has moved from second piston stop 212.

Just as with piston 132, to minimize friction between second position 210 and the interior wall of internal housing 206, second piston 210 also includes low friction bearings 218. Low friction bearings 218 assure that second piston 210 will move freely to nearly equalize the pressure of third gas 202 with the pressure of oil 118 (and first gas 120) whenever it is above the "initial charge pressure" of third gas 202. To minimize the mass inertia of second piston 210, the wall thickness is kept to a minimum. If necessary to further reduce its inertia, second piston 210 may be fashioned from a low density material such as aluminum alloy.

The operation of suspension spring 200 will now be described. Suspension spring 200 operates in the same way as suspension spring 100 except that third gas 202 has a higher initial charging pressure than second gas 126. Just like suspension spring 100, first gas 120 acts as the primary gas volume. Second gas 126 cannot contract unless the pressure within suspension spring 200 exceeds its initial charge pressure. Similarly, third gas 202 cannot contract until the pressure within suspension spring 200 exceeds the initial charge pressure of third gas 202.

As with suspension spring 100, when the load on suspension spring 200 rises so that the pressure on oil exceeds the lower pressure limit of second gas 126, the spring 200 compresses at a rate consistent with the combined volumes of first gas 120 and second gas 126. This is illustrated in Fig. 11, where suspension spring 200 is shown under a medium load. As the load on suspension spring 200 increases to the lower threshold for third gas 202, third gas 202 begins to compress. The compression rate of the spring 200 at that point is 20 dependent upon the total volume of the first gas 120, the second gas 126, and the third gas 202. This is illustrated in Fig. 12.

As with suspension spring 100, in spring 200, first gas 120, second gas 126, and third gas 202 may all be nitrogen.

The deflection versus load curve for suspension spring 100 is illustrated generally in Fig. 13. The deflection versus load curve for suspension spring 200 is illustrated in Fig 14.

Both curves demonstrate an improved characteristic for the spring when compared to the conventional oleo-pneumatic spring with only one independent gas volume (Figure 6).

In suspension spring 100 or suspension spring 200, the second gas 126 and the third gas 202, respectively, need not be contained by a piston. Instead, the gases may be contained within an accumulator by an elastomeric bladder or diaphragm as would be known to those skilled in the art.

It can be seen from the preceding description that this invention provides a simple means for providing an apparatus containing at least two different effective gas quantities.

One quantity of gas contributes to the deflection characteristics of the suspension spring when the truck is empty and the load on the suspension spring is low. The combined gas from each of the effective gas quantities contributes to the deflection characteristics of the suspension spring when the truck is loaded and the loads on the spring are much higher. The apparatus presented by the present invention is much simpler than any previously-known apparatus for obtaining this two (or more) stage gas quantity effect. The simplicity of the suspension spring of the present invention results in lower manufacturing and maintenance costs and also a higher reliability than for previously known suspension springs.

The present invention is not intended to be limited solely to the embodiments and elements described above. While only a few exemplary embodiments are provided, those skilled in the art will readily appreciate that many modifications are possible without deviating from the content and scope of the present invention. Accordingly, all such modifications are intended to be included within the scope of the present invention and the claims appended hereto.

*go• *o o* o 0•0

Claims (12)

1. A suspension spring apparatus, comprising: a housing; a piston rod which slides within the housing; a seal between the housing and the piston rod; wherein the housing, the piston rod, and the seal together define a sealed internal volume with a variable magnitude dependent on an engagement length between the piston rod and the housing; a liquid disposed within the sealed internal volume; a first gas disposed within the sealed internal volume, occupying a first gas volume, wherein the first gas is always subject to the same pressure as the liquid; and a second gas disposed within the sealed internal volume, occupying a second gas volume and being restrained so that pressure of the second gas is greater than or equal to the pressure of the liquid and the first gas.

2. A suspension spring apparatus according to claim 1, further comprising: a third gas disposed within the sealed internal volume, occupying a third gas volume S and being restrained so that the pressure of the third gas is greater than or equal to the pressure of the second gas. go•9

3. The suspension spring apparatus of claim 1, wherein both the first and second gases are nitrogen.

4. The suspension spring apparatus of claim 1, wherein the liquid is substantially 25 incompressible. 9 9 The suspension spring apparatus of claim 4, wherein the liquid is oil.

6. The suspension spring apparatus of claim 1, further comprising: a piston slidably disposed between the second gas and the liquid in the piston rod to restrain the pressure of the second gas so that it is greater than or equal to the pressure of the liquid and the first gas.

7. The suspension spring apparatus of claim 6, further comprising: low friction bearings disposed in the piston to minimize friction between the piston and an interior surface of the piston rod.

8. The suspension apparatus of claim 7, wherein the first piston comprises an aluminum alloy.

9. The suspension spring apparatus of claim 2, further comprising: an internal housing entrapping the third gas; and a piston slidably disposed between the third gas and the liquid outside the internal housing to restrain the pressure of the third gas so that it is greater than or equal to the pressure of the liquid and the first gas. The suspension spring apparatus of claim 9, further comprising: low friction bearing disposed in the piston to minimize friction between the piston and S. an interior surface of the internal housing.

11. The suspension spring apparatus of claim 10, wherein the second piston comprises an aluminum alloy.

12. The suspension spring apparatus of claim 2, wherein the first, second, and third gases are nitrogen.

13. The suspension spring apparatus of claim 2, wherein the liquid is substantially S: 25 incompressible.

14. The suspension spring apparatus of claim 11, wherein the liquid is oil. DATED: 9th June, 2000 PHILLIPS ORMONDE FITZPATRICK Attorneys for: LIEBHERR AMERICA, INC.