GB2282204A - Antibacklash gearing arrangement - Google Patents

Abstract

An antibacklash gearing arrangement for use in a precision drive comprises: first and second spur gears 8, 9 driven by a drive gear 7; first and second pinion gears 10, 11 coupled to respective ones of the first and second spur gears 8, 9 to be rotated therewith, the first pinion gear 10 being fixedly coupled to the first spur gear 8, the second pinion gear 11 being coupled to the second spur gear 9 so as to be capable of a degree of rotational movement relative thereto, and spring biassing means 30, 31, 32 being provided between the second pinion gear 11 and the second spur gear 9; third and fourth spur gears 12, 13 arranged to be driven by respective ones of the first and second pinion gears 10, 11; third and fourth pinion gears 14, 15 fixedly coupled to respective ones of the third and fourth spur gears 12, 13 to be rotated therewith; and a driven gear 6 arranged to be driven by the third and fourth pinion gears 14, 15. <IMAGE>

Description

IMPROVEMENTS RELATING TO ANTIBACKLASH GEARING Field of the Invention: This invention concerns improvements relating to antibacklash gears and has particular, though not exclusive, application to an antibacklash gearing arrangement for use in a drive stage of a mechanical manipulator as described in our International Patent Application No. PCT/GB 92/00613 (WO 92/17313).

Backaround of the Invention: In the mechanical manipulator that is described in WO 92/17313, a platform is suspended by three pairs of supportive legs which can be driven longitudinally to determine the position and orientation of the platform in space. A motor driven rotary stage is mounted on the platform and provides a mounting for a following motor driven pivotal stage which in turn carries a motor driven tool holder. By precise control of the leg drives and the following driven stages, precise positioning of a tool held in the tool holder is possible and the apparatus can be employed for automatic computer aided manufacture.

The motor driven rotary stage that is mounted on the platform has to be capable of being rotated with precision in a repeatable manner so that its position can be precisely determined. In order to achieve this, one of the arrangements described in WO 92/17313 is an indexing stage which provides for rotation to a finite number of precisely determined positions, and another arrangement that is described is a continuously variable precision rotary stage incorporating antibacklash provisions. One such antibacklash provision employs two electric motors with associated optical encoders and motor control systems enabling one of the motors to be retarded with respect to the other so as to take up the backlash of accompanying gear systems. Another antibacklash provision that is proposed in WO 92/17313 employs only one motor to drive the rotary stage and employs two gearboxes acting between the motor output and the driven stage, the output shafts of the two gearboxes acting on a common driven gear which keeps them synchronised and one of the gearboxes incorporating a circumferentially-acting spring to effect a near constant pre-load between the input gear that couples the two gearboxes and the input shaft; because the two gearboxes rotate in unison, this pre-load is maintained through continuous rotation and acts throughout every gear interface thereby minimizing net backlash and enabling the rotary stage to be moved deterministically even for tiny increments.

As described in WO 92/17313 therefore, if continuously variable precision rotary movement is to be achievable, gearing arrangements associated with electric drive motors must include antibacklash provisions. If continuous variability is not a requirement, then the described indexing stage which employs a Kelvin clamp type of positioning arrangement requires no antibacklash provisions. An apparatus such as that described in WO 92/17313 requires the highest orders of precision, and accordingly special antibacklash provisions as described have to be made in the case that continuous adjustment through motor driven gearing arrangements is required. The use of conventional antibacklash gears comprising sprung gearwheel pairs simply would not be adequate for this purpose, inter alia on account of the radial forces that are developed.

Objects and Summarv of the Invention: It is accordingly one object of the present invention to provide an antibacklash gearing arrangement capable of being used with a motor driven rotary stage of a precision apparatus such as, for example, is described in WO 92/17313.

According to the present invention in one of its aspects there is provided an antibacklash gearing arrangement comprising: a drive gear adapted to be rotated by a prime mover; first and second spur gears arranged to be driven by said drive gear; first and second pinion gears coupled to respective ones of said first and second spur gears to be rotated therewith, the first pinion gear being fixedly coupled to the first spur gear, the second pinion gear being coupled to the second spur gear so as to be capable of a degree of rotational movement relative thereto, and spring biassing means being provided between said second pinion gear and said second spur gear; third and fourth spur gears arranged to be driven by respective ones of said first and second pinion gears; third and fourth pinion gears fixedly coupled to respective ones of said third and fourth spur gears to be rotated therewith; and a driven gear arranged to be driven by said third and fourth pinion gears.

In an arrangement according to the present invention as above defined, it is preferred that the first spur gear, the first pinion gear fixedly coupled thereto, the third spur gear arranged to be driven by the first pinion gear and the third pinion gear fixedly coupled to the third spur gear constitute a first gear train coupling the drive gear to the driven gear; the second spur gear, the second pinion gear coupled thereto by way of said spring biassing means, the fourth spur gear arranged to be driven by the second pinion gear and the fourth pinion gear fixedly coupled to the fourth spur gear constitute a second gear train coupling the drive gear to the driven gear; and said first and second gear trains are substantially symmetrical. In application of such an arrangement to a motor driven rotary stage of an apparatus as described in WO 92/17313, it is preferred furthermore that the drive gear comprises an external spur gear and the driven gear comprises an internal spur gear; such an arrangement is particularly convenient for use with a magnetostrictive drive motor as described in our British Patent Application No.

9312243.0.

The spring biassing means that is provided between said second pinion gear and said second spur gear may for example comprise a first component fixedly coupled relative to said second pinion gear for rotation therewith, a second component fixedly coupled relative to said second spur gear for rotation therewith, and means providing a circumferentially directed spring biassing force between said first and second components. Said first and second components preferably each have a portion defining a mounting for respective opposite ends of a spring biassing arrangement, the respective mountings opposing each other, and more preferably each have a plurality of said mounting-defining portions arranged symmetrically with respect to the axis of rotation of said second spur gear and said second pinion gear. In an exemplary arrangement that is described hereinafter, the mountings are defined by recesses having partspherical surfaces, and the ends of said spring biassing arrangement(s) comprise complementarilyshaped part-spherical surfaces adapted to be received in said part-spherical recesses, the arrangement enabling said spring biassing arrangement(s) to move relative to said first and second components with relative rotation therebetween, and said spring biassing arrangements comprise a pin having a partspherical head portion and a shank portion, a slider having a part-spherical external surface and formed with an internal bore slidably receiving the shank of the pin, and spring means interposed between the head portion of the pin and the slider.

As described in detail hereinafter in connection with an exemplary embodiment of the invention, a spur gear connected to a motor shaft bears upon two further spur gears which constitute the lead gears of two largely symmetrical gear trains which are synchronised at their input ends by engagement with the spur gear on the motor shaft and at their output ends by engagement with a common output gear. One of the gear trains constitutes a reference train and is pre-loaded by the other gear train which constitutes a pre-load train and incorporates the spring biassing means aforementioned. The spring biassing means gives rise to a circumferential force between two gears of the pre-load train such that the pre-load train of gears tries to cause the output gear to rotate until its movement is stalled by engagement with the reference train of gears which, in turn, is unable to rotate on account of its engagement with the common input shaft.

In this way the gears are pre-loaded so as to eliminate backlash. The arrangement is particulary advantageous in that only a single pre-load component is required at the input side of the pre-load gear train in order to achieve backlash elimination with the mechanical advantage of the gear trains gearing up the pre-load from a small input side pre-load to a high output pre-load. Furthermore, the arrangement of the two gear trains enables balanced loads to be imposed on the output gear such that under static conditions it has no displacement vector such as would otherwise give rise to a biased load onto its bearings.

The invention also extends to an arrangement for generating a pre-load in a gear train, said arrangement comprising a first component adapted to be fixedly coupled with respect to a first gear of said gear train, a second component adapted to be fixedly coupled with respect to a second gear of said gear train, said first and second gears being arranged to rotate coaxially, and means providing a circumferentially directed spring biassing force between said first and second components.

The foregoing and other features of the invention, together with the advantages thereof, will hereinafter be explained with reference to an exemplary embodiment of the invention which is illustrated in the accompanying drawings.

Brief DescriPtion of the Drawings: Figure 1 is a schematic showing in plan view of an antibacklash gearing arrangement embodying the present invention; Figure 2 is a schematic showing in side elevation view of a motor drive incorporating the gearing arrangement of Figure 1; and Figures 3A to 3H are views described more fully hereinafter of a spring-biassing gear pre-loader that is incorporated into the arrangement of Figure 1.

Detailed Descrintion of the Embodiment: The embodiment of the present invention that will hereinafter be described in detail is an antibacklash gearing arrangement for use with an electric motor, for example a magnetostrictive drive motor as described in our British Patent Application No.

9312243.0, in a rotary precision drive stage mounted on the controllably positionable platform of a computer controlled machine tooling apparatus such as is described in WO 92/17313. For a more complete understanding of the construction and operation of the machine tooling apparatus, reference should be made to WO 92/17313 the disclosure whereof is incorporated herein by way of reference.

Referring to Figure 1 of the accompanying drawings, which may be compared and contrasted with Figure 14A of the drawings of WO 92/17313, the platform of the machine tooling apparatus is designated 1 and includes three Kelvin clamp type mounting arrangements 2 adapted for the precision attachment to the platform of the universal joints which couple the platform to its longitudinally adjustable suspension legs. The platform 1 has an open centre within which there is received an electric drive motor providing for precision rotation relative to the platform by way of a rotary sliding journal arrangement comprising first and second annular discs having mating sets of radially spaced apart annular grooves as is described in WO 92/17313 with reference to Figures 14A and 14B of the drawings thereof. One such annular disc is retained rigidly to one side of the platform 1 which serves as a mounting for the motor, and the other disc is rotatably coupled to the first disc with the groove interface between the two mating discs lubricated with a hydro-dynamic lubricant and has a gear formed around its inner periphery which is arranged to be driven through the gearing arrangement hereinafter described by a gear on the motor output shaft. In the accompanying drawings, the annular disc that is fixedly attached to the platform 1 is designated 3 and the motor is designated 4, and the annular disc that mates with the first disc 3 is designated 5 (see Figure 2) and the gear that is formed around its inner periphery is designated 6 (again see Figure 2).

The output shaft of the motor 4 has affixed thereto a drive gear 7 which engages first and second spur gears 8 and 9 which respectively comprise the lead gears of two largely symmetrical gear trains which are synchronised at their input ends by engagement with the drive gear 7 and at their output ends by engagement with the internal gear 6 formed on annular disc 5. First and second pinion gears 10 and 11 are coupled to the first and second spur gears 8 and 9 respectively, second pinion gear 11 being coupled to second spur gear 9 by way of a spring biassed pre-load mechanism providing for a degree of rotational movement between the second pinion gear 11 and the second spur gear 9 as will be described hereinafter with particular reference to Figures 3A-4.

Third and fourth spur gears 12 and 13 are arranged to be driven by respective ones of the first and second pinion gears 10 and 11, and third and fourth pinion gears 14 and 15 are fixedly coupled to respective ones of the third and fourth spur gears 12 and 13 to be rotated therewith, the third and fourth pinion gears 14 and 15 both engaging with the gear 6 that is formed around the inner periphery of the grooved annular ring 5 that is adapted to be rotated relative to the platform 1. The gears 8, 10, 12 and 14 thus form a first gear train coupling the drive gear 7 on the motor output shaft to the driven gear 6, and the gears 9, 11, 13 and 15 incorporating the pre-load mechanism above-mentioned form a second gear train coupling the drive gear 7 to the driven gear 6, the pre-load mechanism taking up backlash in the two gear trains as will be described hereinafter.

Figure 2 is a schematic showing in side elevation view of a motor driven rotary stage incorporating the gearing arrangement of Figure 1, though not all of the gears can be seen in Figure 2 (the gear train 9, 11, 3 and 15 and its pre-load mechanism in particular being omitted from Figure 2). A motor housing 20 is adapted to be secured to the underside of the platform 1 and serves as a mounting for an electric motor 4, the motor projecting upwardly into the hollow centre of the platform 1, an associated optical encoder arrangement 21 enabling the position of the motor drive shaft to be precisely monitored, and the antibacklash gearing arrangement coupling the motor drive shaft to the rotary output stage. Similarly to the arrangement described in WO 92/17313 with reference to Figures 14A and 14B thereof, the motor housing 20 incorporates an annular portion 22 which is formed with a set of radially spaced-apart annular Vshaped grooves. An annular disc 23 is rotatably journaled with respect to the motor housing 20 by virtue of having an annular portion 24 which is likewise formed with a set or radially spaced-apart annular V-shaped grooves which are complementary to and mate with the grooves of the portion 22 with a high degree of conformance. A flanged ring 25 is secured to the motor housing 20 and serves to hold the disc 23 rotatably thereto, there being a large number of small ball bearings 26 running in a bearing groove defined by the respective parts and such groove being lubricated with a hydrodynamic lubricant retained by means of an O-ring seal 27. The interengaging grooves of the fixed and rotating parts 22 and 23 provide for precision alignment of the axes of the parts with a large contact area between which an internally shearing hydrodynamic lubricant reduces low speed friction and absorbs shock loads and low frequency vibrations, thereby achieving a smooth and stiff axis of rotation of the disc 23.

The rotary disc 23 has an annular recess 28 around the radially outer periphery of which is formed the drive gear 6 aforementioned. The third and fourth pinion gears 14 and 15 locate within this recess 28 and engage with the gear 6 (only the pinion 14 is shown in Figure 2). The gear trains aforementioned couple the input gear 7 mounted on the drive shaft of motor 4 to respective ones of the pinions 14, 15 and shown in Figure 2 are the general locations of the gears 8, 12 and 14 of the gear train 8, 10, 12, 14 shown in Figure 1. The gears 9, 11, 13 and 15 of the second, anti-backlash (pre-load) gear train are not shown in Figure 2, but it will be appreciated that they are arranged within the motor housing 20 on the left-hand side of the motor 4 as it is viewed in Figure 2.

Referring now to Figures 3A-3H, Figure 3A is a top plan view of a first pre-load component 30 which is adapted to be fixedly coupled to the second pinion gear 11, Figure 3B is a top plan view of a second preload component 31 which is adapted to be fixedly coupled to the second spur gear 9 and Figure 3C shows the first and second pre-load components 30, 31 assembled together and with means 32 providing a circumferentially directed spring biassing force between them. Figures 3D and 3E are cross-sectional views of the first component 30 on the lines A-A and B-B in Figure 3A respectively, Figure 3F is a crosssectional view of the second component 31 on the line A-A in Figure 3B, Figures 3G-1 and 3G-2 are crosssectional and end elevation views respectively of a spring support slider 33 forming part of the means 32, and Figures 3H-1 and 3H-2 are cross-sectional and end elevation views respectively of a spring support pin 34 forming another part of the means 32. The views in Figures 3A-3H are all much enlarged as compared to the actual size of the components.

The first pre-load component 30 has a central through bore 35 for enabling the mounting of the component 30 fixedly onto the shaft of pinion 11 and screw-threaded bores 36 communicate with the through bore 35 for receiving lock screws for locking the component 30 onto the shaft. The component 30 is formed with a pair of oppositely directed mountings 37 for receiving the respective ends of the spring biassing means 32, the mountings 37 comprising generally semi-spherical recesses. Screw-threaded bores 38 communicate with the recesses 37 for receiving tension adjustment screws as will be described more fully hereafter. As may be seen from Figure 3D, the component 30 is generally in the form of a flat disc having cut-outs 39 into surfaces of which the recesses 37 are formed. The cut-outs 39 are shaped to enable the component 30 to mate with the component 31 which, as can be seen from Figures 3B and 3F, is constituted by a flat disc 40 having upstands 41 which locate in the cut-outs 39 when the component 30 is placed on top of the component 31. The disc 40 has a through bore 42 larger than that formed through the component 30 for enabling the mounting of the component 31 onto the hollow shaft of spur gear 9 (the shaft of the pinion gear 11 being journaled in this hollow shaft) and screw-threaded bores 43 communicate with the through bore 42 for receiving locking screws enabling the component 31 to be fixed onto the shaft of spur gear 9. The upstands 41 of the component 31 have recesses 44 similar to the recesses 37 that are formed in the component 30 and, as can be seen from Figure 3C, respective pairs of the recesses 37 and 44 co-operate to receive the spring-biassing means 32.

As can be seen from Figures 3C, 3G and 3H and as mentioned hereinbefore, the spring biassing means 32 is constituted by a spring support slider 33 which is adapted to fit slidably onto the shank of a spring support pin 34 with a stack of disc springs supported on the shank of the pin 34 between the flat face of the slider 33 and the flat underside of the head of pin 34. The other side of the slider 33 and the upper side of the head of pin 34 are formed each to have a semi-spherical portion which, when the arrangement is assembled as shown in Figure 3C, is received in a respective one of the recesses 37 and 44 formed in the components 30 and 31. The sphericity of the surfaces of the slider 33, the head of pin 34 and the recesses 37 and 44 ensures that the spring biassing (pre-load) means 32 nests with the components 30 and 31 throughout a range of relative angular coaxial displacement between such components 30, 31, the components being designed to accommodate such displacement at least to the extent of one full gear tooth of the gear train.

In the assembly of the gearing arrangement of Figure 1, the pre-load component 30 is secured to the shaft of the pinion gear 11 and the component 31 is secured to the hollow shaft of spur gear 9, the spring biassing (pre-load) means 32 being fitted between the components 30 and 31. The set screws that bear down on the spring support pins 34 are then tightened until the free slack in the gearing between the motor drive gear 7 and the driven gear 6 is taken up, and the set screws are then tightened a further measured amount, thereby compressing the disc springs and applying a pre-load which attempts to rotate the gears 9, 11 relative to each other.

The arrangement described in the foregoing provides a single pre-load site on the input side of the gear train between the motor drive gear 7 and the driven gear 6, which is advantageous firstly in that it is not necessary to provide a pre-load on every gear in the train and is advantageous furthermore in that the mechanical advantage of the gear train gears up the pre-load so that a relatively small input side pre-load gives a high output side pre-load with the same relative pre-load throughout both gear trains.

A further significant advantage arises in that by virtue of having the two pinion gears 14 and 15 driving the output gear 6, the resultant forces acting on the output gear can be balanced under static conditions. As compared to a corresponding system configured with conventional antibacklash gears comprising sprung gearwheel pairs, the invention also enables lighter single gears to be utilized.

The invention having been described in the foregoing with reference to a specific embodiment, it is to be well understood that the embodiment described is exemplary only and that modifications and variations thereto could be made without departure from the spirit and scope of the invention. In particular, the number of gears in the two gear trains connecting the input (motor drive) gear to the driven output gear is susceptible to variation as is the location of the spring biassing (pre-load) means in the gear trains. Only a single gear train could be provided in a situation where it was not important to balance the static loading upon the output (driven) gear, and not only could the described spring biassing (pre-load) means be utilized with alternative gearing arrangements but also the described spring biassing (pre-load) means could be replaced by an alternative arrangement achieving the same or substantially the same function.

Claims (15)

CLAIMS:

1. An antibacklash gearing arrangement comprising: a drive gear adapted to be rotated by a prime mover; first and second spur gears arranged to be driven by said drive gear; first and second pinion gears coupled to respective ones of said first and second spur gears to be rotated therewith, the first pinion gear being fixedly coupled to the first spur gear, the second pinion gear being coupled to the second spur gear so as to be capable of a degree of rotational movement relative thereto, and spring biassing means being provided between said second pinion gear and said second spur gear; third and fourth spur gears arranged to be driven by respective ones of said first and second pinion gears; third and fourth pinion gears fixedly coupled to respective ones of said third and fourth spur gears to be rotated therewith; and a driven gear arranged to be driven by said third and fourth pinion gears.

2. An antibacklash gearing arrangement as claimed in claim 1 wherein: the first spur gear, the first pillion gear fixedly coupled thereto, the third spur gear arranged to be driven by the first pinion gear anc the third pinion gear fixedly coupled to the third spur gear constitute a first gear train coupling the drive gear to the driven gear; the second spur gear, the second pinion gear coupled thereto by way of ,aid spring biassing means, the fourth spur gear arranged to be driven by the second pinion gear and the fourth pinion gear fixedly coupled to the fourth spur gear constitute a second gear train coupling the drive gear to the driven gear; and said first and second gear trains are substantially symmetrical.

3. An antibacklash gearing arrangement as claimed in claim 1 or 2 wherein the drive gear comprises an external spur gear and the driven gear comprises an internal spur gear.

4. An antibacklash gearing arrangement as claimed in any of the preceding claims wherein said spring biassing means comprises a first component fixedly coupled relative to said second pinion gear for rotation therewith, a second component fixedly coupled relative to said second spur gear for rotation therewith, and means providing a circumferentially directed spring biassing force between said first and second components.

5. An antibacklash gearing arrangement as claimed in claim 4 wherein said first and second components each have a portion defining a mounting for respective opposite ends of a spring biassing arrangement, the respective mountings opposing each other.

6. An antibacklash gearing arrangement as claimed in claim 5 wherein said first and second components each have a plurality of said mounting-defining portions arranged symmetrically with respect to the axis of rotation of said second spur gear and said second pinion gear.

7. An antibacklash gearing arrangement as claimed in claim 5 or 6 wherein the mountings are defined by recesses having part-spherical surfaces, and the ends of said spring biassing arrangement(s) comprise complementari ly-shaped part-spherical surfaces adapted to be received in said part-spherical recesses, the arrangement enabling said spring biassing arrangement(s) to move relative to said first and second components with relative rotation therebetween.

8. An antibacklash gearing arrangement as claimed in claim 7 wherein said spring biassing arrangement(s) comprise(s) a pin having a part-spherical head portion and a shank portion, a slider having a part-spherical external surface and formed with an internal bore slidably receiving the shank of the pin, and spring means interposed between the head portion of the pin and the slider.

9. An antibacklash gearing arrangement as claimed in any of claims 5 to 8 wherein adjustment means are provided in the mounting for at least one end of said spring biassing arrangement(s).

10. An antibacklash gearing arrangement as claimed in claim 9 as dependent upon claim 7 or claim 8 wherein a set screw is provided in a screw-threaded bore communicating with at least one of said recesses, said set screw being adjustable for adjusting the setting of the spring biassing means.

11. An antibacklash gearing arrangement as claimed in any of the preceding claims and wherein the spring biassing means between said second pinion gear and said second spur gear is substantially as herein described with reference to Figures 3A to 3H of the accompanying drawings.

12. An antibacklash gearing arrangement substantially as herein described with reference to the accompanying drawings.

13. An arrangement for generating a pre-load in a gear train, said arrangement comprising a first component adapted to be fixedly coupled with respect to a first gear of said gear train, a second component adapted to be fixedly coupled with respect to a second gear of said gear train, said first and second gears being arranged to rotate coaxially, and means providing a circumferentially directed spring biassing force between said first and second components.

14. An arrangement as claimed in claim 13 wherein said spring biassing means is adapted to provide an adjustable degree of angular displacement between said first and second components.

15. An arrangement as claimed in claim 14 and substantially as herein described with reference to Figures 3A-3H of the accompanying drawings.