Wheel for a Track Laying Vehicle

The present invention relates to wheels for track laying vehicles and, more particularly, to improvements in road wheels for military tanks and other heavy track laying vehicles.

In track assemblies of crawler-type vehicles a plurality of wheels are provided in a tandem row on each side of the vehicle to contact and run upon the associated track and carry the vehicle weight through the associated suspension systems. One type of known track or bogie wheel assembly comprises a dual wheel set in which two wheels, each having a mounting disc or body and a peripheral rim, are mounted back-to-back onto a hub or spindle, and solid elastomeric treads are secured to the outer faces of the dual rims to ride upon the track shoes or cleats. The mutually facing edges of the dual rims are spaced apart axially of the wheel to provide clearance for the usual upwardly protruding track guides of the track shoes as the wheels roll along the track. In this art, it is common to find special wheel designs which are characteristically custom made, heavy and expensive.

Some typical examples of such prior art tank or track vehicle road wheels are disclosed in U.S. Pat. Nos. 3,263,315 and 3,997,217. Also of interest for their showing of tank idler wheels are U.S. Pat. Nos. 2,789,438 and 3,013,843.

The previous art of tank road wheel design and manufacture typically comprised a substantially flat (cylindrical) rim base to which the rubber tire was bonded. Often the axial edges were curved inwardly to reinforce the edges and as a concession to the economic production of such rims. These flat rims were subject to loads imposed by the tire due to the road forces being absorbed. The flat rim, having a small thickness in the direction of the major resultant of the forces and thus a low section modulus or beam strength, was subject to high deformation and stresses.

A further problem in the art arises due to wider rims (axially of the wheel) being used to allow smaller diameter wheels (a function of the carrying capacity of the tires) for greater suspension travel so as to increase mobility over rough terrain. These wider rims accentuate the deflection and thus the required reinforcement for the same. Such reinforcement has been accomplished in a variety of ways, including thicker rim stock, rings attached to the interior of the tire rim, and location of the wheel disc near the high deflection point. All of these methods are aimed at reducing the span between support points to thereby lower overall deflection and stresses. They also add weight and cost.

Another problem in such road wheels for track laying vehicles resides in the high stress levels and stress concentrations in the molded rubber tire affixed to the rim of the wheel. Hysteresis heat generation in the tread rubber due to high loads and speeds contributes to premature rubber and rubber-to-metal bond failures. Also, high compressive and tensile stress concentrations have been found to exist at the bonded edges of such tires. This is where most failures have been seen to initiate in current road wheel tire designs, such as those used on the Ml U.S. military tank. Thus, despite the use of a relatively thick rubber section in the tire, which adds weight and cost to the wheel, tire life has not been as satisfactory as desired in such heavy duty military track laying vehicle applications.

Accordingly, it is an object of the present invention to provide an improved road wheel for a track laying vehicle which is constructed to have a more even distribution of imposed loadings to thereby reduce deflection stresses of the wheel and tire, thus lowering overall stresses on the part and allowing either greater life or a reduction in stock thickness and weight for the same life with respect to both the wheel and tire.

Another object of the present invention is to provide an improved method of manufacturing the aforementioned novel wheel of the invention.

Other objects, as well as features and advantages of the present invention will become apparent from the following detailed description and when taken in conjunction with the accompanying scale drawings, wherein:

FIG. 1 is a side elevational view of one embodiment of a tank road wheel constructed in accordance with the present invention as viewed from the hub mounting side of the wheel;

FIG. 2 is a fragmentary cross sectional view taken on the line 2--2 of FIG. 1;

FIG. 3 is a fragmentary composite diagrammatic layout view illustrating certain progressive steps in the formation of the wheel of FIGS. 1 and 2 pursuant to one embodiment of the method of the invention;

FIG. 4 is a fragmentary view of the wheel of FIGS. 1 and 2 sectioned as in FIG. 2, but without the tire thereon illustrating its mode of deflection under typical load applications;

FIG. 5 is a fragmentary side elevational view of another embodiment of a tank road wheel constructed in accordance with the present invention viewed from the hub-mounting side of the wheel;

FIG. 6 is a radial cross sectional view taken on the line 6--6 of FIG. 5; and

FIG. 7 is a fragmentary view of the wheel of FIGS. 5 and 6, sectioned as in FIG. 6, but without the tire thereon, illustrating its mode of deflection under typical load applications.

Referring to FIGS. 1 and 2, one embodiment of a tank road wheel 20 constructed in accordance with the present invention comprises a one-piece stamped steel disc and rim part 22 including a disc or "backbone" portion 24 having a hub mounting portion 26 provided with a center opening 28 and a circular row of mounting holes 30 for receiving suitable wheel mounting fasteners therethrough for attachment of the wheel to an associated hub or axle spindle. Disc portion 24 also has a reversely curved portion 32 extending radially outwardly from mounting portion 26 which merges with a radially extending outer peripheral margin portion 36, which in turn is integrally joined through a right angle bend 38 to a rim portion 40. Rim 40 extends generally axially of the wheel away from disc portion 24 and merges through a return bend portion 42 at its outboard edge with an in-turned flange portion 44.

Road wheel 20 also includes a solid rubber tire 50 bonded to the outer peripheral surface of rim 40 to form a circumferentially continuous tire surface for the wheel. Tire 50 has a cylindrical outer periphery which forms a track engaging surface 52, and laterally opposite sloping side surfaces 54 and 56 which preferably are tapered to diverge radially inwardly relative to the wheel center axis 58 to accommodate the difference between the required narrower width of tread face 52 (axially of the wheel) and the preferred wider axial dimension of rim 40. The inner peripheral surface 60 of tire 50 exactly matches the exterior contour of the outer surface 62 of rim portion 40, a match up which occurs automatically when tire 50 is molded in situ to rim 40. Preferably side surface 54 terminates tangentially with the curvature of return bend 42, whereas inboard tire surface 56 is axially offset in an outboard direction to intersect bend 38 so as to be slightly spaced axially outwardly from the plane of the exterior surface of disc margin 36.

The disc-rim part 22 is preferably manufactured in accordance with the procedure indicated diagrammatically in FIG. 3. In the first stage of this method a flat sheet stock of suitable metal, such as SAE 950 HSLA steel modified to meet hardenability requirements and having a minimum thickness of, for example, 0.300", is blanked to form a circular blank of appropriate starting diameter. Preferably sequentially in the same die set an initial draw stamping operation is performed to work the blank into a preform having the contour indicated at 70 in FIG. 3. In the next stage a stamping and ironing operation is performed to reshape preform 70 into the configuration illustrated in phantom at 72 in FIG. 3. In a third operation the upper edge portion 74 of the formed edge flange is machined off or otherwise edge conditioned to provide an edge 75 of uniform height and free of burrs, etc. Next, the upright flange portion is bent inwardly to an angle of approximately 45.degree. with the blank axis 58 to thereby tip the flange inwardly to the position shown at 76 in FIG. 3. The final rim forming operation curls flange 76 to the position shown at 78 in FIG. 3 so that the resultant flange 44 will have its final inclination (approximately 55.degree. to axis 58) as shown in FIG. 2. Center hole 28 and bolt mounting holes 30, if not already integrated in a previous operation, can then be pierced and/or punched in the disc-rim part 22 while the same is held fixtured by its rim portion 40 to insure concentricity of the center and bolt holes 28,30 with the axis of rim portion 40. It is to be understood that the residual stresses in part 22 resulting from the aforementioned drawing and forming operations are intentionally concentrated primarily in the tipped-in flange 44 as well as in the central portion of rim 40, and are beneficial relative to part geometry and design load application inasmuch as they tend to prevent yielding under load. That is, work hardening in the finished disc-rim part resulting from the above-described draw forming and die shaping sequence of steps is sufficient to increase the yield strength of the part material in the areas of highest stress so that the stress levels present will not initiate yielding in the part even under theoretical overloads.

Preferably, the radially extending margin portion 36 is surface hardened to Brinell 287-461 and to a minimum depth of 0.120", the hardening method as per MIL-STD-12515 or using interrupted quench after induction hardening, with the part to be 400.degree. F. to 500.degree. F. after quench.

In accordance with one principle feature of the present invention, the forming of the disc-rim part 22 is predesigned to produce a rim section 40 having the shape of an arch as viewed in cross section in FIG. 2. This rim arch preferably has a substantially uniform radius of curvature in the plane of the drawing (perpendicular to the radius of curvature of rim section 40 about the axis 58). The arch of the rim 40 spans from the outermost edge 38 of the disc portion 24 to the junction of the rim 40 with the return bend 42. Rim 40 is thus of convex configuration looking in the direction of the applied load in service. It will thus be understood that the geometry of rim 40 provides the strength of the classic arch, which is one of the strongest architectural forms known to man.

In addition, the arch radius of curvature of rim 40 in the aforementioned axial plane may be substantially equal to the radius of curvature of rim 40 about the wheel axis 58. Thus, in three dimensional terms, the curvature of rim portion 40 forms a section of a sphere and is thus spheroidal, another elemental structure of great strength. Design and structural analysis may require different radii of the arch of the rim 40 versus the outside radius about the wheel axis 58, resulting in a substantially toroidal surface. Nevertheless, if consistent with other design specifications and limitations for a given vehicle, such as overall wheel width and wheel diameter, equal radii of rim curvature and wheel are preferred, i.e., a truly spherical rim geometry. Thus, as used herein "toroidal" may be defined as spherical plus or minus the required deviation in rim arch radius from such equality to meet the vehicle wheel envelope specifications. This toroidal surface may be optimized to minimize stress concentrations due to imposed loadings, thereby causing the entire surface of the toroid to support those loads, thereby distributing stresses and keeping any maximum stress relatively low. Due to this configuration of rim portion 40, in accordance with the present invention, deflection of the rim portion is kept to a minimum, thus lowering overall stresses in the part which in turn results in either greater life, or a reduction in stock thickness and weight for the same life.

Referring to FIG. 4, the aforementioned improvement in uniformity of stress distribution can be seen in the manner in which the disc-rim part 22 deflects under normal service loading as the tank road wheel rolls on its track under the weight of the vehicle loading. Compare the solid line position in the unloaded free state of wheel disc-rim part 22 with the phantom line showing of the part which shows typical part deflection at its maximum rated loading. Note that maximum deflection occurs in the displacement of the in-turned outer edge flange 44. Note also that very little change occurs in the curvature of the rim portion 40 between unloaded and maximum deflection positions. It thus will be seen that rim 40, due to its toroidal section, has the strength to carry the loading out to the outer rim edge 44, which, due to its C-shaped channel section, has a strong section modulus relative to the rim portion 40. Relatively even stress distribution thus results so that the strength of the material is utilized with maximum efficiency.

It has also been found that the angle of the in-turned flange 44 relative to axis 58, or alternatively to the radial plane of the wheel perpendicular to this axis, affects the desired optimum stress loading of the part. In the case of the wheel geometry of the embodiment of FIGS. 1-4 (Ml wheel geometry), an angle of approximately 55.degree. relative to the radial plane has been found to be optimal for this effect. In addition, it has been found that the effect of just changing from the prior art "flat" rim to a toroidal rim, without changing from the usual radial flange, is sufficient to reduce the maximum stress by approximately 16% in this embodiment (design). The rubber thickness of tire 50 and the rim toroidal diameters are also parameters which are variables in optimizing the wheel design. Generally speaking, lower stresses can be obtained utilizing larger toroidal diameters.

Another feature of the design shown in FIGS. 1-4 is the "reverse backbone" at portion 32. This reverse backbone is located at the track lug wear surface-disc interface to provide an open wear surface along disc portion 38. Without this reverse curvature, the wheel would be susceptible to being "notched" as the track guide lugs wore against the disc wear surface. Moreover, it has been found that the provision of the reverse backbone in the disc is not detrimental or significant to the resulting desired reduction in the stress levels in the wheel.

Another important feature of the present invention, and presently believed to be a significant contributor to synergistically improved results obtained thereby, resides in the configuration of the molded rubber tire tread 50 which is vulcanized to rim section 40. Because of the arched and toroidal configuration of the interface between tread 50 and rim portion 40, the stresses in the molded rubber tire 50 are also significantly reduced, thereby reducing stress crack initiation and retarding growth of the same so as to improve the rubber life of the tire tread 50. Basically it has been found that a toroidal supporting surface for the tire tread, whether it be an arch or toroidal section such as rim portion 40 or some other section geometry providing this contour of outer supporting surface, will cause a reduction in maximum stress in the road wheel rubber due to a more uniform stress distribution of loads in and through the rubber tread 50.

Prior art track laying wheels and associated rubber tread designs, such as those used on the Ml tank road wheel tire, have been analyzed and found to indicate high compressive and tensile stress concentrations at the bonded corner where most failures have been seen to initiate. However, with the toroidal supporting surface provided by the rim section 40 of the present invention, much more uniform stress distribution has been obtained, thereby increasing the tread strength-to-weight ratio and allowing greater life for the tread in addition to that of the wheel steel or, due to such greater strength-to-weight ratio, a reduction in thickness of the radial dimension of the tire tread 50 with a concomitant reduction in the weight thereof as well as the wheel. For example, by providing a toroidal support for the tire tread 50 in accordance with the present invention, stress concentrations at the bonded corners of the tire tread are eliminated, tensile stresses on the lateral face of the tire tread are decreased, up to 7.3% less rubber is used in the tread, and the tread retains a stiffness within 5% of prior art designs. Because of the elimination of stress concentrations and reduction in tensile stresses, a more durable product is obtained. By following the aforesaid novel tread geometry, it is also believed that the use of a higher than normal modulus rubber compound would further improve performance of the tire tread and overall performance of the wheel. In the embodiment of FIGS. 1-4, it has been found that the preferred tread geometry may be expressed as a ratio of the minimum thickness of tire tread 50 taken radially of wheel 20 to the maximum radial dimension of tread 50 measured from the cylinder defined by the outer face 52 of tread 50 to the junction of the side walls 54 and 56 of tread 50 with the tim portion 38, 40,42 of the disc-rim part 22, this ratio being approximately 1:2.

FIGS. 5, 6 and 7 illustrate another embodiment of a road wheel 100 for a track laying vehicle constructed in accordance with the present invention, with those elements corresponding to like elements in wheel 20 designated by reference numerals with a prime suffix. Different wheel design specifications have resulted in a variation of the evolution of the wheel geometry of the present invention while adhering to the aforementioned general principles thereof. The wheel 100 of FIGS. 5, 6 and 7 is designed to employ HSLA steel and the dimensional specifications for an armored combat earthmover. It will be seen from viewing these Figures that the toroidal rim concept of the invention is again applied, and weight reduction, which, of course, is desirable, is also obtained. Thus, wheel 100 again consists of a disc-rim part 102 with a molded rubber tire tread 104 having a cross-sectional contour of the present invention and vulcanized to the outer periphery of the rim. Flange 44' has a somewhat smaller inner diameter than that of embodiment 20 due to the different parameters of wheel diameter and loading in this design. As compared to the prior art present wheel configuration used for the aforementioned armored combat earthmover, the embodiment of FIGS. 5 and 6 may be characterized by way of distinction as having a toroidal rim section 40' and the reverse in-turned flange 44' (FIG. 6), preferably having an angle of incidence with the axis 58' of the wheel of 35.degree.. Wheel 100 also has a shallower dish portion 106 than that of the previous prior art design geometry, i.e., a larger radius of curvature and less offset axially relative to the mounting hub portion 26' of the disc-rim part 102.

It has been found that the highest stress location is at the outer flange 44'. The effect of changing the angle of incidence of flange 44' relative to the axis 58' from 0.degree. to the aforementioned 35.degree. is to shift the point of maximum stress from the tip (free edge) of the flange back along the outer side edge of the flange, i.e. toward the return bend portion 42' of the flange. Again, wheel 100 with the disc-rim part 102 may be formed pursuant to the procedure described previously in conjunction with FIG. 3. Likewise, the maximum stress which might be calculated or expected in the wheel embodiment 100 in the steel portion 102 may be mitigated by the residual forming stresses developed in shaping the part 102 according to the method sequence described previously in conjunction with FIG. 3. The resulting work hardening in the part is expected to be sufficient to increase the yield strength so that the stress levels present will not initiate yielding in the material. In addition, weight savings are obtained in this design, mainly from the change in the disc backbone or dish 24'.

Again, it has been found, in accordance with the principle feature of the present invention, that the presence of the toroidal rim 40' is beneficial in that the same strength could not be maintained without it at the same stock thickness. As illustrated in the free state versus loaded conditions in FIG. 7, the presence of the toroidal rim contour prevents the rim section from deflecting inwardly radially under load. Again the toroidal rim 40' of wheel 100 cooperates with the cross-sectional contour of the rubber tire tread 104 as shown in FIG. 6 to improve the performance of tire 104 by reducing high shear stresses present at the bonded corners of the tread and by reducing both maximum and minimum principal stresses along the lateral rim surface. Although tire tread 104 of wheel embodiment 100 is relatively thick radially of the wheel as compared with embodiment 20, thinning of this tread section would provide increased tire life, but not necessarily improve the performance of the metal disc-rim part 102.

From the foregoing description it will now be appreciated that the improved road wheel for a track laying vehicle of the present invention provides several advantages over prior commercial and military road wheels. The toroidal support of the tire treads significantly reduces the maximum stress concentrations in the rubber tire, and the toroidal shape of the rim section likewise minimizes maximum stress concentrations and improves the strength to weight ratio of the metal part of the wheel. Overall reduction in weight or an improvement in strength to weight ratio is thus obtained by the present invention in a very economical fashion.

Although the present invention is particularly designed and adapted for use with military-type armored or heavy track laying vehicles, it should be understood that the invention may be used with other types of crawler vehicles, such as those utilized in earth moving operations.

In one working example of the present invention constructed in accordance with FIGS. 1 through 4 of the drawings, wheel embodiment 20 is made to the following specifications:

In accordance with another working example of the wheel constructed in accordance with the present invention pursuant to the alternate wheel embodiment 100 of FIGS. 5, 6 and 7, the following specifications were employed:

It will also be understood that, although the foregoing description and drawings describe and illustrate in detail successful working embodiments of the present invention, to those skilled in the art to which the present invention relates the present disclosure will suggest many modifications in construction as well as widely differing embodiments and applications without hereby departing from the spirit and scope of the invention. The present invention, therefore, is intended to be limited only by the scope of the appended claims and the applicable prior art.