JP2007247734A - Worm type reduction gear and electric power steering system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a construction which is simple and manufactured at low cost without using specific and large members and, furthermore, prevents jarring noise or jarring vibration from occurring during operation. <P>SOLUTION: A backlash of engaging portion between a worm 9 and a worm wheel 8 is eliminated by pushing the worm 9 toward the worm wheel 8 with a pre-loading device 12. Additionally, the pre-loading device 12 pushes a pushing member 17 toward the worm 9, and in detail pushes a tip of this worm 9 toward the worm wheel 8 based on an engagement between an engaging protrusion 18 and engaging recess 19. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a worm type speed reducer that transmits a rotational speed of a drive shaft that rotates at a relatively high speed to a driven shaft that rotates at a relatively low speed while rotating the driven shaft with a relatively large torque. Related to machine improvements. In particular, the present invention realizes a structure suitable as a worm-type speed reducer for a mechanical device that needs to be operated with low noise and no rattling like an electric power steering device for an automobile.

  As a power steering device for reducing the force required for the driver to operate the steering wheel during steering, an electric power steering device using an electric motor as auxiliary power is widely known. Conventionally, an electric power steering apparatus having a structure as shown in FIGS. 6 to 8 is known. Among these, the one shown in FIG. 6 obtains auxiliary power by the electric motor 2 installed in the steering column 1 portion. Further, in the device shown in FIG. 7, auxiliary power is obtained by the electric motor 2 installed in the steering gear 3 portion. Further, in the apparatus shown in FIG. 8, the rack 4 is driven by the steering gear 3, and at the same time, the rack 4 is driven by the electric motor 2 provided separately from the steering gear 3, thereby obtaining auxiliary power. 6 to 8, the rotational force of the electric motor 2 is transmitted to the driven shaft after increasing the torque at the same time as being decelerated by the worm type reduction gear. In the case of the structure shown in FIG. 6, this driven shaft is a steering shaft 5 (rotational shaft connected to) that is rotatably supported in the steering column 1. In the case of the structure shown in FIG. 7, the rack shaft (or the rotating shaft connected thereto) serving as the output shaft of the steering gear 3 serves as the driven shaft. Further, in the case of the structure shown in FIG. 8, the rotating shaft for driving the pinion meshed with the rack 4 is the driven shaft.

  Since the basic structure of the worm type reduction gear is the same in any structure, it will be described with reference to FIG. 1 showing an example of an embodiment of the present invention. The worm speed reducer 6 includes a casing 7, a worm wheel 8, and a worm 9. Of these, the casing 7 supports and fixes the electric motor 2 on the outer surface thereof, and the driven shaft 10 and the driving shaft 11 are rotatably arranged in a twisted relationship with each other. The worm wheel 8 is supported concentrically with the driven shaft 10 by a key engagement, a spline engagement or the like at an intermediate portion of the driven shaft 10 and rotates together with the driven shaft 10. On the other hand, the worm 9 has a base end portion coupled to the distal end portion of the drive shaft 11 concentrically with the drive shaft 11 (in the case shown, the worm 9 and the drive shaft 11 are integrally formed). . Such a worm 9 rotates together with the drive shaft 11 in a state where it is engaged with the worm wheel 8.

  During operation of the electric power steering apparatus, the electric motor 2 rotates the worm wheel 8 with a predetermined torque in a predetermined direction via the worm 9 based on a signal from a torque sensor (not shown). As a result, auxiliary torque from the electric motor 2 is applied to the driven shaft 10 supporting the worm wheel 8 in addition to torque applied from the steering wheel 23 (see FIGS. 6 to 8). As a result, the driven shaft 10 tends to rotate with a torque larger than the torque applied from the steering wheel 23, and the driver can reduce the force (steering force) required to operate the steering wheel 23. I can plan.

  By the way, generally in the case of a worm type reduction gear, backlash exists in the meshing part of the worm wheel 8 and the worm 9. Based on this backlash, noise and vibration called rattling noise are generated during operation. Such vibrations and noises cause discomfort to passengers including the driver. In particular, when the backlash becomes large due to integration of dimensional errors and assembly errors of each component, wear due to use, and the like, the noise and vibration become more prominent.

  For this reason, conventionally, the worm is pressed against the worm wheel by a preload applying device as shown in Patent Documents 1 to 4, and the preload is applied to the meshing portion between the worm and the worm wheel. If a preload is applied to the meshing portion and the backlash of the meshing portion is eliminated, the generation of noise and vibration as described above can be suppressed. However, among the preload applying devices for worm type speed reducers described in Patent Documents 1 to 4 described above, the structures described in Patent Documents 1, 2, and 4 are complicated in structure and costly. there were. In addition, the structure described in Patent Document 3 uses a housing having an opening and a special and relatively large leaf spring, which not only increases costs, but also facilitates entry of foreign matter into the housing, ensuring durability. Therefore, the structure was not preferable.

JP 2002-145082 A JP 2004-181991 A JP 2004-224303 A JP 2005-42913 A

  In view of the circumstances as described above, the present invention is a worm-type speed reducer that has a simple structure, is low-cost without using special and large members, and does not generate annoying noise or vibration during operation, and The invention was invented to realize an electric power steering apparatus incorporating a worm type reduction gear.

Of the worm type reduction gear and the electric power steering device according to the present invention, the worm type reduction gear described in claim 1 includes a casing, a worm wheel, a worm, and a preload applying device.
Of these, the worm wheel is supported concentrically with the driven shaft on the outer peripheral surface of the driven shaft rotatably supported in the casing, and rotates together with the driven shaft.
Further, the worm is coupled to a drive shaft that exists at a position twisted with respect to the driven shaft in the casing, and rotates together with the drive shaft in a state of being engaged with the worm wheel.
Further, the preload applying device presses the worm toward the worm wheel to apply a preload to the meshing portion between the worm and the worm wheel (eliminates backlash).

  Particularly, in the worm type speed reducer according to claim 1, in order to constitute the preload applying device, a part of the casing is provided with a support hole in a portion facing the tip surface of the worm. It is provided in the axial direction. A pressing member is provided in the support hole so as to be capable of displacement in the axial direction of the worm and pressed toward the worm. Also, an engaging convex portion that is inclined in a direction in which the outer diameter decreases toward the front end portion on one of the front end surface of the worm and one axial side surface of the pressing member facing the front end surface. Is forming. Further, an engaging recess that is inclined in a direction in which the inner diameter increases toward the opening is formed on the other surface of the worm tip surface and the axial one side surface of the pressing member facing the tip surface. ing. Then, friction engagement between a part of the engagement concave part in the circumferential direction and a part of the engagement convex part in the circumferential direction is generated in a state in which a component force directed to the worm wheel is generated at the tip of the worm. I am letting. In addition, the cross-sectional shape regarding the virtual plane orthogonal to the central axis of the said worm of the said engagement recessed part and the said engagement convex part is all circular.

  When carrying out the present invention as described above, for example, as described in claim 2, the center of the engaging convex portion or the engaging concave portion provided on one axial side surface of the pressing member is provided on the tip surface of the worm. The center of the engaging concave portion or the engaging convex portion is biased toward the worm wheel. Further, only the axial displacement of the pressing member is allowed between the inner peripheral surface of the support hole and the outer peripheral surface of the pressing member. In other words, the pressing member rotates inside the support hole. An engaging portion is provided for preventing the above. And the relationship between the center of the engaging convex part and the center of the engaging concave part is always maintained in the state described above. As the engagement portion, various conventionally known anti-rotation structures such as a key engagement portion and a serration engagement portion can be employed.

When carrying out the invention described in claim 2 as described above, for example, as described in claim 3, the engaging convex portion is engaged with the tip surface of the worm and the one axial side surface of the pressing member is engaged. Recesses are provided respectively. And these engagement convex part and engagement concave part are friction-engaged in the part on the opposite side to the said worm wheel.
Alternatively, as described in claim 4, an engagement concave portion is provided on the tip surface of the worm, and an engagement convex portion is provided on one axial side surface of the pressing member. And these engagement recessed parts and engagement convex parts are friction-engaged by the said worm wheel side part.

  When the present invention as described above is carried out, preferably the worm is made of a hard metal such as bearing steel or high carbon steel as described in claim 5. In contrast, the pressing member is made of a low friction material having a low coefficient of friction with the metal constituting the worm, such as a synthetic resin such as polyamide resin or polytetrafluoroethylene resin, or an oil-containing metal. .

  In carrying out the present invention, preferably, as described in claim 6, the tip of the worm is supported inside the support hole via a radial bearing such as a single row deep groove ball bearing. Further, at least one of a portion between the inner peripheral surface of the radial bearing and the outer peripheral surface of the tip portion of the worm and a portion between the outer peripheral surface of the radial bearing and the inner peripheral surface of the support hole. An elastic support member whose thickness dimension in the radial direction is elastically expanded and contracted is provided in the portion. As the elastic support member, an elastomer such as rubber, or a corrugated spring made of metal or synthetic resin can be used.

Preferably, when carrying out the present invention, preferably, an elastic member such as a leaf spring or an elastomer such as rubber is provided between the other side surface in the axial direction of the pressing member and a portion fixed to the casing. Is provided. And the elasticity of the direction which goes to the said worm | warm is provided to the said press member with the elasticity of this elastic member.
Further, when the invention described in claim 7 is carried out, preferably, as described in claim 8, a removable lid member is provided in the opening portion of the support hole opened in the outer surface of the casing. . An elastic member is provided between the lid member and the pressing member. In the case of the invention described in claim 8, the lid member that closes the opening becomes a portion fixed to the casing described in claim 7.
Further, when the invention described in claim 8 is implemented, preferably, as described in claim 9, the lid member is formed on the outer peripheral surface of the male screw and on the inner peripheral surface of the support hole. Based on the threaded engagement with the female screw, it is mounted inside the opening so that the axial position can be adjusted. Then, the axial dimension of the elastic member is adjusted by adjusting the axial position of the lid member, and the magnitude of the force with which the elastic member presses the pressing member toward the worm can be adjusted.

An electric power steering apparatus according to a tenth aspect includes a rotating shaft, an electric motor, and a worm type reduction gear.
Of these, the rotating shaft rotates based on the operation of the steering wheel, and gives the steering wheel a steering angle corresponding to the amount of rotation.
The electric motor is for applying an auxiliary force to the rotating shaft.
Further, the worm speed reducer is provided between the output shaft of the electric motor and the rotating shaft, and transmits the rotation of the output shaft to the rotating shaft.
In particular, in the electric power steering apparatus of the present invention, the worm type speed reducer is the worm type speed reducer of the present invention as described above, and the rotating shaft is driven and the output shaft is driven. Each corresponds to an axis.

According to the worm type reduction gear and the electric power steering device of the present invention, the worm type reduction gear and the electric power steering device incorporating the worm type reduction gear that do not generate annoying noise or vibration during operation can be obtained at low cost. It is done.
That is, in the case of the present invention, for example, as described in claims 2 to 4, the engagement convex portion and the engagement concave portion formed by being distributed to the tip surface of the worm and the one side surface in the axial direction of the pressing member are arranged. Based on this, the tip of the worm is pressed against the worm wheel to eliminate backlash at the meshing portion between the worm and the worm wheel. Such a structure can be easily configured without using special parts, and thus manufacturing costs can be reduced. In addition, since there is no need to provide a hole that allows the inside and outside of the casing to communicate with each other, foreign matter can be prevented from entering the casing, thereby ensuring durability.

According to the fifth aspect of the present invention, the preload applying device is provided while keeping the strength required for the worm and suppressing the frictional force of the friction engagement portion for applying preload to a low level. The reduction in transmission efficiency of the worm type reduction gear can be minimized.
Further, according to the invention described in claim 6, since a large portion of the radial load applied to the tip of the worm can be supported by the radial bearing, the surface pressure of the friction engagement portion for applying the preload is lowered. A reduction in the transmission efficiency of the worm type reduction gear associated with the provision of the preload applying device while keeping the frictional force acting on the friction engagement portion low can be minimized. When the tip portion of the worm tends to be displaced in the radial direction as the preload is applied, the thickness of the elastic support member is elastically expanded / contracted to allow this displacement.

According to the seventh aspect of the present invention, one or both of the worm and the worm wheel are worn with use over a long period of time, and the backlash of the meshing portion between the worm and the worm wheel tends to increase. Even if it becomes, it can continue pushing this worm toward the worm wheel. For this reason, it is possible to suppress abnormal noise and vibration during operation over a long period of time.
In particular, according to the invention described in claims 8 and 9, the lid member is opened to replace the elastic member (in the case of the invention described in claim 8), or the axial position of the lid member is adjusted. By adjusting the amount of elastic deformation of the elastic member (in the case of the invention described in claim 9), the elastic force of the elastic member becomes smaller than the size required for applying the preload to the meshing portion. Even in this case, the necessary elasticity can be recovered easily.

[First example of embodiment]
FIGS. 1-2 has shown the 1st example of embodiment of this invention corresponding to Claims 1-3, 5-10. A feature of the present invention is that a preload applying device 12 that presses the worm 9 against the worm wheel 8 in order to prevent unpleasant noise and vibration from occurring at the meshing portion of the worm wheel 8 and the worm 9. In the structure. The overall configuration of the electric power steering apparatus is the same as a conventionally known structure, for example, as shown in FIGS. Further, the basic structure of the worm type reduction gear 6 is the same as that of a general worm type reduction gear as described above. Therefore, the overlapping description will be omitted or simplified, and the following description will focus on the structure and operation of the preload applying device 12, which is a characteristic part of the present invention.

  In order to configure the preload applying device 12, a support hole 13 is provided in the axial direction of the worm 9 in a part of the casing 7 that faces the tip surface of the worm 9. The support hole 13 and the worm 9 are substantially concentric, but do not have to be strictly concentric. In addition, the shape of the inner peripheral surface of the support hole 13 is different between the inner end portion, the intermediate portion, and the outer end portion with respect to the inner and outer directions of the casing 7. First, the inner end (left end in FIGS. 1 and 2) is a simple cylindrical surface. Next, the intermediate portion has a non-circular shape such as female serration, oval shape, or oval shape. Furthermore, an internal thread is formed at the outer end.

  In such a support hole 13, first, the tip end portion of the worm 9 is rotatably supported at the inner end portion in a state in which a slight displacement in the radial direction is possible. The worm 9 is made of a hard metal such as bearing steel or high carbon steel. In order to support the tip of the worm 9 as described above, in the case of this example, the tip of the worm 9 is placed inside the inner end of the support hole 13 and the single row deep groove ball bearing It supports through such a radial bearing 14. Further, an elastic support in which the thickness dimension in the radial direction is elastically expanded and contracted between the inner peripheral surface of the inner ring constituting the radial bearing 14 and the outer peripheral surface of the small diameter portion 15 formed at the tip of the worm 9. A member 16 is provided to allow the above displacement. The elastic support member 16 is a corrugated spring made of elastomer such as rubber, or metal or synthetic resin, and the thickness dimension in the radial direction is elastically expanded and contracted. The outer diameter of the small-diameter portion 15 is preferably reduced as long as strength can be secured in order to keep the rotational resistance based on the friction of the contact portion between the engagement convex portion 18 and the engagement concave portion 19 described later low. .

  Next, a pressing member 17 is provided at an intermediate portion of the support hole 13 so that the worm 9 can be displaced in the axial direction, but prevented from rotating in the support hole 13. The pressing member 17 is made of a low friction material having a low coefficient of friction with the metal constituting the worm 9 such as a synthetic resin such as polyamide resin or polytetrafluoroethylene resin, or an oil-containing metal. In order to support such a pressing member 17 inside the intermediate portion of the support hole 13 as described above, a key engagement portion is provided between the inner peripheral surface of the support hole 13 and the outer peripheral surface of the press member 17. In addition, an engagement portion such as a serration engagement portion that allows only the axial displacement of the pressing member 17 is provided.

  In the case of this example, the front end surface of the worm 9 (the right end surface in FIGS. 1 and 2) and one axial side surface of the pressing member 17 (the left side surface in FIGS. A force in a direction toward the worm wheel 8 is applied to the tip portion. For this reason, in the case of this example, the tip surface of the small diameter portion 15 formed at the tip of the worm 9 is a partially conical convex surface, and this portion is the engaging convex portion 18. The engaging projection 18 is concentric with the worm 9 and substantially concentric with the support hole 13 (in the neutral state of the elastic support member 16). On the other hand, on one side surface of the pressing member 17 in the axial direction, an engagement recess 19 having a trapezoidal shape (conical concave shape) whose bus bar shape is trapezoid is formed. The central axis of the engaging recess 19 is parallel to the central axis of the support hole 13, but is biased toward the worm wheel 8 side (lower side of FIGS. 1 and 2) with respect to the central axis of the support hole 13. Yes. Therefore, the engagement convex portion 18 and the engagement concave portion 19 are opposite to the worm wheel 8 in a state in which the front end surface of the worm 9 and the one axial side surface of the pressing member 17 are abutted with each other (FIG. 1). Abuts only on the upper part of .about.2. Since the pressing member 17 does not rotate in the support hole 13 as described above, the contact position does not change with respect to the circumferential direction of the engaging convex portion 18 and the engaging concave portion 19. Absent.

Further, a lid member 20 is screwed to the outer end portion of the support hole 13. The lid member 20 is formed with a male screw on the outer peripheral surface and an engagement hole such as a hexagonal hole in order to engage the tip of a tool such as a hexagon wrench on the outer end surface (the right end surface in FIGS. 1 and 2). is doing. Further, a lock nut 21 is screwed onto a portion of the outer peripheral surface of the lid member 20 protruding from the appearance of the casing 7 so that the lid member 20 does not rotate carelessly.
Between the inner end face (left end face in FIGS. 1 and 2) of the lid member 20 and the other axial side face (right side face in FIGS. 1 and 2) of the pressing member 17, a disc spring, rubber or the like is provided. An elastic member 22 such as an elastomer is provided to give the pressing member 17 elasticity in a direction toward the worm 9 (leftward in FIGS. 1 and 2).

  Since the pressing member 17 is given elasticity in the direction by the elastic member 22, the engagement recess 19 is always on the engagement protrusion 18 opposite to the worm wheel 8 (see FIGS. 1 and 2). Pressed at the upper part. Of the pressing force acting on the contact portion between the engagement concave portion 19 and the engagement convex portion 18, the component force in the radial direction of the worm 9 acts in the direction toward the worm wheel 8. 9 is pressed against the worm wheel 8. As a result, the backlash of the meshing portion between the worm 9 and the worm wheel 8 is eliminated (preloading is applied to the meshing portion), and an unpleasant noise or vibration is generated at the meshing portion when the worm type reduction gear 6 is operated. It will not occur.

The magnitude of the force for pressing the worm 9 toward the worm wheel 8 is determined by rotating the lid member 20 with the lock nut 21 loosened, and adjusting the axial position of the lid member 20 to thereby adjust the elasticity. It can be easily adjusted by adjusting the elasticity of the member 22. For this reason, the preload can be adjusted without being affected by the dimensional error of each component. Further, when the elastic member 22 sags, it can be easily replaced with a new elastic member by removing the lid member 20 from the support hole 13.
According to the worm type reduction gear and the electric power steering apparatus of this example as described above, as described above, the electric power steering incorporating the worm type reduction gear and the worm type reduction gear that do not generate annoying noise or vibration during operation. The device can be obtained at low cost.

[Second Example of Embodiment]
FIG. 3 also shows a second example of the embodiment of the present invention corresponding to claims 1 to 5 and 5 to 10. In the case of this example, the tip surface of the small diameter portion 15 provided at the tip of the worm 9 is a partial spherical convex surface, and this partial spherical convex surface is the engaging convex portion 18a. The bus bar shape of the engaging recess 19a formed on one side surface in the axial direction of the pressing member 17a is a convex arc having a large curvature radius. In the case of this example, by making the bus bar shape of the engagement recess 19a and the engagement projection 18a into a convex arc, the friction area between the engagement recess 19a and the engagement projection 18a is reduced. For this reason, in the case of this example, compared with the case of the first example described above, the friction loss associated with the provision of the preload applying device 12a is suppressed, and the worm type speed reducer associated with the provision of the preload applying device 12a. The lowering of the transmission efficiency can be suppressed to a lower level. Since the configuration and operation of the other parts are the same as in the case of the first example described above, the same parts are denoted by the same reference numerals, and redundant description is omitted.

[Third example of embodiment]
FIG. 4 shows a third example of an embodiment of the present invention corresponding to claims 1, 2, 4 to 10. In the case of the preload device 12b of this example, a mortar-shaped engaging recess 19b is provided on the distal end surface of the small diameter portion 15 provided at the distal end portion of the worm 9. On the other hand, a frustoconical engaging projection 18b is provided on one axial side surface of the pressing member 17b. The central axis of the engaging convex part 18b exists at a position deviated to the worm wheel 8 side from the central axis of the engaging concave part 19b. Therefore, the engagement concave portion 19b and the engagement convex portion 18b are frictionally engaged at the worm wheel 8 side portion. Since the configuration and operation of the other parts are the same as in the case of the first example described above, the same parts are denoted by the same reference numerals, and redundant description is omitted.

[Fourth Example of Embodiment]
FIG. 5 also shows a fourth example of an embodiment of the present invention corresponding to claims 1, 2, 4 to 10. In the case of this example, the engaging convex portion 18c formed on one side surface in the axial direction of the pressing member 17c has a partial spherical shape. In the case of this example, the area of the friction between the engaging convex portion 18c and the engaging concave portion 19b provided on the worm 9 side is reduced by making the generatrix of the engaging convex portion 18c a convex arc. For this reason, in the case of this example, compared with the case of the above-mentioned third example, the friction loss associated with the provision of the preload applying device 12c is suppressed, and the worm type speed reducer associated with the provision of the preload applying device 12c. The lowering of the transmission efficiency can be suppressed to a lower level. Since the configuration and operation of the other parts are the same as in the case of the third example described above, the same parts are denoted by the same reference numerals, and redundant description is omitted.

  The worm-type speed reducer of the present invention can effectively be applied as a speed reducer for an electric power steering apparatus because it eliminates backlash at the meshing portion and suppresses noise generated during operation. However, the present invention can be used not only as an electric power steering device but also as a reduction gear for various mechanical devices that require low noise and low vibration.

The figure equivalent to the expanded AA cross section of FIG. 6 which shows an example of embodiment of this invention. The B section enlarged view of FIG. The figure similar to FIG. 2 which shows the 2nd example of embodiment of this invention. The figure similar to FIG. 2 which shows the 3rd example. The figure similar to FIG. 2 which shows the 4th example. 1 is a partially cut side view showing a first example of an electric power steering apparatus that is an object of the present invention. FIG. The partial cutting side view which shows the 2nd example. The partial cutting side view which shows the 3rd example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Steering column 2 Electric motor 3 Steering gear 4 Rack 5 Steering shaft 6 Worm type reduction gear 7 Casing 8 Worm wheel 9 Worm 10 Driven shaft 11 Drive shaft 12, 12a, 12b, 12c Preload application device 13 Support hole 14 Radial bearing 15 Small-diameter portion 16 Elastic support member 17, 17a, 17b, 17c Press member 18, 18a, 18b, 18c Engaging convex portion 19, 19a, 19b Engaging concave portion 20 Lid member 21 Lock nut 22 Elastic member 23 Steering wheel

Claims (10)

  A casing, a worm wheel that is supported concentrically with the driven shaft on an outer peripheral surface of a driven shaft that is rotatably supported in the casing, and rotates with the driven shaft; and the driven shaft within the casing The worm is coupled to the drive shaft that exists in a twisted position with respect to the worm wheel, and rotates with the drive shaft while meshing with the worm wheel. In a worm type speed reducer provided with a preload applying device for applying a preload to a meshing portion with a wheel, the preload applying device is provided on a portion of the casing facing the tip surface of the worm. A support hole provided in the axial direction of the worm, and in the support hole, the worm can be displaced in the axial direction and pressed toward the worm. The outer diameter of the pressing member formed on the worm and the tip end surface of the worm and the one axial side surface of the pressing member facing the leading end surface become smaller toward the leading end. The inner diameter increases toward the opening formed on the other surface of the engagement convex portion inclined in the direction and the distal end surface of the worm and the one axial side surface of the pressing member facing the distal end surface. An engaging recess that is inclined in the direction in which the engaging recess has a circumferential portion of the engaging recess and a circumferential portion of the engaging projection that faces the worm wheel at the tip of the worm. A worm-type speed reducer that is frictionally engaged in a state in which a component force is generated.   The center of the engaging convex part or engaging concave part provided on one side surface of the pressing member in the axial direction is biased toward the worm wheel side and supported from the center of the engaging concave part or engaging convex part provided on the tip surface of the worm. The worm-type speed reducer according to claim 1, wherein an engaging portion that allows only an axial displacement of the pressing member is provided between the inner peripheral surface of the hole and the outer peripheral surface of the pressing member.   An engaging convex portion is provided on the tip surface of the worm, and an engaging concave portion is provided on one side surface of the pressing member in the axial direction. The engaging convex portion and the engaging concave portion are frictionally engaged with each other on the side opposite to the worm wheel. The worm type speed reducer according to claim 2, which is combined.   An engagement recess is provided on the tip surface of the worm, and an engagement protrusion is provided on one side surface of the pressing member in the axial direction. The engagement recess and the engagement protrusion are frictionally engaged at the worm wheel side portion. The worm type reduction gear according to claim 2.   The worm type reduction gear according to any one of claims 1 to 4, wherein the worm is made of metal, and the pressing member is made of a low friction material having a low coefficient of friction with the metal constituting the worm. Machine.   The tip portion of the worm is supported inside the support hole via a radial bearing, the portion between the inner peripheral surface of the radial bearing and the outer peripheral surface of the tip portion of the worm, the outer peripheral surface of the radial bearing, and the above-mentioned The elastic support member which the thickness dimension regarding a radial direction expands / contracts elastically is provided in the part between at least one between the internal peripheral surfaces of a support hole. The worm type reduction gear described in any one of the items.   The elastic member in the direction toward the worm is given to the pressing member by the elastic force of the elastic member provided between the other axial side surface of the pressing member and the portion fixed to the casing. The worm type reduction gear described in item 1.   A detachable lid member is provided at the opening portion of the support hole that is opened on the outer surface of the casing. Between the lid member and the pressing member that is fixed to the casing with the opening portion closed. The worm type speed reducer according to claim 7, further comprising an elastic member.   The lid member is attached to the opening of the support hole so that the axial position can be adjusted based on the screwing of the male screw formed on the outer peripheral surface and the female screw formed on the inner peripheral surface of the support hole. 9. The axial dimension of the elastic member is adjusted by adjusting the axial position of the lid member, and the magnitude of the force with which the elastic member presses the pressing member toward the worm can be adjusted. Worm type reduction gear. A rotating shaft that rotates based on the operation of the steering wheel and gives a steering angle corresponding to the amount of rotation to the steering wheel, an electric motor for giving auxiliary force to the rotating shaft, an output shaft of the electric motor, An electric power steering apparatus provided with a worm-type speed reducer provided between the rotary shaft and transmitting the rotation of the output shaft to the rotary shaft. 9. The electric power steering apparatus according to claim 9, wherein the rotary shaft corresponds to a driven shaft and the output shaft corresponds to a drive shaft.

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