1. Field of the Invention
The present invention relates to the art of vehicle axles. More particularly, the invention relates to the art of raised-center axles for heavy-duty vehicles, such as tractor-trailers or semi-trailers and straight trucks, and processes for forming such axles.
2. Background Art
Heavy-duty vehicles, such as tractor-trailers or semi-trailers and straight trucks, typically include multiple axles that are longitudinally spaced along the vehicle to create ride stability. Each axle usually includes a central tube and a pair of spindles. The spindles are mounted on opposing ends of the central tube and extend outboardly therefrom. The wheels of the vehicle are rotatably mounted on the spindles, and a trailing or leading arm suspension system connects each axle to the vehicle frame. The suspension system and axle often are referred to in combination as an axle/suspension system. For the purpose of convenience, reference herein will be made only to axles, with the understanding that such axles are used in a heavy-duty vehicle axle/suspension system.
Certain types of heavy-duty vehicles, such as rear-discharge tanker trailers, often utilize an axle in which the central tube includes a center portion that is bent upwardly. Such axles are known in the art as raised-center axles, and the upwardly bent portion is referred to as a bend or a hump. When the raised-center axle is in an in-service position, the hump is above the rest of the axle or its unbent horizontally disposed portions. This is in contrast to a drop-center axle, in which the center tube of the axle includes a hump that is below the rest of the axle when the axle is in an in-service position. In the prior art, the processes associated with forming the hump for a raised-center axle have necessitated the use of a thick wall for the central tube, such as about 0.750 inches or greater.
For example, cold-forming processes of the prior art used to fabricate a raised-center axle involve a single-hit bending process, where the hit is performed by a hydraulic press. In such a process, an axle having a straight central tube is inserted into the press and a punch driven by the press hits a center portion of the tube, bending the center portion of the tube in one motion to form the hump. This single-hit process, however, produces residual stresses in the cross-section of the axle that are detrimental to the axle when it is loaded in service conditions. That is, since the hump of the raised-center axle is above the rest of the axle in an in-service position, the “overloading” of the central tube caused by the press in forming the hump produces compressive stresses in the bottom portion of the hump. However, after the punch retracts once the central tube has been bent to form the hump, there is a slight amount of spring-back of the central tube. This spring-back produces a state of residual tensile stress in the bottom portion of the hump of the axle, which combines with load forces experienced by the axle when it is in service that tend to flex the hump and to create additional tensile stresses in the bottom portion of the axle hump. This combination of load-induced tensile stresses and residual tensile stresses in the axle at the bottom portion of the hump has the potential to produce a premature failure of the axle.
The effect of the state of residual tensile stress in the bottom portion of the hump of the raised-center axle is in contrast to the effect of the same state of residual stress in a drop-center axle. More particularly, a drop-center axle includes the same state of residual tensile stress in the same location as the raised-center axle, however the inverted orientation of the drop-center axle causes in-service loading conditions to produce compressive stresses that counterbalance the residual tensile stresses. This contrast substantiates the principle that a one-time overload to form an axle tube produces residual stresses that are favorable to subsequent loading in the same direction, while those same residual stresses are detrimental to subsequent loading in the opposite direction.
To compensate for the residual stresses that contribute to the potential premature yielding of raised-center axles that are cold-formed according to prior-art processes, the axles include large wall thicknesses, on the order of about 0.750 inches. Even with such large wall thicknesses, however, these prior-art raised-center axles are still potentially susceptible to premature structural failure. Post forming treatment processes have been tried in an attempt to reduce such undesirable residual stresses in the prior art raised-center axles, such as by shot peening or needle peening of the axle. However, the addition of such post-forming treatment processes undesirably increases the time and expense to produce a raised-center axle. In addition, surface treatments such as those noted above do not significantly improve the static strength of the axle, which is diminished severely by residual stresses from cold-forming.
Other prior-art raised-center axle forming processes include hot forming of the hump. Hot forming reduces residual stresses, but requires a large wall thickness to maintain the ability to bend the axle central tube to form the hump while maintaining structural integrity. Thus, an axle tube wall thickness of about 0.750 inches is again required. Hot forming also is typically more expensive than cold forming, due in part to the additional time, energy and equipment associated with heating the axle for forming. As a result, raised-center axles that are hot formed tend to be more expensive than those that are cold formed, and still require increased wall thicknesses, which introduce unnecessary weight to the axle.
The large wall thicknesses needed for raised-center axles formed according to prior art processes are undesirable, as large wall thicknesses increase the amount and thus the cost of the raw material needed for the axle. In addition, a large wall thickness increases the weight of the axle, which undesirably contributes to decreased payload for the vehicle on which the axle is incorporated.
Thus, a need exists in the art for a raised-center axle that overcomes the problems of the prior art by being economically formed, while reducing undesirable residual stresses and wall thickness, and maintaining or improving physical properties exhibited by prior art raised-center axles. A need also exists in the art for a process to economically form such a raised-center axle. The present invention provides such a raised-center axle and method for forming the same.
An objective of the present invention is to provide a raised-center axle that replaces a detrimental residual stress state in the humped portion of the axle with a beneficial residual stress state.
Another objective of the present invention is to provide a raised-center axle that has a reduced wall thickness.
Yet another objective of the present invention is to provide a raised-center axle that exhibits physical properties that are comparable to or better than those of prior art raised-center axles.
Still another objective of the present invention is to provide a method for economically forming a raised-center axle having beneficial residual stresses and a reduced wall thickness, yet comparable or improved physical properties as compared to prior art raised-center axles.
These objectives and others are obtained by the cold-formed, raised-center axle for a heavy-duty vehicle of the present invention, the general nature of which may be stated as including a central tube having a pair of ends and a wall thickness of from about 0.500 to about 0.625 inches. Each one of a pair of spindles is attached to a respective one of the central tube ends. A hump is cold-formed in the central tube between the ends and the hump extends generally upwardly when the axle is in an in-service position. The bottom portion of the hump in the in-service position exhibits compressive residual stresses.
These objectives and others are also obtained by the method for cold-forming a raised-center axle of a heavy-duty vehicle of the present invention, which includes the steps of providing a substantially straight axle and inserting the axle into a press. A hump is over-formed in a first direction with the press, producing a camber in the axle. Subsequently, the axle is formed in a second direction with the press to substantially remove the camber while simultaneously replacing detrimental residual stresses in the humped portion of the axle in an in-service position with beneficial residual stresses.
The preferred embodiment of the invention, illustrative of the best mode in which applicants have contemplated applying the principles of the invention, is set forth in the following description and is shown in the drawings, and is particularly and distinctly pointed out and set forth in the appended claims.
Similar numerals refer to similar parts throughout the drawings.
Turning now to the drawings, wherein the illustrations are provided to show a preferred embodiment of the invention and not to limit the same, an axle to be formed into a raised-center axle is shown in
For forming according to the method of the present invention, axle 10 is placed in a press 20, which includes an upper platen 22 and a lower platen 24. A punch 26 is secured to upper platen 22 and extends downwardly therefrom, preferably at the center of the platen, as indicated by centerline C. A pair of lower shoulder blocks 28 are secured to lower platen 24 and extend upwardly therefrom, and the blocks are spaced apart from one another at a distance d with each one of the blocks being equidistant from centerline C. Punch 26 includes a height Ph and a width Pw that cooperate with a height Bh of lower shoulder blocks 28 and distance d to produce desired bend dimensions in central tube 12 for the hump to be formed. Punch 26 includes a curved lower surface 30 and each lower shoulder block 28 includes a radiused upper surface 32 that provide a desired curvature in the hump to be formed. Optionally, lower shoulder blocks 28 may each include an upper channel (not shown) that receives central tube 12, thereby allowing the blocks to hold axle 10 in place. Each one of a pair of upper shoulder blocks 42 is secured to a respective one of ends 14 of central tube 12 adjacent each spindle 18 by a means well known in the art, with each block being equidistant from the centerline C and having a height SBh to produce the desired bend dimensions in the central tube 12, and in particular to control overbend of the axle during the first forming step, as will be described in detail below.
With continuing reference to
Turning now to
In raised-center axle forming processes of the prior art, the goal has been to achieve a zero-degree camber α upon a single forming operation in order to allow the wheels (not shown) that will be mounted on spindles 18 to ride evenly during vehicle operation. For drop-center axles, this single forming operation produces desirable residual stresses because such stresses are favorable to subsequent loading in the same direction during operation of the vehicle, so only one hit is performed, with resulting camber α of about zero degrees. In the raised-center axle forming process of the present invention, however, the first forming operation is an over-forming operation, yielding some amount of positive camber α, such as in a range of about 0.250 to about 1.000 degrees, and preferably in a range of about 0.375 to about 0.500 degrees. Once upper platen 22 and punch 26 are retracted after the first forming operation, axle 10 experiences some spring-back, creating residual tensile stress in bottom portion 36 of hump 40.
With reference now to
This second forming operation, which is a post-forming operation after the initial formation of hump 40, without contact by punch 26, causes a yielding of the axle 10 that reverses the bending moment in the center portion of the axle. This yielding in the center portion of axle 10 induces residual stresses that essentially reverse the residual tensile stresses in bottom portion 36 of hump 40 caused by the first forming operation (
Turning now to
A vertical beaming fatigue test was conducted, in which a sample raised-center axle 44 of the present invention, the sample having wall thickness t of about 0.625 inches, was cyclically loaded to determine the number of loading cycles that would cause the axle to crack through its exterior tube wall. More particularly, raised-center axle 44 and a typical prior-art raised-center axle, the prior-art axle having a wall thickness of 0.750 inches, were each cyclically loaded at spindles 18 with up to about 23,000 pounds per spindle, or about 46,000 total pounds, until each axle cracked through its exterior tube wall. Raised-center axle 44 of the present invention cracked through its exterior tube wall at 104,200 cycles on average, lasting an average of 40 percent longer than the most durable representative sample of the prior art raised center axle which cracked through its exterior tube wall at approximately 74,000 cycles, demonstrating a marked improvement over the prior art despite having a reduced wall thickness. It is worth noting that the most durable representative sample of the raised-center axle 44 of the present invention cracked through its exterior tube wall at about 116,000 cycles.
A vertical beaming static load test was also performed, in which a sample raised-center axle 44 of the present invention, the sample having wall thickness t of about 0.625 inches, was loaded with increasing weight by loading each spindle 18 to simulate in-service loading characteristics, until the axle yielded, thereby measuring the strength of the axle. For comparison, a straight axle having a load rating of 23,000 pounds, which is typically expected by those skilled in the art to be stronger than a raised-center axle, was loaded under the same conditions as the raised-center axle 44 of the present invention. Raised-center axle 44 of the present invention showed no permanent set at a load of about 69,000 total pounds, or about 34,500 pounds per spindle 18, and minor but conclusive set at a load of about 80,500 total pounds, or about 40,250 pounds per spindle. The straight axle showed no permanent set at a load of about 46,000 total pounds, or about 23,000 pounds per spindle, inconclusive or very minor set at a load of about 57,500 total pounds, or about 28,750 pounds per spindle, and minor but conclusive set at a load of about 69,000 total pounds, or about 34,500 pounds per spindle. Comparing the loads at the point of minor but conclusive set, which is indicative of yield, raised-center axle 44 of the present invention unexpectedly is approximately 16 percent stronger than the straight axle rated at 23,000 pounds.
Thus, raised-center axle 44 of the present invention exhibits longer life and improved strength when compared to prior-art axles. Raised-center axle 44 is cold-formed by two forming processes, or hits, in two different directions, thereby imparting favorable residual stresses to the thickness t that is less than that of prior-art raised-center axles, which leads to raw material savings and weight savings.
In this manner, raised-center axle 44 is economically formed and results in improved characteristics and performance over raised-center axles of the prior art. It is to be noted that certain steps, the order of those steps, and the machinery or tools used to carry out such steps, have been described herein by way of example, but are not intended to limit the scope of the invention. For example, the order and the number of steps, and/or the type of equipment used to accomplish those steps, may be altered or adjusted without affecting the overall concept of the invention.
Accordingly, the raised-center axle and method for forming the same of the present invention is simplified, provides an effective, safe, inexpensive, and efficient structure and method which achieves all the enumerated objectives, provides for eliminating difficulties encountered with prior-art raised-center axles and processes for forming the same, and solves problems and obtains new results in the art.
In the foregoing description, certain terms have been used for brevity, clearness and understanding; but no unnecessary limitations are to be implied therefrom beyond the requirements of the prior art, because such terms are used for descriptive purposes and are intended to be broadly construed.
Moreover, the description and illustration of the invention is by way of example, and the scope of the invention is not limited to the exact details shown or described.
Having now described the features, discoveries and principles of the invention, the manner in which the improved raised-center axle is constructed, arranged and used, the characteristics of the construction and arrangement, and the advantageous, new and useful results obtained; the new and useful steps, structures, devices, elements, arrangements, parts and combinations, are set forth in the appended claims.
This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/658,994, filed on Mar. 4, 2005.
Number | Date | Country | |
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60658994 | Mar 2005 | US |