Gear And An Electric Actuator Provided Therewith

Abstract

A gear that can be utilized in an electric actuator has teeth formed on its outer circumference and a central hole formed at its center. An intermediate region is positioned between a peripheral portion near the teeth and a boss near the central hole. The intermediate region has a thickness thinner than the peripheral portion and the boss. A plurality of weight-lightening apertures is circumferentially and equidistantly formed in the intermediate region. A vibration absorbing member of synthetic rubber is formed on both side surfaces of the intermediate region. The vibration absorbing member is integrally connected on each side through the weight-lightening apertures. The vibration absorbing member is attached to the radially outer side rather than an outer diameter of a bearing arranged adjacent to the vibration absorbing member.

Claims

1. A gear comprising: teeth formed on an outer circumference of the gear; a central hole formed at a center of the gear; an intermediate region is positioned between a peripheral portion near the teeth and a boss near the central hole, the intermediate region has a thickness thinner than the peripheral portion and the boss; a plurality of weight-lightening apertures is circumferentially and equidistantly formed in the intermediate region; and a vibration absorbing member, of synthetic rubber, is formed on the gear, the vibration absorbing member includes side surfaces integrally connect with each other through the weight-lightening apertures, the vibration absorbing member is attached to the intermediate region of the gear rather than an outer diameter of a bearing arranged adjacent to the vibration absorbing member.

2. The gear of claim 1, wherein the weight-lightening apertures are arranged at a position near the outer circumference of the intermediate region.

3. The gear of claim 1, wherein each weight-lightening aperture has a rectangle or triangle expanding radially outward configuration.

4. The gear of claim 1, wherein the side surfaces of the vibration absorbing member are configured to be flush with the peripheral portion and the boss.

5. The gear of claim 1, wherein the gear is formed of sintered alloy.

6. An electric actuator comprising: a housing; an electric motor mounted on the housing; a speed reduction mechanism for transmitting rotational force of the motor (M) to a ball screw mechanism via a motor shaft; and the ball screw mechanism converts the rotational motion of the electric motor (M) to axial linear motion of a drive shaft, via the speed reduction mechanism, the speed reduction mechanism includes an output gear on an outer circumference of a nut, the nut is rotationally but axially immovably supported relative to the housing by a pair of supporting bearings mounted on the housing, the nut includes a helical screw groove on its inner circumference; a screw shaft includes an outer circumference with a helical screw groove corresponding to the helical screw groove of the nut, the screw shaft is adapted to be inserted into the nut, via a number of balls, the screw shaft is axially movably and non-rotationally supported relative to the housing; the output gear is secured on the outer circumference of the nut, the output gear is sandwiched by an inner ring of one supporting bearing and a flange portion of the nut; and the output gear is configured by a gear defined by claim 1.

Description

DRAWINGS

[0025] The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure.

[0026] FIG. 1 is a longitudinal section view of a preferable embodiment of an electric actuator.

[0027] FIG. 2 is an enlarged longitudinal section view of the ball screw mechanism of FIG. 1.

[0028] FIG. 3(a) is a perspective view of a configuration of weight-lightening apertures of an output gear.

[0029] FIG. 3(b) is a perspective view of a comparative example of the configuration of weight-lightening apertures of an output gear.

[0030] FIG. 3(c) is a perspective view of another comparative example of the configuration of weight-lightening apertures of an output gear.

[0031] FIG. 4(a) is an explanatory view of a relative arrangement between an output gear and its supporting bearing.

[0032] FIG. 4(b) is a perspective view of a mounted state between an output gear and its supporting bearing.

[0033] FIG. 5 is a longitudinal section view of a prior art electric actuator.

[0034] FIG. 6 is a schematic longitudinal section view of an output gear of the prior art electric actuator.

DETAILED DESCRIPTION

[0035] An electric actuator comprises an aluminum alloy housing. An electric motor is mounted on the housing. A speed reduction mechanism transmits rotational force of the motor to a ball screw mechanism, via a motor shaft. The ball screw mechanism converts the rotational motion of the electric motor to the axial linear motion of a drive shaft, via the speed reduction mechanism. A nut is formed with a helical screw groove on its inner circumference. The nut outer circumference includes an output gear forming part of the speed reduction mechanism. The nut is rotationally but axially immovably supported relative to the housing by a pair of supporting bearings mounted on the housing. A screw shaft is coaxially integrated with the drive shaft. The screw shaft outer circumference has a helical screw groove corresponding to the helical screw groove of the nut. The screw shaft is adapted to be inserted into the nut, via a large number of balls. The screw shaft is non-rotationally but axially movably supported relative to the housing. The output gear is secured on the outer circumference of the nut. The output gear is sandwiched by an inner ring of one supporting bearing and a flange portion of the nut. The output gear includes teeth formed on its outer circumference and a central hole at its center. An intermediate region is between a peripheral portion near the teeth and a boss near the central hole. The intermediate region has a thickness thinner than those of the peripheral portion and the boss. A plurality of weight-lightening apertures, with rectangle expanding radially outward configuration, is formed circumferentially and equidistantly in the intermediate region. A vibration absorbing member, of synthetic rubber, is formed on both side surfaces of the intermediate region. Both sides of the vibration absorbing member integrally connect to each other through the weight-lightening apertures. The vibration absorbing side members are attached to the radially outer sides rather than the outer diameter of a bearing to be arranged adjacent to the vibration absorbing member.

[0036] One preferable embodiment of the present disclosure will be hereinafter described with reference to the drawings.

[0037] FIG. 1 is a longitudinal section view of one preferable embodiment of an electric actuator. FIG. 2 is an enlarged longitudinal section view of the ball screw mechanism of FIG. 1. FIG. 3(a) is a perspective view of a configuration of weight-lightening apertures of an output gear. FIG. 3(b) is a perspective view of a comparative example of a configuration of weight-lightening apertures of an output gear. FIG. 3(c) is a perspective view of another comparative example of a configuration of weight-lightening apertures of an output gear. FIG. 4(a) is an explanatory view of a relative arrangement between an output gear and its supporting bearing. FIG. 4(b) is a perspective view of a mounted state between an output gear and its supporting bearing.

[0038] As shown in FIG. 1, the electric actuator 1 comprises a cylindrical housing 2, an electric motor M mounted on the housing 2, and a speed reduction mechanism 6. The speed reduction mechanism 6 includes an input spur gear 3 secured on a motor shaft 3a of the electric motor M. An intermediate gear 4 mates with the input gear 3. An output gear 5 mates with the intermediate gear 4 and is mounted on the outer circumference of a nut 18. A ball screw mechanism 8 converts rotational motion of the electric motor M to axial linear motion of a drive shaft 7, via the speed reduction mechanism 6.

[0039] The housing 2 is formed from aluminum alloy such as A 6063 TE, ADC 12 etc. It is die casting and includes a first housing 2a and second housing 2b. The electric motor M is mounted on the first housing 2a. The second housing 2b abuts and is bolted to an end face of the first housing 2a by fastening bolts (not shown). The first housing 2a and the second housing 2b are formed with a through bore 11 and a blind bore 12, respectively, to contain the screw shaft 10, as described later.

[0040] The input gear 3 is press-fit onto the end of the motor shaft 3a of the electric motor M. Thus, the input gear is non-rotatable relative to the shaft 3a but is rotationally supported by a rolling bearing 13. The rolling bearing 13 has a deep groove ball bearing mounted on the second housing 2b. The output gear 5 mates with the intermediate spur gear 4. The output gear 5 is integrally secured on the nut 18, via a key 14, that forms part of the ball screw mechanism 8.

[0041] The drive shaft 7 is integrally formed with a screw shaft 10 that forms part of the ball screw mechanism 8. Guide pins 15, 15 are mounted on one end (right-side end in FIG. 1) of the drive shaft 7. A sleeve 17 is fit in the blind bore 12 of the second housing 2b. Axially extending recessed grooves 17a, 17a are formed, by grinding, on the inner circumference of the sleeve 17. The recessed grooves 17a, 17a are arranged circumferentially opposite. The guide pins 15, 15 engage the grooves 17a, 17a to axially removably support the screw shaft 10. Falling-out of the sleeve 17 is prevented by a stopper ring 9 mounted on an opening of the blind bore 12 of the second housing 2b.

[0042] The sleeve 17 is formed from sintered alloy by an injection molding machine that molds plastically prepared metallic powder. In this injection molding, metallic powder and binder, comprising plastics and wax, are first mixed and kneaded by a mixing and kneading machine to form pellets from the mixed and kneaded material. The pellets are fed into a hopper of the injection molding machine. The pellets are then pushed into dies under a heated and melted state and finally form the sleeve by a so-called MIM (Metal Injection Molding) method. The MIM method can easily mold sintered alloy material articles having desirable accurate configurations and dimensions even though the article require high manufacturing technology and have configurations that are hard to form.

[0043] The guide pins 15 are formed of high carbon chromium bearing steel such as SUJ 2 or carburized bearing steel such as SCr 435. The pin surfaces are formed with carbonitrided layer having carbon content more than 0.80% by weight with a hardness of more than HRC 58. In this case, it is possible to adopt needle rollers, used in needle bearings, as guide pins 15. This makes it possible to have the guide pins 15 with a hardness of more than HRC 58 and have excellent anti-wear properties, availability and manufacturing cost.

[0044] As shown in the enlarged view of FIG. 2, the ball screw mechanism 8 includes the screw shaft 10 and the nut 18, inserted on the screw shaft 10, via balls 19. The screw shaft 10 outer circumference includes a helical screw groove 10a. The screw shaft 10 is axially movably supported in the housing. The nut 18 inner circumference includes a screw groove 18a corresponding to the screw groove 10a of the screw shaft 10. A plurality of balls 19 is rollably contained between the screw grooves 10a, 18a. The nut 18 is rotationally and axially immovably supported by two supporting bearings 20, 20 relative to the housings 2a, 2b. A numeral 21 denotes a bridge member to achieve an endless circulating passage of balls 19 through the screw groove 18a of the nut 18.

[0045] The cross-sectional configuration of each screw groove 10a, 18a may be either one of a circular-arc or Gothic-arc configuration. However, the Gothic-arc configuration is adopted in this embodiment. Thus, it can have a large contacting angle with the ball 19 and set a small axial gap. This provides a large rigidity against axial loads and thus suppresses the generation of vibration.

[0046] The nut 18 is formed of case hardened steel such as SCM 415 or SCM 420. The nut surface is hardened to HRC 55 to 62 by vacuum carburizing hardening. This omits treatments, such as buffing for scale removal after heat treatment, to reduce the manufacturing cost. The screw shaft 10 is formed of medium carbon steel such as S55C or case hardened steel such as SCM 415 or SCM 420. The screw shaft surface is hardened to HRC 55 to 62 by induction hardening or carburizing hardening.

[0047] The output gear 5, forming part of the speed reduction mechanism 6 is firmly secured on the outer circumference 18b of the nut 18, via a key 14. The support bearings 20, 20 are press-fit onto the nut 18, via a predetermined interference, at both sides of the output gear 5. More particularly, as shown in FIG. 2, the output gear 5 is secured on the nut 18 by the key 14 fit into a rectangular key way space 14a formed on an outer circumference 18b of the nut 18. A key way 32a is formed on an inner circumference of the output gear 5. The output gear 5 is sandwiched by an inner ring 23 of the supporting bearing 20, arranged at the side of the first housing 2a, and a nut flange portion 18c. The supporting bearing 20, arranged at the side of the second housing 2b, is secured on the outer circumference 18b of the nut 18. It is sandwiched by the nut flange portion 18c and the second housing 2b. This prevents both the supporting bearings 20, 20 and output gear 5 from axially shifting even though strong thrust loads are applied to them from the drive shaft 7. Each supporting bearing 20 comprises a deep groove ball bearing. Shield plates 20a, 20a are mounted on both sides of the balls. The shield plates 20a, 20a prevent lubricating grease sealed within the bearing body from leaking outside. Also, the plates 20a, 20a prevent abrasive debris from entering into the bearing body from outside.

[0048] In the present embodiment, both the supporting bearings 20, 20 are formed by deep groove ball bearing with the same specifications. Thus, it is possible to support both a thrust load applied by the drive shaft 7 and a radial load applied by the output gear 5. Also, this simplifies confirmation work to prevent errors during assembly of the bearing. Further, this improves the assembling operability. In this case, the term “same specifications” means that the deep groove ball bearings have the same inner diameters, outer diameters, width dimensions, rolling element sizes, rolling element numbers and internal clearances.

[0049] The pair of supporting bearings 20, 20 are fit into the first and second housings 2a, 2b, via radial clearance. One support bearing 20, of these paired bearings 20, 20, is mounted on the first housing 2a via a washer 22. The washer 22 includes a ring-shaped elastic member.

[0050] The washer 22 is a wave washer press-formed of austenitic stainless steel (JIS SUS 304 etc.) or preserved cold rolled steel sheet (JIS SPCC etc.). The washer 22 has high strength and wear resistance. An inner diameter D of the washer 22 is larger than an outer diameter d of the inner ring 23, of the supporting bearing 20. The washer 22 urges the supporting bearing 20 toward the adjacent output gear 5. This eliminates axial play of the pair of supporting bearings 20, 20. Thus, rotation of the nut 18 is smooth. In addition, the washer 22 contacts only the outer ring 24 of the supporting bearing 20. The washer 22 does not contact the rotational inner ring 23. This prevents the inner ring 23 of the supporting bearing 20 from contacting the housing 2a even if the nut 18 is urged toward the housing 2a by a reverse-thrust load. Thus, this prevents the nut 18 from being locked by an increase of the frictional force.

[0051] Returning to FIG. 1, a gear shaft 25, of the intermediate gear 4 forming part of the speed reduction mechanism 6, is fit into the first and second housings 2a, 2b. The intermediate gear 4 is rotationally supported on the gear shaft 25 via a rolling bearing 26. One end, first housing 2a-side end, of the gear shaft 25 is press fit into the first housing 2a. This enables assembling misalignment and obtains smooth rotational performance by performing the clearance fitting of the other end, second housing 2b-side end. The rolling bearing 26 is a needle roller bearing of a so-called shell type. It includes an outer ring 27 and a plurality of needle rollers 29. The outer ring 27 is press-formed from a steel sheet. The outer ring is press-fit into an inner circumference of the intermediate gear 4. The plurality of needle rollers 29 is rollably contained in the outer ring 27, via a cage 28. This enables the adoption of easily or readily available bearings or a standard design and thus reduces manufacturing cost.

[0052] Ring-shaped washers 30, 30 are installed on both sides of the intermediate gear 4. The washers 30, 30 prevent direct contact of the intermediate gear 4 against the first and second housings 2a, 2b. In this embodiment, the face width of the teeth 4a of the intermediate gear 4 is formed smaller than an axial width of the gear blank. This reduces the contact area between the intermediate gear 4 and the washers 30, 30. Thus, this reduces their frictional resistance and obtains smooth rotational performance. The washers 30 are flat washers press-formed from austenitic stainless steel sheet or preserved cold rolled steel sheet with high strength and frictional resistance. Alternatively, the washers 30 may be formed of brass, sintered metal or thermoplastic synthetic resin such as PA (polyamide) 66. The thermoplastic synthetic resin is impregnated with a predetermined amount of fiber reinforcing material such as GF (glass fibers).

[0053] The output gear 5 is formed from a sintered alloy. The output gear includes spur teeth 5a, on its circumference, and a central hole 5b. The central hole 5b is a circular hole adapted to be fit onto the outer circumference 18b of the nut 18, as shown in FIG. 3(a). An intermediate region 33 is between a peripheral portion 31, near the teeth 5a, and a boss 32, near the central hole 5b. The peripheral portion 31 has a thickness (a). The boss 32 has a thickness (b). The intermediate region 33 has a thickness (x) thinner than those of the peripheral portion 31 and the boss 32. Thus, a>x and b>x. A plurality of weight-lightening apertures 34 are formed equidistantly in the intermediate region 33 along its circumference. Each weight-lightening aperture 34 has a rectangle expanding radially outward configuration. A key way 32a, engaging with securing key 14, is formed on the inner circumference of the boss 32. Although illustrated with rectangular weight-lightening apertures 34 that are effective for reducing the weight of the gear, the shape of each aperture 34 is not limited to a rectangle or any other shape. An egg-shape or a triangle with an expanding toward radially outward configuration may be possible if the weight-lightening apertures 34 can reduce the weight of the output gear 5 while maintaining strength and rigidity.

[0054] The metallic powder for the sintering alloy includes completely alloyed powder, atomized iron powder of alloyed and melted steel where alloyed components are uniformly distributed in grains, or partially alloyed powder alloyed powder where partially alloyed powder is adhered to pure iron powder of Fe, Mo and Ni. One example of the alloyed powders is a hybrid type alloy powder (trade name JIP 21 SX of JFE steel Co., Japan). Here, the pre-alloy copper powder includes Fe of 2% by weight, Ni of 1% by weight and Mo is adhered to fine Ni powder, Cu powder and graphite powder via binder. This hybrid type alloy powder is able to obtain high mechanical strength, tensioning strength and hardness, due to an increase of the martensite phase ratio to the metallic structure of the sintered body while increasing the cooling speed, higher than 50° C./min, after sintering. This eliminates heat treatment after sintering and provides a high accuracy output gear. It is preferable to have Mo of 0.5 to 1.5% by weight in order to improve the hardenability. Ni of 2 to 4% by weight is added to improve the toughness of the sintered body. Similar to the sleeve 17 described above, the output gear 5 may be formed of sintered alloy by the MIM method.

[0055] According to the present embodiment, the weight-lightening apertures 34 of the output gear 5 are arranged at a position near the outer circumference of the intermediate region 33, as shown in FIG. 3(a). This reduces the moment of inertia that is proportional to the square of the radius of the output gear 5. Also, this improves the strength and durability of the gear compared to where the weight-lightening apertures 34 are arranged at a position near the boss 32 in the intermediate region 33, as shown in FIG. 3(b). This arrangement further contributes to weight reduction as compared to the case where circular weight-lightening apertures 34′ are provided as shown in FIG. 3(c).

[0056] According to the present embodiment, a vibration absorbing member 35 is integrally adhered by vulcanized adhesion to the thin walled intermediate region 33. Thus, synthetic rubber side surfaces 35a and 35b are on both sides of the intermediate region 33. The side surfaces 35a and 35b are connected to each other through the weight-lightening apertures 34, as shown in FIG. 4. The vibration absorbing member 35 is formed of synthetic rubber such as NBR (acrylonitrile-butadiene rubber). It is adhered to the intermediate region 33 in a radially outer region rather than the outer diameter of the outer ring 24 of an adjacent supporting bearing 20, as shown in FIG. 4(a). In addition, the side surfaces 35a and 35b of the vibration absorbing member 35 are configured so that they are substantially flush with the peripheral portion 31 and the boss 32. This makes it easy to form the vibration absorbing member 35 and to assure the desired dimensional accuracy. In addition, the vibration absorbing member 35 is connected on both sides of the intermediate region 33 through the weight-lightening apertures 34. Thus, this improves the reliability and prevents peeling-off or dropping out of the vibration absorbing member 35. Further, it suppresses the generation of abnormal noise, such as teeth hitting sound, while reducing vibration of the teeth and simultaneously reducing the weight of the gear 5. The inner radius r.sub.1 of the side surfaces 35a and 35b is greater than the outer radius r.sub.2 of the support bearing 20 (r.sub.1>r.sub.2). This ensures smooth rotation of the gear 5 while preventing contact of the gear 5 with the outer ring 24 of the bearing 20, as shown in FIG. 4(b). In this specification, the term “substantially flush” means only target values in design and thus errors caused by machining should be naturally allowed.

[0057] Examples of the material of the vibration absorbing member 35, other than previously mentioned NBR, is HNBR (hydrogenation acrylonitric-butadiene rubber) superior in heat resistance, EPM, EPDM, ACM (poly-acrylic rubber) and FKM (fluororubber) superior in heat and chemical resistance.

[0058] The gear of the present disclosure can be used as an output gear of an electric actuator provided with a ball screw mechanism to convert a rotational input motion, from an electric motor, to a linear motion of a drive shaft, via a gear reduction mechanism. Electric motors for general industry use or drive parts of an automobile etc are included.

[0059] The present disclosure has been described with reference to the preferred embodiments. Obviously, modifications and alternations will occur to those of ordinary skill in the art upon reading and understanding the preceding detailed description. It is intended that the present disclosure be construed to include all such alternations and modifications insofar as they come within the scope of the appended claims or their equivalents.