Shaft coupling structure

In a shaft coupling structure in which a second shaft is inserted into a cylindrical portion provided in a first shaft to be coupled with each other, with a bush being interposed between an inner circumference of the first shaft and an outer circumference of the second shaft, the first shaft and the second shaft are coupled with each other by a combination of a key coupling and a clamp coupling; the first shaft has a keyway with which the key engages and a shaft slit which is formed in the cylindrical portion by cutting off a portion thereof in the axial direction; and a fastener for the clamp coupling is mounted on a portion of an outer circumference of the first shaft in which the shaft slit is formed and the bush has a bush slit which corresponds to the keyway.

INCORPORATION BY REFERENCE

Priority is claimed to Japanese Patent Application No. 2012-265852, filed Dec. 4, 2012, the entire content of which is incorporated herein by reference.

BACKGROUND

1. Technical Field

The present invention relates to a shaft coupling structure.

2. Description of the Related Art

The related art discloses a shaft coupling structure where a motor shaft is inserted into a hollow portion provided in an input shaft to be coupled with each other.

In the shaft coupling structure, a keyway is formed in each of the input shaft and the motor shaft and a slit is formed in the input shaft, and the input shaft and the motor shaft are coupled with each other by the combination of a key coupling and a clamp coupling. Power transmission is performed via the key, and the clamp bridges a gap between the key and the keyway, thereby contributing to reduction in vibration and noise.

SUMMARY

According to an embodiment of the present invention, there is provided a shaft coupling structure in which a second shaft is inserted into a cylindrical portion provided in a first shaft to be coupled with each other. With a bush being interposed between an inner circumference of the first shaft and an outer circumference of the second shaft, the first shaft and the second shaft are coupled with each other by a combination of a key coupling and a clamp coupling. The first shaft has a keyway with which the key engages and a shaft slit which is formed in the cylindrical portion by cutting off a portion thereof in the axial direction. A fastener for the clamp coupling is mounted on a portion of an outer circumference of the first shaft in which the shaft slit is formed. The bush has a bush slit which corresponds to the keyway.

DETAILED DESCRIPTION

Since, in the shaft coupling structure described above, for example, an input shaft was required to be prepared for each type of motor shaft, costs were likely to become high.

In certain embodiments of the present invention, it is desirable to provide a shaft coupling structure which can be applied to a coupling between various types of shafts at low costs by the combination of a key coupling and a clamp coupling.

In the embodiment of the present invention, when the second shaft is coupled with the inner circumference of the first shaft, with the bush being interposed between the inner circumference of the first shaft and the outer circumference of the second shaft, the coupling is performed based on the combination of the key coupling and the clamp coupling.

At this time, the first shaft has the keyway and the shaft slit formed in the cylindrical portion by cutting off a portion thereof in the axial direction, and the fastener for the clamp coupling is mounted on a portion of the outer circumference of the first shaft in which the shaft slit is formed. On the other hand, the bush has the bush slit which corresponds to the keyway of the first shaft.

Accordingly, with merits of both of the key coupling and the clamp coupling being enjoyed, a first shaft and a second shaft of various shapes or sizes can be coupled with each other at low costs when a bush, which corresponds to the shafts of various shapes or sizes, is only interposed therebetween.

FIG. 2is a cross-sectional view illustrating an exemplary configuration in which a shaft coupling structure according to an exemplary embodiment of the present invention is applied to a coupling between a motor shaft of a motor and an input shaft of a speed reducer.

A coupling structure between an input shaft12(a first shaft) of a speed reducer10and a motor shaft16(a second shaft) of a motor14will be described later, and, first, a schematic configuration of the power transmission system of the speed reducer10will be briefly described with reference toFIG. 2.

The speed reducer10is a speed reducer which is called an eccentric oscillation type and is widely used in the joint drive of a robot and the drive system of a machine tool. The input shaft12of the speed reducer10is arranged in the position of a shaft center O1 of an internal gear22. An eccentric body26is integrally formed with the input shaft12. An external gear20is assembled onto the outer circumference of the eccentric body26via a roller28. The external gear20internally meshes with the internal gear22. The internal gear22is integrated with a casing30. The external gear20has the number of teeth just slightly less (as less as 1 in this example) than the number of teeth of the internal gear22.

A pin-shaped member32passes through each of the external gears20. On both sides of the external gears20in the axial direction, a pair of a first carrier34and a second carrier36are rotatably supported by the casing30via bearings38and40. The first carrier34and the second carrier36are coupled with each other via the pin-shaped member32and a bolt42. A driven member not illustrated is coupled with the first carrier34via a tap hole34A.

An operation of the power transmission system of the speed reducer10will be briefly described.

When the input shaft12is rotated, the eccentric body26integrated with the input shaft12is rotated and the external gear20are oscillated via the roller28. As a result, a phenomenon occurs in which a meshing position of the external gear20sequentially shifts with respect to the internal gear22. Since the number of teeth of the external gear20is at least one fewer than the number of teeth of the internal gear22, the external gear20shifts at least one tooth out of phase with respect to the internal gear22every time when the input shaft12is rotated (makes a rotation). The rotational component is transmitted to the first carrier34and the second carrier36via the pin-shaped member32, thereby driving the driven member coupled with the drive member via the first carrier34and the tap hole34A.

Subsequently, with reference toFIGS. 1 to 3B, a coupling structure between the input shaft12(the first shaft) of the speed reducer10and the motor shaft16(the second shaft) of the motor14according to the embodiment will be described in detail. The dimensions of gaps and the likes do not necessarily coincide with the actual dimensions thereof such that the embodiment of the present invention can be easily understood.

In the coupling structure, the input shaft12and the motor shaft16are coupled with each other by inserting the tip of the motor shaft16into a hollow portion (cylindrical portion)18provided in the input shaft12. A bush52is interposed between an inner circumference12A of the input shaft12and an outer circumference16A of the motor shaft16, and the input shaft12and the motor shaft16are coupled with each other by the combination of a coupling by a key54and a clamp coupling by a fastener58.

The motor shaft16of the motor14has a keyway56formed in the outer circumference16A thereof along the axial direction. In the embodiment, a general-purpose motor is used as the motor14, and the motor shaft16and the keyway56of the general-purpose motor14are used as originally configured.

The input shaft12of the speed reducer10is supported by the first carrier34and the second carrier36via ball bearings44and46. The input shaft12has the hollow portion18on a side opposite to a load side in the axial direction (on the motor14side). The input shaft12has a stepped down portion on the motor14side thereof formed from substantially axial center position of the hollow portion18via two steps12K1and12K2, and the stepped down portion has a thinner wall thickness than the remainder of the hollow portion18.

The input shaft12has a keyway formed from an end surface12E on the motor14side thereof to an axial position slightly exceeding the steps12K1and12K2, where the key54engages with the keyway, and has a clamping shaft slit formed by cutting off a portion of the hollow portion18in the axial direction. However, in the embodiment, a single shaft groove62serves both as the keyway and the shaft slit (the shaft groove62, the keyway and the shaft slit indicate the same single portion). That is, in appearance, the input shaft12has the only one shaft groove62which serves both as the keyway and the shaft slit.

In the specification, for convenience of explanation, the physical “shaft groove62” is suitably called the keyway when the shaft groove is focused on serving as a keyway and the shaft slit when the shaft groove is focused on serving as a shaft slit.

On the other hand, the bush52has a cylindrical shape, a collar52P and a bush slit64formed to pass therethrough in the axial direction, and the bush slit64corresponds to the keyway (shaft groove62) of the input shaft12. The bush slit64is intended to be made in such a manner that the key54can go beyond the bush52in the radial direction and engage with both of the keyway56of the motor shaft16and the keyway of the input shaft12. The collar52P is pinched between the end surface12E of the input shaft12and a snap ring68locked on the motor shaft16, thereby locating the bush52in the axial direction.

The fastener58for clamp coupling is mounted on a portion of an outer circumference12B of the input shaft12in which the shaft slit is formed. The fastener58has slit portions70and71for diameter reduction of the shaft and tightens the input shaft12from the radial outside by tightening a bolt72, thereby reducing the diameter of the input shaft12. In the embodiment, tightening is performed in a state where the slit portions70and71of the fastener58are placed in positions P2and P3at substantially 90° to a position P1where the shaft groove62for serving as both the keyway and the shaft slit is placed.

The bush52has an axial length L1 (axial length of the bush slit64) longer than an axial length L2 of the fastener (refer toFIG. 1).

Subsequently, with reference toFIGS. 3A and 3B, a magnitude relation between each part in the embodiment will be described.

Herein, signs are defined as follows.

Bf1: the width of the bush slit64of the bush52in a free state (before clamping is performed)

Bc1: the width of the bush slit64of the bush52after clamping is completed

Sf1: the width of the shaft groove62(=the keyway−the shaft slit) of the input shaft12in a free state.

Sc1: the width of the shaft groove62(=the keyway−the shaft slit) of the input shaft12after clamping is completed

K1: the width of a key

M1: the width of the keyway56of the motor shaft16

Since the shaft groove62of the input shaft12becomes small due to diameter reduction of the input shaft12after clamping is completed compared to in a free state (before clamping), the width Sc1 is smaller than the width Sf1 where Sc1 is the width of the shaft groove62after clamping is completed and Sf1 is the width of the shaft groove62in a free state (Sc1<Sf1). Similarly, the width Bc1 is smaller than the width Bf1 where Bc1 is the width of the bush slit64after clamping is completed and Bf1 is the width of the bush slit64in a free state (Bc1<Bf1).

In the embodiment, after clamp coupling is completed, the bush slit64of the bush52has the width Bc1 larger than the width Sc1 of the keyway of the input shaft12(Bc1>Sc1).

In addition, even after clamp coupling is completed, the keyway of the input shaft12has the width Sc1 larger than the width K1 of the key54(Sc1>K1).

Furthermore, the width Bc1 is larger than the width Sf1 where Bc1 is the width of the bush slit64after clamping is completed and Sf1 is the width of the shaft groove62of the input shaft12in a free state (Bc1>Sf1).

Consequently, as can be visually recognized fromFIG. 3A, a relation of Bf1>Bc1>Sf1>Sc1>K1 is established.

In the embodiment, after clamping is completed, the shaft groove62is set to have the width Sc1 identical with the width M1 of the keyway56of the motor shaft16(Sc1≅M1). However, after the clamping is completed, the shaft groove62is not necessarily required to have the width Sc1 identical with the width M1 of the keyway56of the motor shaft16, for example, may be set to have a width larger than the width M1 of the keyway56of the motor shaft16.

Subsequently, an operation of the shaft coupling structure will be described.

In the shaft coupling structure according to the embodiment, the bush52and the key54are interposed between the inner circumference12A of the input shaft12and the outer circumference16A of the motor shaft16. The bush52and the key54are pre-assembled onto any of the input shaft12and the motor shaft16, and, in the pre-assembled state, the motor shaft16is fitted into the hollow portion18of the input shaft12.

For example, first, the position (the position P1inFIG. 1) of the bush slit64of the bush52is aligned with the shaft groove62(as the keyway) of the input shaft12and then the bush52is assembled onto the inner circumference12A of the input shaft12. In this state, the key54is assembled into the keyway56of the motor shaft16, the position of the key54is aligned with the position P1of the shaft groove62of the input shaft12, and then the motor shaft16together with the key54is inserted into the hollow Portion18of the input shaft12.

The fastener58is mounted on a portion of the outer circumference of the input shaft12in which the shaft groove (as the shaft slit) is formed, and the input shaft12is tightened by screwing the bolt72of the fastener58(diameter reduction).

At this time, the shaft groove62of the input shaft12has a width reduced from Sf1 to Sc1, but, since the shaft groove62is set to have the width Sc1 larger than the width K1 of the key54even after clamp coupling is completed (in a free state as well), there is no possibility that the shaft groove62of the input shaft12comes into contact with the key54in the middle of clamping being performed, thereby inhibiting the diameter of the input shaft12from being further reduced.

In addition, since the bush slit64of the bush52always has the width Bc1 larger than the width Sc1 of the shaft groove62after clamping is completed, there is also no possibility that the bush slit64of the bush52comes into contact with the key54while the shaft groove62of the input shaft12is reduced in diameter, and thus the diameter of the input shaft12cannot be further smoothly reduced.

That is, regardless of the bush52being interposed between the input shaft12and the motor shaft16, diameter reduction of the input shaft12can be reliably performed by the fastener58without being affected by the existence of the key54and the existence of the bush52.

As is apparent from the operation of the coupling described above, in the embodiment, gaps exist between the key54and the keyway and between the key54and the bush slit even after clamping is completed. Accordingly, power transmission is performed relying on the clamp coupling. Specifically, the power transmission is performed by frictional tightening force between the outer circumference16A of the motor shaft16and the inner circumference of the bush52, and by frictional tightening force between the outer circumference of the bush52and the inner circumference12A of the input shaft12. The key54does not contribute to the power transmission between the motor shaft16and the input shaft12(during a normal operation).

In this respect, for example, the embodiment greatly differs in the technical ideas of the coupling from a structure with a combination of the key and the clamping in the related art described above. That is, in the shaft coupling structure of the related art, “power transmission is basically performed via the key, and the clamping bridges a gap between the key and the keyway, thereby preventing vibration and noise from occurring”, and the key takes the role of the power transmission. However, when this configuration is applied to a configuration where the bush is interposed between the motor shaft and the input shaft, the diameter reductions of the input shaft and the bush are not only prone to interfere with each other via the key while diameter reduction is performed, but also (since the bush exists between the keyway of the motor shaft and the keyway of the input shaft) gaps in any of the keyways are prone to be incompletely bridged and to remain. That is, the structure according to the related art described above is not basically prone to be applied to the structure with the bush being interposed.

In contrast, even in a free state or even when clamping is performed or even after clamping is completed, the embodiment is configured in such a manner that there are basically no interferences between the key54and the shaft groove62and between the key54and the bush slit64, and thus the clamping (diameter reduction) of the input shaft12can be smoothly performed even with the bush52being interposed. In addition, since power transmission is performed basically relying on the clamp coupling, the merits of the clamp coupling such as the non-existence of backlash, small vibration and noise can be fully enjoyed.

In addition, even in case frictional transmission force is lost due to unknown causes, a coupling by the key54serves as backup, thereby preventing a driven machine coupled with the drive member from unexpectedly behaving.

In such a configuration where the key coupling and the clamp coupling are combined, the configuration can be applied to a coupling between various types of shafts at low costs only by suitably designing the bush. More specifically, when a coupling between the input shaft12(the first shaft) of the speed reducer10and the motor shaft16(the second shaft) is described as an example, the motor shaft16may have a diameter of various sizes, but one type of the input shaft12can handle the motor shaft16with a diameter of such various sizes by preparing a plurality of types of the bush52with an inner diameter corresponding to the diameter of the motor shaft16. In addition, as the motor shaft may have a cylindrical shape, the motor shaft may have a taper shape. Even in such a case, the input shaft12with a cylindrical hollow portion can handle the motor shaft of any shape by preparing the bush with a tapered inner circumference.

Subsequently, a second embodiment of the present invention will be described. The same reference numerals and signs will be assigned to the same or similar portions to the first embodiment.

FIG. 4illustrates an example in which a shaft slit62S1is added to the input shaft12(the first shaft) and, as a result, a plurality of the shaft slits (2 pieces) are formed in conjunction with the same shaft slit (also serving as a keyway) as in the first embodiment. The additional shaft slit62S1is formed at a position opposite to the shaft slit (180° opposite position) which also serves as a keyway.

However, in the embodiment, regardless of the plurality of shaft slits and62S1being formed in the input shaft12, the bush52does not have a plurality of the bush slits64but has the only one bush slit64where the bush slit64also serves as the same shaft slit as in the first embodiment.

An intention of the configuration is as follows. That is, when the plurality of shaft slits and62S1are formed in the input shaft12, a merit is obtained that the uniform diameter reduction of the input shaft12is much easily achieved compared to when only one shaft slit is formed. Accordingly, an off-center between the input shaft12and the shaft groove62can be suppressed, and stress concentration in the vicinity of the shaft groove62can be mitigated. In addition, the shaft slits and62S1formed in the input shaft (the first shaft) can be relatively easily processed by chucking a portion of the input shaft12on a side opposite to the motor side in the axial direction. On the other hand, when the plurality of bush slits are formed in the bush52, the bush is not prone to be chucked in many cases when processing is performed, and thus the bush slits are not necessarily formed (processing) easily. In addition, since the bush52receives diameter reduction force from the fastener58via the input shaft12, the uniform diameter reduction force of the input shaft12basically enables relatively uniform diameter reduction of the bush52. Accordingly, when a configuration is made in such a manner that the input shaft12(the first shaft) has the shaft slits and62S1and the bush52has the only one bush slit64, it can be said that the configuration has realistic processability and uniform clamping compatible with each other.

The input shaft12may have three or more shaft slits (as far as strength necessary for power transmission is ensured). Qualitatively, the more shaft slits the input shaft12has, the more uniformly the diameter thereof can be reduced.

In addition, in the example ofFIG. 4, similar to the first embodiment, one shaft groove62for serving both as the keyway and the shaft slit is formed and one shaft slit62S1(not serving as a keyway) with a width smaller than the keyway is additionally formed, but, for example, inFIG. 4, the additional shaft slit may be also formed to have such a width (the width Sf1 in a free state and the width Sc1 after clamping is completed) that the additional shaft slit can also serve as the same keyway as the shaft groove62. In this case, since the shaft slits can be continuously processed using the same tool, manufacturing is much easily facilitated.

FIG. 5illustrates a modification example of the bush.

As described above, due to challenging chucking or the like, it is not necessarily easy in practicality that the bush slit is formed in the bush to pass therethrough in the axial direction or the plurality of bush slits of a long length are formed in the bush.

In the bush52illustrated inFIG. 5, a plurality of the bush slit64and a bush slit64A have axial lengths L4 and L5 changed, respectively. That is, since the bush slit64passes through the bush52in the axial direction, the bush slit64has the axial length L4 identical with the axial length L1 of the bush52. However, the bush slit64A does not pass through the bush52in the axial direction, and the bush slit64A has the axial length L5 smaller than the axial length L4 of the bush slit64(L4>L5).

Accordingly, since rigidity of the bush52necessary for the chucking thereof can be ensured when processing is performed, easy processability and uniform clamping that are compatible with each other can be achieved.

In the bush52according to the embodiment, when the bush slit64passes through the bush52and is a long one, the bush slit64becomes a wide bush slit with which the key54(not illustrated inFIG. 5) can engage, and when the bush slit64does not pass through the bush52and is a short one, the bush slit64becomes a slender bush slit with which the key54cannot engage. However, similar to the modification example, the additional bush slit may also become a wide bush slit with which the key54can engage. The bush may have three or more bush slits.

As such, in the plurality of bush slits, even when there is a bush slit which does not pass through, the bush slit preferably has an axial length (L5 in this case) longer than the axial length L2 of the fastener58(not illustrated inFIG. 5and refer toFIG. 2). Accordingly, partial contact of the fastener58can be prevented when clamping is performed.

Herein, with reference toFIGS. 3A and 3Bagain, a description of a magnitude relation between the widths of a bush slit, a keyway and a key will be supplemented.

As described above, inFIG. 3A, after clamping is completed, the bush slit64is set to have the width Bc1 larger than the width Sf1 of the shaft groove62of the input shaft in a free state (Bc1>Sf1). However, for example, the magnitude relation is not necessarily required as illustrated inFIG. 3B, in conclusion, when a relation is maintained “that, after the clamp coupling is completed, the bush slit64has a width Bc2 larger than a width Sc2 of the keyway of the input shaft12”, there is hardly any concerns in practicality. That is, for example, even though the bush slit64in a free state has a width Bf2 smaller than a width Sf2 of the keyway in a free state, there is no concerns as far as the bush slit64after the clamping is completed has the width Bc2 larger than the width Sc2 of the keyway after the clamping is completed.

In any ofFIGS. 3A and 3B, it is required to maintain a configuration that, even after the clamping is completed, the keyway of the input shaft12has the width Sc1 or Sc2 larger than the width K1 of the key54(Sc1>K1 or Sc2>K1) and thus the clamping is reliably performed all the way through the operation.

However, in certain embodiments of the present invention, a reversal of the magnitude relation (Sc1>K1 or Sc2>K1) is not completely prohibited. That is, after clamping is completed, the keyway of the input shaft12may be designed to have the width Sc1 smaller than the width K1 of the key54(Sc1<K1). In this case, the clamping by the fastener58serves to remove a gap between the key54and the keyway. Accordingly, power transmission involving power transmission by the key54can be achieved. Even in this case, when frictional tightening force itself by the fastener58is perfectly maintained and the power transmission is designed to be persistently performed on the basis of clamping force, the appropriate setting of the dimensions of each member can be simply controlled. In addition, after clamping is completed, the keyway may have the width Sc1 (Sc2) identical with the width Bc1 of the bush slit or may have Sc1 (Sc2)<Bc1.

In the embodiments described above, the slit of the input shaft serves as the keyway of the input shaft, but, in certain embodiments of the present invention, the shaft slit is not necessarily required to serve as the keyway. For example, a groove serving only as a keyway may be individually formed in a circumferential position different from a circumferential position where a slit serving only as a slit is formed, otherwise the keyway may be formed at the same circumferential position and the slit may be formed outside the circumference of the keyway to have a width smaller than the keyway.

In addition, in the embodiments described above, certain embodiments of the present invention is applied to a coupling between a motor shaft and an input shaft of a speed reducer, but are not limited to the coupling between the motor shaft and the input shaft of the speed reducer. Certainly, a configuration of the speed reducer is not limited to the configuration described above. For example, in an eccentric oscillation-type speed reducer having a plurality of eccentric body shafts (having an eccentric body) at positions offset from a shaft center of an internal gear or called a distribution type, for example, certain embodiments of the present invention can be applied to a portion where the input shaft of the speed reducer is coupled with a motor. In addition, certain embodiments of the present invention is not exactly limited to the speed reducer, in conclusion, can be widely applied to a shaft coupling structure where a second shaft is inserted into a cylindrical portion provided in a first shaft to be coupled with each other.