BALANCE SHAFT FORMED FROM SHEET METAL

FIELD OF THE INVENTION

[0001] The present invention relates generally to internal combustion engines. More particularly, the present invention relates to a balance shaft for use in a reciprocating piston internal combustion engine.

BACKGROUND OF THE INVENTION

[0002] Balance shafts are often provided in reciprocating piston internal combustion engines that, due to their design asymmetry, incur undesirable vibrations that cannot be eliminated no matter how well the engine components are balanced. Often, balance shafts are produced by means of hot forming processes, i.e., a drop forgoing process, from rod-shaped metal blanks. As such, typical balance shafts are solid metal components which add unwanted weight to the engine. As well, such processes require heavy equipment and multiple steps which contribute to the expense of producing the balance shaft. As such, it is desirable to develop a lighter weight balance shaft that can be produced with the less material and lighter equipment than existing balance shifts.

[0003] The present invention recognizes and addresses considerations of prior art constructions and methods. SUMMARY OF THE INVENTION

[0004] One embodiment of the present invention provides a method of forming a balance shaft assembly comprising the steps of providing a sheet metal blank including a first bearing portion having a first width, a second bearing portion having the first width, and an unbalanced portion having a second width, wherein the first width and the second width both extend along axes that are transverse to a longitudinal center axis of the sheet metal blank, forming a cylindrical first bearing seat from the first bearing portion; forming a cylindrical second bearing seat from the second bearing portion, forming an unbalanced lobe portion from the unbalanced portion; wherein the first bearing seat and the second bearing seat are both concentric about a longitudinal center axis of the balance shaft; and a longitudinal center axis of the unbalanced load portion is radially offset from the longitudinal center axis of the first and second bearing seats.

[0005] Another embodiment of the present invention provides a method of forming a balance shaft assembly including the steps of providing a cylindrical metal tube including a first end portion, a second end portion, and a central portion extending therebetween, forming a cylindrical first bearing seat on the first end portion of the metal tube, forming a cylindrical second bearing seat on the second end portion of the metal tube, forming an unbalanced portion of the central portion of the tube, wherein the unbalanced portion is formed by lancing the central portion of the tube thereby creating a lanced portion of material, and pressing the lanced portion of the material inwardly into an interior of the tube. [0006] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one or more embodiments of the invention and, together with the description, serve to explain the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended drawings, in which:

[0008] Figures 1A and IB are perspective views of a sheet metal blank and balance shaft formed with the blank, respectively, in accordance with an embodiment of the present invention; [0009] Figures 2A and 2B are side and end views, respectively, of a balance shaft formed from metal tube stock in accordance with an alternate embodiment of the present invention; and [0010] Figures 3A and 3B are side and end views, respectively, of a balance shaft formed from metal strip stock in accordance with an alternate embodiment of the present invention.

[0011] Repeat use of reference characters in the present specification and drawings is intended to represent same or analogous features or elements of the invention according to the disclosure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [0012] While one or more preferred embodiments of the invention are described above, it should be appreciated by those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope and spirit thereof. It is intended that the present invention cover such modifications and variations as come within the scope and spirit of the appended claims and their equivalents. [0013] Referring now to the figures, a balance shaft 100 in accordance with the present invention is formed on a press from a sheet metal blank 102, as shown in Figures 1A and IB, respectively. Sheet metal blank 102 includes the first bearing portion 104, a second bearing portion 106, and an unbalanced portion 108 disposed therebetween. Note, the number of bearing portions and unbalanced portions, as well as their configuration with respect to each other, is dependent upon the desired configuration of balance shaft 100 and its intended use. The embodiment shown in Figure 1A is only an example, with alternate embodiments being able to have multiple unbalanced portions and more than two bearing portions. Sheet metal blank 102 may be formed from sheet metal stock material such as, but not limited to, low or high carbon steel.

[0014] Referring specifically to Figure IB, balance shaft 100, formed from sheet metal blank 102, includes an elongated body 110, a first bearing seat 104a formed from first bearing portion 104, a second bearing seat 106a, formed from the second bearing portion 106, and a unbalanced lobe portion 108a formed from unbalanced portion 108. In order to maintain the desired shape of elongated body 110 of balance shaft 100, welds 125 may be used after the desired forming operations are performed, such as on a press. As shown, first bearing seat 104a and second bearing seat 106a are cylindrical and concentric about a longitudinal center axis 112 of elongated body 110. Longitudinal center axis 112 of elongated body 110 corresponds to the axis of rotation of balance shaft 100. First bearing seat 104a and second bearing seat 106a are configured to slidably receive a first bearing race 124 and a second bearing race 126, respectively. First and second bearing races 124 and 126 are preferably cylindrical sleeves that are formed by a drawing process and undergo hardening processes such as, but not limited to, carburizing, so that they function as hardened bearing race surfaces. Because first and second bearing races 124 and 126 serve as the actual raceway surfaces for the corresponding bearings, it is not necessary to subject the elongated body 110 of balance shaft 100 to subsequent surface treatment operations after the forming operations are complete.

[0015] As shown in Figure IB, a longitudinal center axis 114 of unbalanced lobe portion 108a is radially disposed from longitudinal center axis 112 of balance shaft 100. In the embodiment shown, longitudinal center axis 114 of unbalanced lobe portion 108a and longitudinal center axis of balance shaft 100 both lie in the plane of symmetry of the balance shaft. This is not necessarily the case in alternate embodiments wherein more than one unbalanced lobe portion may be utilized. As well, the extent to which longitudinal center axis of unbalanced lobe portion 108a is offset from longitudinal center axis of balance shaft 100 is related to the amount of material disposed in unbalanced lobe portion 108a and the extent to which that material extends outwardly beyond the outer surfaces of first and second bearing races 124 and 126.

[0016] Referring now to Figures 2A and 2B, an alternate embodiment of bearing shaft 200 in accordance with the present invention is shown. Similarly to the previously discussed embodiment (Figures 1A and IB), balance shaft 200 includes a first bearing seat 204, a second bearing seat 206, and an unbalanced portion 208 disposed therebetween. However, bearing shaft 200 is formed from metal tube stock rather than sheet metal stock. As best seen in Figure 2A, the first and second bearing seats 204 and 206 may be formed by either expanding the end portions of the tube stock generally outwardly from a longitudinal center axis 212 of the tube or, conversely, by swaging the central port of the tube stock radially inwardly toward longitudinal center axis 212. Alternatively, the end portions of the tube stock may be swaged radially inwardly so that they have smaller diameters than the central portion. Using either method, a first transition region 214 and a second transition region 216 are preferably formed between first and second bearing seats 204 and 206 and unbalanced portion 208, respectively. As shown, first and second bearing seats 204 and 206 may be used as raceway surfaces for corresponding bearings or, in the alternative, first and second bearing seats 204 and 206 may slidably receive drawn and hardened cylindrical bearing races thereon, similar to those shown in Figure IB (124 and 126). In the event that separate hardened bearing races are utilized, it is not necessary to subject first and second bearing seats 206 and 208 of balance shaft 200 to surface to hardening operations.

[0017] Unlike the embodiment shown in Figures 1A and IB, wherein unbalanced lobe portion 108a extends generally outwardly beyond first and second bearing races 124 and 126, unbalanced portion 208 of balance shaft 200 is disposed within the interior of balance shaft 200. Preferably, unbalanced portion 208 is created utilizing press operations in which lancing and forming operations are used to create the imbalance of the metal material. Specifically, with balance shaft 200 oriented as shown in Figures 2A and 2B, a lancing operation is performed on the upper portion 218 of balance shaft 200 and the displaced material is pressed inwardly. As shown, upper portion 218 is pressed downwardly until the inner surface of upper portion 218 is adjacent to the inner surface 219 of the bottom portion of the balance shaft. As is expected, the concentration of material forming central portion of the balance shaft 200 causes the center of mass 215 of unbalanced portion 208 to move downwardly with respect to longitudinal center axis 212 of bearing shaft 200, with both longitudinal center axis 212 and center of mass 215 being disposed in a plain of symmetry of bearing shaft 200.

[0018] Referring now to Figures 3 A and 3B, an alternate embodiment of bearing shaft 300 in accordance with the present invention is shown. Similarly to the previously discussed embodiments (Figures 1A-1B and 2A-2B), balance shaft 300 includes a first bearing seat 304, a second bearing seat 306, and an unbalanced portion 308 disposed therebetween. However, bearing shaft 300 is formed from metal strip stock rather than tube stock. As best seen in Figure 3A, the first and second bearing seats 304 and 306 are formed by first cutting the strip stock to the desired shape in a press operation. For example, as first and second bearing seats 304 and 306 are ultimately formed by bending and curling portions of the cut strip stock, the portions of the strip stock that are used should preferably be cut such that their dimensions that are transverse to the longitudinal center axis 312 are substantially equal to the circumferences of the resulting first and second bearing seats 304 and 306. In an alternate embodiment, the portions of the cut strip stock forming the first and second bearing seats 304 and 306 may be formed by either expanding the end portions of the strip stock generally outwardly from longitudinal center axis 312 of the tube after the bending and curling operations, or swaging them inwardly. Conversely, they may be formed by swaging the central portion of the strip stock radially- inwardly toward longitudinal center axis 312 after the central portion has been formed by bending and curling operations.

[0019] Using either of the noted methods, a first transition region 314 and a second transition region 316 are preferably formed between first and second bearing seats 304 and 306 and unbalanced portion 308, respectively. As shown, first and second bearing seats 304 and 306 may be used as split raceway surfaces for corresponding bearings or, in the alternative, first and second bearing seats 304 and 306 may slidably receive drawn and hardened cylindrical bearing races thereon, similar to those shown in Figure IB (124 and 126). Alternatively, transition regions 314 and 316 (inward or outward) can be formed in the stock prior to curling. Another variation can be curling and welding closed the gap shown as 317. In the event that separate hardened bearing races are utilized, it is not necessary to subject first and second bearing seats 306 and 308 of balance shaft 300 to surface to hardening operations. Another alternative to hardened bearing raceway sleeves over bearing seats 204, 206, 304, and 306 can be raceway sleeves inside these seats.

[0020] Unlike the embodiment shown in Figure 1A and IB, wherein unbalanced lobe portion 108a extends generally outwardly beyond first and second bearing races 124 and 126, unbalanced portion 308 of balance shaft 300 is disposed within the interior of balance shaft 300, similarly to balance shaft 200 shown in Figures 2A and 2B. Preferably, unbalanced portion 308 is created by leaving the desired amount, or weight, of strip stock material at the central region after the cutting operation. As shown, the remaining strip stock material is curled inwardly and pressed downwardly until both portions of the strip stock material have portions adjacent the inner surface 319 of the bottom portion of the balance shaft, as well as portions that abut each other forming an unbalanced portion 308 in the form of an internal flange 318. As is expected, the concentration of material forming central portion of the balance shaft 300 causes the center of mass 315 of unbalanced portion 308 to move downwardly with respect to longitudinal center axis 312 of bearing shaft 300, with both longitudinal center axis 312 and center of mass 315 being disposed in a plain of symmetry of bearing shaft 300, along which flange 318 extends. [0021] Forming operations lend themselves to drive feature attachments such as splines, crimps, roll forms, staking, etc. Additionally, some drive features could be made integral with forming operations producing pulleys, gears, sprockets, etc. Likewise, forming processes readily afford the ability to expand bearing diameters outward or swage them inwardly smaller. The same can be said of the imbalance diameters. The relative diameter magnitudes of bearings and imbalances are a function of the application not a limitation of the invention. Forming processes allow 1, 2, 3, or more bearing positions and imbalances located axially along the length of the shafts as needed.

[0022] These and other modifications and variations to the invention may be practiced by those of ordinary skill in the art without departing from the spirit and scope of the invention, which is more particularly set forth in the appended claims. In addition, it should be understood that aspects of the various embodiments may be interchanged in whole or in part. Furthermore, those of ordinary skill in the art will appreciate that the foregoing description is by way of example only, and it is not intended to limit the invention as further described in such appended claims. Therefore, the spirit and scope of the appended claims should not be limited to the exemplary description of the versions contained herein.