JP2012110913A - Worm wheel, manufacturing method thereof, and electric power steering device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a worm wheel which is compact, cheep, and is excellent in durability.SOLUTION: The worm wheel 31 is a forged product. After forming a helical shaped tooth face by rough forging of a metal material, a plurality of slide cores for finishing forging arranged radially is forged so as to reduce its diameter and moved so that the side faces of these slide cores contact closely mutually, and a worm wheel tooth face is finished up in a tooth portion. Corresponding to a tooth crest 34 of each tooth portion 32, the mating portion of the tooth crest forming face of the adjacent slide core for finishing forging is positioned during forging, thereby, a trace 35a of a parting line 35 during forging is formed in the tooth crest 34.

Description

  The present invention relates to a worm wheel, a manufacturing method thereof, and an electric power steering apparatus.

For example, as a method of manufacturing a worm wheel that is used in a reduction mechanism of an electric power steering device and at least a tooth portion is made of a synthetic resin, a preform having a tooth portion of the same tooth shape as a helical gear is molded with a synthetic resin. Thereafter, a manufacturing method for forming the tooth portion of the worm wheel by hobbing the tooth portion of the preform has been proposed (see, for example, Patent Document 1).
In addition, it has been proposed that the worm wheel is composed of a core metal formed by a cold forging process and a synthetic resin gear portion fixed to the outer periphery of the core metal (see, for example, Patent Document 2). .

JP 2003-334724 A JP 2007-255524 A

With the recent increase in output of electric power steering devices, the speed reduction mechanism has become larger and the layout on the vehicle has become very strict. When resin is used as the worm wheel, the worm wheel tends to be large in order to satisfy the strength. In addition, a resin that can withstand high strength is expensive, and the manufacturing cost increases.
Even if the worm wheel is formed by forging using a metal material, if there is a trace of the parting line during forging on the meshing surface of the teeth, there is a new problem that the meshing accuracy will be affected and the durability will deteriorate. is expected.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a worm wheel that is small, inexpensive and excellent in durability, a manufacturing method thereof, and an electric power steering device.

  In order to achieve the above object, the present invention is a worm wheel (31) made of a forged molded product, corresponding to the tooth tip surface (34) of each tooth portion (32), and a slide core ( The worm wheel in which the mark (35a) of the parting line (35) at the time of forging is formed on the tooth tip surface by positioning the mating portion of the tooth tip forming surfaces (39c, 39c) of 39, 39) (Claim 1).

  According to the present invention, since a forged molded product made of a metal material is used without using a resin molded product that is expensive and tends to increase in size, a small, inexpensive, and high-strength worm wheel can be realized. Further, the surface roughness of the tooth surface can be improved, and the durability can be improved through the suppression of wear. Furthermore, since there is no trace of the parting line at the time of forging on the meshing surface (tooth surface) of the teeth, the meshing accuracy can be increased, and as a result, the durability can be increased.

  The present invention also includes a rough forging step of rough forging a metal material (43) to obtain a manufacturing intermediate (44) having a tooth portion having a rough shape, and a radial shape around a central axis (C1). A plurality of slide cores (39) arranged in the same manner are reduced in diameter in the radial direction of the intermediate for manufacture so that the side surfaces (39a) of the slide cores are in close contact with each other, and the tooth portion is provided with a worm wheel tooth surface. And a finish forging step for finishing the worm wheel.

  According to the present invention, a metal worm wheel can be formed by forging. That is, forging using a metal material without using an expensive and large-sized resin, a small, inexpensive, and high-strength worm wheel can be manufactured. Further, the surface roughness of the tooth surface can be improved, and the durability can be improved through the suppression of wear. Furthermore, since there is no trace of the parting line at the time of forging on the meshing surface (tooth surface) of the teeth, the meshing accuracy can be increased, and as a result, the durability can be increased.

If the rough shape is a helical shape (Claim 3), the worm wheel tooth surface can be easily formed in the next finishing step.
The electric power steering device (1) that decelerates the power of the steering assisting electric motor (18) using the worm wheel as a reduction gear (claim 4) is small, inexpensive, and excellent in durability.

  In the above description, numbers in parentheses represent reference numerals of corresponding components in the embodiments described later, but the scope of the claims is not limited by these reference numerals.

It is a mimetic diagram showing a schematic structure of an electric power steering device containing a worm wheel of one embodiment of the present invention. It is a schematic perspective view of a worm wheel. It is a schematic sectional drawing of the metal for rough forging in a rough forging process. It is a schematic plan view of the fixed die for rough forging. It is a schematic sectional drawing of the intermediate body for manufacture for which rough forging was carried out. It is a schematic sectional drawing of the metal mold | die for final forging in the state just before the start of a final forging process. It is a schematic sectional drawing of the metal for final forging in a final forging process. It is a schematic sectional drawing of the slide core for finish forging in a finish forge process. It is a front view of the slide core for finish forging. It is a schematic sectional drawing which shows the matching part of the slide core for finish forging at the time of finish forging. FIG. 3 is a schematic cross-sectional view of a finish forging die in a state where the die is opened in order to remove the worm wheel from the finish forging die.

Embodiments of the present invention will be specifically described below with reference to the drawings.
FIG. 1 is a schematic view of an electric power steering apparatus including a worm wheel according to an embodiment of the present invention. Referring to FIG. 1, an electric power steering device 1 assists steering by a steering wheel 2 as a steering member, a steering mechanism 4 that steers the steered wheels 3 in conjunction with rotation of the steering wheel 2, and a driver. A steering assist mechanism 5 is provided. The steering wheel 2 and the steering mechanism 4 are mechanically connected via a steering shaft 6 and an intermediate shaft 7.

In the present embodiment, a description will be given according to an example in which the steering assist mechanism 5 applies assist force (steering assist force) to the steering shaft 6. However, the present invention can also be applied to a structure in which the steering assist mechanism 5 applies assist force to the pinion shaft described later.
The steering shaft 6 includes an input shaft 8 connected to the steering wheel 2 and an output shaft 9 connected to the intermediate shaft 7. The input shaft 8 and the output shaft 9 are connected via a torsion bar 10 so as to be relatively rotatable on the same axis.

  A torque sensor 11 disposed around the steering shaft 6 detects the steering torque input to the steering wheel 2 based on the relative rotational displacement amounts of the input shaft 8 and the output shaft 9. The torque detection result of the torque sensor 11 is input to an ECU (Electronic Control Unit) 12 as a motor control device for assisting steering. Further, the vehicle speed detection result from the vehicle speed sensor 90 is input to the ECU 12. The intermediate shaft 7 connects the steering shaft 6 and the steering mechanism 4.

The steering mechanism 4 includes a rack and pinion mechanism including a pinion shaft 13 and a rack shaft 14 as a steered shaft. A steered wheel 3 is connected to each end of the rack shaft 14 via a tie rod 15 and a knuckle arm (not shown).
The pinion shaft 13 is connected to the intermediate shaft 7. The pinion shaft 13 rotates in conjunction with the steering of the steering wheel 2. A pinion 16 is provided at the tip (lower end in FIG. 1) of the pinion shaft 13.

  The rack shaft 14 extends linearly along the left-right direction of the automobile. A rack 17 that meshes with the pinion 16 is formed in the middle of the rack shaft 14 in the axial direction. By the pinion 16 and the rack 17, the rotation of the pinion shaft 13 is converted into the axial movement of the rack shaft 14. The steered wheel 3 can be steered by moving the rack shaft 14 in the axial direction.

When the steering wheel 2 is steered (rotated), this rotation is transmitted to the pinion shaft 13 via the steering shaft 6 and the intermediate shaft 7. The rotation of the pinion shaft 13 is converted into an axial movement of the rack shaft 14 by the pinion 16 and the rack 17. Thereby, the steered wheel 3 is steered.
The steering assist mechanism 5 includes a steering assist electric motor 18 and a speed reduction mechanism 19 as a transmission mechanism for transmitting the output torque of the electric motor 18 to the steering mechanism 4. The speed reduction mechanism 19 includes a worm shaft 20 as a drive gear and a worm wheel 31 as a driven gear that meshes with the worm shaft 20. The speed reduction mechanism 19 is accommodated in the gear housing 21.

The worm shaft 20 is connected to a rotating shaft (not shown) of the electric motor 18 through a joint (not shown). The worm shaft 20 is rotationally driven by the electric motor 18. Further, the worm wheel 31 is connected to the steering shaft 6 so as to be integrally rotatable.
When the electric motor 18 rotationally drives the worm shaft 20, the worm wheel 31 is rotationally driven by the worm shaft 20, and the worm wheel 31 and the steering shaft 6 rotate integrally. The rotation of the steering shaft 6 is transmitted to the pinion shaft 13 via the intermediate shaft 7. The rotation of the pinion shaft 13 is converted into the axial movement of the rack shaft 14. Thereby, the steered wheel 3 is steered. That is, the steered wheels 3 are steered by rotating the worm shaft 20 by the electric motor 18.

  The electric motor 18 is a three-phase brushless motor, and is controlled by the ECU 12 as a motor control device. The ECU 12 controls the electric motor 18 based on the torque detection result from the torque sensor 11, the vehicle speed detection result from the vehicle speed sensor 90, and the like. Specifically, the ECU 12 determines a target assist amount using a map in which the relationship between the torque and the target assist amount is stored for each vehicle speed, and controls the assist force generated by the electric motor 18 to approach the target assist amount. To do.

  FIG. 2 is a schematic perspective view of a worm wheel 31 according to an embodiment of the present invention. The worm wheel 31 is made of metal and is formed by forging. The worm wheel 31 has a center hole 31a. The worm wheel 31 has a large number of teeth 32 having an advance angle A1 on its outer periphery 31b. A tooth gap 33 is formed between adjacent tooth portions 32, 32. On the tooth tip surface 34 of each tooth portion 32, the tooth tip forming surfaces 39 c, 39 c of the finish forging slide core 39 (see FIG. 10) are positioned, and as shown in FIG. On the surface 34, a mark 35a of the parting line 35 at the time of finish forging is formed. The mark 35 a of the parting line 35 extends along the longitudinal direction of the tooth tip surface 34.

It is manufactured using a rough forging die 36 whose schematic configuration is shown in FIG. 3 and a finish forging die 37 whose schematic configuration is shown in FIG. In the rough forging die 36, an annular rough forging fixing die 38 as shown in FIG. 4 is used. On the other hand, in the finish forging die 37, slide cores 39 for finish forging arranged radially about the central axis C1 are used.
As shown in FIG. 4, radially-arranged groove-forming projections 61 are formed on the inner periphery of the rough forging fixed die 38 of the rough forging die 36. As shown in FIG. 3, the metal material 43 is pushed into the rough forging fixed die 38 by the movable die 62, and the rough tooth groove forming protrusion 61 is digged into the outer periphery of the metal material 43. As a result, as shown in FIG. 5, a manufacturing intermediate 44 having a tooth portion in which a rough tooth groove 44b is formed, that is, a helical shape as a rough shape is formed. The production intermediate 44 is taken out from the rough forging die 36 by pushing the knockout 63 shown in FIG.

As shown in FIG. 6, the finish forging die 37 includes a base 40, a guide shaft 41 that is fixed to the base 40 and has the center axis C <b> 1, and an annular guide tube 42 that is centered on the guide shaft 41. I have.
A center hole 44a of the production intermediate 44 is fitted to the guide shaft 41, and the production intermediate 44 is guided in the vertical direction (clamping direction) by the guide shaft 41.

A conical taper surface 45 is formed on the inner peripheral surface 42a of the guide tube 42. The center of the conical tapered surface 45 coincides with the central axis C <b> 1 of the guide shaft 41. Due to the conical taper surface 45, a plurality of finish forging slide cores 39, which are slide-type, are reduced in diameter while being guided in the vertical direction.
Specifically, guide grooves 46 extending vertically along the conical tapered surface 45 are formed radially. That is, each guide groove 46 extends along the generatrix of the conical tapered surface 45. As shown in FIG. 8, guided projections 49 that fit into the corresponding guide grooves 46 (see FIGS. 6 and 7) of the conical tapered surface 45 are formed on the outer surface of each finish forging slide core 39. Is provided. Further, on the inner surface of each finish forging slide core 39, a finish tooth groove forming projection 50 (finish shape tooth profile forming projection) for forming a worm wheel tooth surface shape as a finished shape on the manufacturing intermediate 44 is provided. Respectively).

  As shown in FIG. 9, the finish tooth groove forming protrusion 50 is inclined at an angle corresponding to the advance angle A1 of the worm wheel 31 with respect to the vertical direction. The finish tooth groove forming protrusion 50 has a pair of tooth surface forming surfaces 50a and a tooth bottom forming surface 50b interposed between the pair of tooth surface forming surfaces 50a. Further, the inner surface of the finish forging slide core 39 has a pair of tooth tip forming surfaces 39 c on both sides of the pair of tooth surface forming surfaces 50 a of the finish tooth groove forming projection 50.

As shown in FIG. 10, the opposing side surfaces 39 a of the finish forging slide cores 39 adjacent to each other at the time of diameter reduction face each other. By positioning the joint portions of the side surfaces 39a and 39a, a mark 35a (see also FIG. 2) of the parting line 35 at the time of finish forging is formed on the tooth tip surface of the manufacturing intermediate 44.
Referring to FIGS. 6 and 7, the finish forging die 37 has a receiving plate 51 that collectively receives the manufacturing intermediate 44 and a plurality of finish forging slide cores 39, and the receiving plate 51 is elastic. An elastic member 52 to be supported, and first to third pressing members 53 to 55 for pushing the manufacturing intermediate body 44 and the plurality of finish forging slide cores 39 downward against the elastic member 52 are upper molds. It is provided as a part.

  After the manufacturing intermediate body 44 having a tooth portion in which a helical shape as a rough shape is formed by the rough forging die 36 is set in the finishing forging die 37, a third press is performed by a press (not shown). The member 55 is pushed and the respective pressing members 53 to 55 are lowered at a time. As shown in FIG. 6, the first and second pressing members 53 and 54 are made into the production intermediate 44 and the finish forging slide core 39, respectively. Abut.

  Next, as shown in FIG. 7, as the pressing members 53 and 55 are further lowered, the production intermediate 44 and the finish forging slide core 39 are further lowered integrally, thereby being radially arranged. The finished forging slide core 39 is reduced in diameter in the radial direction of the manufacturing intermediate 44, and a finished-shaped tooth groove 33 is formed in the manufacturing intermediate 44. That is, the tooth surface of the tooth portion of the manufacturing intermediate 44 is given a worm wheel tooth surface shape as a finished shape.

Next, when each of the pressing members 53 to 55 is raised and the mold is opened, as shown in FIG. 11, the worm wheel 31 having the tooth portion 32 formed with the worm wheel tooth surface shape as the finished shape is used for finish forging. The mold 37 can be taken out. Thereafter, a predetermined heat treatment is performed on the surface of the tooth portion 32 of the worm wheel 31 to ensure the necessary hardness.
According to the worm wheel 31 and the manufacturing method thereof of the present embodiment, since the forged molded product made of the metal material 43 is used without using the resin molded product that is expensive and tends to increase in size, the worm wheel 31 is small, inexpensive, and has high strength. The wheel 31 can be realized. Further, the surface roughness of the tooth surface can be improved, and the durability can be improved through the suppression of wear. Furthermore, since the mark 35a of the parting line 35 does not exist on the meshing surface (tooth surface) of the tooth portion 32, the meshing accuracy can be increased.

In particular, since the shape of the tooth surface formed by rough forging is a helical shape, the tooth surface of the worm wheel 31 can be easily formed in the next finish forging step.
In the above embodiment, each finish forging slide core 39 forms a single tooth groove, but may form a plurality of tooth grooves.
In addition, various modifications can be made within the scope of the claims of the present invention.

  DESCRIPTION OF SYMBOLS 1 ... Electric power steering device, 4 ... Steering mechanism, 5 ... Steering assist mechanism, 6 ... Steering shaft, 18 ... Electric motor, 19 ... Deceleration mechanism, 20 ... Worm shaft, 31 ... Worm wheel, 32 ... Tooth part, 33 ... Tooth gap, 34 ... tooth tip surface, 35 ... parting line, 35a ... (parting line) trace, 36 ... rough forging die, 37 ... finish forging die, 38 ... rough forging fixed die, 39 ... Slide core for finish forging, 39c ... Tooth tip forming surface, 40 ... Base, 41 ... Guide shaft, 42 ... Guide cylinder, 43 ... Metal material, 44 ... Intermediate for production, 45 ... Conical tapered surface, 46 ... Guide grooves 49... Guided protrusions 50. Finishing tooth groove forming protrusions 51. Receiving members 52. Elastic members 53. First pressing members 54 54 Second pressing members 55 55 Third pressing members 61 ... Coarse groove forming protrusion, C ... center axis

Claims (4)

  It is a worm wheel made of a forged molded product, and the forging is performed on the above-mentioned tooth tip surface by positioning a matching portion of the tooth tip forming surface of the slide core adjacent to each other at the time of forging corresponding to the tooth tip surface of each tooth portion. A worm wheel with traces of the parting line at the time. A rough forging step of rough forging a metal material to obtain a manufacturing intermediate having a tooth portion in which a rough shape is formed;
A plurality of slide cores arranged radially around the central axis are reduced in diameter in the radial direction of the intermediate for manufacturing so that the side surfaces of the slide cores are in close contact with each other, and the worm wheel tooth surface is moved to the tooth portion. A finish forging process for finishing the worm wheel.   3. The method for manufacturing a worm wheel according to claim 2, wherein the rough shape is a helical shape.   An electric power steering apparatus that decelerates the power of an electric motor for steering assistance using the worm wheel according to claim 1 as a reduction gear.

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