TWO-STAGE ACTUATING GEAR MECHANISM WITH ADJUSTABLE GEAR BACKLASH

Abstract

A two-stage actuating gear mechanism comprises a spur gear transmission as the first transmission stage and a harmonic drive as the second transmission stage. A spur gear transmission gear with inward facing teeth is designed simultaneously as a component of a harmonic generator of the harmonic drive. A gear with outward facing teeth that meshes with the gear with inward facing teeth is rigidly connected to a rotor of an electric motor, the motor housing of which is secured to the transmission housing of the harmonic drive. The motor housing is positioned relative to the transmission housing by means of a centering device which is eccentric to the rotational axis (A.sub.E) of the rotor. An adjustable backlash between the gear with outward facing teeth and the gear with inward facing teeth is provided by a pivoting capability between the motor housing and the transmission housing.

Claims

1. A two-stage actuating gear mechanism comprising: a spur gear transmission as a first transmission stage, a harmonic drive as a second transmission stage, a first gear having inward facing teeth is configured as a component of both the spur gear transmission and the harmonic drive, a second gear having outward facing teeth is configured to mesh with the first gear, the second gear rigidly connected to a rotor of an electric motor, and a motor housing of the electric motor secured to a transmission housing of the harmonic drive, and the motor housing positioned relative to the transmission housing by a centering device eccentric to a rotational axis of the rotor, and the motor housing configured to pivot relative to the transmission housing about a pivot axis to adjust a backlash between the second gear and the first gear.

2. The two-stage actuating gear mechanism of claim 1, wherein the centering device comprises a centering pin and the pivot axis is defined by a central axis of the centering pin.

3. The two-stage actuating gear mechanism of claim 2, wherein an elongated hole is formed in one of the motor housing or the transmission housing, the elongated hole configured to secure the motor housing to the transmission housing in variable positions.

4. The two-stage actuating gear mechanism of claim 1, wherein the centering device is formed by a receptacle in the transmission housing, the motor housing disposed within the transmission housing.

5. The two-stage actuating gear mechanism of claim 4, wherein a first distance between the pivot axis and the rotational axis of the electric motor is less than a second distance between the rotational axis of the electric motor and a central axis of the harmonic drive.

6. The two-stage actuating gear mechanism of claim 1, wherein the first gear is configured as an inner ring of a rolling bearing of the harmonic drive.

7. The two-stage actuating gear mechanism of claim 6, wherein the rolling bearing further comprises an outer ring, the outer ring configured for deforming a cup-shaped harmonic drive transmission element with outward facing teeth.

8. A method of assembling a two-stage actuating gear mechanism, comprising: providing a harmonic drive including a transmission housing having a first centering contour, and an input-side component of the harmonic drive is configured as a first gear with inward facing teeth, providing an electric motor including a motor shaft rigidly connected to a second gear with outward facing teeth the motor shaft protruding from a motor housing, the motor housing having a second centering contour arranged eccentrically on the motor housing, assembling the transmission housing and the motor housing such that the first and second centering contours form a pivot axis between the transmission housing and the motor housing, setting of an angular position between the transmission housing and the motor housing such that a spur gear transmission with pre-defined backlash is formed by the first gear and the second gear, and fixing of the transmission housing relative to the motor housing at the angular position.

9. The method of claim 8, wherein, in the setting of the angular position, the motor housing is first rotated relative to the transmission housing to a zero backlash position, and then rotated back by a pre-defined angle.

10. The two-stage actuating gear mechanism of claim 1, wherein the two-stage actuating gear mechanism is used as a device for varying a compression ration of a reciprocating piston engine.

11. The two-stage actuating gear mechanism of claim 1, wherein the first gear is configured as an input-side component of the harmonic drive.

12. The two-stage actuating gear mechanism of claim 6, wherein the inner ring is non-circular.

13. The two-stage actuating gear mechanism of claim 7, wherein the outer ring is flexible.

14. The two-stage actuating gear mechanism of claim 1, wherein the rotational axis of the electric motor is offset from a central axis of the harmonic drive.

15. The two-stage actuating gear mechanism of claim 14, wherein the pivot axis is: i) parallel to and offset from the rotational axis of the electric motor, and ii) parallel to and offset from the central axis of the harmonic drive.

16. A two-stage actuating gear mechanism, comprising: a spur gear transmission, a harmonic drive having a transmission housing, a first gear configured as a component of both the spur gear transmission and the harmonic drive, a second gear configured to mesh with the first gear, the second gear rigidly connected to a rotor of an electric motor, and a motor housing of the electric motor positioned relative to the transmission housing by a centering device eccentric to a rotational axis of the rotor, the motor housing configured to pivot relative to the transmission housing about a pivot axis to adjust a backlash between the second gear and the first gear.

17. The two-stage actuating gear mechanism of claim 16, wherein the centering device is formed by a receptacle arranged within the transmission housing.

18. The two-stage actuating gear mechanism of claim 16, wherein the centering device comprises a centering pin, and the pivot axis is defined by a central axis of the centering pin.

19. The two-stage actuating gear mechanism of claim 16, wherein the pivot axis is: i) offset a first distance from the rotational axis of the electric motor, and ii) offset a second distance from a central axis of the harmonic drive.

20. The two-stage actuating gear mechanism of claim 19, wherein the first distance is less than the second distance.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Below, two exemplary embodiments of the disclosure are explained in more detail by means of a drawing. In the figures:

[0024] FIG. 1 shows a first exemplary embodiment of a two-stage actuating gear mechanism in a sectional view,

[0025] FIG. 2 shows a schematic front view of the features of the actuating gear mechanism according to FIG. 1, and

[0026] FIGS. 3 and 4 show an electric motor of a further two-stage actuating gear mechanism in front and rear views.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0027] Unless otherwise stated, the following explanations relate to both exemplary embodiments. Parts or geometric details that correspond to each other or have basically the same effect are marked with the same reference symbols in all figures.

[0028] A two-stage actuating gear mechanism, identified overall by reference number 1, is used in a VCR device (not shown), i.e., a device for varying the compression ratio of a reciprocating piston engine. With regard to the principle function of the VCR device, reference is made to the prior art cited at the outset.

[0029] The actuating gear mechanism 1 comprises a spur gear transmission 2 as the first transmission stage and a harmonic drive 3 as the second transmission stage. Here, a gear 4 with inward facing teeth is a component of both transmission stages 2, 3.

[0030] The gear 4 with inward facing teeth meshes with a smaller gear 5 with outward facing teeth, which is rigidly connected to a motor shaft 6 and a rotor 7 of an electric motor 8. The associated stator of the electric motor is denoted by 9. The stator 9 is located in a motor housing 10 of the electric motor 8. In the exemplary embodiments, the electric motor 8 is designed as a brushless DC motor. A connector housing 30 is part of the electric motor 8.

[0031] The motor housing 10 is adjustable in both exemplary embodiments, as will be explained in more detail below, and connected to the transmission housing of the harmonic drive 3, which is denoted by 11.

[0032] The gear 4 with inward facing teeth is an input-side component of the harmonic drive 3. The toothing of the gear 4 is denoted by 33, the toothing of the smaller gear 5 with outward facing teeth is denoted by 32. The gear 4 with inward facing teeth is also designed as an inner ring 12 of a rolling bearing 13, namely a ball bearing. The pitch radius of the toothing 33 of the gear 4 is denoted by TR.

[0033] The rolling bearing 13 is part of a harmonic generator, denoted by 29, of the harmonic drive 3. The outer contour of the inner ring 12 is non-circular, namely elliptical, is shaped and forms a rolling bearing raceway 14 on which balls 15 roll as rolling elements. In contrast to the inner ring 12, the associated outer ring 16 of the rolling bearing 13 is flexible and permanently adapts to the non-circular shape of the inner ring 12.

[0034] The outer ring 16 is surrounded by a cup-shaped elastic transmission element 17 with outward facing teeth, which is referred to as a flexible cup. The external toothing of the transmission element 17, denoted by 18, engages partially, namely at two diametrically opposite points, in an internal toothing 19 which is formed by a toothing ring 20 arranged rigidly in the transmission housing 11. The number of teeth of the external toothing 18 deviates slightly, namely by two, from the number of teeth of the internal toothing 19. This has the effect that a full rotation of the inner ring 12 in relation to the transmission housing 11 is converted into a slight pivoting between the elastic transmission element 17 and the transmission housing 11.

[0035] The transmission housing 11 is rigidly connected to the engine block of the reciprocating piston engine (not shown). The elastic transmission element 17 is connected with the aid of a screw 21 to an eccentric shaft 22, which enables the compression ratio of the reciprocating piston engine to be adjusted in a manner known per se.

[0036] The central axis of the harmonic drive 3 is denoted by A.sub.W and is identical to the central axis of the eccentric shaft 22. The rotational axis of the electric motor 8 denoted by A.sub.E is parallel and spaced apart from the central axis A.sub.W. When installing the actuating gear mechanism 1, the motor housing 10 is positioned relative to the transmission housing 11 in such a way that a defined backlash of the spur gear transmission 2 occurs. In particular, the spur gear transmission 2 can be adjusted without backlash. For this purpose, a centering device 23 is provided which comprises centering contours 24, 25 on the transmission side and on the electric motor side.

[0037] In the exemplary embodiment according to FIGS. 1 and 2, the motor housing 10 of the electric motor 8 is inserted into a receptacle 31 in the transmission housing 11. The gear-side centering contour 24 is thus formed directly by the circular receptacle 31. On sides of the electric motor, the motor housing 10, which likewise has a circular cross section in this area, forms the associated centering contour 25. A pivot axis A.sub.S is given by the axis of symmetry of the receptacle 31 and thus the centering device 23. The effect of a seal denoted by 28 between the housings 10, 11 is practically not dependent on the angular position of the motor housing 10 relative to the transmission housing 11.

[0038] The distance between the pivot axis A.sub.S, about which the motor housing 10 can be rotated within the receptacle 31, and the rotational axis A.sub.E of the electric motor 8 is denoted by EE and is less than the distance denoted by AD between the rotational axis A.sub.E and the central axis A.sub.W of the harmonic drive 3. All axes A.sub.S, A.sub.E, A.sub.W lie, as can be seen from FIG. 2, within the circular cross section of the receptacle 31.

[0039] In the exemplary embodiment according to FIGS. 3 and 4, the centering device 23 has a centering pin 26 which is inserted into the centering contours 24, 25 and couples the motor housing 10 to the transmission housing 11 in such a way that the pivot axis A.sub.S is formed between the transmission housing 11 and the motor housing 10, which in this case is defined by the central axis of the centering pin 26.

[0040] When mounting the actuating gear mechanism 1 according to FIGS. 3 and 4, the motor housing 10 is first secured to the transmission housing 11 and the centering pin 26 is inserted. In this state, there remains at least a slight pivoting capability between the two housings 10, 11 of the actuating gear mechanism 1.

[0041] The centering pin 26 is arranged eccentrically with respect to the rotational axis A.sub.E. The centering pin 26 is also spaced apart from the central axis A.sub.W of the harmonic drive 3. When the motor housing 10 is secured to the transmission housing 11, the spur gear transmission 2 is in principle already functional.

[0042] In this state, the motor housing 10 is rotated about the centering pin 26 until the backlash of the spur gear transmission 2 is removed, i.e., the spur gear transmission 2 is set to block. The motor housing 10 is then turned back by a certain pre-defined angle in order to set a pre-defined backlash of the spur gear transmission 2. In this setting, the motor housing 10 is finally fixed relative to the transmission housing 11. In order to enable fixation in variable positioning, elongated holes 27 are located on the transmission housing 11. Thus, a readjustment of the backlash of the spur gear transmission 2 is possible. In an analogous manner, elongated holes 27 could be located on the motor housing 10 instead of on the transmission housing 11.

[0043] The backlash adjustment of the spur gear transmission formed from the gears 4, 5 is carried out in a corresponding manner in the case of the actuating gear mechanism 1: The motor housing 10 is pivoted as far as possible within the receptacle 31, i.e., up to the stop. Then a pre-defined backlash is set by pivoting the motor housing 10 back about the pivot axis A.sub.S.

LIST OF REFERENCE SYMBOLS

[0044] 1 Actuating gear mechanism

[0045] 2 Spur gear transmission

[0046] 3 Harmonic drive

[0047] 4 Gear with inward facing teeth

[0048] 5 Gear with outward facing teeth

[0049] 6 Motor shaft

[0050] 7 Rotor

[0051] 8 Electric motor

[0052] 9 Stator

[0053] 10 Motor housing

[0054] 11 Transmission housing

[0055] 12 Inner ring

[0056] 13 Rolling bearing

[0057] 14 Rolling bearing raceway

[0058] 15 Ball

[0059] 16 Outer ring

[0060] 17 Elastic transmission element, Flextopf

[0061] 18 External toothing

[0062] 19 Internal toothing

[0063] 20 Gear ring

[0064] 21 Screw

[0065] 22 Eccentric shaft

[0066] 23 Centering device

[0067] 24 Centering contour

[0068] 25 Centering contour

[0069] 26 Centering pin

[0070] 27 Elongated hole

[0071] 28 Seal

[0072] 29 Harmonic generator

[0073] 30 Connector housing

[0074] 31 Receptacle

[0075] 32 Toothing of the gear 5

[0076] 33 Toothing of the gear 4

[0077] AD Distance between the axes A.sub.W and A.sub.S

[0078] A.sub.W Central axis of the harmonic drive

[0079] A.sub.E Rotational axis of the electric motor

[0080] A.sub.S Pivot axis

[0081] EE Distance between the axes A.sub.S and A.sub.E

[0082] TR Pitch radius of the gear 4