WO2011029472A1

WO2011029472A1 - Drive assembly, industrial robot, robot boom and robot joint

Info

Publication number: WO2011029472A1
Application number: PCT/EP2009/061702
Authority: WO
Inventors: Ivan Lundberg; Jan Larsson
Original assignee: Abb Research Ltd
Priority date: 2009-09-09
Filing date: 2009-09-09
Publication date: 2011-03-17
Also published as: CN102484410A; EP2476186A1

Abstract

A drive assembly comprising a motor (10) including a motor housing (20), a rotor (44), a rotor shaft (40), and a rear bearing (50) for supporting the rotor shaft in the motor housing at a rear side of the rotor;and a strain wave gearing (60) including a circular spline (70) secured to the motor housing, a flex spline (80) engaging the circular spline, a wave generator (90) engaging the flex spline and secured to a drive end (42) of the rotor shaft, and a wave generator bearing between the circular spline and the wave generator. According to the invention, the wave generator bearing (100) serves as an exclusive drive end bearing for supporting the rotor shaft (40) in the motor housing (20) at a front side of the rotor (30).

Description

Drive assembly, industrial robot, robot boom and robot joint

TECHNICAL FIELD

This invention relates to a drive assembly comprising a motor including a motor housing, a rotor, a rotor shaft, and a rear bearing for supporting the rotor shaft in the motor housing at a rear side of the rotor; and a strain wave gearing including a circular spline secured to the motor housing, a flex spline engaging the circular spline, a wave generator engaging the flex spline and secured to a drive end of the rotor shaft, and a wave generator bearing between the circular spline and the wave generator. The invention also relates to an industrial robot, a robot boom and a robot joint provided with such a drive assembly.

BACKGROUND

When mounting a motor and a wave generator in a harmonic / strain wave drive it is important that the motor and wave generator axes are accurately aligned with the circular spline axis of the harmonic drive.

The concentricity demand for these parts is typically in the range of 10 - 20 Mm and is very hard to meet in practice, since it requires strict tolerances on both the motor and the mounting surface for the motor.

If the wave generator concentricity is not met, the friction of the harmonic drive increases and the running becomes unsmooth, typically with two friction peaks per motor revolution, when the elliptic lobes of the wave generator align with the eccentricity.

To overcome this problem, harmonic drive gearboxes can be fitted with an Oldham coupling between the motor shaft and the wave generator. Using the Oldham coupling roughly doubles the allowed eccentricity and makes it possible to meet the tolerance requirements with "standard" machining operations.

There are some drawbacks associated with using Oldham couplings, e.g. slightly increased backlash, cost and weight. Also, Oldham couplings are not available for the smallest harmonic drive gearboxes (size 3 & 5).

DISCLOSURE OF THE INVENTION

An object of the invention is to provide an alternative solution to the concentricity problem in a drive assembly of the kind defined above. This object is obtained by the features in the appended claims.

According to an aspect of the invention, the wave generator bearing serves as an exclusive drive end bearing for supporting the rotor shaft in the motor housing at a front side of the rotor. The rotor shaft may then have only two bearings, where the wave generator bearing is a single bearing for both the strain wave gearing and the drive end of the rotor shaft. In other words, the invention is to use the wave generator elliptic bearing directly as a motor bearing The overconstrained situation in the traditional harmonic drive train design with three bearings on the same axis is thereby eliminated, thus solving the concentricity problem discussed above.

Benefits of the invention compared to the overconstrained design are lower tolerance demands on the assembly, longer life of the harmonic drive due to reduced wear from overconstrained bearings and reduced gearbox friction and ripple.

In an embodiment of the invention, the rear bearing is a ball bearing, e.g. a groove ball bearing. Thereby, the rotor shaft will allow for the possible misalignment - typically up to 0.2 mm - introduced when mounting the strain wave gearing to the motor.

While the groove ball bearing may be a single row groove ball bearing, in another embodiment, the rear bearing is a spherical double-row groove ball bearing. Such a double row bearing may allow for smooth running under relative large axial misalignments.

While the strain wave gearing may conventionally be mounted to the drive end plate of the motor, in another embodiment of the invention, the circular spline may be designed to form the drive end plate of the motor housing. Thereby, the drive assembly may be simplified, resulting in reduced length, weight and cost.

To avoid axial play in the rotor shaft resulting from the absence of the conventional front end bearing of the rotor shaft, a spring may be provided for axially biasing the rotor shaft.

Specifically, the spring may be a compression spring located between a rearward face of the flex spline and a front face of the rotor shaft. The spring will thereby be concealed in the otherwise dead space within the cup-shaped flex spline.

The drive assembly may further have a sealing between the strain wave gearing and the motor to prevent ingress of lubricant from the gearing to the motor.

According to further aspects of the invention are defined an industrial ro- bot provided with a drive assembly having any one of the above defined features, a robot boom provided with a drive assembly having any one of the above defined features, and a robot joint provided with a drive assembly having any one of the above defined features.

Other features and advantages of the invention may be apparent from the appended claims and the following detailed description of embodiments.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a view with parts broken away of a drive assembly according to the invention;

FIG. 2 is a diagrammatic sectional side view of a drive assembly according to the invention;

FIG. 3 is a cross-sectional view taken along line 3-3 of an assembly according to FIG. 2; and

FIG. 4 is a rearward side view of a robot wrist provided with a couple of drive assemblies according to the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

The drive assembly shown in FIG. 1 comprises a motor 10 and a strain wave gearing 60. In the example shown, the motor 10 is an electric motor, such as a servomotor of the type that can be used together with gearing 60, for example to actuate a pitch and/or roll mechanism in an industrial robot (not shown) connected to an output end 84 of gearing 60.

As apparent from FIG. 1 and 2, the motor 10 has a rotor shaft 40 rota- tionally supported in a motor housing 20. In a well-known manner, a rotor 44 on the shaft 40 is electromagnetically energized by a stator 22 in the housing 20 to rotate shaft 40. As further indicated in FIG. 2, the motor 10 may also have a rear end re- solver unit 30 for controlling motor operation.

Also in a well-known manner, the strain wave gearing 60 has a circular spline 70, a cup-shaped flex spline 80, a wave generator comprising an elliptical wave generator plug 90 connected to a drive end 42 of shaft 40, and a wave generator bearing 100. As can be understood from FIG. 3, flex spline 80 and wave generator bearing 100 are elastically deformed by plug 90 to an elliptical shape. The elliptical shape will rotate inside the circular spline 70 when the plug 90 is rotated by the rotor shaft drive end 42. External teeth 82 of the flex spline 80 mesh with internal teeth 72 of the circular spline 70 at opposite major axis ends of the elliptical shape. As a result, since the number, e.g. fifty-six, of external teeth 82 is smaller than the number, e.g. sixty, of internal teeth 72, in operation the flex spline 80 will rotate in an opposite direction to the elliptical shape with a ratio determined by the difference between the respective number of teeth 72 and 82, all in a well-known manner.

According to the invention, the wave generator bearing 100 serves as an exclusive drive end bearing. As shown in FIG. 2, the shaft 40 is accordingly supported only by a rear bearing 50 and the wave generator bearing 100 in the assem- bly.

The rear bearing 50 may be a single-row groove ball bearing, as indicated in full line in FIG. 2. The rear bearing 50 may, however, also be a spherical double-row groove ball bearing, as diagrammatically indicated in phantom in FIG. 2.

While the circular spline 70 may equally well have, for example, a con- ventional annular shape that is bolted to a motor front end plate, in the embodiments shown, the circular spline 70 is shaped to function also as the motor front end plate. The circular spline 70 is then secured to the motor housing 20 for example by bolts through bores 74 (FIG.3) as a conventional motor end plate.

To eliminate axial play of the rotating parts in the assembly, the rotor shaft 40 is biased in an axial direction. In the example shown in FIG. 2, a compression spring comprising a conical coil spring 86 is located between a rear bottom face 88 of the cup-shaped flex spline 80 and a front face of the drive end 42 of rotor shaft 40. Both bearings 50 and 100 will thereby be independently subjected to forces that eliminate axial play. A ball 48 received in a groove 46 of drive end 42 may be pro- vided to keep the spring 86 in place and to reduce friction between spring 86 and drive end 42.

To avoid leakage of grease from the strain wave gearing 60 into the motor housing and to other components such as a mechanical brake (not shown) engaging the rotor shaft 40, a sealing 24 such as a labyrinth sealing acting on the rotor shaft 40 may be provided between the gearing 60 and the motor 10, as diagrammatically indicated in FIG. 2.

In the example shown in FIG. 4, a pair of drive assemblies 1 14 according to the invention are shown transversely mounted to a robot joint or wrist 1 12 having an end effector 120. The wrist 1 12 is connected to a boom 122 of a diagrammatically depicted industrial robot 1 10. Drive ends of drive assemblies 1 14 are connected to respective belt transmissions 1 16 that in turn are connected to a differential gear 1 18. In a well-known manner, the output of each drive assembly 1 14 may be independently controlled for controlling respective pitch and roll movements of the end effector 120 through the differential gear 118.

The foregoing detailed description is given primarily for clearness of understanding and no unnecessary limitations are to be understood therefrom. Modifications will become obvious to those skilled in the art upon reading this disclosure and may be made without departing from the spirit of the invention or the scope of the appended claims.

Claims

1 . A drive assembly comprising:

a motor (10) including:

a motor housing (20),

a rotor (44),

a rotor shaft (40), and

a rear bearing (50) for supporting the rotor shaft in the motor housing at a rear side of the rotor; and

a strain wave gearing (60) including:

a circular spline (70) secured to the motor housing,

a flex spline (80) engaging the circular spline,

a wave generator (90) engaging the flex spline and secured (90) to a drive end (42) of the rotor shaft, and

a wave generator bearing (100) between the circular spline and the wave generator;

characterized by

the wave generator bearing (100) serving as an exclusive drive end bearing for supporting the rotor shaft (40) in the motor housing (20) at a front side of the rotor (30).

2. The drive assembly according to claim 1 , wherein the rear bearing is a ball bearing (50).

3. The drive assembly according to claim 2, wherein the rear bearing is a spherical double-row ball bearing (50).

4. The drive assembly according to any of the previous claims, wherein the circular spline (70) forming a drive end plate of the motor housing (20).

5. The drive assembly according to any of the previous claims, comprising a spring (86) for axially biasing the rotor shaft (40) to reduce play in said bearings (50, 100).

6. The drive assembly according to claim 5, wherein the spring being a compression spring (86) located between a rearward face of the flex spline (80) and a front face of the rotor shaft (40).

7. The drive assembly according to any of the previous claims, comprising a sealing (24) between the gearing (60) and the motor (10) to prevent ingress of lubricant from the gearing (60) to the motor.

8. An industrial robot (1 0) provided with a drive assembly (114) according to any of the previous claims.

9. A robot boom (122) provided with a drive assembly (1 14) according to any of claims 1 -7. 0. A robot joint ( 2) provided with a drive assembly (114) according to any of claims 1 -7.