Shaft drive

Abstract

Wave gear (1), comprising a wave generator (8), a flexible, externally toothed gear component (14) deformable by the wave generator, and at least one internally toothed gear component (4, 5) meshing with the flexible, externally toothed gear component (14), characterized in that the flexible, externally toothed gear component (14) is designed as a flex ring or a flex pot and has a non-circular, convex basic shape with respect to its mechanically unloaded state.

Description

[0001] The invention relates to a wave gear which has a wave generator and a flexible externally toothed gear component in the form of a flex ring or flex pot which can be deformed by the wave generator, wherein the flexible, externally toothed gear component meshes with at least one internally toothed, typically rigid gear component.

[0002] Such a wave gear is known, for example, from DE 10 2014 202 060 A1. It is a component of an electric camshaft adjuster. An adjusting shaft, which drives a wave generator of the wave gear, has several preferred positions.

[0003] Another electric camshaft adjuster, which also operates with a wave gear, is disclosed, for example, in DE 10 2007 049 072 A1. The wave gear is actuated by an adjustment motor via a compensating clutch, namely an Oldham clutch. DE 101 22 660 A1 shows a wave gear with a flexible sleeve that is not rigid but sags partially due to gravity.

[0004] DE 10 2006 042 786 A1 discloses a hollow shaft transmission with a plurality of movable tooth segments. DE 10 2013 216 182 A1 and DE 10 2009 053 728 A1 disclose further shaft transmissions. DE 10 2011 004 070 A1 describes a camshaft adjuster whose rotation angle is limited by stops. According to DE 102 07 760 A1, a brake is provided in a camshaft adjuster, which, for example, allows a startable emergency running position to be reached in the event of a malfunction.

[0005] The invention is based on the objective of providing a wave gear that is further developed compared to the aforementioned prior art and is particularly suitable for use in an electric camshaft adjuster and in a device for adjusting the compression ratio of an internal combustion engine.

[0006] This problem is solved according to the invention by a wave gear with the features of claim 1. This wave gear has, in a basic structure known per se, a wave generator and a flexible, externally toothed gear component, which is deformable by the wave generator and which is a flex ring or a flex cup. In each case, the flexible, externally toothed gear component meshes with at least one internally toothed gear component, which can also have a ring shape or a cup shape. The flexible, externally toothed gear component differs from the prior art in that it has a non-circular, convex basic shape when in its mechanically unloaded state.Unlike flexible rings or flexible pots, which are circular when mechanically unloaded, the spring energy stored in the flexible, externally toothed gear component of the wave gear according to the invention is subject to constant fluctuations during operation of the wave generator. These fluctuations in the stored spring energy are suitable, as will be explained in more detail below, for improving the controllability of the wave gear. The convex shape ensures that the externally toothed gear component can actually store the spring energy without local overloading. Furthermore, such a ring-shaped, closed gear component is easy to manufacture.

[0007] The stored spring energy of a flexible gear component designed according to the invention and installed in a wave gear is thus dependent on the angular position. The flexible gear component therefore tends to rotate into certain preferred positions. This property generates a snap torque. According to one embodiment of the invention, the snap torque is used to hold the wave generator of the wave gear in a preferred position. This reduces the energy required to hold a specific angular position because the self-aligning torques of the system are utilized. If necessary, at a high gear ratio, the snap torque may be sufficient to generate adequate self-locking of the gear, such that no additional holding torque needs to be applied via the wave generator. The externally required energy then decreases the longer the system is operated in constant angular positions.

[0008] In one embodiment, the flexible, externally toothed gear component has the shape of a polygon. The corners of the polygon are generally rounded to keep the stresses in the material below a required threshold. In a preferred embodiment, the polygon is a regular polygon.

[0009] In a preferred embodiment, the flexible, externally toothed gear component has an elliptical basic shape, such that the spring energy stored in the gear component exhibits two minima and two maxima during one full revolution of the wave generator. As long as no external force acts on the flexible, externally toothed gear component, it thus has a shape similar to that of a rigid, elliptical gear component of the wave generator. The rigid, elliptical component can, in particular, be an inner ring of a rolling bearing, especially a ball bearing. The ratio between the semi-axes of the rigid elliptical component of the wave generator is not necessarily identical to the ratio of the semi-axes of the flexible, also elliptical, gear component.

[0010] Preferably, the flexible elliptical gear component is designed such that it always remains in a mechanically stressed state during rotation of the wave generator, the degree of mechanical stress and thus the stored spring energy depending on the angular position of an adjusting shaft that drives the wave generator. The spring energy stored in the flexible, externally toothed gear component is minimal when the major semi-axis of this gear component, relative to the shape of the gear component in its uninstalled state (i.e., not deformed by the wave generator), coincides with the major semi-axis of the rigid elliptical component of the wave generator. Similarly, the spring energy stored in the flexible, externally toothed gear component is maximal when its major semi-axis is aligned with the minor semi-axis of the rigid elliptical component of the wave generator.If the major semi-axles, and thus also the minor semi-axles, of the flexible externally toothed gear component, in particular the flex ring, on the one hand, and of the rigid elliptical component, in particular the inner ring, of the shaft generator, on the other hand, are identically aligned, then a stable equilibrium exists. However, in the 90° rotated state of the shaft generator, where one major semi-axle and one minor semi-axle of the two aforementioned components coincide, an unstable equilibrium exists.

[0011] The gear ratio of the wave gear is preferably selected such that the transition between one stable equilibrium state and the next stable equilibrium state occurs within a required control quality, specified, for example, in degrees or arcminutes. The gear ratio of the wave gear results from the differing numbers of teeth on the external teeth of the flexible gear component and the internal teeth of the meshing, rigid gear component. Typically, the number of teeth on the external teeth is two fewer than the number of teeth on the meshing internal teeth.

[0012] For example, the flexible external gear component has 180 teeth, while the rigid internal gear component has 182 teeth. With one full rotation of the wave generator, the rigid internal gear component is rotated by two teeth, corresponding to a gear ratio of 90:1. Since the angular distance between two teeth in this case is two degrees, the rotation of two teeth is equivalent to a rotation of 4°. A 90° adjustment of the wave generator's adjustment shaft thus results in a 1° adjustment of the rigid internal gear component. The rigid internal gear component is either identical to or rigidly connected to an output element of the wave generator.If this output element is to be adjusted with an accuracy of ± 1°, then, given a gear ratio of 90:1 for the wave gear, a stable state must be achieved within an interval of ± 90° of the wave gear's adjustment shaft. At least one such stable state exists within this ± 90° interval, relative to the wave generator's adjustment shaft and thus also to the circumference of the flexible, externally toothed gear component.

[0013] If an adjustment accuracy of ± 1.5° for the output element of the wave gear is required, a gear ratio of 60:1 is sufficient to achieve any desired angular position of the output element with a minimum of the spring energy stored in the flexible gear component.

[0014] In general, the following relationship exists between the transmission ratio of the wave gear, denoted by l, and the required control quality r, expressed in degrees, with reference to the angular position of the output shaft: l≥90 / r

[0015] The energy minima of the flexible gear component correspond to preferred positions of the wave gear, which is preferably used as an actuator. In an advantageous embodiment, the spring preload of the non-circular, flexible gear component is strong enough to cause automatic movement to the preferred positions, also known as snapping. Compared to conventional wave gears, the preferred positions can be held not only precisely, but also with particularly low energy expenditure, in particular without any energy expenditure at all.

[0016] In a preferred embodiment, the difference between the maximum and minimum spring energy stored in the flexible, externally toothed gear component during operation of the wave gear is greater than the minimum spring energy stored in the flexible gear component. The wave gear is preferably designed as a self-locking gear, as is the case with conventional wave gears. This means that the adjustment shaft of the wave generator cannot be moved by rotating the output element of the wave gear. This assumes that another shaft, typically fixed to the housing, of the wave gear, which is fundamentally designed as a three-shaft gear, is locked in place.In actual operation of the wave gear, either two shafts rotate—namely, the adjusting shaft, which actuates the wave generator, and the output shaft—or all three shafts rotate, in which case the output shaft rotates at the speed of the other shaft as long as the adjusting shaft also rotates at that speed. The first case (exactly two rotating shafts) occurs, for example, in a device for adjusting the compression ratio of a reciprocating engine, while the second case is found particularly in an electric camshaft adjuster.

[0017] In general, the wave gear is suitable for fixing an output shaft with minimal effort regarding the torque to be introduced into an adjusting shaft. For the sake of simplicity, self-locking effects are not considered in the following: A gear ratio of 50:1 is assumed for the wave gear. The output shaft is to be fixed with a holding torque of 50 Nm. In this case, a slight out-of-roundness of the flexible, externally toothed gear component, which provides a snap torque of at least 1 Nm, is sufficient for reliably fixing the output shaft. An adjustment motor does not need to generate any torque. In fact, due to the inherent self-locking properties of the wave gear, even a lower snap torque of the flexible, externally toothed gear component is sufficient.

[0018] An embodiment of the invention is explained in more detail below with reference to a drawing. The drawing shows: Fig. 1 a wave gear in a symbolic sectional view, Fig. 2 states of a flexible gear component of the wave gear, Fig. 3 in a diagram: Changes in state of the flexible gear component.

[0019] In Fig. Figure 1 outlines the structure of a wave gear unit designated as reference numeral 1, with regard to its basic function referenced to the prior art cited at the beginning.

[0020] The wave gear 1 has a housing 2 which is rigidly connected to a drive wheel 3. The drive wheel 3 can be driven, for example, by means of a traction element or a gear. A drive gear 4, which has internal teeth, is also rigidly connected to the housing 2 as a gear component. Next to the drive gear 4 is an output component 5, which also has internal teeth and is rigidly coupled to an output shaft 7 via an output disc 6.

[0021] Radially within the two internally toothed gear components 4, 5 is a wave generator 8, which is driven by an adjusting shaft 9. The adjusting shaft 9 is coupled via a compensating clutch 10 to an adjusting motor (not shown), in particular an electric motor. An inner ring 11 of the wave generator 8 has a rigid, elliptical shape. A flexible outer ring 12 of the wave generator 8 continuously adapts to the elliptical shape of the inner ring 11 during rotation of the adjusting shaft 9, with balls 13 rolling elements between the inner ring 11 and the outer ring 12.

[0022] A flexible, externally toothed gear component 14, namely a flex ring, is positioned directly around the outer ring 12. During rotation of the adjusting shaft 9 of the wave generator 8, the flex ring 14 permanently assumes the shape of the outer ring 12. In doing so, the external teeth of the flex ring 14 engage with the internal teeth of the gear components 4 and 5 at two diametrically opposed points. Due to the slightly different number of teeth on the aforementioned gear components 4, 5, and 14, a high gear ratio of the wave gear 1 is achieved, in this case a gear ratio of 90:1.

[0023] In Fig. Figure 2 shows the shape of the flex ring 14 in its mechanically unloaded state, i.e., not installed in the wave gear 1. A first outer dimension d1 is given along the major semi-axes, and a second outer dimension d2 along the minor semi-axes. Circles tangent to the flex ring 14 at the corresponding points are designated as outer circles K. a and as inner circle K i The difference between the semi-axes of the flex ring 14, denoted by x0, corresponds to the difference between the radius of the outer circle K. a and the radius of the inner circle K i and is also referred to as roundness deviation.

[0024] In Fig. 3 is denoted by α as the angle of the adjusting shaft 9 and by β as the angle of the output shaft 7. α0 is a target angle of the adjusting shaft 9 and β0 is a target angle of the output shaft 7. The energy E, given in joules, is the spring energy stored in the flex ring 14, which depends on the angular position of the adjusting shaft 9. As shown from Fig. As can be seen from Figure 3, the spring energy stored in the flex ring 14 fluctuates sinusoidally between a minimum value E(dw_g-d1) and a maximum value E(dw_g-d2) during the rotation of the adjusting shaft 9. The minimum energy E(dw_g-d1) is stored in the flex ring 14 when the major semi-axis of the elliptical inner ring 11 is parallel to the major semi-axis of the flex ring 14, relative to its in Fig. The 2 sketched state is aligned.

[0025] States of minimum energy are designated A1 and A2, and a state of maximum energy is designated B. In state B, the flex ring 14 is maximally deformed compared to its mechanically unloaded state. The transition from state B to one of the lower-energy states A1 or A2 is equivalent to an adjustment of the adjusting shaft 9 by ± 90° and an adjustment of the output shaft 7 by ± 1°. With a required adjustment accuracy, i.e., control quality, of ± 1° for the output shaft 7, a state of minimum energy A1 or A2 can always be reached, with the transition being possible solely due to the snap torque of the flex ring 14. In state A1 or A2, the wave gear 1 can be held with minimal energy expenditure, even without applying torque to the adjusting shaft 9. Reference symbol list 1 wave gear 2 Drive element, housing 3 drive wheel 4 Drive gear, internally toothed gear component 5 Output gear, internally toothed gear component 6 Output disc 7 Output shaft 8-wave generator 9 Adjustment shaft 10 compensating clutch 11 Inner ring 12 Outer ring 13 rolling elements 14 flexible, externally toothed gear component, flex ring, flex pot α Angle of the adjusting shaft β Angle of the output shaft α0 Target angle of the adjustment shaft β0 Target angle of the output shaft A1,A2 states of minimum energy B state of maximum energy d1,d2 External dimensions E stored spring energy E(dw_g-d1) Minimum value of energy E(dw_g-d2) Maximum value of energy l Translation ratio of the wave gear K a outer circle K i inner circle r standard quality (in degrees) x0 Roundness deviation, difference of the semi-axes

Claims

[1] Wave gear (1), comprising a wave generator (8), a flexible, externally toothed gear component (14) deformable by the wave generator, and at least one internally toothed gear component (4, 5) meshing with the flexible, externally toothed gear component (14), characterized by , that the flexible, externally toothed gear component (14) is designed as a flex ring or a flex pot and has a non-circular, convex basic shape with respect to its mechanically unloaded state. [2] Wave gear (1) according to claim 1, characterized by , that the flexible, externally toothed gear component (14) has an elliptical basic shape, wherein the spring energy (E) stored in this gear component (14), with reference to one full revolution of the shaft generator (8), has two minima E(dw_g-d1) and two maxima E(dw_g-d2). [3] Wave gear (1) according to claim 2, characterized by, that the amount of the difference between the number of teeth of the external teeth of the flexible gear component (14) and the number of teeth of the internal teeth of the internally toothed gear component (4,5) corresponds to at most 1 / 60, in particular at most 1 / 90, of the number of teeth of the external teeth of the flexible gear component (14). [4] Wave gear (1) according to claim 2 or 3, characterized by , that the minimum spring energy E(dw_g-d1) stored in the flexible, externally toothed gear component (14) is less than half of the maximum spring energy E(dw_g-d2) stored in this gear component (14). [5] Wave gear (1) according to claim 1, characterized by , that the flexible, externally toothed gear component (14) has the basic shape of a polygon with rounded corners. [6] Wave gear (1) according to claim 1, characterized by , that one of two internally toothed gear components (4,5) is designed as an output gear (5). [7] Use of a wave gear (1) according to claim 1 as an actuating gear of an electric camshaft adjuster or a device for adjusting the compression ratio of an internal combustion engine. [8] Method for operating a wave gear (1) according to claim 1, comprising a flexible, externally toothed gear component (14) deformable by a wave generator (8) and at least one internally toothed gear component (4, 5) meshing with the flexible, externally toothed gear component (14), wherein the flexible, externally toothed gear component (14) has a non-circular, convex basic shape with respect to its mechanically unloaded state, whereby, during a period in which no relative adjustment is to take place, the flexible, externally toothed gear component (14) is predominantly adjusted into a preferred position in which the holding torque to be applied as a result of the self-holding torque for a drive of the wave generator (8) is reduced.

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