Description

APPARATUS FOR FORMING TAPER SERRATION ON THE

PIVOT SHAFT

Technical Field

[1] The present invention relates to a pivot shaft that is used to connect shafts for power transmission, and more particularly, to an apparatus for forming a taper serration on a pivot shaft that can very accurately form a serration on the outer periphery of a taper shaft, so that a pivot shaft is firmly press-fitted to a driven member and accurately transmits power from a driving member to the driven member without slip (idling). Background Art

[2] In general, Key-shaft connection, spline-shaft connection, serration- shaft connection, and the like have been widely used as shaft connection that is used to transmit power of a driving member to a driven member in various mechanical tools.

[3] In the serration-shaft connection among them, the number of teeth is large and a pitch and tooth height are small, so that pressure strength is large. Accordingly, it is possible to transmit large torque as compared to when a spline shaft having the same diameter is used. Further, since a fastening force to the driven member is large and free movement does not occur, it is possible to obtain high accuracy in power transmission. Therefore, the serration- shaft connection has been widely used in various mechanical tools where accurate power transmission is required regardless of the magnitude of transmitted power

[4] In particular, serration is formed on a tapered surface by using the characteristic where a tooth of serration has a triangular cross-section. Therefore, a so-called taper serration shaft, which is to be firmly fixed by press-fitting without forming serration on a boss of a driven member, is manufactured and used to accurately transmit relatively large power.

[5] For example, a pivot shaft for transmitting power of a wiper motor to a wiper arm has been used in a wiper system of an automobile.

[6] That is, as shown in FIG. 1, in the wiper system, a rotational motion of a wiper motor

M is converted into a linear motion of a link mechanism L, and the linear motion is converted into an angular motion by a lever plate P. Accordingly, the wiper system angularly moves a pivot shaft S to which the wiper arm A is fixed by press-fitting, at a predetermined angle, thereby driving the wiper arm A.

[7] The pivot shaft S has a spur serration (not shown) at a lower end thereof, and is connected to the lever plate P. Further, the pivot shaft successively has a taper serration T and a male thread B at an upper end thereof. The taper serration T is press-fitted into

a boss hole H of the wiper arm A and a nut N is tightened to the male thread B, so that the lever plate P and the wiper arm A are movably connected to each other.

[8] In the related art, as shown in FIG. 2, a roller-shaped taper spur serration die 1, which is rotatably mounted to a holder 2, is perpendicularly pressed against a tapered portion Ts of a rotating pivot shaft S in order to form a serration. For this reason, there is a problem in that the taper serration T is not accurately formed. Further, there is a problem in that a large fastening force caused by pressing is not obtained since the small- and large-diameter portions have the same valley depth.

[9] That is, since circumferential lengths of both ends (large- and small-diameter portions) of the tapered portion Ts are different from each other, circumferential speeds thereof are different from each other. The taper spur serration die 1, which has the same height and angle of teeth at both ends, is used to form a serration. For this reason, the number of teeth of the small- and large-diameter portions is not uniformed, slip and twisting are caused at both ends of the tapered portion Ts, and the crush occurs at the tip of a tooth of the serration, so that the serration is not accurately and precisely formed. Further, the small- and large-diameter portions are formed to have the same valley depth.

[10] In addition, since the taper spur serration die 1 presses the tapered portion Ts of the pivot shaft S in one direction in order to form a taper serration T, the pivot shaft S does not stand and is eccentrically rotated. For this reason, there are problems in that it is difficult to form a taper serration T or the pivot shaft is bent.

[11] Therefore, a fastening force is decreased due to the lack of the press-fitting of the wiper arm A, so that torque transmission performance deteriorates. When the pivot shaft has been used for a long time, the pivot shaft S is idled in the boss hole H of the wiper arm A.

[12] The following technology has been known as another method of forming a serration.

As shown in FIG. 3, three taper serration dies 3, which have the same taper angle as a tapered portion Ts of a pivot shaft S, are mounted on a holder 4 at regular angular intervals so as to face the tapered portion Ts of the pivot shaft S. Then, the holder 4 is moved so that a rotating pivot shaft S is inserted into a space among the taper serration dies 3.

[13] However, this method cannot also form an accurate taper serration T, and the small- and large-diameter portions of the tapered portion Ts have the same valley depth of the serration. For this reason, there are problems in that torque transmission performance deteriorates due to the lack of the press-fitting of the pivot shaft to the wiper arm A and the pivot shaft S is idled.

[14] That is, as shown in FIGS. 4(a) and 4(b), in each of the three taper serration dies 3, addendum angles α and β are gradually increased from a small-diameter portion

toward a large-diameter portion (α<β), and the small- and large-diameter portions have the same tooth height (valley depth) h. In the related art, the taper serration dies 3 and the tapered portion Ts of the pivot shaft S are rotated while reversely coming in contact with each other. Accordingly, as shown in FIGS. 5 to 6(b), a serration is relatively well formed on the small-diameter portion of the tapered portion Ts. However, due to the speed difference between the taper serration dies and the pivot shaft, the large-diameter portion comes in contact with small-diameter portions of the taper serration dies 3 that have the same the number of the serrations as the small-diameter portion and a small pitch. For this reason, a blunt serration is formed with the crush of the tip.

[15] Further, the small-diameter portion of the pivot shaft S has a large addendum angle β due to the contact with the large-diameter portions of the taper serration dies 3, and the large-diameter portion thereof has an addendum angle γ larger than the addendum angle of the small-diameter portion due to the tapered shape. For this reason, the press- fitting of the pivot shaft to the wiper arm A significantly deteriorates, so that it is difficult to accurately transmit torque and the idling of the pivot shaft S does not occur.

[16] Furthermore, three taper serration dies 3 are supported by the holder 4 through supporting pins 5 on three axes. Accordingly, when the tapered portion Ts of the pivot shaft S is pressed and inserted into a space among the three taper serration dies 3, the three axes of the taper serration dies 3 are deviated while the supporting pins 5 are ruptured and bent due to a force for widening a space among the three taper serration dies 3. As a result, there is a problem in that it is difficult to form the taper serration T.

[17] For this reason, the taper serration T may be formed by forging. In this case, the taper serration T is good to some extent. However, there is a problem in that the structure of a product is changed, so that a breakage danger is increased at a specific portion due to a metal flow and the strength deteriorates.

[18] Further, post- forming may be needed according to circumstances (the entire shape of a shaft), so that manufacturing costs are increase. Furthermore, there is also a drawback in that a life span is short

[19]

Disclosure of Invention Technical Problem

[20] Accordingly, the present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide an apparatus for forming a taper serration on a pivot shaft that very accurately forms a serration on the outer periphery of a pivot shaft without the crush of the serration, so that the pivot shaft is firmly press-fitted to a driven member and accurately transmits the torque from a driving member to the driven member without slip.

[21] Further, another object of the present invention is to provide an apparatus for forming a taper serration on a pivot shaft where two taper serration dies are positioned on one axis so as to face each other and a serration is easily formed on the tapered portion of the pivot shaft to be inserted between the taper serration dies, thereby preventing the occurrence of problems, such as the deviation between the axes of the taper serration dies and the bending of the taper serration dies, easily forming a taper serration on the pivot shaft, preventing the occurrence of a deformation problem such as the bending of the supporting shaft for supporting the pivot shaft. Technical Solution

[22] In order to accomplish the above objects, according to an embodiment of the present invention, there is provided an apparatus for forming a taper serration on a pivot shaft. The apparatus includes two taper serration dies, supporting shafts, and bolts. Each of the two taper serration dies includes a taper serration portion formed at a front edge thereof and a taper hole formed therein. One ends of the supporting shafts are inserted into the taper holes of the two taper serration dies at a predetermined depth, respectively, and the other ends thereof are rotatably supported by objects, respectively. Each of the bolts fixes the taper serration die to the supporting shaft with a washer, which is inserted into a front portion of the taper serration die by a predetermined depth and interposed between the bolt and the taper serration die.

[23] In the apparatus, the two taper serration dies may be supported by objects through the supporting shafts so as to face each other with a predetermined distance therebetween, and the taper serration portions positioned with a predetermined distance therebetween may come in contact with a tapered portion of a pivot shaft so as to form a taper serration.

[24] In the apparatus, each of the two taper serration dies may include: the taper serration portion that is formed at a front edge of the taper serration die, and corresponds to a taper angle of the tapered portion of the pivot shaft; the taper hole that has a taper angle corresponding to the outer diameter of the supporting shaft, and is increased in diameter from the front side toward the rear side; and the receiving portion that is formed at a front portion of the taper serration die, and has an inner diameter larger than the diameter of the taper hole so as to receive the washer and the head of the bolt therein.

[25] In the apparatus, the taper serration portion may be formed to have one selected from a bevel shape and a helical shape.

[26] In the apparatus, each of the supporting shafts may include a first tapered surface of which diameter is decreased from the middle of the supporting shaft toward the front end thereof, a second tapered surface of which diameter is decreased from the middle

of the supporting shaft toward the rear end thereof, and a threaded hole that is formed on the front face of the supporting shaft and have a predetermined depth so that a threaded portion of the bolt is fastened to the threaded hole.

[27] As described above, the apparatus for forming a taper serration on a pivot shaft according to the present invention can accurately form a sharp serration on the outer periphery of a pivot shaft without the crush of the serration. Therefore, it is possible to significantly improve the reliability, torque transmission performance, and durability of the pivot shaft.

[28] Further, two taper serration dies are positioned on one axis so as to face each other, and a serration is easily formed on the tapered portion of the pivot shaft to be inserted between the taper serration dies. Therefore, it is possible to prevent the occurrence of problems, such as the deviation between the axes of the taper serration dies and the bending of the taper serration dies. Further, it is possible to easily form the taper serration on the pivot shaft, and to prevent the occurrence of a deformation problem such as the bending of the supporting shaft for supporting the pivot shaft.

[29]

Advantageous Effects

[30] As described above, the apparatus for forming a taper serration on a pivot shaft according to the present invention can accurately form a sharp serration on the outer periphery of a pivot shaft without the crush of the serration. Therefore, it is possible to significantly improve the reliability, torque transmission performance, and durability of the pivot shaft.

[31] Further, two taper serration dies are positioned on one axis so as to face each other, and a serration is easily formed on the tapered portion of the pivot shaft to be inserted between the taper serration dies. Therefore, it is possible to prevent the occurrence of problems, such as the deviation between the axes of the taper serration dies and the bending of the taper serration dies. Further, it is possible to easily form the taper serration on the pivot shaft, and to prevent the occurrence of a deformation problem such as the bending of the supporting shaft for supporting the pivot shaft.

[32]

Brief Description of the Drawings

[33] FIG. 1 is a partially enlarged plan view schematically showing a wiper system for an automobile;

[34] FIG. 2 is a schematic front view of an example of an apparatus for forming a taper serration in the related art;

[35] FIG. 3 is a schematic partial cross-sectional view of another example of an apparatus for forming a taper serration in the related art;

[36] FIG. 4(a) is a side view of a taper serration die of the apparatus shown in FIG. 3;

[37] FIG. 4(b) is a cross-sectional view of the taper serration die of the apparatus shown in FIG. 3; [38] FIG. 5 is a side view of a pivot shaft on which a taper serration is formed by a method in the related art;

[39] FIG. 6(a) is a cross-sectional view taken along a line A-A of FIG. 5;

[40] FIG. 6(b) is a cross-sectional view taken along a line B-B of FIG. 5;

[41] FIG. 7 is an exploded perspective view of an apparatus for forming a taper serration according to the present invention; [42] FIG. 8 is a cross-sectional view of the assembled apparatus for forming a taper serration according to the present invention; [43] FIG. 9(a) is a front view of a taper serration die having a bevel type taper serration portion that is formed by the apparatus according to the present invention; [44] FIG. 9(b) is a front view of a taper serration die having a helical type taper serration portion that is formed by the apparatus according to the present invention; [45] FIG. 10(a) is an enlarged front view of main portions of a taper serration portion of the taper serration die that is formed by the apparatus according to the present invention; [46] FIG. 10(b) is a side cross-sectional view of main portions of the taper serration portion of the taper serration die that is formed by the apparatus according to the present invention; [47] FIG. 11 is a schematic view showing that a taper serration is formed at a tapered portion of a pivot shaft by left and right apparatuses for forming a serration according to the present invention; [48] FIG. 12 is a side view of a pivot shaft on which a taper serration is formed by the method according to the present invention;

[49] FIG. 13(a) is a cross-sectional view taken along a line C-C of FIG. 12; and

[50] FIG. 13(b) is a cross-sectional view taken along a line D-D of FIG. 12.

Mode for the Invention [51] An embodiment of the present invention will be described in detail below with reference to FIGS. 7 to 11. [52] The same components as those in the related art have the same reference numerals, and detailed description thereof will be omitted. [53] An apparatus for forming a taper serration on a pivot shaft according to an embodiment of the present invention includes two taper serration dies 10, supporting shafts 20, and bolts 30. Each of the taper serration dies includes a taper serration portion 12 formed at a front edge thereof and a taper hole 14 formed therein. One ends

of the supporting shafts are inserted into the taper holes 14 of the two taper serration dies 10, respectively. The other ends thereof are rotatably supported by objects, respectively. Each of the bolts fixes the taper serration die 10 to the supporting shaft 20 with a washer 32, which is inserted into a front portion of the taper serration die 10 by a predetermined depth and interposed between the bolt and the taper serration die.

[54] That is, the two taper serration dies 10 are supported by objects (for example, a headstock and a tailstock) through the supporting shafts 20 so as to face each other with a predetermined distance therebetween. The taper serration portions 12 positioned with a predetermined distance therebetween come in contact with a tapered portion Ts of a pivot shaft S so as to form a taper serration T.

[55] In this case, each of the two taper serration dies 10 includes the taper serration portion 12, the taper hole 14, and a receiving portion 16. The taper serration portion is formed at a front edge of the taper serration die, and corresponds to a taper angle of the tapered portion Ts of the pivot shaft S. The taper hole has a taper angle corresponding to the outer diameter of the supporting shaft 20, and is increased in diameter from the front side toward the rear side. The receiving portion is formed at the front portion of the taper serration die, and has an inner diameter larger than the diameter of the taper hole 14 so as to receive the washer 32 and the head of the bolt 30 therein.

[56] As shown in FIG. 9(a) and 9(b), the taper serration portion 12 may be formed in a bevel shape or a helical shape. While the axis of each taper serration portion 12 and the axis of the tapered portion Ts of the pivot shaft S cross at a right angle (90°), the taper serration portions come in contact with the pivot shaft. Accordingly, small-diameter portions of the taper serration portion and the pivot shaft come in contact with each other, and large-diameter portions of the taper serration portion and the pivot shaft come in contact with each other.

[57] Each of the supporting shafts 20 includes a first tapered surface 22 of which diameter is decreased from the middle of the supporting shaft toward the front end thereof, a second tapered surface 24 of which diameter is decreased from the middle of the supporting shaft toward the rear end thereof, and a threaded hole 26 that is formed on the front face of the supporting shaft and have a predetermined depth so that a threaded portion of the bolt 30 is fastened to the threaded hole.

[58] The taper angle of the taper hole 14 of the taper serration dies 10 corresponds to the taper angle of the first tapered surface 22 of the supporting shaft 20. The taper angle is formed so that the front end (front face) of the first tapered surface 22 is spaced apart from a boundary (step line) between the taper hole 14 and the receiving portion 16 toward the front side by a predetermined distance I as shown in FIG. 8 when the supporting shaft 20 is fitted to the taper serration die 10.

[59] Meanwhile, only with the above-mentioned structure, it is possible to form a better

taper serration on the outer periphery of the tapered portion Ts of the pivot shaft S as compared to the related art. However, according to the embodiment of the present invention, since the small- and the large-diameter portions of the taper serration portion 12 of each taper serration die 10 are formed to have the same addendum angle θ as shown in FIG. 10(a) and 10(b), a valley depth (tooth height, hi <h2) is increased toward the large-diameter portion.

[60] That is, the addendum angle θ of the serration is generally increased toward the large-diameter portion due to the difference between the diameters of the small- and large-diameter portions of a taper shaft. However, since the small- and the large- diameter portions are formed to have the same addendum angle θ in the present invention, the valley depth is increased toward the large-diameter portion, so that the deviation of the addendum angles θ of the small- and large-diameter portions is compensated.

[61] Next, the operation and effect of the above-mentioned apparatus for forming a taper serration on a pivot shaft according to the embodiment of the present invention will be described.

[62] First, as shown in FIG. 11, the two taper serration dies 10 are rotatably fixed to, for example, a headstock (not shown) and a tailstock (not shown), respectively, through the supporting shafts 20 so as to be positioned on the same axis as the headstock and the tailstock. Further, the pivot shaft S is rotatably supported by a clamp (not shown), and is fixed to a carriage (not shown) at a right angle with respect to the taper serration dies 10.

[63] In this state, if the pivot shaft S shown by a two-dot chain line of FIG. 11 is moved to a space between the taper serration portions 12 of the taper serration dies 10 facing each other, the small-diameter portion of the tapered portion Ts of the pivot shaft S comes in close contact with the small-diameter portions of the taper serration portions 12 of the taper serration dies 10 at the same angle, and the large-diameter portion of the tapered portion Ts comes in close contact with the large-diameter portions of the taper serration portions at the same angle.

[64] In this case, the tapered portion Ts of the pivot shaft S and the taper serration die 10 to be fixed to the headstock are rotated, for example, in a direction indicated in FIG. 11 due to the friction therebetween, and the taper serration die 10 to be fixed to the tailstock is rotated due to the friction between the pivot shaft S and itself. Accordingly, a serration T is formed on the tapered portion Ts of the pivot shaft S.

[65] As described above, according to the present invention, the pivot shaft S is inserted into a space between the taper serration dies 10 that are positioned at 180°so as to be perpendicular to the taper serration dies, and the taper serration portions 12 of the taper serration dies 10 come in contact with the tapered portion Ts of the pivot shaft S at a

predetermined taper angle. Therefore, while pressure applied from the taper serration dies 10 is directly applied to the tapered portion Ts of the pivot shaft S, the serration T can be accurately formed on the tapered portion Ts of the pivot shaft S in the same shape as the taper serration portions 12 of the taper serration dies 10 without the crush thereof.

[66] That is, two taper serration dies 10 are positioned on one axis so as to face each other and the tapered portion Ts of the pivot shaft S comes in contact with the two taper serration dies at a position between the taper serration dies so that the taper serration T is formed. Therefore, it is possible to prevent the occurrence of problems, such as the deviation between the axes of the taper serration dies 10 and the bending of the taper serration dies. Further, it is possible to easily form the taper serration T on the pivot shaft S, and to prevent the occurrence of a deformation problem such as the bending of the supporting shaft 20 for supporting the pivot shaft S.

[67] In particular, according to the embodiment of the present invention, since the small- and the large-diameter portions of the taper serration portion 12 of each taper serration die 10 are formed to have the same addendum angle θ, a valley depth (hl < h2) is increased from the small-diameter portion toward the large-diameter portion. Accordingly, it is possible to accurately form the serration T on the tapered portion Ts of the pivot shaft S.

[68] Therefore, in the pivot shaft S where the serration T is formed on the tapered portion

Ts by the apparatus for forming a taper serration according to the embodiment of the present invention, as shown in FIGS. 12 to 13(b), the small- and the large-diameter portions of the taper serration T have the same addendum angle θ, the tip of the tooth is formed to be sharp without the crush of the tip, and the valley depth h2 of the large- diameter portion is larger than the valley depth hi of the small-diameter portion. Therefore, when the taper serration T of the pivot shaft S is press-fitted into the boss hole H of the wiper arm A like in the related art (see FIG. 1), the serration T uniformly and quickly penetrates the inner surface of the boss hole H. Since the valley depths hi and h2 of the serration T are gradually increased toward the large-diameter portion, the serration more deeply penetrates the inner surface of the boss hole as the nut N is tightened. Therefore, the serration has a large fastening force.

[69] As a result, the pivot shaft S, on which a serration is formed by the apparatus for forming a taper serration according to the embodiment of the present invention, is firmly press-fitted to a driven member and accurately transmits the torque from a driving member to the driven member without slip (idling). Even though a long time has passed, idling does not occur.

[70] Meanwhile, the present invention is not limited to the above-mentioned embodiment, and may have modifications and changes without departing from the scope and spirit

of the invention. It should be understood that the modifications and changes are included in the accompanying claims. [71] [72]