The present invention relates to an improvement in a harmonic drive mechanism.

The advantage of harmonic drive gearbox is high holding torque and near zero backlash. Harmonic drives incorporate a flexible gear that is key to its operation. However, the flexible gear is prone to jamming.

In some designs, the flexible gear is mounted to a flexible cup form to prevent jamming. In other designs, the flexible gear is unconstrained and a chain and sprocket tooth profile is used to prevent teeth from jamming.

It is an object of the invention to provide an

improved harmonic drive.

In essence, embodiments of the present invention add flanges to involute profiled gears in the harmonic drive to prevent jamming.

According to the invention there is provided a

harmonic drive constructed from a concentric gear assembly comprising of input gear at the centre which connects to one or more planetary gears which then connect to a flanged flexible ring gear with internal and external teeth, which then connects to a flanged ring gear.

The input gear may connect to a plurality of planetary gears which then connect to the flanged flexible ring gear.

The flexible ring gear may be constrained to prevent its rotation about its axis of rotation. The constraint may be of a pin form. The pin form constraint may comprise pins that are constrained to slide in radial slots cut into a gear housing. The pins may be radially pushed or pulled by a spring mechanism substantially inward towards the centre of the gear assembly. The one or more planetary gears may be flanged.

The input gear may be flanged.

The flange diameter may be set to the pitch circle diameter of the involute gear tooth profile. This permits flanges of a gear to touch and rotate in synchronisation with opposing gear flanges without rubbing and wearing away the flanges. The said flanges maintain a separation

distance between flexible gear teeth and rigid gear teeth preventing the flexible gear from jamming.

Flanges may be provided on one face only of the gears, but it is preferred to provide flanges on both of the opposite faces as in the embodiments of the invention described below.

Tooth height of flexible gear and gears that contact the flexible gear is adjusted advantageously to reduce jamming. Some adjustment of the tooth width of flexible gear and gears that contact the flexible gear is

advantageous for reducing jamming at the expense of

increased wear rate of the teeth. Thus the teeth height of one or more gears may differ from its nominal ideal value to reduce gear jam.

The present invention is suitable for making harmonic drive with five layers of sheet material. The entire mechanism may be composed substantially of five layers of flat material, starting with a gear layer in the centre, followed by a pair of flange layers on either side of the gear layer, followed by a pair of gear housing layers on either side of the gear assembly. Thus, the innermost layer is the gear layer, two flange layers sandwich the gear layer, and another two layers on either side make up the gearbox housing. For the said five layer design, the flexible gear may advantageously be fitted with a plurality of pins to constrain it from rotating in undesirable modes of operation.

A plurality of slots may be cut into the flanged flexible ring gear to improve the flexing properties of the gear .

The wave generator comprising input gear and

connecting planetary gears may be substituted with a wave generator that contains an oval race track filled with balls engaging the flexible gear which has an opposing race track. The ball and race in the wave generator may be substituted with a plurality of wheels and a guide slot on the flexible gear to constrain the wheels.

The drive may be constrained by a set of planetary gears. The planetary gears may be around the outer

circumference of the flanged ring gear.

The drive may be cut with a slot form at the centre to receive mechanical power from a rotating axle.

According to the invention there is further provided a harmonic drive comprising a wave generator that engages an inner circumferential part of a flexible ring gear carrying teeth around an outer circumference which engage teeth around an inner circumferential part of an outer ring gear, characterized in that flanges on the interengaging toothed portions of the flexible ring gear and the outer ring gear limit the movement of the interengaging toothed portions towards one another.

The wave generator may comprise a central input gear and one or more planetary gears between the central input gear and the flexible ring gear. Alternatively, the wave generator may comprise a non-circular member rotatable about the axis of the outer ring gear and engaging the inner circumferential part of the flexible ring gear.

There will be described, by way of example only, materials according to the present invention, and methods according to the present invention, of constructing an improved harmonic drive.

References will be made to the following schematic figures, of which;

FIGURE 1 shows a basic design of a known harmonic drive,

FIGURE 2 shows a design of improved harmonic drive constructed with flanged gears,

FIGURE 3 shows a construction of a gearbox housing and flexible gear rotation prevention mechanism that uses pins, FIGURE 4 shows a side view of a five layer sandwich cross section for a flat harmonic drive design,

FIGURE 5 shows construction of a wave generator variant with ball raceway between flanged flexible gear and wave generator, and

FIGURE 6 shows construction of a wave generator variant with wheels between flanged flexible gear and wave generator .

Referring to the accompanying drawings, and initially to Figure 1, a standard harmonic drive construction is shown in Figure 1, wherein a central input gear 2 is connected to planetary gears 3 that then connect to a flexible gear 4, which then connects to a rigid output gear 5. Numerous ways to constrain output gear 5 are known and one such mechanism is the use of a further planetary gear set 6. The gears 6 could alternatively be simple wheels . Figure 2 shows an improved harmonic drive comprising flanged input gear 7, flanged planetary gear 8, flanged flexible gear 9, flanged output gear 10 and flanged

planetary gears 11 that constrain flanged output gear 10. The flanges in flexible gear 9 are also flexible. The flanges are of the same diameter as the pitch circle diameter of the gear teeth which allows them to rotate in synchronisation with other gears without rubbing and wearing down the flanges.

The flanges advantageously prevent teeth from flexible gear 9 penetrating too far into the output gear 10 or into the planetary gears 8. In normal use, the flanges, which in the illustrated embodiment are provided on both sides of the gears, also prevent the gear assembly comprising 7, 8, 9 and 10 from being taken apart after construction. For this reason the flanges have to be removable in some of the gears to allow the gearbox to be assembled from components.

Larger and thinner gear designs allow the entire assembly to become flexible. The said flexible assembly could be snapped together by bending the gears and slotting them together without having to remove the flanges.

The gear assembly 7, 8, 9 and 10 made with flanged gears form a self supporting interlocked assembly. With the said interlocked assembly, a way to constrain output gear 10, without use of planetary gears 11, is to put an axle through gear 7 and then mount the axle to the gearbox housing .

With planetary gears 11 fitted, the power to the input gear 7 can be from a simple coupling such as a slot 12 milled into input gear 7.

The planetary gears 8 can be reduced in number to one gear if gear 7 is supported in its central position by means of supports such as an axle. For increased

stability, the planetary gears 8 can be increased in number to three gears or more.

Figure 3 shows a construction of a gearbox housing and flexible gear rotation prevention mechanism that uses pins.

Torque on the output gear 10 could potentially drive the entire assembly consisting of 8, 9, and 10, around gear 7. This can be overcome by using a plurality of rotation inhibitor pins 15 that constrain rotation of flexible gear 9 relative to the gear housing 13. The gear assembly consisting of 7, 8, 9, 10 and 11 has in this case a pair of gear housing plates such as 13 on either side of the gearbox. The prevention of rotation of gear assembly 8, 9, and 10 around gear 7 is achieved by insertion of pins 15 through the housing 13 and through holes 16 in the flexible gear 9. The pins 15 protrude into the gearbox housing 13 through linear slots 17 radially cut to allow portions of the flexible gear 9 to move in the radial direction as the gear changes shape, but not to rotate. When gear wheels 8 turn and push at the flexible gear 9, the radial slots allow the flexible gear 9 to flex radially as guided by pin 15 sliding in slot 17. With a plurality of pins 15 in place to constrain flexible gear 9, the flexible gear 9 cannot rotate in normal operation with application of torque on the input gear 7 or on the output gear 10.

The flexible gear 9 can stretch and jam in unpredict ^¬ able ways if it cannot pull itself into an oval shape to feed the teeth gradually into the points where the teeth from 9 mesh with gears 8 and 10. To assist with oval shape retention, the flexible gear 9 can be engineered with suitable materials such as plastic of the correct stiffness to pull itself into a smaller shape. An alternative is to mount springs between pins 15 and body 13 such that the pin is pulled or pushed radially inward to the centre of the gearbox so that the flexible gear can take the shape of an oval advantageous for better meshing of teeth.

Figure 4 shows a side view of a five layer sandwich cross section of a harmonic drive constructed from flat parts. Viewing the sandwich edge wise, the two outer gear housing layers 13 are rigidly held apart by a plurality of spacers 21. There are two flange layers 22 and one gear layer 23 that make up a five layer harmonic drive. It will be understood that for example the gear layer 23 shown in side view in Figure 4 may comprise the gears 7,8,9 and 10 of Figure 2.

The flange layer 22 and the outer layer 13 may rub when the gearbox is running. The friction can be reduced by using dissimilar bearing materials between the two

surfaces. The two layers can also be prevented from

touching by constructing the planetary gears 11 and its bearing and axle assembly such that the gear is constrained in central position between the two plates 13. The flanges then constrain the other gears to prevent the flange layer 22 from touching gearbox casing layer 13.

In Figure 4, which is schematic, the flange layers 22 are shown spaced from the gear layer 23. Usually the flange layers will be in contact with the gear layer 13 with flanges of gears fixed to them. It is, however, possible for the flanges to be separate and indeed there may even be some freedom of movement of a flange relative to an associated gear; for example a gear flange may be rotatably coupled to rotate with the gear but free for some limited axial movement relative to the gear. The principal gearbox components can be rough cut from sheet material through numerous tools such as a laser cutter, stamping machine, plasma torch, water jet, or a photo etching system. With flanges fitted, the rough cut gears are less likely to jam. Use of rough cut gears and flanges together reduce machining steps needed to make a working harmonic drive.

The flange ring for the flexible gear 9 could make the flexible gear too stiff to be useful. Reducing the flange width is one possible way to solve the problem. Increasing the number of teeth in the flexible gear and reducing gear teeth travel is a second way to solve the problem. Reducing the thickness of the flange and gear is a third way to solve the problem.

The stiffness of gear 9 can also be reduced by cutting numerous holes 20 through the flange and gear. Slots and other geometric shapes such as laser cut arc slits or spiral segment slits will also work.

The involute gear tooth profile height of gear teeth 18 and 19 of flexible gear 9 can be varied to make the gears more readily mesh without jamming. The involute tooth profile height in gears 8 and 10 can also be varied to reduce likelihood of jamming. Widths of gear teeth 18 and 19, and of teeth in gears 8 and internal teeth in gear 10 can be adjusted to a limited extent to reduce jamming at the expense of increased teeth wear.

The involute tooth profile pressure angles are correct for teeth mounted on a circular form. The tooth profile can be modified to take into consideration the oval shape of flexible gear to maintain the pressure angle. The tooth can also be reshaped to reduce rubbing action taking account of the oval form on which they are mounted. In the drives described above, gears 7 and 8 form the wave generator mechanism of the harmonic drive. Alternative wave generator designs are possible.

Figure 5 shows construction of a wave generator variant with a ball raceway between a flanged flexible gear and the wave generator. The wave generator 24 has race tracks cut into it to house balls 25, one of which is shown in outline form in Figure 5. The race track is also cut into the flexible gear 26 such that the balls 25 are constrained between the race tracks. In normal ball bearings with a race track, there are also separating elements that keep the balls apart inside the race track. Separating elements that are flexible are fitted in the said wave plate generator (not shown) . The oval race track is constructed such that the balls do not fall out during normal operation. The flanges on flexible gear 26 prevent its teeth from penetrating too far into gear 10. The flexible gear 26 is constrained to not rotate by the same methods as described earlier for flexible gear 9.

FIGURE 6 shows a construction of a wave generator variant with wheels between a flanged flexible gear and a wave generator. The wheels 28 are fitted to the wave plate 27. The axles 29 for the wheels 28 are retained by the wave plate 27. There is a slot milled into the flexible gear 30 to allow the wheel 28 to sit inside the gear 30. The slot functions as a guide and a support means for the wheel 28 and the wave plate 27 as it rotates. The wheels 28 are of a bearing material that does not wear down when it turns and rubs against wave plate 27 and flexible gear 30.

The wave plate can be made from two plates with a thickness of that of the combined flange and gear layers, while the wheel can be made with a thickness of that of the gear layer. The wave plate and wheel comprise a three layer mechanism which is suitable for incorporating into the five layer harmonic drive design

The improved harmonic drives described above have advantages of high holding torque, near zero backlash, and in minimal construction, are made from five layers of sheet material cut with tools such as laser, stamping machine, water jet, plasma torch or photo etching.

Where in the foregoing description, integers or elements are mentioned which have known, obvious or

foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be

appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims.