GB2064685A - Friction Gears - Google Patents

Abstract

A harmonic drive friction gear comprises a cam-shaped body of rotation 3 connected to a drive shaft (1), and a rolling bearing having a resiliently deformable outer race 9 and inner race 8, with the inner race 8 resting against the body of rotation. A hollow deformable inner cylinder 11 connected to a driven shaft is disposed concentrically with the outer race 9 and lies against the outer race. A non-rotatable, but deformable, hollow outer cylinder 12 is disposed concentrically with the inner cylinder 11 and lies against its outer surface, such that the inner cylinder 11 presses against the outer cylinder in the cam region of the body of rotation 3. <IMAGE>

Description

SPECIFICATION Friction Gears The present invention relates to friction gears.

Friction gears are frequently used for medium step-up ratios, where only moderate requirements are set in terms of efficiency and life. In certain previously proposed constructions, cylindrical roilers are pressed against each other, one side being used as the drive wheel and the other side as the driven wheel. The type of construction mostly used is one in which the contact pressure automatically rises as the torque increases.

Planetary gearing in friction gear form has also been proposed. Tapered-roller gears and similar constructions are used for speed reduction. For high step-up ratios, friction gears are used which operate in accordance with the harmonic drive principle, wherein a conical thin-walled cylinder is elliptically deformed and pressed against a thickwalled cylindrical ring, which is chamfered conically at the contact position. This construction is also provided with conical annular grooves in order to increase the friction contact areas.

The drawback of all these previously proposed constructions is that only line contact is set up at the friction zone, and thus high Hertzian pressures arise which make high-quality steels necessary.

On the other hand, high-quaiity steels have low friction factors, and this naturally prejudices the efficiency and overall size of the friction gear.

According to the present invention, there is provided in a friction gear, a drive shaft, a camshaped body of rotation connected to the drive shaft, a rolling bearing having a resiliently deformable outer race and inner race, the inner race resting against the body of rotation, a driven shaft, a hollow inner cylinder connected to the driven shaft, the inner cylinder being disposed concentrically with the outer race and lying against the outer race, and the inner cylinder being deformable, and a non-rotatable, but deformable, hollow outer cylinder disposed concentrically with the inner cylinder and lying against its outer surface, such that the inner cylinder presses against the outer cylinder in the cam region of the body of rotation.

Preferably, the hollow outer cylinder is disposed in a cup-shaped housing, in which one of the bearings for the drive shaft is provided, the other bearing thereof being disposed in a recess in the driven shaft.

The pressing action between the two hollow cylinders arises due to the elastic force of the outer cylinder. This force can be determined by means of a simple equation. In this, the.diameter of the two hollow cylinders, their wall thicknesses and the deformation play a deciding role. The step-up ratio is calculated from the difference between the diameters of the two hollow cylinders, and a wall thickness factor which takes account of the non-circuiar deformation of the two hollow cylinders, consideration being given to the fact that the outer strand of the hollow inner cylinder is stretched whereas the inner strand of the hollow cylinder is compressed. By this means, the two surfaces are repelled from each other as the body of rotation rotates, and in this manner a relative rotation occurs between the hollow inner cylinder and the stationary hollow outer cylinder.

The direction of rotation depends in this case on the ratio of the wall thicknesses, while the step-up ratio depends on the wall thickness ratio and the degree of deformation. Tests have shown that even thin-walled hollow cylinders pressed into each other slide relative to each other during non-circular rotary deformation. Thus, by continuously adjusting the deformation, a continuously variable gear for high step-up ratios can be obtained. The continuous adjustment of the deformation can be easily attained by providing devices which provide an infinitely variable adjustment of the distance between the cams of the body of rotation.

Surface contact can also be attained if one of the two cylinders is constructed with a larger wall thickness, so that it substantially maintains its circular form, the thin-walled inner or outer hollow cylinder being so deformed that the radii of deformation coincide with the centre of the rigid hollow cylinder. The wall thickness factor is then approximately zero, so that the step-up ratio can be easily calculated.

Further according to the invention, there is provided in a friction gear, a rotatable drive member, a rotatable driven member, an oval body fast for rotation with one of said members, a resiliently deformable cylinder fast for rotation with the other of said members, said cylinder surrounding the oval body, rolling bearing means interposed between the oval body and the cylinder, said bearing means being resiliently deformable, and a resiliently deformable stationary cylinder surrounding the rotatable cylinder, said rotatable cylinder pressing against the stationary cylinder in zones adjacent to the major axis of the oval body.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a vertical section through a friction gear according to the invention; Figure 2 is a section along line Il-Il of Figure 1; Figure 3 is a representation corresponding to Figure 2, but showing only the two hollow cylinders in their deformed state; Figures 4 and 5 are schematic sections through the two hollow cylinders in the nondeformed and deformed state, respectively; Figure 6 is a vertical section, to a reduced scale, showing a modified form of the hollow inner cylinder in its non-deformed state; Figure 7 is a fragmentary section showing the hollow inner cylinder of Figure 6 in its deformed state; and Figures 8 and 9 are fragmentary sections showing further modified forms of the hollow inner cylinder.

The reference numeral 1 (Figure 1) indicates the drive shaft of the friction gear and 2 indicates the driven shaft. The drive shaft 2 is here connected to a cam-shaped, or oval, body of rotation 3, which in the illustrated embodiment consists of two parts 4, 5 in the form of cams which can be adjusted relative to each other in a radial direction, and are supported in opposite directions. In the illustrated embodiment, the adjustment mechanism is in the form of a screw unit provided with two screws 6, 7, one of which comprises a left-hand thread and the other a right-hand thread. The two screws 6, 7 can be screwed at one end into a thickend part I a of the drive shaft 1. At their other end, the two screws 6, 7 rest against the parts 4, 5 of the body of rotation 3.By correspondingly turning the two screws 6, 7, the two parts 4, 5 of the body of rotation 3 can be moved either towards or away from each other. It is clear that for this purpose another type of adjustment mechanism, for example, automatic, can be used which can be operated not only during shut-downs, but also during the operation of the body of rotation 3, and thus during the operation of the friction gear.

As can be seen from Figure 2, each part 4, 5 has a contour in the form of a circular segment, the centre of each circular segment lying on the longitudinal axis of the drive shaft 1.

A rolling bearing with a resiliently deformable inner race 8 and a resiliently deformable outer race 9 is provided, with rolling bodies 10 being disposed therebetween. As is clear from Figures 1 and 2 of the drawing, the resiliently deformable inner race 8 rests against the two parts 4, 5 of the body of rotation 3.

A hollow inner cylinder lilies against the outer race 9 of the rolling bearing and concentric therewith, it being made deformable by suitably choosing its material or its construction. A nonrotatable, but deformable, hollow outer cylinder 1 2 is disposed concentrically to the hollow inner cylinder 11 and lying against its outer surface, such that the hollow inner cylinder 11 is pressed against the hollow outer cylinder 12 in the cam region of the body of rotation 3.

In the embodiment shown in Figure 1, the hollow outer cylinder 12 is disposed in a cupshaped housing 13, in which one bearing 14 is of the drive shaft is provided. A second bearing 1 5 of the drive shaft is disposed in a recess 1 6 in the drive shaft 2.

As shown in Figure 1 of the drawing, recesses 17 are provided in the hollow inner cylinder 11 in order to increase its deformability.

Figure 2 of the drawing shows clearly the large-area contact between the two hollow cylinders 11 and 12 over the angle 56. By this means, the contact pressure is reduced to a minimum in comparison with line pressure.

Figure 3 is a simplified cross-section through the two hollow cylinders 11, 12 when in their deformed state. The reference numeral 18 indicates an imaginary enveloping circle which enables the deformation to be seen more clearly.

The body of rotation 3 has not been shown in order to improve visibility. It can be seen that the hollow inner cylinder 1 which when in its unstressed, that is non-deformed state, has a smaller diameter than the hollow outer cylinder 12, and has a common radius of curvature rover the range d due to the cam-shaped body of roatation 3, this radius having its centre M displaced outwards on both sides through a distance x from the centre m. The outer surface of the hollow inner cylinder 11 separates from the inner surface of the hollow outer cylinder 12 over the angle 90--2.

Figures 4 and 5 clarify the kinematics involved in the mutual repulsion of two thin-walled cylinders 11, 12 slid into each other, as the noncircular deformation rotates. The hollow inner cylinder 11 is slid into the hollow outer cylinder 12 so that they both have the same centre m, the outer diameter d2 of the hollow inner cylinder 11 being substantially the same as the inner diameter of the hollow outer cylinder 12.

Figure 5 shows the deformation produced by the cam-comprising body of rotation 3 (not shown in this Figure) the centre M of the body of rotation 3 being displaced outwards by the distances y from the centre m. The two hollow cylinders 11, 12 have wall thicknesses s2 and s1.

The neutral layers are indicated by dashed and dotted lines. The common radius R is also indicated with a broken line. The outer layer of the hollow inner cylinder 11 and the inner layer of the hollow outer cylinder 12 are stretched through a distance AU2, and the inner layer of the hollow outer cylinder 12 is compressed through a distance AUl. As the body of rotation 3 rotates, the two layers are repelled from each other by a distance U1 +AU2. Even hollow cylinders which are tightly forced into each other undergo phasedisplaced relative rotary motion as the body of rotation 3 provided with the cams rotates.

In the embodiment of Figure 6, the recesses 1 7 are in the form of slots disposed in the periphery of the hollow inner cylinder 11 and mutually staggered in an axial direction.

Figure 7 shows that the deformation of the hollow inner cylinder 11 decreases in parallel steps towards the driven shaft 2.

In the embodiment shown in Figure 8 of the drawing, the wall of the hollow inner cylinder 11 consists of several cylinder-shaped parts slid into each other. It is desirable for each cylinder-shaped part to be slotted, and these parts to be constructed of different materials.

The difference between the hollow inner cylinder 11 and that of the remaining embodiments is that the recesses 1 7 are in the form of longitudinal slots, which ensure easy deformability.

The friction gear described above is particularly suitable for high step-up ratios. The drive wheel and driven wheel are in flat contact with each other. Because of this, low specific contact pressures can be used, which make the use of materials of high friction coefficients possible. The result of this is a friction gear with small overall dimensions and a high efficiency.

Claims (11)

Claims

1. A friction gear comprising a drive shaft, a cam-shaped body of rotation connected to the drive shaft, a rolling bearing having a resiliently deformable outer race and inner race, the inner race resting against the body of rotation, a driven shaft, a hollow inner cylinder connected to the driven shaft, the inner cylinder being disposed concentrically with the outer race and lying against the outer race, and the inner cylinder being deformable, and a non-rotatable, but deformable, hollow outer cylinder disposed concentrically with the inner cylinder and lying against its outer surface, such that the inner cylinder presses against the outer cylinder in the cam region of the body of rotation.

2. A gear as claimed in claim 1, further comprising a cup-shaped housing, said hollow outer cylinder being disposed in said cup-shaped housing, and a bearing for the drive shaft, said bearing being mounted in the housing.

3. A gear as claimed in claim 1 or claim 2, wherein the drive shaft is journalled in a bearing mounted in a recess in the driven shaft.

4. A gear as claimed in any one of claims 1 to 3, comprising recesses in the hollow inner cylinder in order to increase deformability.

5. A gear as claimed in claim 4, wherein the recesses are in the form of longitudinal slots.

6. A gear as claimed in claim 4, wherein the recesses are in the form of slots disposed at the periphery of the hollow inner cylinder and mutually staggered in an axial direction.

7. A gear as claimed in any one of claims 1 to 6, wherein the body of rotation comprises two cam-forming parts which can be adjusted relative to each other in a radial direction and which are supported in opposing directions, and of which each has a contour in the form of a circular segment.

8. A gear as claimed in any one of claims 1 to 3, wherein the hollow cylinder consists of several cylinder-shaped parts slid one into another.

9. A gear as claimed in claim 8, wherein each cylinder-shaped part is slotted, and these parts consist of different materials.

10. A friction gear comprising a rotatable drive member, a rotatable driven member, an oval body fast for rotation with one of said members, a resiliently deformable cylinder fast for rotation with the other of said members, said cylinder surrounding the oval body, rolling bearing means interposed between the oval body and the cylinder, said bearing means being resiliently deformable, and a resiliently deformable stationary cylinder surrounding the rotatable cylinder, said rotatable cylinder pressing against the stationary cylinder in zones adjacent to the major axis of the oval body.

11. A friction gear substantially as hereinbefore described with reference to the accompanying drawings.