GB2496455A

GB2496455A - Harmonic drive having gears with flanges

Info

Publication number: GB2496455A
Application number: GB1119633.4A
Authority: GB
Inventors: Joseph Michael
Original assignee: Individual
Current assignee: Individual
Priority date: 2011-11-14
Filing date: 2011-11-14
Publication date: 2013-05-15
Also published as: GB201409342D0; GB2514926A; GB201119633D0; WO2013072673A1

Abstract

A harmonic drive comprises a plurality of flanged planetary gears 8 driven by a flanged input gear 7 and meshing with inner teeth on a flanged flexible ring gear 9 which has outer teeth that mesh with internal teeth on a flanged output ring gear 10. The output ring gear 10 has external teeth that mesh with a plurality of flanged planetary gears 11 which constrain the output ring gear 10. The drive is made with five layers of sheet material of which the inner most layer is the gears, then two flange layers sandwich the gears, and then another two layers make up a gearbox housing. The flexible gear 9 has a plurality of pins (15, fig 3) to constrain it from rotating in undesirable modes of operation. The harmonic drive gearbox has high holding torque, near zero backlash, minimal construction and is made from five layers of sheet material cut with tools such as laser, stamping machine, water jet, plasma torch or photo etching.

Description

IMPROVED HARMONIC DRIVE

The present invention relates to an improvement in 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.

The present invention adds flanges to involute profiled gears in the harmonic drive to prevent jamming.

The flange diameter is 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.

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.

The present invention is suitable for making harmonic drive with five layers of sheet material. The inner most 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 is advantageously fitted with a plurality of pins to constrain it from rotating in undesirable modes of operation.

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 figures, of which; FIGURE 1 shows basic design of harmonic drive.

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

FIGURE 3 shows construction of a gearbox housing and flexible gear rotation prevention mechanism that uses pins FIGURE 4 shows 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.

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 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 further planetary gear set 6. The gears 6 could alternatively be simple wheels.

Figure 2 shows the improved harmonic drive comprising flanged input gear 7, flanged planetary gear 8, flanged flexible gear 9, flanged output gear 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 falling into the output gear 10 or into the planetary gears 8. In normal use, the flanges 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 simple coupling such as 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 numbers to three gears or more.

Figure 3 shows 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 constrains rotation of flexible gear 9 relative to the gear housing 13.

The gear assembly consisting of 7, 8, 9, 10 and 11 will require 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 requires 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 the flexible gear 9 to move in the radial direction but not 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 unpredictable 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 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.

The flange layer 22 and outer layer 13 may rub when 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 to prevent the flange layer 22 from touching gearbox casing layer 13.

The principle 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. Rough cut gears and flanges used 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 gear in 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.

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 ball raceway between flanged flexible gear and 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 falling into gear 10. The flexible gear 26 is constrained to not rotate by same methods described earlier for flexible gear 9.

FIGURE 6 shows construction of a wave generator variant with wheels between flanged flexible gear and 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 wheel 28 to sit inside gear 30. The slot functions as a guide and a support means for wheel 28 and wave plate 27 as it rotates. The wheels 28 needs to be 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 thickness that of flange layer, while the wheel can be made with thickness that of gear layer.

The wave plate and wheel comprise a three layer mechanism which is suitable for incorporating into the five layer harmonic drive design.

Claims

<claim-text>CLAIMS1. A harmonic drive constructed from a concentric gear assembly comprising of input gear at the centre which connects to a plurality of planetary gears that which then connects to a flanged flexible ring gear with internal and external teeth, that which then connects to a flanged ring gear.</claim-text> <claim-text>2. A machine according to claim 1 wherein the flexible ring gear is constrained to prevent its rotation about its axis of rotation.</claim-text> <claim-text>3. A machine according to claims 1 and 2 wherein the plurality of planetary gears are flanged.</claim-text> <claim-text>4. A machine according to claims 1 and 2 wherein the input gear is flanged.</claim-text> <claim-text>5. A machine according to claims 1 and 2 wherein the constraint is of a pin form.</claim-text> <claim-text>6. A machine according to claims 1, 2 and 5 wherein the pin forms are constrained to slide in radial slots cut into the gear housing.</claim-text> <claim-text>7. A machine according to claims 1, 2, 5 and 6 wherein the pin forms are advantageously radially pushed or pulled by spring mechanism substantially inward to the centre of the gearbox.</claim-text> <claim-text>8. A machine according to claim 1 and 2 wherein the entire mechanism is composed substantially of five layers of flat material, starting with 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 gearbox.</claim-text> <claim-text>9. A machine according to claims 1 and 2 wherein a plurality of slots are cut into the flexible gear and its flange ring to improve the flexing properties of the gear.</claim-text> <claim-text>10. A machine according to claims 1 and 2 wherein the teeth height of one or more gears has been changed from its nominal value to reduce gear jam.</claim-text> <claim-text>11. A machine substantially as claimed in 1 and 2 wherein the wave generator comprising input gear and connecting planetary gears is substituted for a wave generator that contains an oval race track filled with balls engaging the flexible gear which has an opposing race track.</claim-text> <claim-text>12. A machine substantially as claimed in 11 wherein the ball and race in the wave generator is substituted with a plurality of wheels and a guide slot on flexible gear to constrain the wheels.</claim-text> <claim-text>13. A machine substantially as claimed in ito 12 which has been constrained by a set of planetary gears.</claim-text> <claim-text>14. A machine substantially as claimed in ito 13 which has been cut with a slot form at the centre to receive mechanical power from a rotating axle.</claim-text>