METHOD AND APPARATUS FOR PLASTIC WORKING OF HOLLOW RACK BAR AND HOLLOW RACK BAR
A preparatory working device by which a work piece to be fed to a rack bar forging device is obtained. The device is provided with a die sets for obtaining a diameter reduction at a location of a blank pipe, which becomes a toothed part of a rack bar, while the wall thickness being substantially unchanged or slightly increased. The device is further provided with a die set for reducing a wall thickness at a location of the blank pipe, which becomes a connection part of the rack bar. The wall thickness reduction is one half of the initial thickness, which causes the length of the work piece to be elongated. Then a rack bar forging by a die set is done.
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1. Field of Invention
The present invention relates to a production of rack bar in a steering mechanism of a vehicle from a pipe shaped blank by plastic working, and, more particularly, to its improvement, by which an increased reduction in the weight of the product is obtained in comparison with the prior art.
2. Description of Related Art
A rack bar is a main part of a steering system of a vehicle and has conventionally been produced by machining from a solid bar of a rounded cross-sectional shape by using originally a hobbing machine and recently a broaching machine. However, such a machining from a solid bar of rounded cross-sectional shape makes the product to be heavy. Therefore, a solution has been proposed, wherein the machined product is subjected to a subsequent boring by using a gun drill to obtain a hollow structure, thereby reducing the weight. However, this solution makes the production cost to be increased on one hand and, on the other hand, makes the resource consumption efficiency to be reduced.
Thus, the applicants of this application et al have proposed an improvement, wherein a plastic working (forging) of a pipe shaped blank is done for obtaining a rack bar of a reduced weight while keeping a reduced resource consumption as well as an increased vehicle performance. In this rack forging technology, a pipe shaped blank is held by a die set having toothed portions at a side facing the blank and a mandrel is inserted into the hollow space of the blank, so that toothed portions corresponding to those of the die set are copied to the blank. See specifications of Japanese Patent No. 3547378, Japanese Patent No. 3607204 and Japanese Patent No. 3607205 and Japanese Un-Examined Patent Publication (Kokai) No. 2006-026703.
In the prior art disclosed in above patents, pipe shaped blanks just supplied by manufacturers of predetermined fixed wall thickness of a value of about 4 mm are used. This predetermined wall thickness of the blank pipe is decided by the tooth height desired for the particular rack bar. In order to obtain an increased tooth height of a rack bare, a blank pipe of in increased wall thickness is needed at locations where the toothed portions are created. In the prior art, a blank pipe of a uniform value of wall thickness was used, resulting in an excessive wall thickness at the locations other than the toothed portions. Thus, the prior art is defective in an increased product weight and reduced material consumption efficiency. Thus, a long felt need has been existed as far as a reduced production cost, increased material consumption efficiency and increased performance are concerned.
SUMMARY OF THE INVENTIONThe present invention aims to overcome the above difficulties in the prior art and, in particular, to provide a technology, by which an effective use of the material along the entire length of the blank is realized, a desired performance as a rack bar is obtained and increased degrees of a weight reduction, a cost performance, a resource saving and a vehicle performance is realized.
According to the present invention, a method for forging a of a rack bar of radially outwardly copied type is provided, wherein a pipe shaped work piece is held by a die set, the material of the work piece is plastically flown to the die set located diametrically outward from the work piece in a manner that a toothed shape of the die set is copied to the work piece. The present invention features that an adjustment of the cross sectional area of the work piece is preliminary done by a local increase or decrease of wall thickness of the work piece prior to the execution of the rack forging process of radial expanded copied type. Namely, upon the execution of the preliminary adjustment of the cross-sectional area, a work piece is subjected to plastic workings for desirably increasing or decreasing the wall thickness at the respective axial positions of the work piece along the length thereof. As a result, desired values of cross-sectional area corresponding to desired functions at the respective portions of the work piece along its length are obtained.
The above mentioned adjustment of the wall thickness of work piece at the respective portions along its length is carried out by so-called swaging or ironing process under a single stage or a plurality of divided stages.
An adjustment of the cross-sectional area according to the present invention is carried out by an apparatus, which is constructed by a combined die set having an elongated mandrel and a plurality of dies and a vertical or horizontal hydraulic cylinder mechanism, which is cooperated with the die set in a manner that a desired local increase or decrease of the wall thickness of the blank pipe is obtained at respective positions of the blank pipe along its length. In place of the hydraulic cylinder mechanism, any suitable reciprocating mechanism, such as a ball screw type, where a rotating movement is converted to a linear movement, may be employed.
In a power steering mechanism of an electric operated type, a rack bar of VGR (variable gear ratio) type is usually used. In this type of mechanism, it is needed that values of the pitch as well as the inclination angle of toothed portions are varied along the length. Such a construction of the VGR rack bar makes it difficult that it is machined from a solid work piece. Therefore, a forging from a pipe shaped blank is usually employed. Furthermore, unlike the hydraulic operated type, the electric operated type requires an increased bending strength of toothed portion, i.e., an increased size of tooth part as well as an increased thickness.
The present invention can achieve this goal, while obtaining the maximum reduction of the weight of a rack bar. Namely, according to the present invention, a forging of toothed portions is done by a radially outwardly directed plastic flow of metal of a pipe shaped blank while preventing fibrous metals from being broken. Under such principle of rack formation, an adjustment of the cross sectional area of the blank pipe is essential and has been done by a grinding. In contrast, according to the present invention, a plastic working is done for obtaining a desired increase or decrease in the wall thickness at locations along the length of the work piece, so as to obtain desired values of cross-sectional area, corresponding to the desired functions required at the respective locations. In more detail, at the location where a toothed form is to be formed, a wall thickness corresponding to the wall thickness of the blank pipe is basically maintained, so that a desired toothed shape is obtained under the plastic flow, although a wall thickness increase by a diameter reduction can be taken when it is necessary.
In comparison, at locations other than the location for the formation of the toothed portions, a desired strength is obtained, even when the wall thickness is smaller than the wall thickness of the blank pipe. Therefore, according to the present invention, a wall thickness reduction is done so long as a desired strength within a permissible range is obtained. A wall thickness reduction may be done by ironing, which causes the work piece to be axially elongated. Namely, in the present invention, the length of the product must be decided while an axial elongation as generated by the reduction of the wall thickness of the blank pipe is taken into the consideration. In other words, in the present invention, a blank pipe of a reduced axial length for a value corresponding the axial elongation can be used, resulting in a corresponding reduction of the weight of product. Namely, in the prior art as in '378, '204, '205 or '703 patent, the weight of a product is almost equal to that of the blank pipe and is about 1,236 kg when the blank of wall thickness of 4 mm and of a diameter of 25 mm has a length of 615 mm. In contrast, in the present invention, the value of wall thickness at toothed portions is maintained to that of the blank pipe, i.e., 4 mm and the wall thickness of the portion other than the toothed portion is reduced to 2 mm by an ironing process, while keeping the desired strength of the product. Thus, according to the present invention, it is possible to use a blank pipe of a reduced length for a value corresponding to the lengthwise elongation as generated by the ironing. The inventor has found that a desired performance is obtained even when the length of a blank pipe is reduced so that the weight of a blank pipe, i.e., the weight of a product is reduced to 920 g, although the weight reduction depends on a length of toothed portions having a value of a wall thickness, corresponding to that of the blank pipe. In this example, a weight reduction of the present invention is 25.6% over the prior art as in '378, '204, '205 or '703 patent and is, even, better than 60% over the prior art where a solid blank rather than the pipe shaped blank is used. Irrespective of such an additional process like a swaging or ironing for obtaining a cross sectional area control, a substantial reduction of the cost is obtained. Assuming that a material cost is 170 yen/kg, a resultant reduction of the material cost would be (1.236−0.920)×170=53.7 yen per one product. In addition, the redemption cost of the die set would be, at the most, 10 yen per one product. It will thus be understood that a material cost reduction of about 48 yen per one product is possible. Furthermore, as a result of a reduction of a weight of a product, i.e., a rack bar, a resultant reduction in the weight of a vehicle is obtained, which assists in an improvement of a drive ability of the vehicle, on one hand, and, on the other hand, in a reduction in a fuel consumption efficiency. The reduction in the fuel consumption efficiency per one car would be minimal but is significant in view of a huge automobile market, which might be amount to 70,000,000 cars in worldwide.
Furthermore, as a result of the ironing after the swaging, a highly improved surface roughness is obtained not only at the inner surface but also at the outer surface, which allows a dimension to be controlled within a tolerance of microns, thereby allowing a increased product quality and precision to be easily obtained.
At a second step as shown in
At a third step as shown in
In the third step shown in
Also in the third step in
At a fourth step shown in
A fifth step is shown in
After the completion of the forging of the rack bar, conventional processes for finishing to a product are done, which includes a correction of a curving, additional swaging at both ends, a working of grooves, a working of width-across-flat portion, an inner diameter tapping, a quench-and-temper process, an outer diameter machining, a test of meshed condition and anti-corrosive treatment, et al. The thus obtained final product is wrapped and shipped. Any more detailed explanation of these processes are omitted, since they are not directly related with the essence of the present invention.
Now, a diameter reduction process as schematically illustrated in
A diameter reduction process as practiced by the device shown in
A further forward movement of the blank 10 causes the latter to be introduced into the drawing die 23 via its tapered inlet portion 23A, so that the blank 10 is subjected to a diameter reduction in a manner that a clearance with respect to the front portion 26-1 of the mandrel 26 is diminished. In
In this embodiment, the mandrel 26 is finally and additionally moved in the forward direction as shown in
During the further forward movement of the mandrel 30, its diameter expanded portion 30-2 is faced with the wall thickness reduction die 32 via the portion 10″ of values of the wall thickness and the outer diameter, which are substantially unchanged with respect to those of the initial blank pipe. The portion 10″ has a value of an outer diameter, which is properly larger than that of an inner diameter of the die 32 for a wall thickness of the work piece. The difference of these values is, for example, about one half of the wall thickness of the work piece. As a result, the portion 10″ (
At the next step as shown in
Then, a switching of the introduction of the hydraulic cylinder mechanism is done in a manner that the mandrel 30 is retracted in the right-handed direction as shown by an arrow c. Due to the fact that the rear end portion 10″ of the blank 10 is contacted with a shoulder portion 30-4 of the mandrel located between the diameter expanded part 30-2 and the inner diameter setting part 30-3, the blank 30 is entrained by the movement of the mandrel 30 until the part 10″ is made contacted with the striker 34. However, when the rearward movement of the mandrel 30 in the direction as shown by the arrow c is done until the portion 10″ is made contact with the striker 34, the entrained movement of the blank 10 is ceased due to the fact that the blank is engaged with the striker 34, on one hand and, on the other hand, the rearward movement of the mandrel 30 in the direction as shown by the arrow c is continued. During such a rearward movement of the mandrel 30 in the direction as shown by the arrow c, the large diameter part 30-2 is engaged with the part 10″ of the work piece, so that the latter is radially outwardly displaced. Thus, at the rear end of the work piece, a part 10-3 is formed as shown in
At the following stage, the work piece in
An introduction of a hydraulic pressure to the hydraulic cylinder causes the mandrel 36 to be moved forward, so that the work piece on the mandrel 36 is introduced into the die 37. As a result of the introduction of the mandrel into the die 37, the portions 10′″ and 10′ of the work are subjected to a diameter reduction for a value corresponding to a difference between the inner diameter of die 37 and the outer diameter of the work.
The mandrel 39 is formed with a rear large diameter part 50, a middle diameter part 52 and a front small diameter part 52-2. The mandrel 39 is connected with a hydraulic cylinder mechanism (not shown) for obtaining a reciprocated movement of the mandrel 39. The middle diameter part 52 is formed with a tapered part 52-1, from which the small diameter part 52-2 is axially forwardly projected. Finally, the plastic working device is further provided with a blank knockout pin 54 for removing a product after completion of the designated processes.
An operation of the second embodiment of the present invention will now be explained. As illustrated with reference to
An introduction of the hydraulic pressure into the hydraulic cylinder device for operating the mandrel is continued from the condition as shown in
When the mandrel 39 is further moved in the forward direction from the condition as shown in
In short, this second embodiment is advantageous in that a worked blank for forging a rack bar is obtained under one shot process from a straight blank pipe while preparatory works are eliminated except for a squeezing process at the tip end, resulting in a high efficiency of the process.
In the embodiments as explained, the hydraulic cylinder device for operating a mandrel is illustrated as horizontal type. However, a hydraulic cylinder device of vertical type may also be used. In addition, in place of such a hydraulic cylinder device, a mechanism such as an elongated ball screw operated by a servomotor of a variable velocity may also be used.
Claims
1. Method for producing a rack bar comprising the steps of:
- providing a work piece of pipe shape;
- subjecting the pipe shaped work piece to plastic workings for adjusting wall thickness of the work piece to desired values at the respective positions along the length of the work piece corresponding to functions as required, and;
- forging linearly spaced toothed portions on the work piece after subjected to said wall thickness adjustment, thereby obtaining a rack bar.
2. Method according to claim 1, wherein the adjustment of the wall thickness values is done by a single integrated step.
3. Method according to claim 1, wherein the adjustment of the wall thickness values is done by a plurality of divided steps.
4. Method for producing a rack bar comprising the steps of:
- providing a work piece of pipe shape;
- subjecting the pipe shaped work piece to plastic workings for reducing the wall thickness of the work piece except at a location where the rack is to be formed, while freeing the corresponding plastic extension of the work piece along the length of the work piece, and;
- forging linearly spaced toothed portions on the portion of the work piece of unchanged wall thickness or slightly increased wall thickness after subjected to said local wall thickness adjustment, thereby obtaining a rack bar.
5. Method according to claim 4, wherein the adjustment of the wall thickness values is done by a single integrated step.
6. Method according to claim 4, wherein the adjustment of the wall thickness values is done by a plurality of divided steps.
7. In an apparatus for producing a rack bar from a pipe shaped work piece, wherein the work piece is held by a die set having linearly spaced toothed portions, and a mandrel is inserted to the work piece so that toothed portion corresponding to the toothed portions on the die set are forged to the pipe shaped work piece, the improvement comprising a device for effecting an adjustment of the wall thickness of the work piece to desired values at the respective positions along the length of the work piece corresponding to functions as required, prior to the execution of the forging of the rack bar.
8. An apparatus according to claim 7, wherein said device comprising:
- a die set including an elongated mandrel and a plurality of dies, and;
- a driving means for moving the work piece to the first die set for effecting the desired adjusting wall thickness of the work piece.
9. A hollow rack bar used as a steering part of a vehicle, which is forged from a pipe shaped blank, including a first portion of a wall thickness, which substantially the same as or slightly increased from the wall thickness of the blank pipe and a second portion of a wall thickness, which is highly reduced from the wall thickness of the blank pipe and which is elongated for a length corresponding to the reduction of the wall thickness, the rack portion being formed on the first portion of the blank pipe.
Type: Application
Filed: Mar 15, 2007
Publication Date: Sep 6, 2007
Applicants: (Mishima-shi), (Tokyo-to), (Tokyo-to)
Inventor: Seiji Shiokawa (Mishima-shi)
Application Number: 11/686,705
International Classification: B21C 37/30 (20060101);