EP3175142A2

EP3175142A2 - Gear mechanism

Info

Publication number: EP3175142A2
Application number: EP15756974.0A
Authority: EP
Inventors: Raymond J HICKS
Original assignee: Involution Technologies Ltd
Current assignee: Involution Technologies Ltd
Priority date: 2014-07-30
Filing date: 2015-07-29
Publication date: 2017-06-07
Also published as: WO2016016645A2; WO2016016645A3; GB201413458D0

Abstract

A gear mechanism, such as may non-exclusively be used with a scroll centrifuge, sugar mill drive or winch drive, the gear mechanism comprising a housing; and a first gear arrangement comprising: a first annular gear having a first axis about which it is rotatable in the housing; a second annular gear, being rotatable in the housing about the first axis; and a set of planet gears which each engage the first annular gear and the second annular gear. Such a gear mechanism can achieve high ratios and high torques with few components and be of compact size.

Description

GEAR MECHANISM

This invention relates to a gear mechanism such as may, in some embodiments, non- exclusively be used with a scroll centrifuge, a scroll centrifuge using such a gear mechanism and a corresponding method.

Scroll centrifuges are well known, and comprise a rotating bowl in which there is mounted a scroll of the form of an elongate screw. It is necessary to rotate the scroll in the bowl in order to move the contents of the bowl. As such, this requires the scroll to be driven at a high speed, but close to that of the bowl.

Conventionally, a centrifuge comprises an outer bowl driven at high speed, geared to drive a co-axial spiral threaded scroll in the same sense but at a somewhat different speed, dependent on the particular application. If the scroll is rotating faster it is said to be leading and if slower, it is lagging.

When both rotate at the same speed, no torque is generated between them but when their respective speeds are changed, the relative speed difference generates large equal and opposite axial forces and torques between the bowl and scroll. Thus, when their speeds are no longer equal, the disparity in the respective powers requires a secondary power input/output. Typically, a differential gear is therefore used, which has a constant speed input driven by the bowl and a variable speed output driving the scroll with the ratio between input and output controlled by a reaction member driven by a control motor.

With conventional solutions the mass of the gears is significant, increasing cost, reducing reliability, and giving problems due to the fact that the gears are a cantilevered mass operating in an apparent high gravitational field (the apparent centrifugal force). Balance and whirling speeds become sensitive.

Furthermore, it is generally desirable to produce compact gear mechanisms which can be used in high-torque situations.

According to a first aspect of the invention, we provide a gear mechanism, the gear mechanism comprising: • a housing; and

• a first gear arrangement comprising:

o a first annular gear having a first axis about which it is rotatable in the housing;

o a second annular gear, being rotatable in the housing about the first axis; and

o a set of planet gears which each engage the first annular gear and the second annular gear. Thus, this provides for the high torque stage of a gear mechanism that can, for example, be used to drive a centrifuge scroll. The benefit of such an arrangement is that the loads on the common planet carrier are less than in prior art arrangements, which allows the gearbox to be lighter. Typically, the gear mechanism will be for a scroll centrifuge comprising a rotatable bowl and a scroll rotatably mounted in the bowl. The gear mechanism may comprise a bowl input member arranged to be coupled to the bowl so as to be driven by rotary motion of the bowl. The gear mechanism may also comprise an output member arranged to be coupled to the scroll so as to cause the scroll to rotate . The first annular gear may be coupled to the bowl input member, and the second annular gear may be coupled to the output member. The first gear arrangement may comprise a common planet carrier, the common planet carrier being rotatable in the housing about the first axis, the common planet carrier carrying the set of planet gears. It is to be noted that there is no need for a sun gear in the first gear arrangement. Indeed, the first gear arrangement can be provided surrounding the output (where a sun gear would otherwise be necessary), allowing for a more compact mechanism.

Typically, the gear arrangement will further comprise :

· a control input member;

a housing;

a second gear arrangement comprising:

o a first epicyclic gear comprising a sun gear fixed relative to the housing, an annular gear and planet gears, the planet gears being carried on a planet carrier coupled to the bowl input member; and o a second epicyclic gear comprising a sun gear coupled to the control input, an annular gear coupled to the annular gear of the first epicyclic gear and planet gears, the planet gears being carried on a planet carrier; in which the mechanism comprises a transmission member that couples the common planet carrier of the first gear arrangement to the planet carrier of the second epicyclic gear.

Thus, if the output member is used to drive the scroll of a centrifuge, the control input member can be used to control the relative angular speed of the scroll relative to a bowl coupled to the input member. Effectively, the first gear arrangement acts as a high torque stage of the gear mechanism, and the second gear arrangement as a low torque stage.

Typically, there will be provided a pair of second annular gears in the first gear arrangement, each of the pair typically being coupled to the output member and engaging the set of planet gears. This allows the load on the carrier to be symmetric, as the pair of second annular gears can be arranged symmetrically about the first annular gear. Typically, the first annular gear and each second annular gear will be parallel to each other.

Each of the first and second annular gears may have teeth by means of which they engage the planet gears. The first annular gear may have a different number of teeth to each second annular gear, even if the first and second annular gears have the same internal radius. This can be achieved by the teeth of the first annular gear having a different correction factor to the teeth of each second annular gear. The difference in number of teeth may be equal to the number of planet gears; in the preferred embodiment, there are 135 teeth on the first annular gear and 138 teeth on each second annular gear. The housing may be provided with containment means, by means of which each of the set of planet gears is contained against outwards radial movement relative to the first axis. The containment means may comprise at least one rolling ring, which may engage a groove in each of the set of planet gears. The housing may provide an oil reservoir tapped by a pitot tube. Typically, the housing will have a fixed part, to which the sun gear of the first epicyclic gear is fixed, and a rotating part that can rotate relative to the fixed part, and which will typically rotate with the bowl input member, the oil reservoir being formed in the rotating part. The Pitot tube may be closer to the first axis than the reservoir. As such, the Pitot tube can be used to draw oil from the reservoir and to direct it where it is needed for lubrication purposes, with the oil then returning to the oil reservoir by action of centrifugal forces. In an alternative embodiment, there is no common planet carrier; as such, the set of planet gears may lack a carrier that joins them together. Instead, the first gear arrangement may comprise a location means, which acts to space the set of planet gears radially from the first axis. In such a case, the mutual circumferential spacing of the set of planet gears can be maintained by the common rotation of the planet gears within the first and second annual gears.

This arrangement has been found to provide good performance for a lower speed, but high torque and high gear ratio gearbox. Such an embodiment would be useful, for example, in a sugar mill drive, or in winch drives. It can be made compact, with a low component count.

The location means may comprise at least one rolling ring, which is positioned radially inwards of each of the set of planet gears relative to the first axis. Such rolling rings are easy to implement and will keep the set of planet gears at an appropriate radius from the first axis, so that each of the set of planet gears engages the first and second annular gears.

In this embodiment, the first annular gear may be fixed relative to the housing. This is useful where there is no need for any differential motion. Furthermore, the first gear arrangement may comprise a sun gear engaging each of the set of planet gears, with the sun gear being rotatable about the first axis.

This gear mechanism will typically comprise an input member. As such, this embodiment may comprise an alternative second gear arrangement, comprising:

· a first sun gear coupled to the input member; • a second sun gear coupled to the sun gear of the first gear arrangement; and

• a set of planet gears which each engage the first sun gear and the second sun gear. In such a case, the gear mechanism will comprise an output member coupled to the second annular gear of the first gear arrangement. Where the first gear arrangement comprises a pair of second annular gears, each of the pair of second annular gears may be coupled to the output member. In one embodiment, each of the pair of second annular gears will be provided with a flange, which are of approximately equal stiffness relative to the first axis (to within 10%, 5% or 1 %), and which meet between the pair of second annular gears, preferably at or around a midpoint between the pair of second annular gears. The output member may be a shaft, with the flanges meeting together on the shaft. This provides a gear arrangement with largely symmetric stiffness.

Alternatively, each of the pair of second annular gears may be provided with a flange to which the respective second annular gear is coupled, with the output member being a shaft, and the flanges being coupled to the shaft at different points along the shaft. This is potentially more compact than the symmetric embodiment described above .

According to a second aspect of the invention, there is provided a scroll centrifuge, comprising a bowl, a scroll inside the bowl and arranged for relative rotation within the bowl, and a gear mechanism in accordance with the first aspect of the invention, in which the bowl is coupled to the bowl input member and the scroll is coupled to the output member.

Typically, the centrifuge would also comprise a control means coupled to the control input member, the control means being arranged to drive the control input member at a variable speed, so as to control the relative rotational speeds of the scroll and bowl.

The centrifuge may also comprise a drive means, arranged to drive the bowl for rotation relative to the housing of the gear mechanism. According to a third aspect of the invention, there is provided method of using a centrifuge, the centrifuge being in accordance with the second aspect of the invention, the method comprising driving the bowl for rotation, and then causing the scroll to rotate relative to the bowl by driving the control input member.

The method may also comprise reversing the sense of rotation of the scroll relative to the bowl by reversing the sense of rotation of the control input member.

There now follows, by way of example only, description of embodiments of the invention, described with reference to the accompanying drawings, in which:

Figure 1 shows a schematic view of a gear mechanism in accordance with a first embodiment of the invention; Figure 2 shows a cross section through the gear mechanism of Figure 1 ;

Figure 3 shows a cross-sectional view of a portion of the engagement of the first annular gear of the mechanism of Figure 1 , engaging with one of the common planet gears;

Figure 4 shows a cross-sectional view of a portion of the engagement of one of the second annular gears of the mechanism of Figure 1 , engaging with one of the common planet gears; Figure 5 shows a schematic view of a gear mechanism in accordance with a second embodiment of the invention;

Figure 6 shows a cross section through the gear mechanism of Figure 5 ; and Figure 7 shows a cross section through a gear mechanism in accordance with a third embodiment of the invention.

Figures 1 to 4 of the accompanying drawings show a gear mechanism for use with a scroll centrifuge in accordance with a first embodiment of the invention, which comprises a rotating bowl having a scroll inside which can rotate relative to the bowl so as to move the contents of the bowl in the manner of an Archimedes screw.

The gear mechanism comprises two inputs: a hub acting as a bowl input member 3 , which is coupled to the bowl of a scroll centrifuge, and a shaft acting as a control input member 5. Furthermore, a further hub acts as an output member 4, which will be coupled to the scroll of the centrifuge . The gear mechanism is mounted within housing 6. The gear arrangement is arranged so as to drive the scroll at a controllable rotational speed close to, but slightly different to that of the bowl (either leading or lagging) . As such, it comprises a first, high torque stage 1 , and a second, low torque stage 2.

The high torque stage 1 comprises a first annular gear 7 coupled to the bowl input member 3. The annular gear 7 is rotatable in the housing 6 about axis 16. The high torque stage 1 also comprises a pair of second annular gears 8, mounted either side of the first annular gear 7, which are also rotatable in the housing 6 about axis 16. The second annular gears 8 are both coupled to the output member 4. Inside the annular gears 7, 8 there are provided three common planet gears 9 rotatably mounted on planet carrier 17 and which mesh with the internal gearing of both first and second annular gears 7, 8. The three planet wheels are mounted in a lightweight straddle carrier 17 which serves as a primary reaction member. Each of the planet gears 9 can rotate relative to the carrier 17 about an axis 18 parallel to axis 16, the axes 18 (and so the planet gears 9) being equally spaced around the axis 17.

The low torque stage 2 comprises a common annular gear 10 again mounted for rotation in the housing 6 about axis 16. Inside the annular gear 10, there are provided two side-by-side planet carriers 1 1 , 12, each carrying a set of planet gears which mesh with the internal gearing of the annular gear 10, and which are both independently rotatable about axis 16 in the housing 6. Inside the first planet carrier 1 1 , where is provided a sun gear 13 that is fixed relative to the housing 6. Inside the second planet carrier 12 there is a sun gear 14 which is coupled to the control input member 5. The first planet carrier 1 1 is coupled to the bowl input member 3, whereas the second planet carrier 12 is coupled to the planet carrier 17 of the high torque stage 1.

As such, this arrangement will drive the output member 4 at the rotational speed of the bowl input member 3, with the addition or subtraction of the speed of rotation of the control input member 5 (depending on the sense of that input) . As such, a motor coupled to the control input member 5 can be used to drive the scroll relative to the bowl. The significance of this arrangement, is that the primary stage planet wheel bearing loads are very low and symmetrically balanced to eliminate torsional wind up in what consequently is a very light carrier assembly. In addition, the conformal contacts between annuli and planet teeth minimises their volumes and increases efficiency with much lower relative gear tooth sliding velocities. Furthermore, it is quite feasible to achieve overall ratios between the control sun wheel and scroll speed increments in excess of 250/1.

Rolling rings 19 housed in the first annular gear 7 either side of its teeth, engage with mating surfaces on the three planet wheels 9 to contain the centrifugal forces imposed by the carrier 17 speed. The planet rolling surfaces are located in the same grooves as the journal bearings 20 which support the planet tangential tooth loads. This requires appropriate tolerances of the respective rolling and bearing diameters of the planets to ensure that the bearings are only subjected to the net tooth loads which generate the carrier torque reaction.

The torques transmitted from the second annular gears 8 to the scroll are equalised by symmetrically located coupling teeth which engage with the scroll drive shaft.

The rotating gear case provides a pressurised oil reservoir tapped by a pitot tube 15 mounted on the stationary torque reaction of the first stage control gear. The Pitot tube 15 is designed to operate irrespective of the direction of rotation. The Pitot tube 15 acts to draw oil from the reservoir to where it is needed, the oil then draining back into the oil reservoir by the action of centrifugal forces. When the high torque stage planet carrier 17 is stationary, the ratio between the speeds of bowl and scroll is inversely proportional to the associated annulus tooth numbers. For example, when the first annular gear 7 has 135 teeth (shown in Figure 3 of the accompanying drawings) and the second annular gears 8 each have 138 teeth (shown in Figure 4 of the accompanying drawings) then the ratio is 46/45. Incidentally, for assembly purposes the number of equi-spaced planets is 3 i.e. the difference in tooth numbers. However, the actual number of teeth in the planets has no influence on the ratio. To simplify manufacture and timing, the planet teeth meshing with the first and second annular gears are all the same. This is achieved by having different annulus tooth correction factors to keep the respective planet/annulus centre distances the same (in the present embodiment, -0.45 for the first annular gear, and 0.915 for the second annular gears), so changing the operating pressure angle.

The ratio of the torques in the first and second annular gears is, conversely, reciprocal to the speed ratio i.e. the bowl/scroll torque is 45/46. The overall torque balance is maintained by the primary planet carrier 17 in turn driven by the bowl via the secondary control train. Thus the overall bowl and scroll torques are effectively equal.

The primary train planet carrier 17 is a simple cylinder with three lugs at either end to support the planet gears 9 through journal bearings. As shown, this is accommodated in the space between the inner tips of the planets and the drives from the first and second annular gears 7, 8. The carrier torque reaction is then transmitted from the mid-point of the cylinder to the second stage planet carrier 12 of the low torque stage 2.

The foregoing features ensure that the resulting transmission is lighter, more efficient and reliable than current state of the art alternatives. Weight is particularly important due to the dynamic problems associated with the higher operating speeds required by modern centrifuges. Furthermore, dependent on the planet carrier/sun wheel ratio of the low torque, it is possible to achieve overall gear ratios of up to 250: 1 or possibly even higher.

A second embodiment of the invention is shown in Figures 5 and 6 of the accompanying drawings. This gear mechanism provides a reduction of the input speed and multiplication of the input torque. The gear mechanism is suitable for applications requiring high torque and high gear ratio keeping the overall space envelope relatively small; typically, this might be for a sugar mill drive or a winch. The mechanism comprises input member 53, low torque stage 52, high torque stage 5 1 and output member 54. The input member is coupled to a driving machine (for example, an electric motor) and is driven by rotary motion of the machine.

The low torque stage 52 comprises of a sun wheel 60, coupled to the input member 53, and several low torque planet wheels 61 , 62, each having a first 61 and second 62 side, which are fixed rotationally relative to each other about planet wheel axis 58. The low torque sun wheel 60 is coupled to the input member 53 and rotates around central axis 56. Torque and motion are transmitted through teeth mesh to low torque planet wheels at first side 61 , which are mounted in the output flange 71. The planets 61 , 62 rotate around low torque planet axis 58 and the planets as a whole 61 , 62 rotate with the output flange 71 around central axis 66.

The minimum number of planet gears 61 , 62 is one, however preferably there are three or more. The first 61 and second 62 sides of the each planet gear 61 , 62 can have the same number of teeth; however, this is not compulsory and teeth numbers may vary. The output flange 71 acts as planet carrier. Torque and motion is then transmitted to a high torque sun wheel (63, 64) by teeth mesh; a first side 63 of the high torque sun wheel is within the low torque stage 52 and a second side 64 of the high torque sun wheel is within the high torque stage 5 1. There is no annular gear in the low torque stage 52. The low torque stage 52 is supported by bearings 59 which are mounted on a supporting flange 70 and housed by a reaction housing (73).

The high torque stage comprises the second side of the high torque sun wheel 64, at least one common planet gear 66, a pair of output annular gears 65 and a reaction annular gear 67. The reaction annulus 67is coupled to the reaction housing 73. The reaction forces and moments are transmitted through the housing 73 to the ground point. The output annular gears 65 are coupled to output flanges 71 , 72 which rotate in the housing 73 around central axis 66. First output flange 71 is coupled to output member 54 which is at a different point spaced along that member 54 coupled to the second output flange 72. Each common planet gear 66 is driven by the high torque sun wheel 64. The first 63 and second 64 side of the high torque sun wheel can have the same number of teeth however this is not compulsory and teeth numbers may vary. Each common planet gear 66 is supported in radial direction (relative to axis 66) by rolling rings 68, which are mounted in a groove in each common planet gear 66. The rolling rings 68 engage with the groove and contain resulting radial forces caused by meshing between the high torque sun gear 64, the output and reaction annular gears 65, 67 and each common planet gear 66. Each common planet gear 66 engages with output 65 and reaction 67 annular gears. Each common planet gear 66 rotates around a high torque planet axis 57 and common planet gears 66 rotate about the sun wheel 64 around central axis 66. There is no planet carrier. The rolling rings 68 also provide axial support for the high torque sun wheel 64.

Each of the output and reaction annular gears 65, 67 may have teeth by means of which they engage the planet gears. The reaction annular gear 67 may have a different number of teeth to each output annular gear 65, even if the output and reaction annular gears 65, 67 have the same internal radius. This can be achieved by the teeth of the reaction annular gear 67 having a different correction factor to the teeth of each output annular gear 65. The difference in number of teeth may be equal to the number of common planet gears 66. Each common planet gear 66 can be made out either as one part or multiple parts coupled together as shown in the accompanying drawings. The benefits of such an arrangement are that the there is no planet carrier, lower amount of bearings and smaller space envelope, which allows the gearbox to be lighter and cheaper. The gearbox housing 73 can act as an oil reservoir.

This embodiment can be made typically more compact than the embodiment described below with respect to Figure 7 of the accompanying drawings. The main supporting bearings in this embodiment can be cheaper and the design is not as complicated as the following embodiment. However, the torsional stiffness of the output member 54 will potentially cause non equal angular position between left and right output annular gears 65 under load. Each common planet gear 66 will have to compensate this difference and thus will tip. This will introduce uneven load sharing across the face width on engaging common planet gears 66 which will can be compensated by complex common planet wheel tooth modifications, such as using microgeometry.

Figure 7 of the accompanying drawings depicts a third embodiment of the invention, which functions similarly to that of Figures 5 and 6. Corresponding features to the embodiment of Figures 5 and 6 have been shown with the corresponding reference numerals, raised by 50.

This embodiment functions in largely the same manner as the second embodiment; indeed, the kinematic scheme shown in Figure 5 is entirely applicable. However, the output flanges 121 , 122 are coupled to output member 104 at a central point.

This embodiment is typically not as compact as the second embodiment as shown in Figure 6. The main supporting bearings in this embodiment are more expensive and the design is more complicated than the previous embodiment. However, torsional stiffness of the first 121 and second 122 output flanges is approximately equal. As there no through shaft as the previous embodiment, the angular position of these flanges will be relatively the same under load and thus the tooth load sharing across face width will be almost even and no complex common planet wheel tooth modification is needed.

Claims

1. A gear mechanism, the gear mechanism comprising:

• a housing; and

· a first gear arrangement comprising:

o a set of planet gears which each engage the first annular gear and the second annular gear.

2. The gear mechanism of claim 1 , being for a scroll centrifuge comprising a rotatable bowl and a scroll rotatably mounted in the bowl.

3. The gear mechanism of claim 2, comprising a bowl input member arranged to be coupled to the bowl so as to be driven by rotary motion of the bowl.

4. The gear mechanism of claim 2 or claim 3, comprising an output member arranged to be coupled to the scroll so as to cause the scroll to rotate .

5. The gear mechanism of claim 4 as it depends from claim 3, in which the first annular gear is coupled to the bowl input member and the second annular gear is coupled to the output member.

6. The gear mechanism of any of claims 2 to 5, in which the first gear arrangement comprises a common planet carrier, the common planet carrier being rotatable in the housing about the first axis, the common planet carrier carrying the set of planet gears.

7. The gear mechanism of any preceding claim, comprising:

• a control input member;

• a housing;

• a second gear arrangement comprising: o a first epicyclic gear comprising a sun gear fixed relative to the housing, an annular gear and planet gears, the planet gears being carried on a planet carrier coupled to the bowl input member; and o a second epicyclic gear comprising a sun gear coupled to the control input, an annular gear coupled to the annular gear of the first epicyclic gear and planet gears, the planet gears being carried on a planet carrier; in which the gear mechanism comprises a transmission member that couples the common planet carrier of the first gear arrangement to the planet carrier of the second epicyclic gear.

8. The gear mechanism of any preceding claim, in which the first gear arrangement is provided with a pair of second annular gears.

9. The gear mechanism of claim 8, in which each of the pair engage the set of planet gears.

10. The gear mechanism of claim 9, in which each of the pair is coupled to the output member.

1 1. The gear mechanism of any preceding claim, in which the housing provides an oil reservoir tapped by a pitot tube .

12. The gear mechanism of claim 1 1 , in which the housing has a fixed part, to which the sun gear of the first gear arrangement is fixed, and a rotating part that can rotate relative to the fixed part, and which will typically rotate with the bowl input member, the oil reservoir being formed in the rotating part.

13. The gear mechanism of any preceding claim, in which the housing is provided with containment means, by means of which each of the set of planet gears is contained against outwards radial movement relative to the first axis.

14. The gear mechanism of claim 13 , in which the containment means comprises at least one rolling ring, which may engage a groove in each of the set of planet gears.

15. The gear mechanism of claim 1 , in which, in the first gear arrangement, the set of planet gears lack a carrier that joins them together.

16. The gear mechanism of claim 15, in which the first gear arrangement comprises a location means, which acts to space the set of planet gears radially from the first axis.

17. The gear mechanism of claim 16, in which the location means comprises at least one rolling ring, which is positioned radially inwards of each of the set of planet gears relative to the first axis.

18. The gear mechanism of any of claims 15 to 17, in which the first annular gear is fixed relative to the housing.

19, The gear mechanism of claim 18, in which the first gear arrangement comprises a sun gear engaging each of the set of planet gears, with the sun gear being rotatable about the first axis .

20. The gear mechanism of any of claims 15 to 19, comprising an input member and a second gear arrangement, comprising:

• a first sun gear coupled to the input member;

• a second sun gear coupled to the sun gear of the first gear arrangement; and

• a set of planet gears which each engage the first sun gear and the second sun gear.

21. The gear mechanism of claim 18, comprising an output member coupled to the second annular gear of the first gear arrangement.

22. The gear mechanism of claim 21 , in which the first gear arrangement comprises a pair of second annular gears, each of the pair of second annular gears being coupled to the output member and engaging each of the set of planet gears.

23. The gear mechanism of claim 22, in which each of the pair of second annular gears is provided with a flange, which are of approximately equal stiffness relative to the first axis and which meet between the pair of second annular gears

24. The gear mechanism of claim 23, in which the flanges meet on the output member at or around a midpoint between the pair of second annular gears.

25. The gear mechanism of claim 24, in which the output member is a shaft, with the flanges meeting together on the shaft.

26. The gear mechanism of claim 22, in which each of the pair of second annular gears is provided with a flange to which the respective second annular gear is coupled, with the output member being a shaft, and the flanges being coupled to the shaft at different points along the shaft.

27. A scroll centrifuge, comprising a bowl, a scroll inside the bowl and arranged for relative rotation within the bowl, and a gear mechanism in accordance with claim 6, in which the bowl is coupled to the bowl input member and the scroll is coupled to the output member.

28. The scroll centrifuge of claim 27, in which the gear mechanism was in accordance with claim 7 and the centrifuge comprises a control means coupled to the control input member, the control means being arranged to drive the control input member at a variable speed, so as to control the relative rotational speeds of the scroll and bowl.

29. The scroll centrifuge of claim 27 or claim 28, comprising a drive means, arranged to drive the bowl for rotation relative to the housing of the gear mechanism.

30. A method of using a centrifuge, the centrifuge being in accordance with claim 28, the method comprising driving the bowl for rotation, and then causing the scroll to rotate relative to the bowl by driving the control input member.

3 1. The method of claim 30, comprising reversing the sense of rotation of the scroll relative to the bowl by reversing the sense of rotation of the control input member.