Lightweight wheel for a track system

Abstract

An improved lightweight wheel (300) for a track system suitable for tracked vehicles such as found on agricultural vehicles, comprising a thermoplastic material and having a cylindrical metallic compression limiter (380) having an axial length that is 2% less than the thickness of the surrounding thermoplastic material. The wheel (300) may be composed of a fiber reinforcement and, in at least one embodiment, is composed of nylon. The wheel (300) may have a truss structure (310,360). The wheel (300) may be an idler wheel or a midroller, with specific minimum diameters and nominal loads.

Description

LIGHTWEIGHT WHEEL FOR A TRACK SYSTEMFIELD

[0001] This disclosure relates to lightweight wheels for track systems, and, more generally, to vehicles such as agricultural vehicles or industrial vehicles.BACKGROUND

[0002] Track systems are used in a variety of industries, particularly in off-road vehicles needing high traction. These can include construction vehicles such as loaders and excavators, military vehicles such as tanks, and agricultural vehicles such as tractors and sprayers. Track systems may enhance traction and / or flotation on soft, slippery, and / or irregular grounds.

[0003] Track systems often carry heavy loads. Conversely, size constraints are often placed on track systems. If packaged in a smaller volume, the track system may be more maneuverable and / or less expensive. Consequently, wheels in track systems may be required to be relatively small yet support large loads. Traditionally, this design problem has been solved by using metal wheels. High strength steel has been a common material of choice for track system wheels.

[0004] Steel, however, is dense. Steel wheels are therefore heavy. Since a track system may comprise multiple wheels, steel wheels may represent a large percentage of the weight of the track system. This may impose limits to the utility and function of the track system. For example, some vehicles have roll-over requirements, which mandate a roll cage or reinforced structure that protect the vehicle operator in case of a roll-over. A heavy track system may make it very difficult to meet this requirement. In such cases, the vehicle may be constrained to use pneumatic tires instead of a track system. This may not be optimal for normal vehicle operation. In such cases, the use of light-weight wheels may enable the track system to be used.SUMMARY

[0005] According to an aspect of the invention, there is provided a wheel for a track system comprising a thermoplastic material. A wheel mass divided by a wheel diameter divided by a wheel width is no more than 0.0003 kg I mm2; in other cases, no more than 0.00025 kg / mm2; in other cases, no more than 0.00020 kg / mm2, and in other cases, even less.

[0006] According to an aspect of another invention, there is provided a wheel for a track system comprising a thermoplastic material. A fixation of the wheel to a track system comprises a compression limiter, the compression limiter being characterized by:- comprising a metallic cylinder that is fixed within the thermoplastic material and configured to enable fixation of the wheel to the track system;- the metallic cylinder having an axial length that is at least 2% less than the thickness of the thermoplastic material.

[0007] According to an aspect of yet another invention, there is provided a wheel for a track system comprising a thermoplastic material. The wheel comprises a truss structure between a hub and an outer diameter. The truss structure has a wall thickness no greater than 10 mm.

[0008] For all of the above, a nominal load for a midroller is at least 1000 N, and a minimum outer diameter for a midroller is 180 mm. A minimum diameter for an idler wheel is 250 mm.BRIEF DESCRIPTION OF THE DRAWINGS

[0009] A detailed description of embodiments is provided below, by way of example only, with reference to the accompanying drawings, in which:

[0010] Figure 1 is perspective outside view of atrack system comprising a lightweight wheel as a midroller and a lightweight wheel as an idler wheel.

[0011] Figure 2 is a perspective inside view of a track system comprising a lightweight wheel as a midroller and a lightweight wheel as an idler wheel.

[0012] Figure 3 is an outside view of a track system comprising lightweight wheels.

[0013] Figure 4 shows a track system with prior art wheels.

[0014] Figure 5 shows a prior art midroller.

[0015] Figure 6 shows a prior at idler wheel.

[0016] Figure 7 shows a stress vs. strain curve for an exemplary glass reinforced nylon polymer comprised in a lightweight wheel.

[0017] Figure 8 shows how to estimate a yield strength of the exemplary glass reinforced nylon polymer.

[0018] Figure 9 is an perspective view of an exemplary lightweight idler wheel showing the truss structure.

[0019] Figure 10 is another perspective veiw of the exemplary lightweight idler wheel from the opposite side as Figure 9.

[0020] Figure 1 1 shows an exemplary example of a lightweight midroller

[0021] Figure 12 is another perspective view of the exemplary lightweight midroller

[0022] Figure 13 is a side view of the lightweight midroller

[0023] Figure 14 shows a schematic of a cross section in the R-Y plane of a wheel used in a track system.

[0024] Figure 15 shows a midroller assembly.

[0025] Figure 16 show a sideview of a midroller.

[0026] Figure 17 shows a section view of the midroller taken along line 17-17 inFigure 16 showing the overmolding detail.

[0027] Figure 18 shows a perspective section view of the midroller assembly shown in Figure 17.

[0028] Figure 19 shows a section view of an idler wheel assembly.

[0029] Figure 20 shows a partial section view showing the compression limiter detail for the idler wheel.

[0030] Figure 21 shows alternative embodiment of truss structures.

[0031] Figure 22 shows another alternative embodiment of truss structures.

[0032] Figure 23 shows another alternative embodiment of truss structures.

[0033] Figure 24 shows yet another alternative embodiment of truss structures.

[0034] Figure 25 shows an exemplary lightweight midroller that has been reduced to practice.

[0035] Figure 26 shows an exemplary lightweight idler wheel that has been reduced to practice.DEFINITION OF TERMS

[0036] The following terms are defined as follows for this disclosure, with material properties referring to those at ambient temperature, unless otherwise noted:

[0037] “Radial” (R), “axial” (Y), “circumferential” (q) refer to wheel coordinates as shown in Figure 9.

[0038] “Hub” refers to any structure for supporting the tire and capable of attachment to a vehicle axis.

[0039] “Vertical”(Z), “lateral” (Y) and “longitudinal” (X) refer to tire and vehicle coordinates as shown in Figure 9. The +X direction is the direction of usual vehicle travel.

[0040] The “transverse profile” of a crown profile of a midroller is the profile of the outer radial extent of the midroller in the R - Y plane.

[0041] When referring to a reinforced thermoplastic elastomer, “modulus” means Young’s tensile modulus of elasticity measured per ASTM D638. The tensile modulus may be calculated as the secant modulus at a tensile strain of 0.2%.

[0042] “Ultimate strength” is the tensile stress at which a material fails.

[0043] “Yield strength” is the tensile stress to which a material may be loaded without plastic deformation. For steel, this value is generally the stress at which the strain is 0.2% more than the strain obtained at the same stress but assuming Young’s modulus. For a reinforced thermoplastic resin, the yield strength may be estimated in a similar fashion but using an offset strain of 0.5%, as shown in Figure 8. If the offset strain is less than 0.5% when the ultimate strength is reached, the yield strength and ultimate strength are the same.

[0044] “Design Load” of a wheel is operating load of the wheel when the track system is loaded to a maximum load.

[0045] “Midroller” refers to a wheel position between the idler wheels in a track system as shown in Figure 1 .

[0046] “Idler wheel” refers to a wheel position in a track system as shown in Figure 1.DETAILED DESCRIPTION OF EMBODIMENTS

[0047] Figure 1 and Figure 2 shows the internal structural members of a track system. The track itself is not shown so as to clearly see the idler wheels and midrollers under carriage frame 130. In this track system, these wheels are exemplary embodiments of light-weight wheels. Figure 1 provides a view as would be viewed from the outside, while Figure 2 shows a view from the side facing the vehicle.

[0048] Figure 3 shows a side view of the track system, again without the endless track shown. The lightweight wheels comprise complex geometries that resemble elements in a truss structure. These complex geometries may be formed in a thermoplastic injection molding operation.

[0049] Figure 4 shows a track system with prior art midroller and idler wheels. These wheels are predominantly comprised of metallic materials, such as steel.

[0050] Figures 1 - 4 show track systems employed for vehicles having relatively high load carrying capacity. Specifically, the scope of this invention relates to track systems rated for high load capacity. As a primary metric, a midroller may have a maximum load of 1000 N / midroller may result when the vehicle is operating at maximum permissible load. Furthermore, a midroller according to an aspect of this invention may have an outer diameter no less than 180 mm. An idler wheel according to an aspect of this invention may have an outer diameter no less than 250 mm.

[0051] Figure 5 shows a prior art midroller. The prior art midroller may comprise ductile cast iron as well as structural steel S275. The manufacturing process may include casting as well as welding. While very robust, in terms of load carrying and impact resistance, wheels made of these ferrous materials are heavy.

[0052] Figure 6 shows a prior art idler wheel. Materials and processes are like those for the midroller.

[0053] Structural steel S275 has a yield strength of 275 MPa. With a specific gravity of 7.8, the density is 7800 kg I m3. A material performance index may be calculated as:MS275 = —PWhere oy= yield stress p = density

[0054] Thus, steel S276 has M = 35300 N-m I kg. This index should be maximized to enable a lighter wheel.

[0055] Those skilled in the art of track system wheel design are familiar with higher strength steel alloys. Yield strengths more than 500 MPa are possible, with no changes in density. However, such steel alloys may be more expensive or more difficult to process. Aluminum would also be a possibility, as it has a much lower density than steel. However, the wear characteristics of aluminum are much worse than steel. Additionally, aluminum is much less ductile than steel and may fail in a brittle manner during impact. Furthermore, aluminum is more expensive.

[0056] Thermoplastic composites are a material family known to comprise high strength to weight materials. However, track system engineers may not be skilled in the art of designing wheels using thermoplastic materials, particularly those resins that comprise reinforcing materials such as carbon or glass fiber. The thermoplastic injection process requires thin wall sections. As such, a track system engineer may be loath to consider such materials because current wheels may be much visually much different and use engineering principles with when he is not familiar.

[0057] The inventors have encountered and overcome these mental roadblocks. They have applied truss mechanics in a novel fashion to track system wheels. As such, they have been able to leverage materials that have a higher strength to weight ratio, as well as designing wheel structures that are themselves more efficient.

[0058] Figure 7 shows stress-strain curves for an exemplary thermoplastic resin, Radistrong A RV500 W, comprised in a lightweight wheel according to various aspects of the invention. These curves are after humidity conditioning, in which the resin is exposed to a 50% relative humidity environment at 23C, until equilibrium. Nylon will absorb moisture, which reduces stiffness. These stress-strain curves account for this reduction.

[0059] This resin comprises a nylon resin, PA6, and glass fiber reinforcement. The level of glass fiber reinforcement is 50% on a volume basis. Using PA6 or PA66 as the base resin, the inventors have found that 20% glass fiber may be required to obtain the required strength. In other cases, at least 30%; in other cases, at least 40%, and in other cases, up to 50%.

[0060] This exemplary resin obtains an ultimate strength of 195 MPa at 3% strain when tested at 23C. Furthermore:- The yield strength at 23C is 150 MPa at 23C.- The high cycle fatigue strength is 60 MPa at 23C.- The specific gravity is 1 .5, for a density of 1500 kg I m3.

[0061] The way yield strength is estimated for this reinforced nylon is shown in Figure 7. This is the stress at the intersection of a line intersecting the X axis at a strain of 0.5% with a slope equal to the Young’s modulus of the resin, and the stress - strain curve of the resin.

[0062] Therefore, the material index MRVSOOW = 100,000 N-m I kg. This glass reinforced nylon is almost three times as performant as S275 steel in strength to weight ratio when tested at 23C. As shown in Figure 7, RV500W is affected by temperature. The ultimate strength and therefore yield strength decrease. The inventors have taken this into account in specifying the design of the exemplary lightweight wheels shown in various aspects of the invention.

[0063] The inventors have found that a suitable thermoplastic resin for the invention should have an ultimate strength of at least 90 MPa; in other cases, at least 140 MPa; in other cases, at least 190 MPa; and in other cases, even more.

[0064] Several companies manufacture and market thermoplastic resins that may be suitable for the invention. These include the Akulon brand sold by DSM and the Zytel and Minion brands sold by DuPont. The resin family may be PA6, PA66, PET, or other suitable chemistries. The reinforcement may comprise glass fiber, glass beads, carbon fiber, or a combination of these reinforcements.

[0065] Use of a thermoplastic resin mandates use of a thermoplastic injection process for forming a lightweight wheel. As such, thin wall sections of 10 mm or less are generally required. This presented the inventors with challenges.

[0066] Virtual and rapid prototyping work resulted in an exemplary lightweight idler wheel 300 geometry as shown in Figure 9. The geometry comprises truss structures 310. The hub area 350 comprises truss-type geometry 360. Further, radially oriented arms 320, 370 comprise truss geometries. The truss geometry is characterized by having thin walls. All walls within the truss structure are equal to or less than 10 mm in thickness; in some cases, an exemplary wheel may have truss geometries with wall thickness equal to or less than 8 mm; in other cases, equal to or less than 6 mm; and in other cases, even less. This exemplary idler wheel 300 has a truss structure 310, 360 in which the wall thickness is equal to or less than 7 mm. The wall thickness “T” is measured from a first face 330 to a second face 335, opposite the first face as shown in Figure 9.

[0067] Figure 10 shows another perspective of the exemplary idler wheel 300. This is the inward face 340 of the idler wheel, which is adjacent to the track 1 10 drive lugs 120. A lateral extent near a radial extent on the drive lug side comprises a circumferentially smooth and continuous surface. This enhances and optimizes idler wheel wear as it serves the function of “guiding” and “aligning” the track via contact with the lugs 120.

[0068] Figure 11 shows an exemplary example of a lightweight midroller 200. This wheel also comprises a truss structure 260. Like the idler wheel, no wall sections within the truss structures have a thickness 265 of more than 10 mm.

[0069] Figure 12 shows another perspective of the exemplary midroller.

[0070] Figure 13 shows a side view of the exemplary midroller.

[0071] Figure 14 illustrates a cross section of a track system wheel. The wheel has an outer diameter OD and a width W. The width is the maximum width at or near the outer diameter.

[0072] Figure 15 shows assembly elements comprised in the exemplary midroller: a hub 270, a ring 275, a washer 280, a lock nut 285, and a glass reinforced nylon structure 260.

[0073] Figure 16 and shows a side view and Figure 17 shows a section view of the midroller taken along line 17-17 of Figure 16.

[0074] Figure 18 shows an isometric cut view of the midroller assembly shown in Figure 16 and Figure 17.

[0075] Figures 15 - 18 together provide details of the design of the midroller.

[0076] Figure 19 shows the exemplary idler wheel 300 along with a metal disk that enhances fixation to the track system. In at least one embodiment the metal disk is steel.

[0077] The idler wheel 300 also comprises compression limiters 380, as shown in Figure 20. These compression limiters are inserted into the holes used to mount and affix the wheel onto the studs. The compression limiter 380 comprises a metal cylinder that is dimensioned slightly smaller than the wheel material. When a nut is tightened to a torquespecification to firmly anchor the wheel onto the vehicle mounting stud, the wheel material compresses a specific amount. Then, additional compressive stress is taken by the metal cylinder. In this manner, the compression in the wheel material is limited.

[0078] The compression limiter axial width is 14.7 mm, while the axial width of the wheel material is 15.0 mm. Therefore, the wheel material is compressed 2%. The inventors have found that a compression of at least 1.0% is needed to eliminate wheel I compression limiter chafing during on-vehicle operation; in other cases, at least 2.0% is needed. In other cases, at least 3.0%, and in other cases, even more.

[0079] The inventors have found that exemplary glass reinforced nylon resin may creep if exposed to long-term compressive stress that would result if no compression limiter were used. Even though of relatively high strength and stiffness, this negative performance could result in loosening of the nuts that affix the wheel to the vehicle mounting studs.

[0080] Even when accounting for the entire assembly mass of the exemplary idler wheel and midrollers, the inventors have been able to make surprisingly large reductions in mass. The inventors have established a wheel mass efficiency metric as follows:Where Mass = wheel mass in kgOD = wheel outer diameter in mmW = wheel width in mm

[0081] The following table provides related data for exemplary and prior art wheels:

[0082] The inventors have found that lightweight wheels with Mett of no more than 0.00030 kg / mm2meet performance specifications for track systems; in other cases, no more than 0.00025; in other cases, no more than 0.00020; and in other cases, even less.

[0083] Figures 21 -24 shows several alternative truss geometries intended in the scope of the invention. Truss geometries may comprise triangles; hexagons; parallelograms - any general polygon.

[0084] Exemplary lightweight wheels have been reduced to practice for both midroller and idler wheels. A photograph of exemplary midrollers mounted on a track system fitted to a new Holland T5 Series 105 is shown in Figure 20. These midrollers have been tested over 1000 hours in actual field use and in extreme conditions. This has included on-road and off-road performance.

[0085] Machines such as the New Holland T5 may be used on road to travel to job sites. The on-road speed may be up to 40 kph for extended periods of time. Exemplary wheels according to the invention have been tested at such conditions. Wheel temperature measurements showed that the glass reinforced nylon resin truss structure maintained a temperature that was only 3 - 5 C above ambient, even though the inner periphery of the track was 40+C above than ambient. Reasons for this include the low thermal conductivity of the wheel material. For this exemplary material, K = 0.25 W / m- K. Combined with wheel geometry that has a high surface to volume ratio, the truss structure is largely thermally insensitive to high track temperatures.

[0086] The inventors have found this is an important design attribute, as it further enables the use of materials that may have property evolution with temperature. This includes the exemplary material. As shown in Figure 7, strength and stiffness of plasticbased resins may decrease with increasing temperature. The use of a high surface area to volume truss structure, combined with low thermal conductivity, mitigates or eliminates performance issues associated with higher speed operation.

[0087] Figure 25 shows an exemplary idler wheel that was reduced to practice. The idler wheel was also tested in severe service conditions on a track system installed on a New Holland T5 Series 105.

Claims

aim:1 . A wheel for a track system, the wheel comprised of a thermoplastic material, the wheel a configured for fixation to a vehicle, wherein the fixation of the wheel to a track system comprises a compression limiter, the compression limiter being characterized by: comprising a metallic cylinder that is fixed within the thermoplastic material and configured to enable fixation of the wheel to the track system; wherein the metallic cylinder has an axial length and the thermoplastic material has a thickness adjacent to the metallic cylinder measured in the direction of the metallic cylinder’s axial length; the metallic cylinder’s axial length is at least 2% less than the thickness of the thermoplastic material.

2. The wheel of claim 1 , wherein the metallic cylinder is made of steel.

3. The wheel of claim 1 , wherein the metallic cylinder is made of aluminum.

4. The wheel of claim 1 , wherein the metallic cylinder is made of titanium.

5. The wheel of any one of the above claims, wherein the thermoplastic material comprises a fiber reinforcement selected form the group consisting of glass fiber, carbon fiber, and combinations thereof.

6. The wheel of any one of the above claims, wherein the thermoplastic material comprises a nylon resin.

7. The wheel of any one of the above claims, wherein the wheel is an idler wheel.

8. The wheel of any one of claims 1 -6, wherein the wheel is a midroller.

9. The wheel of claim 8, wherein a nominal load for a midroller is at least 1000 N.

10. The wheel of any one of claim 8 or 9, wherein a minimum outer diameter for a midroller is 180 mm.1 1 .The wheel of claim 7, wherein a minimum diameter for an idler wheel is 250 mm.

12. The wheel of any one of the above claims, wherein the wheel further comprises a metal disk positioned concentrically with the thermoplastic wheel and configured to engage a first end of the metallic cylinder.

13. The wheel of any one of the above claims, wherein track system in which the wheel is used is for an agricultural vehicle.

Images