Background Art Worm gears are used in a variety of industries and generally in situations where it is necessary to transfer drive from a first direction to a direction generally perpendicular to the first direction. Worm gear assemblies generally include a worm gear having a helical thread provided about the outer periphery of a shaft to form the worm gear. The helical thread may be a single entry helical thread or a multiple entry helical thread.

In single entry helical threads a single thread extends about the shaft from one point on the shaft to another point on the shaft to define the worm gear. In multiple entry threads a number of helical threads run in side by side relationship from one position to another position along the length of the shaft.

The worm gear is engaged with an output gear which has teeth which mesh with the helical thread of the worm gear so that when the helical worm gear is rotated about its longitudinal axis advancing movement of the helical thread on the worm gear causes the output gear to rotate. The transmission of motion from the helical thread of the worm gear to the output gear involves a sliding contact between the worm gear and the output gear and this generally makes worm gear assemblies very inefficient in operation.

Summary of the Invention An object of a first aspect of the invention is to provide a worm gear arrangement that is more efficient than conventional arrangements.

The invention, in a first aspect, may be said to reside in a worm gear arrangement including: a worm gear including a thread extending from a first position to a second position along the length of the worm gear; an output gear having a gear body and having teeth on an outer periphery of the gear body, the teeth engaging the thread of the worm; the gear teeth being formed by a rotary member rotatable relative to the output gear body so that when the worm gear rotates about its longitudinal axis and transmit drive to the output gear the teeth are able to rotate thereby reducing the sliding contact between the teeth of the output gear and the thread of the worm gear.

Since the present invention reduces the sliding contact by providing gear teeth in the from of rotary members the efficiency of the transmission can be improved.

In one embodiment of the invention the rotary teeth comprise a tooth member mounted on a pin which extends in substantially the radial direction of the output gear body so that the tooth member can rotate on the pin drive is transmitted from the worm gear to the output gear.

In this embodiment of the invention the member is preferably in the form of a truncated cone having a reduced diameter end remote from the periphery of the gear body and a large diameter end adjacent the gear body.

In a second embodiment of the invention the gear body includes a plurality of cut outs which extend about the periphery of the gear body, each cut out having a rotatable wheel arranged in the cut out for rotation relative to the gear body.

In the first embodiment of the invention the teeth

therefore rotate about an axis on the gear body which is generally radial with respect to the gear body and in the second embodiment the teeth rotate about an axis which is generally tangential with respect to the gear body.

In a second aspect of the invention a worm gear variable transmission is provided which enables transmission of drive from the worm gear to an output gear between a minimum ratio and a maximum ratio.

This aspect of the invention may be said to reside in a transmission including : a worm gear having a thread extending about the worm gear from one position to a second position; output means including an output gear for engaging the worm gear so that upon rotation of the worm gear drive is transmitted from the worm gear to the output means ; moving means for moving the worm gear relative to the output gear in the longitudinal direction of the worm gear; the thread of the worm gear having a changing characteristic between said one position and the second position so as to impart a different drive ratio dependant upon the position of the output means between said one position and the second position; and whereupon a variable drive ratio between the worm gear and the output means can be provided by the moving means moving the worm gear relative to the output means so that the output means is caused to engage the thread of the worm gear at different locations between the said one position and the second position to thereby change the drive ratio of the transmission.

In one embodiment of the invention the output means may include only the output gear which is--in engagement-with the thread of the worm gear.

Preferably the moving means comprises a rotary rack connected to the worm gear or formed integral with the worm gear, a control gear in engagement with the rotary rack and actuating means for rotating the control gear to thereby linearly move the rotary rack and therefore move the worm gear in the longitudinal direction of the worm gear.

In one embodiment of the invention the changing characteristic of the thread of the worm gear comprises a change in the number of threads per unit length, providing a change in pitch of the thread along the length of the worm gear.

In anther embodiment of the invention the changing characteristic comprises a change in the angle the thread of the worm gear makes with the longitudinal axis of the worm gear.

The change in the number of threads per unit length and the change in helical angle may be regarded as describing the same feature in that the change in the helical angle would necessarily involve a change in the spacing or pitch of the thread. This is particularly the case if the change is continuous along the length of the worm. For ease of explanation, the invention will be described by reference to either the characteristic of the thread changing in pitch (ie. spacing between the threads) or helix angle. Most preferably, the width or thickness of the thread is the same regardless of the change in pitch or helix angle.

In the first embodiment of this aspect of the invention the output means comprises an output gear and the output gear has a gear body and teeth in the form of a cone having a reduced diameter end remote from the periphery of the gear body and a large diameter end adjacent the gear

body so that the cone can engage with thread of a different pitch by the cone engaging the thread at different positions between the reduced diameter end and the large diameter end of the cone.

Preferably the cone is mounted for rotation on a pin relative to the gear body to reduce sliding movement of the cone relative to the thread.

In the second embodiment according to this aspect of the invention the output means comprises the output gear and the output gear includes a body having a plurality of cut- outs which extend about the periphery of the gear body, each cut-out having a rotatable wheel arranged in the cut- out for rotation relative to the gear body and for engaging the thread of the worm gear.

A more preferred form of the output means according to the second embodiment of this aspect of the invention includes a transfer gear arrangement and an output gear, the transfer gear arrangement having at least a first gear and a second gear independently rotatable relative to one another, the first gear having a different number of teeth to the second gear and the first and second gears being out of phase relationship so that the teeth of the first and second gears define a pitch of a first value at one position where they engage the worm gear and a pitch of a second value-at another-position along the worm gear.

In one embodiment of the invention the pitch transfer gear includes a third gear, the third gear having a different number of gear teeth to the number of gear teeth of each of the first and second gears.

In this embodiment of the invention the transfer gear assembly has particular application in transferring-motion from a worm having a helical thread which changes in its

helical angle as the thread extends along the worm.

According to this embodiment of the invention at least two of the first gear, second gear and third gear engage with the helical thread and as the angle of the helical thread changes different one of the first gear, second gear or third gear are able to engage with the helical thread to receive drive from the worm gear and transmit the drive to the output gear.

In one embodiment of the invention, each of the gears which form the transfer gear arrangement are arranged in side-by-side relationship for rotation about a common centre.

However, in a different embodiment of the invention, each of the gears are arranged separate from one another and rotate about different centres.

Brief Description of the Drawings Preferred embodiments of the invention will be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a view of a worm gear according to one embodiment of the invention ; Figure 2 is a view of a worm gear assembly according to a first embodiment of the invention; Figure 3 is a cross-sectional view through part of the mechanism shown in Figure 2; Figure 4 is a view of an output gear of a worm gear assembly according to a second embodiment of the invention ; Figure 5 is a cross-sectional view along the line V-V of Figure 4; Figure 6 is a view of a worm gear arrangement according to the second embodiment of the invention; Figure 7 is a view of a transmission utilising a worm gear arrangement according one embodiment of the invention ;

Figure 8 is a view of a first gear of a transfer gear assembly used in the embodiment of Figure 7; Figure 9 shows a second gear of the transfer gear assembly; Figure 10 shows the first and second gears of Figures 8 and 9 in an assembled condition ; Figure 11 is a view illustrating the principles of the transfer gear arranged assembly according to the preferred embodiment of the invention; Figure 12 is a cross-sectional view along the line XII-XII of Figure 11; Figure 13 is a view of a pitch transfer gear according to a second embodiment of the invention ; Figure 14 is a side view of the gear of Figure 13 from the direction of arrow F in Figure 13; and Figure 15 is a side view of the pitch transfer gear of Figure 13 from the direction of arrow G in Figure 13; Figure 16 shows a still further embodiment of the invention ; and Figure 17 is a cross-sectional view along the line XVII-XVII of Figure 16.

Description of the Preferred Embodiment With reference to Figure 1 a worm gear of a worm gear arrangement according to one embodiment of the invention is shown. The worm gear of Figure 1 is intended to be used in worm gear transmissions in which--a-variable-drive ratio between a maximum and minimum ratio can be achieved.

In the first embodiment of the invention the worm gear 11 has a helical thread 13 formed along its length from end 15 to end 17. The helical thread has a varying pitch or spacing between the threads and at the end 15 there may be fifteen threads per inch and at the end 17 the threads may be much further apart such as provide a pitch of ten threads per inch. Between the ends 15 and 17 the pitch of

the helical thread 11 can gradually change from fifteen threads per inch down to ten threads per inch.

As will be explained in more detail hereinafter, in order to provide a transmission with a variable drive ratio the worm gear 11 is moved in a longitudinal direction of the worm gear 11 relative to an output gear (not shown in Figure 1) so that the output gear is caused to engage with the thread 11 at a particular distance along the length of the worm gear 11. This will cause the drive supplied from the worm gear 11 to the output gear to change depending on the pitch of the thread 13 at the place of engagement with the output gear.

Thus, the output drive ratio of the output gear compared to the worm gear-13 can-be changed-by moving the worm gear relative to the output gear.

In a second embodiment of the invention and, possibly a more preferred embodiment of the invention, the worm gear 11 may be provided with a thread 13 which has constant pitch along its length. However, in order to provide for variable drive ratio the helical angle the thread makes with respect to the longitudinal axis A of the worm gear 11 changes from end 15 to the other end 17. For example, the angle of the thread 13 at the end 15 may form an angle a with respect to the longitudinal axis A and the angle the thread--at the end 17 makes with the axis A may be an angle ß. Once again, by changing the position of the output gear along the length of the worm gear a different drive ratio can be achieved because of the change in helical angle the thread makes with respect to the axis A of the worm gear. As the angle changes the amount the thread 13 at the position of engagement with the output gear advances upon a rotation of the worm gear changes.

Thus, for large angles the amount of advancement of the thread at the position of the larger angle is greater than

the amount of advancement at the position of smaller angle and therefore the output gear is caused to advance more at a position adjacent the larger angle than the smaller angle to thereby cause the output gear to rotate faster at the larger angle.

In order to provide a worm gear arrangement which can use the worm gear configurations described above, it is necessary to provide output gear assemblies which can accommodate a change in the pitch size of the thread or a change in the helical angle of the thread.

Figure 2 shows a first embodiment of the invention in which a worm gear arrangement 21 is shown including the worm 11 which has a thread 13 at which the helical angle of the thread 13 changes with respect to the longitudinal axis of the worm 11 along the length of the worm 11. It should be noted that for ease of illustration, the entire length of the worm 11 is not shown.

The worm 11 has an output gear 31 engaged with it. The output gear 31 includes a gear body 33 having an outer periphery 34. Arranged on the outer periphery 35 are a plurality of gear teeth 37 which are formed by truncated conical members. As is best shown in Figure 3, the conical members 37 are connected to the gear body 33 by a pin 39 which is embedded in the gear body 33 and secured by welding or any other suitable fashion and which-extends outwardly radially with respect to the gear body 33. The conical member 37 has a central bore 41 having an enlarged diameter portion 43 at its outer most end. The pin 39 has a head 45 so that the pin 39 can be inserted through the bore 41 and then secured into the gear body 33 so as to support the conical member 37 for rotation on the pin.

Alternatively, the conical member 37 could be deformed over the head 45 by pushing the head through the bore 41 so that the head eventually seats in the enlarged diameter

recess 43 to secure the conical member 37 in place on the pin 39.

When the worm gear 11 rotates about its longitudinal axis the conical thread 13 advances so as to drive the gear 31 in the direction of arrow B for example. During the transfer of motion from the worm gear 11 to the output gear 31, the conical gear teeth 37 are able to rotate on the pins 39 so that the sliding movement between the gear teeth formed by the conical gears 37 is reduced because of the ability of the conical members 37 to rotate on the pins 39. Thus, as drive is transmitted the conical gear teeth 37 can rotate to provide a rolling contact between the thread 13 and the gear teeth 37 rather than a sliding movement thereby improving the efficiency of the worm gear arrangement.

Furthermore, because of the conical gear teeth 37 and the conical angle of those teeth the gear teeth 37 are able to locate in the thread 13 and form generally point contact with the thread 13 where the diameter of the conical members 37 matches the thread spacing of the thread 13.

For small thread spacing, that is a thread which has a large number of threads per inch, the engagement between the thread 13 and the conical members 37 will be higher on the conical members 37 adjacent the free ends or reduced diameter ends of the conical members 37. At wider pitch spacing or where the number of threads per inch is less, the thread 13 can engage lower on the cone 37 towards the periphery of the gear body 31. Thus, the arrangement shown in Figure 2 not only reduces sliding movement and therefore improves the efficiency of the transmission, but also enables engagement with a worm gear which has a varying number of pitches per unit length along the length of the worm gear 11.

Figure 4 shows a further embodiment of the worm gear

assembly. This embodiment also reduces sliding contact and therefore improves efficiency but is intended for environments in which the pitch of the thread 13 is constant along the length of the worm gear. This embodiment can be used in situations where the helical angle of the thread changes.

With reference to Figures 4,5 and 6 output gear 51 includes a body 53 having a plurality of radial extending slots 55 cut in the body and extending from the periphery 57 of the body 53. Each slot 55 contains a rotary wheel 61 which is mounted on an axel 63 (see Figures 5 and 6).

In this embodiment of the invention the body 51 is preferably formed in two halves so that the axel 63 and wheels 61 can be mounted in place between the halves before the halves are secured together. This embodiment of the invention operates in a similar manner to that previously described in which the wheels 61 form the gear teeth and engage in the thread 13 of the worm 11. As the worm 11 is rotated the thread 13 advances so as to push against the wheels 61 to rotate the gear 51 in the direction of, for example, arrow B in Figures 4 and 6.

However, because the wheels 61 are mounted for rotation during engagement with the thread the wheels 61 can rotate so as to reduce an sliding movement between the gear 51 and the worm gear 11 and thereby improve the efficiency of the worm gear arrangement.

Obviously in the embodiments of Figures 2 and Figure 6 output drive is taken off a shaft (not shown) which is secured to the gears 31 and 51 respectively.

Figure 7 shows a first embodiment of a transmission which can provide a variable drive ratio between the minimum ratio and a maximum ratio which employs a worm gear.

As shown in Figure 7 worm gear 11 is provided on an input shaft 71 and has a thread 13. In this embodiment it is preferred that the thread 13 have constant pitch but that the angle the thread 13 makes with the longitudinal axis of the gear 11 changes from end 15 to end 17 as previously described. The change in angle of the helical thread 13 is apparent from the end 13 to the end 17 s shown in Figure 7. The worm gear 11 has a rotary rack 73 formed integral with it and a shaft 71 can be mounted in a casing 75 by suitable bearing schematically illustrated at 77.

A control gear 79 is mounted in the casing 75 and is mesh with the rotary rack 73. The gear 79 has a control handle 81. The worm gear 11 can be moved in the direction of double headed arrow D in Figure 7 by moving the handle 81 in the direction of double headed arrow E so as to rotate the control gear 79. This will pull the rack 73 in the direction of double headed arrow D and therefore the worm gear 11 in the direction of double headed arrow E because of the integral coupling of the rack 73 with the worm gear 11. The worm gear 11 and rack 73 may be mounted on the shaft 71 for sliding movement by splines or any other suitable way which would allow the worm gear 11 and rack 73 to move along the input shaft 71 whilst at the same time enabling rotation of the shaft 71 to be imparted to the worm gear 11 and rack 73. In other embodiments, if the form of input rotary power such as from a motor or the like could accommodate longitudinal-movement of the shaft 71 the shaft 71 can be made integral with the worm gear 11 and rack 73 so that the shaft 71 as well as the worm gear 11 and rack 73 move upon operation of the handle 81.

Although in the embodiment shown in Figure 7 the change in drive ratio is achieved by a change in the helical angle of the thread 11, the arrangement described with reference to Figure 1 in which the number of threads per inch changes could also be used. If the latter form of worm 11

is used then it is preferred that the output gear be in the form of the gear 31 shown in Figure 2 which can accommodate a change in pitch of the worm 11.

If the pitch is the same and the helical angle of the thread 13 changes the output gear could be in the form of the gear 51 shown in Figures 4,5 and 6.

The embodiment shown in Figure 7 utilises an output gear assembly 91 which includes a transfer gear assembly 30 and an output gear 34. The arrangement shown in Figure 7 has particular application in environments in which the pitch of the teeth of the output gear 34 is different to the pitch of the thread 13 of the worm 11. The transfer gear 30 enables engagement with both the worm 11 and output gear 34 notwithstanding the different pitch of-the thread 13 and gear teeth on the output gear 34 as will be described in more detail hereinafter.

Thus, in order to provide output rotation to the output gear 34 the input shaft 71 is rotated to rotate the worm 11. Rotation of the worm 11 effectively advances the thread 13 which causes the transfer gear 30 to rotate and engagement of the transfer gear 30 with the output gear 34 rotates the output gear. In order to change the drive ratio the arm 81 is moved so as to draw the rack 73 in the direction of double headed arrow D (and for purpose of illustration towards-the left in Figure 7) so that the worm gear 11 also moves to the left in Figure 7. This will cause the thread 11 to engage with the transfer gear 30 at a location on the worm gear 11 where the thread 13 has a smaller angle than the position shown in the drawing (such as at a position adjacent end 15 of the worm gear 11). Because the angle of the thread 13 is smaller at the end 15 the amount of advancement--of the thread 13 upon one rotation of the input shaft 71 will therefore be less than at the position shown in Figure 7 and therefore the gear

17 will be rotated a smaller amount for each rotation of the input shaft 71. Thus, the output gear 34 is rotated a small amount thereby effectively increasing the drive ratio of the transmission or placing the transmission into a lower gear.

If the angle the thread 13 makes with the longitudinal axis of the gear 13 gradually changes from end 15 to end 17 a continuously variable output drive ratio can be obtained in which the drive ratio can vary between a maximum and minimum value set by the thread angle at the ends 15 and 17.

The manner in which the transfer gear 30 operates so as to accommodate engagement with two"bodies"which have different--pitches, will be described with reference to Figures 8 to 15. Further details in respect of the pitch transfer gear and its mode of operation are described in our co-pending International patent application number (claiming priority from Australian Provisional Application PR3303), filed concurrently herewith and titled Pitch Transfer Gear and Transmission the contents of which is incorporated into this specification by this reference.

With reference to Figures 8 and 9 a first gear and a second gear of a transfer gear assembly are shown. The first gear 12 has gear teeth 14 and the second gear 16 has gear teeth 18. The gear teeth 14 on the first gear 12 are of a different number to the gear teeth 18 on the second gear 16. Preferably the first gear 12 includes fourteen gear teeth and the second gear 16 has fifteen gear teeth.

The gears 12 and 16 have the same pitch circle diameter and in order to assemble the transfer gear assembly 30 the gears 12 and 14 are arranged in side by side relationship (ie one on top of the other), as shown in Figure 10.

Because the gears 12 and 16 have different numbers of

teeth, when the gears are assembled as shown in Figure 10 the teeth are out of phase with one another.

As shown in Figures 8,9 and 10 the gears 12 and 16 have a central opening 15 so that the gears can be mounted on a shaft (not shown in Figures 8,9 and 10) and so that the gear 12 and gear 16 can rotate on the shaft independently and relative to one another.

When the transfer gear assembly 30 is assembled as shown in Figure 10 at one point, labeled A in Figure 10, at the circumference of the gears 12 and 16 a gear tooth 16'of the gear 16 is at maximum phase difference with respect to adjacent gear teeth 14'and 14''of the gear 12. As one advances in the circumferential direction from the gear tooth 16'it will be seen that the phase relationship between teeth 18 of the gear 16 and the teeth 14 of the gear 12 slightly change their phase relationship to a position at point B (which is diametrically opposite point A) where gear teeth 14a and 18a and teeth 14b and 18b are substantially in phase or registration with one another.

It will be seen that the teeth 14a and 18a are slightly out of registration in view of the different spacing between the teeth on the gear 16 compared with the teeth 14 and the gear 12.

At point A the teeth 16'and 14 define a pitch or modulus of a first value and-at point B the teeth 14a, 18a and 14b, 18b define a pitch or modulus of a second value.

For example, the pitch or modulus at point A can be 1.5 and the pitch or modulus at point B 3. Thus, at point A the gear assembly 30 can engage a gear which has a pitch or modulus of 1.5 and at point B the gear assembly 30 can engage a gear having a pitch or modulus of 3. The teeth of the gears 12 and 16 between the points A and B defined, between adjacent teeth, pitches or modulus of values between 1.5 and 3. For example, at points C and D the

pitch or modulus may be 2.

The transfer gear assembly shown in Figure 10 therefore enables rotation be to transferred from a gear having a first pitch or modulus to a gear having a second pitch or modulus by arranging the first and second gears at appropriate points on the circumference shown in Figure 10 where the modulus of one gear will enable meshing with the gear assembly 30 and the modulus of the other gear will enable meshing with the gear assembly 30.

For example, in Figure 11 an worm gear 11 having a modulus of 1.5 is shown in mesh with the gear assembly 30 and an output gear 34 having a modulus of 3 is also in mesh with the gear assembly 30. As previously mentioned, transfer gear assembly 30 is therefore able to transfer motion from gear 32 having a first pitch or modulus to a gear 34 having a second pitch or modulus.

As is apparent from Figure 11, a portion 32'of thread 13 the gear 11 is able to engage between tooth 14''of first gear 14 and tooth 18'of second gear 16. As the gear 11 rotates in the direction of arrow A gear tooth 32''is able to engage between tooth 18'of the second gear 16 and tooth 14'of the gear 12. As the transfer gear assembly 30 rotates in the direction of arrow B the gear 12 advances relative to the gear 16 so that by the time the tooth 14''has reached point B in Figure 10 the tooth is no longer at maximum phase difference with the tooth 18' but has in fact now caught up with the tooth 18'so that it is in registry with the tooth 18'as shown by the teeth 14b and 18b in Figure 3. The differential in speed is accommodated by the fact that the gears 12 and 16 are able to rotate relative to one another and caused by the fact that the gears 12 and 16 have different numbers of teeth.

The tooth 1411 will advanced one complete tooth spacing of the teeth on the gear 12 with respect to tooth 18''shown

in Figure 4 for each revolution of the gear 12. That is, in other words if the transfer gear 30 is rotated one revolution so that the tooth 14''returns to the exact position shown in Figure 4 then the tooth 18''would have advanced only as far as the tooth marked 18'in Figure 11.

As is also apparent in Figure 11 because tooth 34a of gear 34 is in mesh between teeth 14a, 18a and 14b, 18b of the gear assembly 30, rotation is imparted to the gear 34 so that the gear 34 rotates in the direction of arrow C.

The gears 34 and 12 and 16 are supported on shaft 31, and 35 respectively as best shown in Figure 12. As is also apparent from Figure 12, the gears 12 and 16 are half the thickness of the gear 34 so that when the gears 12 and 16 are assembled one on top of the other as shown in Figure 5 then the thickness of the assembly 30 is the same as the thickness of the gear 34.

Figures 13,14 and 15 show a transfer gear 300 according to a further embodiment of the invention. The transfer gear of this embodiment of the invention has particular application in engaging a straight cut gear at one position about its periphery and a helical gear such as a worm gear at another place about its location. The transfer gear 300 includes three gears 302,304 and 306.

Whilst a transfer gear of the type described with reference to Figures 8 to 12 is suitable for transferring motion from a helical gear having a constant helical angle along the length of the gear, to a straight cut gear, if the helical angle of the gear changes along the length of the gear the embodiment of Figure 300 is preferred.

The gears 302,304 and 306 are designed in the similar manner to that previously described. The gear 302 has, for example, fourteen teeth 302', the gear 304 fifteen

teeth 304'and the gear 306 sixteen teeth 306'. At one position on the periphery of the transfer gear 300 such as that marked by arrow F the gears 302,304 and 306 will enable engagement with a straight cut gear having a predetermined modulus. As shown in Figure 15 a tooth designated To, (shown by extended dashed lines for ease of illustration) couples at least two of the teeth 302s-306x, such as the teeth labelled 302''and 306''in Figure 14.

At a different pitch spacing at another point two different teeth may contact a tooth of the output gear.

At another position such as that labeled by arrow G in Figure 13 the gears will form a space which can receive a helical gear or thread of a worm gear or other helical cut gear. In order to transfer load from the worm gear or helical gear to the transfer gear 300 it is preferred that at least two of the three gears 302 to 306 engage-with the helical thread of the helical gear. In Figure 15 all three gears have teeth in engagement with the helical thread schematically shown by reference T'in Figure 15.

However, as the helical angle of the thread T'changes with respect to the horizontal as shown by reference T'' in Figure 15 only two of the gears may engage the thread T"such as tooth 30211 and 30611 of the gears 302 and 306. The use of the three gears ensures that at least two of the gears will always engage the thread T'regardless of the change in helical angle of the thread T'with respect to the horizontal as depicted in Figure 15.

As in the earlier embodiment, the gears with less teeth will advance relative to the gears with more teeth as the gear rotates.

Figures 16 and 17 show a still further embodiment of the invention in which the transfer gear assembly is formed from two gears which rotate about different centres, rather than about a common centre, and in side-by-side relationship as in the earlier embodiment.

Like reference numerals indicate like parts to those described with reference to Figure 11.

In the earlier embodiments, because the gears which make up the pitch transfer gear arrangement 30 are in side-by- side relationship, the gears are effectively in different planes or engage on different pitch circle diameters with respect to the worm gear 11. In this embodiment, the pitch transfer arrangement is formed from gears 401 and 402 which are separate from one another and which rotate on different centres. The gears 401 and 402 are configured in the same manner as the gears which make up the arrangement 30 previously described and have a different number of gear teeth which are therefore out of phase--with one another, and therefore when combined, effectively form gears of different modulus. The gears 401 and 402 are mounted on shafts 403 and the shafts 403 carry pinions 405. The pinions 405 mesh with sun gear 407 on output shaft 409.

Thus, when the worm 11 is rotated by input rotary power, the gears 401 and 403 are rotated to in turn rotate their respective shafts 403. Rotation of the shafts 403 rotate the pinions 405 which in turn rotate the sun gear 407 and the output shaft 409.

In order to change the drive ratio of the transmission, the worm 11 is moved in the same manner as described in the earlier embodiments, so that the gears 401 and 402 engage at a different position along the worm 11 where the helical thread on the worm 11 has a different angle to thereby change the drive ratio in the manner previously described.

The different helical angle is accommodated by the gears 401 and 402 being able to slightly rotate relative to one

another so that the teeth of the gear 401 and teeth of the gear 402 have a phase relationship so that drive can be transmitted from the worm 11 to those gears in the same manner as previously described.

Since modifications within the spirit and scope of the invention may readily be effected by persons skilled within the art, it is to be understood that this invention is not limited to the particular embodiment described by way of example hereinabove.