GB2218181A - Gearing device - Google Patents

Abstract

A precision gearing device, e.g. a step-down gearing device, has a support body (10) in which are rotatably mounted in bearings (22, 24) input shafts (18, 20) with their rotational axes parallel. Each shaft (18, 20) is provided with a spur gear (28) which is rotatable by means of an input spur gear (30). Each shaft (18, 20) is provided with three cylindrical, eccentric portions (31, 33, 35). The first eccentric portions (31) of the two shafts are arranged to be in phase, and are rotatably mounted by means of bearings (40) and bushings (44) in diametrically opposed lobes of a first orbital ring gear (36). Similarly, the second eccentric portions (33) of the two shafts are in phase, and the third eccentric portions (35) are in phase, and are similarly mounted in diametrically opposed lobes of respective second and third orbital ring gears (37, 38). Each of the eccentrics (31, 33, 35) is 120 DEG out of phase with the other two. When the shafts (18, 20) are rotated, the movement of the eccentric portions causes each of the ring gears (36, 37, 38) to perform an orbital motion about a fixed axis. The orbital motion of the ring gears is used to drive an output gear (54).

Description

DESCRIPTION "GEARING DEVICE" The present invention relates to gearing devices, and in particular, but not exclusively, to gearing devices for use in precision actuation mechanisms.

In precision actuation mechanisms, the rotational speed of a prime mover, usually an electric motor, is often too high to be of use, and it is thus necessary to reduce the speed of the motor to an acceptable value. The simplest way of doing this is to use conventional gears, but the gearing ratios involved, sometimes in'the region of 500:1, mean that multi-stage gears must be used to obtain the correct ratio. This increases the bulk and the weight of the gearing device, which is often not acceptable in precision actuation mechanisms. An alternative way of producing the correct ratio is to use epicyclic gears, which are more compact than ordinary gears, but which still often require several step-down stages to achieve high ratios and hence require a large number of components with their attendant costs.

Epicyclic gears also involve difficult problems of load sharing and distribution. Both multistage gear trains and epicyclic gears suffer from disadvantages that the large number of gear stages which are necessary produce relatively large amounts of backlash, resulting in a relatively high breakaway torque (the torque required to overcome friction) and are not particularly efficient. They also have the disadvantage that if it is desired to change- the overall gear ratio, it is necessary to replace several gears, which is both time consuming and expensive.

It is also possible to produce a gearing device having orbital gears. Such a device is disclosed in U.S.1,260,243 and comprises two parallel shafts rotatably mounted on a support, each shaft having first and second eccentric portions, the two first eccentric portions being in phase and the second eccentric portions being in phase. Two orbital ring gears are received on the first and second eccentric portions respectively and mesh with a common output gear over part of their load-bearing surface. Upon rotation of the shafts the eccentrics cause the orbital ring gears to perform an orbital motion about the output gear, thereby causing the output gear to rotate.

The above device is described as being for use with a hoisting mechanism. It is possible to produce a device which is suitable for use as a precision gearing device using the principles disclosed in the document, by refining the design and using accurate construction techniques.

However it is found that a special situation occurs when the eccentric throws are in-line with the line of eccentric centres. At the in-line situation referred to, the orbital gear and the two rotating shafts are instantaneously in primary kinematic relationship but have a secondary degree of freedom. Conversely this means that if the dimensions are not exact some accommodation, either by elastic distortion or by bearing clearances, is required to enable the orbital gear to pass through the in-line position. As a result there can be an irregularity in the motion of the gear.

Consideration must be given as to how the tooth load between the orbital gear and the output gear is carried by the eccentric bearings. As the output gear rotates, the tooth load rotates round the gear rim of the orbital gear and the loads on the eccentric bearings rotate and vary in magnitude, their vector sum equalling the tooth load. The variation in magnitude depends on the gear's proportions but is typically of the order of four to one. In going through the in-line position one or other of the eccentrics can be momentarily unloaded. The in-line position is close to that in which one eccentric carries its most heavy load and the other its lightest load, and results in a "flickover" and an associated relatively large, rapid change in load on the eccentrics and their bearings and on the input gears.

Such irregularity, if not controlled, reduces the accuracy of the device, reduces its life in service, and results in noisy operation of the device.

It is an object of the present invention to provide a gearing device which overcomes the above-mentioned disadvantages.

In accordance with the present invention, there is provided a gearing device comprising a support body, two substantially parallel shafts, each rotatably mounted in the support body, each shaft having at least first, second and third cylindrical portions eccentric with the shaft, with their longitudinal axes substantially parallel thereto, the first, second and third eccentric portions of one shaft being in phase with the respective first, second and third eccentric portions of the other shaft and being rotatably mounted in respective diametrically opposed cylindrical apertures in respective at least first, second and third orbital gears, and an additional gear in mesh with each orbital gear, whereby rotation of the shafts or of the additional gear induces orbital motion of the orbital gears, thereby causing the other of the additional gear or the shafts to rotate relative thereto.

In one embodiment, each orbital gear is an internally toothed orbital gear which meshes with the output gear.

A gearing device such as that described above has the advantage of having relatively few components, thus reducing backlash, weight and cost of the device, and the use of orbital gears permits a large gearing down ratio using a gearing device which is very compact, with high mechanical stiffness. The use of three or more orbital gears removes any significant "flickover" and produces smoother, quieter running, and an improved "quality" of noise.

The device also has the advantage that the breakaway torque is relatively low. The breakaway torque and frictional losses are further reduced if the eccentric shafts are mounted in rolling element bearings, e.g. ball or needle roller bearings, in the respective orbital gears and in the support body. The backdrive torque is also very low, which is important when the motor is being driven backwards, e.g. as a gearing where the additional gear acts as an input gear, or in push-pull applications comprising two gear units, wherein one unit is used for moving an output gear in one direction only and idles backwards when the other unit is driving.

In one embodiment, the device is in the form of a modular unit which may be connected to existing input and output shafts. The device may, however, be part of a complete system, including input and output shafts.

By way of example only, specific embodiments of the present invention will now be described, with reference to the accompanying drawings, in which: Fig.l is a cross-sectional side elevation of a first embodiment of orbital gear system, in accordance with the present invention; Fig.2 is a front view of the system of Fig.l; Fig.3 is an end elevation of an orbital gear of the gear system of Fig. 1 in one position; Fig.4 is a cross-sectional side elevation of a second embodiment of orbital gear system, in accordance with the present invention; and Fig.5 is a front view of the system of Fig.4.

Referring firstly to Fig.l, the gear system comprises a housing 10 which consists of front and rear housing plates, 12,14 respectively, which are separated by, and secured to, a central housing portion 16. In use, the housing 10 is securely fixed, e.g. by clamping, to a fixed frame. Two shafts 18,20 are mounted equidistant from, either side of, and in line with, the central axis of symmetry 21 of the housing.

Each shaft is mounted in bearings 22 in the front housing plate 12 and needle roller bearings, or other suitable bearings, 24 in the rear housing plate 14. A needle bearing bush 26 is interposed between the needle roller bearing 24 and the rear housing- plate 14. A spur gear 28 is mounted at one end of each of the shafts 18,20 on the end of the shaft extending out of the housing 10. The gears 28 are rotatable by means of a further spur gear 30, which forms part of the equipment to which the gear system, infuse, is connected.

Each shaft 18,20 has first, second and third eccentric cylindrical portions 31,33,35 whose longitudinal axes are parallel to, and eccentric with, the rotational axes of the shafts. Each of the eccentric portions of each shaft is 120 out of phase with the other two, and the two shafts 18,20 are synchronised such that the first eccentrics are in phase, the second eccentrics are in phase and the third eccentrics are in phase. First, second and third orbital ring gears 36,37,38 are mounted on the first, second and third cylindrical portions 31,33,35 respectively of each shaft 18,20 by means of needle roller bearings 40 and needle bearing bushes 44.

Each orbital ring gear 36,37,38 (as best seen in Fig. 3) is in the form of a bilobate plate having a central internally toothed annular portion 48 and two lobes 50,52 with respect to which the eccentric portions 31,33,35 are rotatably mounted. The centre of the annular portion and the axes of rotation of the eccentric portions of the shaft lie on the longitudinal axis of symmetry X of each orbital ring gear, which axis is hereinafter referred to as the major axis of the orbital ring gear.

As the spur gear 30 rotates, meshing with the two spur gears 28 and rotating the shafts 18,20, the three eccentric cylindrical portions 31,33,35 perform an orbital motion about the rotational axis of the shafts. Since the eccentric portions 31 of the two shafts are in phase, the orbital ring gear 36 is moved eccentrically about the central axis of symmetry 21 in an orbital manner. Similarly, the other two orbital ring gears 37,38 are moved eccentrically in an orbital manner about the axis 21, but 1209 out of phase with the orbital gear 36 and with each other.

The teeth of the internally toothed portions 48 of the ring gears 36,37,38 mesh over a limited angular range with the teeth of an output gear 54 which is mounted with its rotational axis coaxial with the central axis of symmetry 21 and whose diameter is smaller than the internal diameter of the orbital ring gears. As the orbital ring gears 36,37,38 perform their orbital motion about the axis 21, the area of contact between each of the ring gears 36,37,38 and the output gear 54 moves around the periphery of the output gear 54, and hence the output gear is forced to rotate at a slower speed than the orbital speed of the orbital ring gears. The output gear is provided with a bore 56 into which the shaft of a device to be driven may be secured.In Fig.3, the orbital ring gear 38 is seen to be meshing with the gear 54, but it will be appreciated that the teeth of the other two orbital ring gears 36,37 mesh with the gear 54, but at angular spacings of 120 from the first orbital ring gear 38. That is to say, the three orbital ring gears have the same motion, but each is 120U out of phase with the other two.

The embodiment illustrated in Fig. 1 is suitable for use in-a modular form, i.e. for installation as a unit for gearing down from an existing input shaft to an existing output shaft, or for gearing up, as will be explained later. The embodiment illustrated in Fig.4 is more suitable for installation with the input and output as a complete unit.

The embodiment of Fig.4 is very similar to the embodiment of Figs. 1 to 3, and operates in an identical way. Thus, corresponding items are identified with the same reference numerals. The housing 10 has an additional component in the form of an end plate 58 which partially surrounds the spur gears 28. As in the first embodiment, the spur gears 28 are rotated by means of an input spur gear 30. The spur gear 30 may be secured to a motor to which the gearing device is to be permanently attached. The input gear 30 meshes with the spur gears 28, thereby rotating the shafts 18,20. The output gear 54 is driven as before.

In this embodiment, an output shaft 68 is secured to the output gear 54 by means of a pin 70. A tubular extension 72 is secured to the output of the housing by means of one or more bolts 74 passing into the housing through a peripheral flange 76 on the end of the extension. The output shaft is supported by two bearings 78, on the extension 72. The bearings 78, 80 are held in position by a bearing nut 82 and lip seal 83 and bearing spacers 84, and by the bearing spacers 84 and a circlip 86 respectively. A second lip seal 88 is attached to the end of the extension remote from the housing 10, and bears sealingly against the shaft 68.

This embodiment is more suitable for installation as a complete unit, with the output shaft (and possibly the input gear 30) as an integral part.

The "flick-over" which occurs at the in-line position with the devices having only two eccentric portions, as discussed previously, does not occur in the present invention, to any significant degree.

The use of the three eccentric portions results in a smoother running gear unit, and also reduces the backlash. Furthermore, the breakaway torque is lower than for the unit having two eccentric portions, possibly because the forces are more evenly distributed. Despite the increase in friction which is present using three pairs of eccentrics, the unit runs more smoothly and therefore would appear to compensate for the increased friction from the provision of the third eccentric portion on each shaft 18', 20'.

The unit is less noisy than devices having two orbital gears, and also the quality of noise is improved. Whereas previously the noise was a harsh, rattling noise, the noise of the present unit is less jarring. Also, the back drive torque of the present invention is much improved. This is important in, for example, push-pull applications where each of two motors drives a ring gear in one direction only, and is driven backwards in an idling mode when the other motor is in operation.

The gear ratio is variable between wide range by varying the sizes of the spur gear 30 and the driven spur gears 28, the orbital ring gears 36, 37, 38, and the output gear 54. It is envisaged that the gear ratio can be varied between 20:1 and 500:1, while retaining the advantages of high efficiency, low backlash, low breakaway torque, high mechanical stiffness and compact construction.

The invention is not restricted to the details of the foregoing embodiments. For example, it is envisaged that there could be more than three eccentric portions on each shaft 18,20, each with a respective ring gear. Best results would be obtained with the areas of contact of the ring gears with the output gear being equally angularly spaced about the central axis of symmetry 21.

Claims (5)

1. A gearing device comprising a support body, two substantially parallel shafts, each rotatably mounted in the support body, each shaft having at least first, second and third cylindrical portions eccentric with the shaft, with their longitudinal axes substantially parallel thereto, the first, second and third eccentric portions of one shaft being in phase with the respective first, second and third eccentric portions of the other shaft and being rotatably mounted in respective diametrically opposed cylindrical apertures in respective at least first, second and third orbital gears, and an additional gear in mesh with each orbital gear, whereby rotation of the shafts or of the additional gear induces orbital motion of one of the orbital gears, thereby causing the other of the additional gear or the shafts to rotate relative thereto.

2. A gearing device as claimed in claim 1, wherein the device is in the form of a modular unit provided with means adapted to be connected to an input shaft and an output shaft.

3. A gearing device as claimed in 1 or claim 2, wherein the device is part of a complete system which includes input and output shafts.

4. A gearing device as claimed in any of claims 1 to 3, wherein each orbital gear is an internally toothed orbital gear which meshes with the pinion.

5. A precision gearing device substantially as herein described, with reference to, and as illustrated in, Figs. 1 to 3, Fig. 4 and Fig. 5 of the accompanying drawings.