Compact multi-stage gear with a planetary gear and a strain wave gear adjacent to said multi-stage gear

A multi-stage gear is provided with a planetary gear and a strain wave gear adjacent to the multi-stage gear, wherein a output of the planetary gear maintains a drive connection with a wave generator of the strain wave gear, and wherein the planetary gear is arranged at least partially radially within the strain wave gear. It is provided that the wave generator is mounted radially on a ring gear of the planetary gear.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to foreign European patent application No. EP 17152884.7, filed on Jan. 24, 2017, the disclosure of which is incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a multi-stage gear with a planetary gear and a strain wave gear adjacent to and/or downstream of said multi-stage gear according to the generic term of the independent Claim1.

BACKGROUND

In such a multi-stage gear, an output of the planetary gear stands in a drive connection with a wave generator of the strain wave gear. The planetary gear is arranged at least partially in a radial way within the strain wave gear.

Multi-stage gears of the category-specific type offer the advantage of a very short axial structural length while the gear has zero backlash. Pure planetary gears, however, are usually not free of backlash. Through the combination of a planetary gear with a strain wave gear, high gear reduction conditions can be achieved and high angular torques can be created. Such gears are particularly interesting for drives in the field of wheel drives and robotic applications, where both freedom of backlash as well as a high angular torque are required.

A multi-stage gear according to the generic term of the independent Claim1is described for example in DE 10333951 B3. In this gear, a single-stage planetary gear is used, whose planetary wheels mesh on one hand with the sun wheel and on the other hand support themselves on an internal, externally toothed support disc. The wave generator of the strain wave gear is supported exclusively on the single planetary carrier of the planetary gear.

A further multi-stage gear according to the generic term of the independent Claim1is known from DE 10151878 A1. Also in this gear, the wave generator of the strain wave gear is supported exclusively on the single planetary carrier of the planetary gear. The planetary wheels of the planetary gear mesh with the sun wheel and an internally toothed ring gear that is, when viewed in an axial direction, arranged next to the strain wave gear so that the overall structure of this gear is not quite as compact as in the drive from DE 10333951 B3.

SUMMARY OF THE INVENTION

The purpose of the present invention is to indicate a multi-stage gear with a planetary gear and a strain wave gear adjacent to said multi-stage gear, which is in particular in the axial direction particularly compact, low-wear, intrinsically stable and at the same time suitable to absorb high tilting torques around an axis that is perpendicular to the longitudinal axis of the gear.

The problem is solved by the features of the independent Claim1. According to said features, a solution of the problem according to the invention exists in a multi-stage gear according to the generic term of Claim1when the wave generator is installed radially on a ring gear of the planetary gear. The ring gear preferably has an internal teething. Further preferably, planetary wheels of the planetary gear mesh with the internal teething of the ring gear. Further preferably, the ring gear forms a bearing of the planetary gear as it is the case in conventional planetary gears. During construction of the multi-stage gear according to the invention, recourse can therefore be made to standard planetary drives that only have to be modified in a minor way where required. The planetary gear and the strain wave gear have the same longitudinal axis in the gear according to the invention.

Preferred embodiments of the present invention are the object of the sub-claims.

According to a particularly preferred embodiment of the present invention, the output of the planetary gear is a planetary carrier of the planetary gear, wherein this planetary carrier is defined as one part with the wave generator. Therefore, a particularly compact integration of the planetary gear in the strain wave gear becomes possible. Further, the number of the required components is reduced this way, which, in turn, lowers the manufacturing workload and the respective costs.

According to a further particularly preferred embodiment of the present invention, the wave generator is installed with at least one roller bearing on the ring gear of the planetary gear. Therefore, the multi-stage gear according to the invention becomes particularly stable and low-wear. In this, it is particularly preferable for the wave generator to be installed with two roller bearings, which are axially spaced from one another, on the ring gear of the planetary gear. Further preferably, the roller bearing or the roller bearings are ball bearings. Further preferably, the roller bearings are arranged on the axial ends of the wave generator, which leads to a particularly stable and low-wear mounting. In this context, it is particularly advantageous if the effective axial distance between the two roller bearing amounts at least to half of the axial overlap of the wave generator and the ring gear. Further preferably, the effective axial distance between the two roller bearings amounts to at least 70% of the axial overlap of the wave generator and the ring gear.

According to a further embodiment of the present invention, the planetary gear is defined in a multi-stage way, wherein the wave generator encloses the ring gear of the planetary gear over a maximum of half of the axial length of the ring gear. This constitutes a good compromise between stable mounting and a light structural design.

According to an alternative embodiment of the present invention, the planetary gear is designed in a single-stage way, wherein the wave generator encloses the ring gear over the whole axial length of the ring gear.

According to a further preferred embodiment of the present invention, the axial overlap of the planetary gear and the strain wave gear amounts to at least 80% of the axial length of the planetary gear. This leads to a compact structural design. Particularly preferably, the planetary drive is enclosed over its complete axial length by the strain wave gear.

Further preferably, an external ring gear of the strain wave gear, which is equipped with an internal teething, forms the output of the multi-stage gear. Therefore, particularly high angular torques can be transmitted. In this, the flexspline of the strain wave gear, which is arranged between the wave generator and the external ring gear of the strain wave gear, is fixed. It can for example be firmly connected to the ring gear of the planetary gear. Alternatively, also the flexspline of the strain wave gear can form the output. In this, the external ring gear of the strain wave gear is formed in a fixed way. Particularly preferably, the strain wave gear is a harmonic drive gear.

The present invention also provides a drive unit with a multi-stage gear according to the invention and an electric motor. In this, the planetary gear is driven by the electric motor, wherein the electric motor is preferably designed as an internal rotor motor.

DETAILED DESCRIPTION

The planetary carrier2of the last planetary stage C forms the output of the three-stage planetary gear. It is formed as one part with the wave generator3of the downstream strain wave gear. Said strain wave gear, in turn, is installed radially on the ring gear4of the planetary gear with two ball drives8and9that are spaced axially from one another. As becomes clear from the drawing, the wave generator3encloses the ring gear4approximately over half of the axial length of the ring gear4. The two ball bearings8and9are positioned in such a way that they are arranged essentially on the opposite ends of the wave generator3.

The wave generator3has a well-known elliptic form and causes during a rotation a circulating deformation of the external flexible spur gear11of the strain wave gear. Between the wave generator3and the flexspline11, a respective roller bearing21is arranged in the known way. In the shown embodiment, the flexspline11is connected in a torque-proof way with the ring gear4of the planetary gear. For this purpose, it can for example be screwed with an extension12of the ring gear4, which, however, cannot be counted as part of the planetary gear but which is part of the external strain wave gear. Here, also a separate component can be used instead of an extension of the ring gear4.

The flexspline11has an external teething that, during rotation of the wave generator, is set to encroach with an internal teething of the external ring gear10of the strain wave gear in a sectionally circulating way. The ring gear10forms the output of the multi-stage gear according to the invention. It is installed flexibly on the extension12of the ring gear4of the planetary gear by means of two ball bearings13and14. In a similar way as the two ball bearings8and9, the two ball bearings13and14are arranged in a spaced way from one another in order to ensure a particularly stable mounting of the ring gear10.