Related Applications

This application claims the benefit of U.S. Provisional Application No. 60/020,267, filed June 19, 1996.

Background of the Invention

The present invention is directed to a pneumatic spring and in particular to a pneumatic spring having a flexible unreinforced monolithic side wall formed from an elastomeric material.

As defined in the Spring Design Manual. Society of Automotive Engineers, Part 4, Chapter 2, "A pneumatic spring is basically a column of confined gas in a container designed to utilize the pressure of the gas as the force medium of the spring. The compressibility of the gas provides the desired elasticity. . ." A pneumatic spring can take the form of a cylinder with a piston, but such a design has a number of problems, including sliding friction, air leakage, and piston and rod guide wear. Accordingly, the overwhelming design approach to pneumatic air springs involves the use of a reinforced flexible member together with a rigid plate at the load bearing end and a rigid plate or piston at the other end. The two principal designs of such pneumatic springs employing reinforced flexible members are the rolling lobe spring and the convoluted spring.

The load bearing capacity of a pneumatic spring is related to the product of the load bearing area of the spring multiplied by the pressure of the gas within the spring. However, the actual load bearing capacity of a pneumatic spring is normally less than that calculation would imply - and sometimes is substantially less. The diminution in load bearing capacity results from internal forces within the spring which oppose its load bearing ability. Further, in a typical pneumatic spring, the load bearing area itself is less than it would seem, since the

actual shape of the spring is more complex, than, say, a simple cylindrical column. The actual load bearing capability of the spring is measured in terms of its "effective area", and that is determined empirically by dividing the magnitude of a load the spring is known to carry by the gas pressure within the spring that is created to carry that load. As will be shown, one of the objectives of this invention is to provide a design which has a larger effective area within a given space than can be achieved by prior pneumatic spring designs. Pneumatic springs typically are subjected to high circumferential loads since, of course, the gas they contain exerts an equal pressure in all directions, but only in the longitudinal axial direction is the force resulting from that pressure reacted by the load being borne. To the extent that the shape of the pneumatic spring resembles a cylinder, the circumferential force its side wall must bear may be approximated by the formula for circumferential force in a cylinder, as follows:

F _c - P - h τ where F _c - force in the circumferential direction,

P - internal pressure, h - cylinder height, and r - cylinder radius.

This force can be very large. For example in applications such as heavy-duty trucking, a pneumatic spring might have a height of about twelve inches, a radius of about six inches, and a normal operating pressure of about ninety pounds per square inch. These parameters lead to circumferential force of 6,480 pounds, which actually is very routine. Since the material in the flexible side wall member must be rather thin to be sufficiently flexible, an unreinforced elastomer would be incapable of bearing this circumferential force. For this reason, the flexible side wall members of prior pneumatic springs are constructed with at least two plies of cord-reinforced, rubber-coated fabric.

The manufacturing process of prior pneumatic springs is similar to that which was used for bias ply tires. First a building drum (or mandrel) is covered with a layer of rubber which serves as a gas-impermeable liner. Next, the cord-reinforced fabric is laid over the liner at equal but opposite angles. Then an outer cover of rubber is applied. If metal beads are to be used to attach the flexible member to the end pieces of the spring, these beads are positioned over the ends of the multi-layered tube, and the ends are turned back to enclose the beads. The composition is then removed from the mandrel and cured in an autoclave.

Springs so constructed offer superb performance and have found excellent market acceptance. However, they do suffer from two deficiencies which this invention is intended to resolve. First, the rnanufacturing process is labor intensive and costly. Second, while the biased cords successfully withstand the circumferential load imposed by pressure on the walls of the spring, they do so by reacting that load along the bias angle of the cords. That reactive force has a downward, longitudinal axial component, which offsets a portion of the load bearing capability of the spring. The result may be an effective area of the spring which is little more than half its actual area at either end. To compensate for this effect, the size of the spring required to carry a given load is made larger. This increased size not only adds to the expense of the spring, but also makes such springs more difficult to design into applications where space is restricted as in many automotive applications.

Pneumatic springs are typically shaped as generally cylindrical columns. Like other columns, when under load mey are subject to columnar instability, or buckling. While the stability of rolling lobe pneumatic springs is enhanced by the plies of biased cords employed in their construction, ύiis enhanced stability is attained at the cost of reduced load bearing capacity. Convoluted pneumatic springs are less stable than rolling lobe springs and, hence, tend to have a more limited ratio of height to diameter and tend to be used in applications where potential instability is less of a problem.

Summary of the Invention

A pneumatic spring including a top load transfer member, a spaced apart bottom load

transfer member, and a side wall extending between the top and bottom load transfer members. The side wall is formed from and consists solely of a flexible unreinforced monolithic sheet or web of elastomeric material. The web of the side wall forms a plurality

of convolutions having a minor diameter in the longitudinal direction of the spring and a major diameter in the transverse direction of the spring. One or more circular hoops formed from a high strength material, and having a generally circular cross section, extend around the side wall and are respectively located between adjacent pairs of the convolutions. The pneumatic spring may include a helical coil spring stabilizer attached to the side wall. The stabilizer may be located internally or externally relative to the side wall member. One end of the stabilizer is adapted to engage the top load transfer member and the other end of the stabilizer is adapted to engage the bottom load transfer member of the spring. The stabilizer is attached to the side wall at three or more locations located around the circumference of the side wall at approximately the middle of the side wall between the top member and bottom

member.

Brief Description of the Drawing Figures

Figure 1 is a cross-sectional view of the pneumatic spring of the present invention.

Figure 2 is a cross-sectional view of the pneumatic spring of the present invention including a stabilizer member.

Figure 3 is a cross-sectional view taken along lines 3-3 of Figure 2.

Figure 4 is a side-elevational view of an alternate embodiment of the stabilizer member.

Figure 5 is a cross-sectional view of the stabilizer member taken along lines 5-5 of Figure 4.

Detailed Description of the Preferred Embodiments The pneumatic spring 10 of the present invention, as shown in Figure 1, is generally cylindrical and includes a longitudinal central axis 12 that extends from the bottom end 14 to the top end 16 of the spring 10. The pneumatic spring 10 includes a rigid generally circular top load transfer plate 20 having an inner surface 22, an outer surface 24 and a peripheral edge surface 26. The pneumatic spring 10 also includes a rigid generally circular bottom load transfer plate 30 that is spaced apart from and generally parallel to the top load transfer plate 20. The bottom load transfer plate 30 includes an inner surface 32, an outer surface 34 and a peripheral edge surface 36. The bottom load transfer plate 30 is approximately the same size, and has approximately the same diameter, as the top load transfer plate 20. The top and bottom load transfer plates 20 and 30 may be formed from metal or other high strength rigid materials.

The pneumatic spring 10 includes a flexible generally cylindrical side wall 40. The side wall 40 is connected at its top end to the top load transfer plate 20 and is connected at its bottom end to the bottom load transfer plate 30. A gas tight chamber 42 is located within the side wall 42 and is adapted to receive and contain a compressible fluid such as air or other gases. The side wall 40 has a thickness that extends between an inner surface 44 and an outer surface 46 of the side wall 40. The side wall 40 consists of a single flexible unreinforced monolithic sheet or web 48 of an elastomeric material. The top and bottom ends of the side wall may include metal fittings and the like to facilitate attachment to the top and bottom plates 20 and 30. The web 48 is formed through well known molding processes such as blow molding or injection molding. A preferred elastomeric material well suited to molding the side wall 40 is a copolyester thermoplastic elastomer such as manufactured by DuPont under the trademark HYTREL. This material is available in various grades of greater

or lesser flexibility, wherein greater flexibility is accompanied by reduced tensile strength. If a elastomeric material with greater flexibility and lesser tensile strength is selected, the tensile strength of the side wall 40 may be increased by increasing the wall thickness of the side wall 40, but thicker walls are intrinsically less flexible.

The side wall 40 includes a plurality of convolutions 50 that are located one adjacent another along the longitudinal length of the pneumatic spring 10. Each convolution 50 includes a major diameter "D" that extends transversely from the outer peripheral surface of the convolution 50 at one side of the side wall 40, through the longitudinal axis 12, to the outer peripheral surface of the convolution 50 at an opposite side of the side wall 40. The circumference of each convolution 50 is generally semi-circular in cross section. Each convolution 50 has a minor diameter "d" that extends from a bottom end of the convolution 50 to a top end of the convolution 50 generally parallel to the longitudinal axis 12. A convolution 50 of the pneumatic spring 10 may have a minor diameter "d" while an adjacent convolution 50 may have a minor diameter ^α d'" that is either longer or shorter dian the minor diameter ^u d".

The pneumatic spring 10 includes a plurality of hoops 60. Each hoop 60 is generally circular having a diameter that is less than the major diameter "D" of the convolutions 50. The hoops 60 may be circular in cross section and formed from metal, glass-fiber composite, or other high tensile strength materials. A hoop 60 is respectively located between each adjacent pair of convolutions 50 adjacent the outer surface 46 of the side wall 40. The hoops 60 limit the stresses which must be borne by the elastomeric material of the side wall 40.

The tensile strength required in the side wall 40 is a function both of the pressure of the gas within the chamber 42 that the spring 10 must accommodate and the minor diameter

"d" of the convolutions 50. Thus, the determination of the convolution minor diameter "d"

is another design option. It may be shown that the maximum stress encountered by the elastomeric material of the side wall 40 is determined by the equation: σ - Pd/2h where σ - the maximum stress in pounds per square inch (psi),

P - internal gauge pressure of the gas in pounds per square inch (psi), d - convolution minor diameter in inches (in), and h * wall thickness of the side wall 40 in inches (in).

Significantly, the radius or major diameter "D" of the spring 10 itself is not a design

consideration.

To illustrate a design possibility which takes into account the choice of material, wall thickness, and convolution minor diameter, let us consider a HYTREL elastomeric material grade which has maximum flexibility, and which, according to DuPont, can sustain a tensile

stress of 800 pounds per square inch without undergoing creep. With this material choice, the somewhat arbitrary choice of a spring side wall 40 thickness of one-eighth inch, and the assumption that the spring must tolerate an internal gas pressure of 150 pounds per square inch upon compression of the spring 10, we can rearrange the above equation to find the maximum minor radius "d" of the convolution 50 which fits these parameters. Thus we have d - (2 • σ ^• h)/P

- (2 ^• 800 psi ^• Vβ in)/150 psi

- lVa in

Accordingly, a pneumatic spring 10 with an overall height of twelve inches between the top and bottom plates 20 and 30 might comprise nine convolutions 50, each having a minor diameter "d" of approximately 1-1/3 inches. Of course, many other combinations of material strength, side wall thickness, and convolution minor diameter are possible.

This invention has two primary objectives. First, by greatly simplifying the manufacturing process, it will reduce costs. Second, since it eliminates the use of biased reinforcing cords, it eliminates the vertical component of cord tension which counteracts a part of the lifting capacity of the spring 10. This permits a more compact spring design. A third benefit also accrues to this design when the spring is manufactured from a thermoplastic elastomer, such as a copolyester like the HYTREL material. That benefit is that the material may be recycled, in contrast to the vulcanized rubber in previous designs, which is thermosetting and so may not be reused.

To enhance the columnar stability of pneumatic springs, and of convoluted springs in particular, the pneumatic spring 10 of the invention may include a helical coil compression spring stabilizer 70 within the side wall 40 and chamber 42 of the pneumatic spring 10 as shown in Figures 2 and 3. Alternatively, the stabilizer 70 may be located externally to the side wall 40. The coil spring stabilizer 70 extends in a longitudinal direction between a bottom end 72 that is adapted to engage, and may be attached to, the bottom load transfer plate 30 and a top end 74 that is adapted to engage, and may be attached to, the top load transfer plate 20 of the pneumatic spring 10. The stabilizer 70 is located concentrically about the longitudinal axis 12 and within the side wall 40. The stabilizer 70 includes a plurality of coils 76, wherein each coil extends approximately 360 ^* . As shown in Figure 2, the stabilizer 70 is generally cylindrical such that each coil 76 has a generally uniform diameter.

The coils 76 of the stabilizer 70 are spaced apart from the inner surface 44 of the side wall 40. The stabilizer 70 is attached to the interior surface 44 of the side wall 40 at approximately the longitudinal midpoint of the side wall 40 between the top plate 20 and bottom plate 30 as shown in Figure 2, or if desired, at a plurality of locations along the longitudinal length of the side wall 40. At any location along the longitudinal length of the

side wall 40 where the stabilizer 70 is connected to the side wall 40, three attachments between the stabilizer 70 and side wall 40 are made at a spacing of approximately 120 ^* relative to one another about the longitudinal axis 12. Alternatively, four attachments may be made at a longitudinal location at a spacing of approximately 90' relative to one another about the longitudinal axis 12. Additional attachments can be made if desired. A ligature 80, such as a cord, wire, strap, or similar connecting device, may be tied, affixed to, or otherwise engage the stabilizer 70 at one end of the ligature 80, and may be attached to the inner surface 44 of the side wall 40 at the other end of the ligature 80, to attach the stabilizer 70 to the side wall 40. The ligature 80 may be attached to the inner surface 44 of the side wall 40 by electronic welding, by an adhesive, or other attachment means. The ligature 80 may also be attached to the side wall 40 by extending through a loop molded into the side wall 40.

Columnar stability of the helical coil spring stabilizer 70 will be enhanced if its transverse diameter is maximized, given the confines of the interior diameter of the chamber 42 of the pneumatic spring 10. A still further enhancement to columnar stability will be obtained by attaching the ends 72 and 74 of the stabilizer 70 respectively to the top and bottom plates 20 and 30 of the pneumatic spring 10 by means in common use for fixing ends of helical coil springs.

Figure 4 shows a modified helical coil spring stabilizer identified with the numeral 90. The stabilizer 90 includes a bottom end 92 and a top end 94 and a plurality of coils 96. The stabilizer 90 is tapered inwardly along its longitudinal axis so that it has the approximate profile of an hour-glass. Thus the coils 96 at the bottom end 92 and top end 94 of the stabilizer 90 have a diameter which is larger than the coils 96 in the middle of the stabilizer 90. The diameter of the coils 96 become progressively smaller from the end coils to the

middle coil. As shown in Figure 5, each coil 96 of the stabilizer 90 is generally elliptical in cross section. The cross section of the coil 96 includes a major axis 100 extending through the wide portion of the cross section and a minor axis 102 extending transversely to the major axis 100 through the narrow portion of the cross section. The minor axis 102 extends generally parallel to the central longitudinal axis 104 of the stabilizer 90. Alternatively, the cross section of the coil 96 may be circular, but die elliptical cross section enhances the columnar stability of the stabilizer 90 over a circular cross section.

Various features of the invention have been particularly shown and described in connection with the illustrated embodiments of the invention, however, it must be understood that these particular arrangements merely illustrate, and that the invention is to be given its fullest interpretation within the terms of the appended claims.