WO2004097255A2

WO2004097255A2 - Gearbox, and centrifuge incorporating same

Info

Publication number: WO2004097255A2
Application number: PCT/GB2004/001918
Authority: WO
Inventors: Jan Michaelsen; Raymond John Hicks
Original assignee: INCENTRA Ltd
Current assignee: INCENTRA Ltd
Priority date: 2003-05-02
Filing date: 2004-04-30
Publication date: 2004-11-11
Anticipated expiration: 2005-11-02
Also published as: WO2004097255B1; EP1623141A2; WO2004097255A3

Abstract

A gearbox for a centrifuge, such as a decanter centrifuge, comprising rotatable bowl (1) connected to a housing (31) of the gearbox drivable at a first speed by a first external driving means (11) and a helical conveyer (2) coaxially arranged for rotating therein at a second rotational speed, wherein the gearbox comprises a compound planet epicyclic gear train, and compound planet wheels (29) of the epicyclic gear train are radially supported by one or more rings (36) to counteract the centrifugal forces acting on them. The invention also includes a centrifuge provided with such a gearbox.

Description

Gearbox, and centrifuge incorporating same

Field of the Invention

The present invention relates to a gearbox and to a centrifuge incorporating such a gearbox, and in particular to a decanter type of centrifuge with a conveyer and a bowl interconnected by the gearbox used for rotation of the conveyer.

Background of the invention

Decanter centrifuges are centrifugal machines employed for separating particulate solids (and/or different density liquids) from liquids. They comprise a tubular bowl which is rotated at high speed to effect separation, forming an outer layer of solids against the inner surface of the bowl, and an internally therein mounted screw conveyer which is rotated relative to the bowl to displace the solids axially towards an outlet at one end of the bowl, while the cleaned liquid flows to an outlet in the opposite end of the bowl. The present invention is concerned with a gearbox for coupling the screw conveyer of a decanter centrifuge to the bowl to provide the desired relative rotation between the bowl and the conveyer.

It has long been known to employ two-stage epicyclic (i.e. planetary) gear boxes for this purpose, but it has been accepted that this imposes the limitation that the screw conveyer will rotate slower than the bowl when the first stage sun gear is stationary or is rotated at less than the bowl speed. The transmission comprises at least two gear stages, because one planetary stage alone is less capable of dealing with the high torques and high tooth speeds than two stages.

Such gearboxes comprise three members, an input member, an output member and a reaction member, the latter of which is either fixed to the machine frame or rotated at a slower speed than the rotor to adjust the relative speed of the conveyer to the bowl. In the latter case, because the rotation of the reaction member is in the same direction as the torque on the reaction member, energy is removed from the rotor system, and thus the reaction member is being braked. This braking can be provided by either a pulley drive connected to the centrifuge drive motor, an electrical brake, i.e. an eddy-current brake, or by an electrical motor with a frequency converter attached to control the speed and the possibility to regain some of the brake energy.

A consequence of the traditional use of a two-stage planetary gearbox is that the screw conveyer turns slower than the bowl, and the restriction to two gear stages limits the maximum transmission ratio of the gearbox.

The conveyer rotating slower than the bowl influences the interior flow of the suspension inside the bowl when the incoming suspension is accelerated up to the speed of the bowl. This influence is positive, if the suspension is fed into the bowl at a position near the solids outlet end of the bowl, but it is negative, if the feed point is near the liquid outlet, which is desirable for certain applications.

A solution to this problem of relative motion could be to use a three-stage planetary gearbox, which would make the conveyer rotate faster than the bowl, as well as providing a higher transmission ratio, but such a solution is not practicable because such a gearbox would be heavier and longer and would therefore penalise the maximum operational speed of the centrifuge.

There exists another epicyclic gearbox design using curved discs by the german company Cyclo, which can produce a high transmission ratio similar to a two-stage planetary in one stage, by which the conveyer is rotating faster than the bowl, and which has advantages for centrifuges fed nearer to the liquid outlet end, and for non-dewatering applications as well.

It would therefore seem advantageous to have a compact design that would offer both opportunities by a simple change or repositioning of a few components. A prior art is shown in WO 91/10846, which describes a planetary gearbox comprising a double planetary gear train having a first stage comprising a sun gear, a ring gear coupled to the bowl for rotation therewith and planet gears mounted on a carrier, the second stage comprising a sun gear fast with the first stage carrier, a ring gear coupled to the bowl for rotation therewith, and planet gears mounted on a carrier which is coupled to the screw conveyer, and wherein one of the said stages comprises idler gears which couple the sun gear of that stage to the corresponding planet gears. With this arrangement, it is possible to drive the screw conveyer at a faster speed than the bowl.

For very stable processes, i.e. for processes with high flow rates like thickening or classification, the conveying speed can be kept constant. Such centrifuges have a simple conveyer drive comprising an epicyclic (i.e. planetary) type of gear transmission between the conveyer and the bowl, with the reaction member (i.e. sun wheel shaft) being fixed to the machine frame. In case of torque overload, a maximum load mechanism releases the fixture and lets the reaction member rotate with the bowl, thus reducing the conveying speed to zero, and the centrifuge can be cleaned and restarted after re-engagement of the release mechanism.

As an improvement to the fixed reaction member concept above, the reaction member may be engaged by a fixed ratio pulley transmission, thus giving a means of changing the conveying speed by changing the pulleys. This adjustment can, however, only be done, when the centrifuge is at a stand-still.

A disadvantage of the above mentioned fixed-ratio solutions is that it cannot be changed while the centrifuge is operating, and that if the speed of the bowl is decreased (in order to ease the G-force on the solids and thereby ease the conveying-out of the solids), the speed of the conveyer decreases at the same rate as the bowl speed, thus reducing the speed of removal of solids from the bowl. It is desirable to have the conveying speed constant, or to increase, during such relief-actions, and a solution to this is provided by applying a controlled braking mechanism to the sun wheel.

Such a controlled brake mechanism causes the reaction member (sun wheel) to rotate at a speed between zero and the speed of the bowl, thereby adjusting the conveying speed between maximum speed (given by the transmission ratio of the gearbox) and zero. The braking, however, causes power to leave the rotor system, and it will dissipate from the braking component as heat. If a conveying speed close to zero is needed, the speed of the reaction sun wheel will be high (close to the bowl speed), and the resulting power loss in the brake will be similarly high.

The braking power, however, is depending on the gearbox ratio and the torque generated between the conveyer and the bowl. If the generated torque is of the order of 8000 Nm at a bowl speed of 4000 rpm, and the ratio is 80:1 (which is a quite common ratio found in the market today), the resulting relative speed of the conveyer to the bowl is 50 rpm with fixed sunwheel. The maximum needed relative speed, however, is about 15 rpm, and the maximum torque would be occurring at about 1 rpm. The torque at the sunwheel would be the generated torque divided by the ratio = 100 Nm, and the sunwheel would be turning at 4000 - 80 = 3920 rpm, which would extract about 39 kW from the system.

A maximum needed relative speed of 15 rpm at a bowl speed of 4000 rpm would indicate that a ratio of 267:1 would be optimum because the speed of the sun wheel would then be 4000 - 267 = 3733 rpm and the torque would be 30 Nm at 1 rpm relative speed, equivalent to a braking power of 11.2 kW being removed from the system. The power loss is therefore much less than above, and the drive motor for the centrifuge can be reduced in size by the difference.

It is, however, not possible to make a two-stage planetary gear design with this size ratio within the limitations envelope described above, the limit ratio is somewhere between 120 and 150, and thus the optimum gearbox layout is not possible within the frames of the present technology.

Object of the invention

A basic object of the invention is the provision of a gearbox and a centrifuge incorporating such a gearbox with which it is possible to establish a relative speed difference between the bowl and the conveyer of a centrifuge with smaller losses than previously known.

Summary of a First Aspect of the Invention

According to a first aspect of the invention, there is provided a gearbox for a centrifuge, such as a decanter centrifuge, comprising a rotatable bowl connected to a housing of the gearbox drivable at a first speed by a first external driving means and a helical conveyer coaxially arranged for rotating therein at a second rotational speed, wherein the gearbox comprises a compound planet epicyclic gear train, and compound planet wheels of the epicyclic gear train are radially supported by one or more rings to counteract the centrifugal forces acting on them.

Summary of a Second Aspect of the Invention

According to a second aspect of the invention there is provided a centrifuge having a gearbox as defined in accordance with the first aspect. Advantages of the Invention

The present invention provides an efficient, compact and uncomplicated gearbox for operating a decanter centrifuge, because the efficiency of the gearbox of the invention is higher than the conventionally applied gearboxes, and because the number of parts needed for the duty is kept at a minimum. By a simple exchange of parts, it can be made to provide a faster or slower movement of the conveyer relative to the bowl.

The present invention comprises a compound-planet epicyclic gearbox, "compound" referring to the fact that each planet is meshing with two internal gears and therefore has a set of (usually different) gears at each end, in which a number of compound planet gears are supported by bearings on a common carrier, and each set of gears on the compound planets are meshed with a ring gear, of which one is fast with the housing which is attached to the bowl, and the other is fast with a shaft connected to the conveyer. One of the sets of gears on the compound planets are meshed with a sun wheel acting as the reaction member. The planet gears are designed to be as light as possible to reduce the large centrifugal forces created by the rotation of the gearbox, when the centrifuge operates. To further reduce the forces acting on the planet bearings, the planet wheels are supported by a ring acting as support on roller diameters on the planet gears in the same way as rollers are supported by the outer race ring in a roller bearing. This feature makes it possible to use rolling element bearings that only need a small amount of lubrication compared with journal bearings and thus is less vulnerable and less complicated than a similar design with journal bearings.

A very important feature that enable the ring gears to mesh with the compound planet gears without mesh sharing conflicts and high stresses occurring due to non-concentricity of the support ring, the carrier and/or the output shaft, is added to the conveyer output shaft. A section of the (hollow) shaft is made thin in

order to make it flexible, thus making it possible for the ring gear connected thereto to move radially as demanded by the mesh forces between the gears. This

is most important in the design according to the invention, because the planet

wheel radial positions are dominated by the support ring, and thus are unable to

perform the self-centering that normally occurs in epicyclic gears with three

planets.

Furthermore, (both) ring gears are designed with as thin sections as possible with regard to the fatigue strength of the material. This in combination

with the flexible shaft would enable the use of more than three planets without the

well-known load sharing problems of this method. The requirement of a large ratio,

however, makes the use of more than 3 planets speculative.

The planet wheels and the ring gears are exchangeable and can be

replaced by sets of gears creating faster as well as slower motion of the conveyer

relative to the bowl. In a preferred embodiment, the two rows of gears on the compound planets are identical, in which case the problem of timing the gears during manufacture and mounting is avoided, and the gearbox can be changed

from faster to slower movement of the conveyer or vice versa by swapping the internal gears.

An optional built-in oil pump is either based on the centripetal pump ("pitot tube") principle, and is therefore dependent on the direction of rotation to build up lubrication pressure, or it is a set of positive displacement pumps incorporated into the carrier utilising the relative motion of the planet wheel shafts in relation to the carrier. Brief description of the drawings

One form of the decanter centrifuge gearbox will now be described, by way of example only, with reference to the accompanying drawings, in which:

Fig. 1 shows a general arrangement of a decanter centrifuge with a differential gear conveyor drive according to the invention. Fig. 2 shows a first embodiment of the invention as a diagrammatic axial cross-section of the gearbox; and

Fig. 3 shows a second embodiment of the invention with a built-in pitot tube oil pump; and

Fig. 4 shows a third embodiment of the invention with built-in gear type oil pumps driven by the movement of the planets relative to the carrier; and

Fig. 5 shows a fourth embodiment of the invention with a built-in Gerotor type oil pump driven by the movement of the carrier relative to the housing.

Fig. 6 shows a detailed cross-section of one embodiment of the invention to clearly illustrate the key feature in the design: the light weight planet wheels supported by the support ring, the flexible conveyer shaft to enable self-alignment of the gear meshes, and the attachment of the ring gears to housing and conveyer shaft, resp., with screws, enabling a simple swap of the ring gears. Detailed description of the drawings

Fig. 1 shows a general arrangement of a decanter centrifuge with gearbox conveyer drive, comprising a bowl 1 rotatably supported by bearings 12, and a helical conveyer 2 mounted therein, supported by bearings 13. At one end a shaft 23 extending from the conveyor 2 is connected to the bowl 1 through a planetary gear transmission 7, the reaction member 15 of which is connected via a pulley transmission 20 to a braking motor 10 controlled by a frequency converter 8. The bowl 1 is driven by a motor 11 via a pulley transmission 9 providing a fixed speed of the bowl 1.

By this arrangement the conveyor 2 is brought to rotate within the bowl 1 at a speed slightly different to the bowl, thus creating a screw conveying action towards the conical end 5 of the bowl 1 to any matter 14 deposited onto the inner wall of the bowl 1. The conveying action requires quite a high driving torque which is provided by the gear transmission 7.

The decanter centrifuge functions in the following way: The slurry or suspension 21 that is to be separated is fed to the centrifuge through a feed pipe 3 and a feed chamber 4 in the interior of the conveyor. By centrifugal force, the feed is contained within the bowl 1 forming an annular liquid ring 19, from which the solids particles are precipitated towards the inner face of the bowl 1 forming a cake 14, which is subsequently conveyed towards the conical end 5 of the bowl 1 by the conveyor 2. Here, the cake compresses and dewaters during passage of the dry part of the bowl and leaves the centrifuge through apertures 16 in the bowl wall. At the other end of the bowl 1 the cleaned liquid 6 leaves the bowl 1 through another set of apertures 17.

As the cake content in the bowl 1 increases, the torque on the gearbox transmission reaction member and brake motor 10 increases, causing the inverter 8 to control the conveyor speed relative to the bowl in such a way as to stabilise the cake quality. Fig. 2 shows a cross-sectional view of a gearbox housing 31 attached to the bowl of the centrifuge 1 with a ring gear 28, and a second ring gear 27 fast with a shaft 23 attached to the conveyer of the centrifuge. A number of compound planet wheels 29 mesh with the ring gears and are supported by a common carrier 34. One of the gears on the compound planets 29 mesh with a sun wheel 15, which works as the reaction member. A ring 36 supports the planet wheels 29 against the centrifugal forces acting on the planet wheels. To further reduce the centrifugal forces, the planet wheels are made lightweight by making them hollow and only with adequate material to carry the loads. The supporting action of the ring 36 makes it possible to use rolling element bearings for the planets, as they only have to withstand the forces from the torque load. As rolling element bearings do not need pressurised oil for lubrication, the splashing from the teeth will provide adequate lubrication for all the moving elements inside the housing 31. The shaft 23 has a thin section 24 which enables the self-centering forces from the gear mesh 39 to align the ring gear 27 relative to the planet wheel mesh, which in turn is governed by the planet wheel contact with the support ring 36.

In a conventional 2-stage epicyclic gearbox, the gearbox is filled to a level (when the gearbox rotates) inside of the planet centres in order to provide lubricant to the highly loaded planet Qournal) bearings. Journal bearings are used because rolling element bearings are not able to withstand the comparatively high forces from the centrifugal loads on the planets. The oil is therefore subject to centrifugal forces, creating a high pressure near the peripheral wall of the gearbox. When a pair of teeth in a ring gear and a planet gear engages, the oil has to be pumped to the sides (distance: half a gear width) against this pressure, thereby causing a considerable loss that is dissipated as heat.

The compound planet gears 29 have the same number of teeth on the two gear meshes 39 and 40. The two ring gears having different number of teeth, so that one revolution of the carrier causes the output shaft 23 to move relative to the housing 7 by an amount given by the difference of teeth numbers on the gear meshes 39 and 40. The difference in working pitch circle diameters caused by the different numbers of teeth on the two ring gears are compensated by different correction factors in the manufacturing of the gears.

To explain the function of the gearbox, first imagine that the housing 31 and is kept stationary.

Turning of the reaction member shaft 15 will cause the compound planet wheel 29 to turn the opposite way at a smaller rate given by the geometries, thus turning the carrier 34. If the two ring gears had the same number of teeth, the planets would just rotate within both ring gears, and there would be no movement of the output shaft 23 in relation to the housing 31. If there is a difference in tooth numbers for the two ring gears, one will move relative to the other, the direction depending on the number of teeth in ring gear 27 being larger or smaller than ring gear 28. In figure 2 is illustrated the situation where ring gear 27 has a larger number of teeth than ring gear 28, which results in the direction of rotation of the output shaft 23 to be opposite to the direction of rotation of the reaction member 15. It is clear that if the two ring gears are swapped, the direction of rotation of the output shaft 23 will be the same as the direction of the reaction shaft 15.

Next, imagine that the housing 31 is rotating at high speed, while the reaction shaft 15 is kept stationary. The ring gear 28 will then move at the same speed as the housing 31 , and the output shaft 23 will turn in the opposite direction to the housing, thus moving the conveyer slightly slower than the bowl.

Fig. 3 shows a gearbox similar to fig. 2, but with a built-in oil scoop type oil

pump.

A particular detail which is important to the reduction of the internal losses in the gear transmission is the oil pump 38.

The oil pump 38 shown is a scoop-type pump exploiting the high rotational speeds of the housing. The scoop 37, which skims the surface of the oil shown as

a dashed line 46 in figure 3, is attached to a reaction shaft 35 kept stationary by

being fixed to the machine frame (not shown) outside the gearbox housing 31. The oil scooped into the pump aperture will be channelled towards the centre of the

gearbox by a bore 41 , and via holes in the sun wheel 15 distributed through other

bores (not shown) to provide lubrication for all bearings and gear meshes.

In the present design, the lubrication is provided by a centripetal pump 38, which takes advantage of the high rotating speed difference between the

stationary scoop arm and the rotating gearbox housing containing the lubricant by scooping the oil into a channel according to a well established technology. The

centripetal pump delivers the pressurised oil via an oil transfer bushing/bearing to

bores through the central shaft(G), from which it is directed to different supply

points for lubrication of journal as well as roller- and ball bearings, and gear

meshes (by spray). In this way, the oil level can be kept near or outside the pitch

diameter of the ring gears, thus eliminating the considerable power loss caused by the gear teeth displacing the oil from the gear mesh.

Fig. 4 shows a gearbox similar to fig. 2, but with a built-in gear type oil pump

driven by the movement of the planets relative to the carrier. A disadvantage of the gearbox shown in figure 3 is that the oil pump only works when the gearbox is turning in the right direction, and the oil pressure is

related to the speed of the centrifuge. As the centrifuge speed is high, the pressure generated is also high, i.e. about 30 bar, which is much more than

needed, and thus needs a bleeding valve or the like. An alternative, but more expensive solution is shown is figure 4. A set of gears 42, 43 are mounted in a pump housing 44 attached to the end face of the carrier 34. One of the gears, 43 is connected to a planet wheel and rotates at the same speed as the planet wheel,

thus creating a pumping action as known from gear pumps. There is one set of gears for each planet, for balancing reasons. Some of the pumps could, however,

be replaced by static dummies, only leaving one active pump, for cost reasons.

The advantage of this arrangement is that by adjusting the size of the gears

it is possible to get just the amount of oil needed from the pumps, thus reducing

the relatively high losses from bleeding off too high pressures. It still, though, has

to be adapted to the direction of rotation of the gearbox. Therefore the whole

housing of the pump is designed in such a way that by dismantling it, turning it 180 degrees and mounting it using the other side as a mounting face, the pumping

action can be adapted to the opposite direction of rotation.

Fig. 5 shows a gearbox similar to fig. 2, but with a built-in gerotor type oil

pump driven by the movement of the carrier relative to the housing .

A better solution than shown in fig. 4 can be obtained by using only one,

centrally mounted pump. The gerotor pump principle offers such a possibility, and

can either be designed as shown with one inner lobe rotor and one outer lobe

rotor, in which case a separate balancing will have to be done on the housing

before mounting, or it can be designed with one inner lobe rotor and to outer, oppositely housed lobe rotors to provide ideal balance when rotating with the gear housing.

A set of lobe rotors 52, 53 are mounted in a pump housing 54 attached to the end face of the housing 31. The inner lobe rotor, 53 is connected to the carrier 34 and rotates at the same speed as that, while the outer lobe rotor 54 is mounted in an eccentrically placed cavity 55 in the housing 54 and rotates therein driven by the inner lobe rotor 53, thus creating a pumping action as shown by arrows 61 (suction) and 62 (pressure). The flow is guided by pockets 56 (suction) and 57 (pressure side) in the housing at either side of the lobe rotor cavity.

The advantage of this arrangement is that only one set of pump rotors is needed. It still, though, has to be adapted to the direction of rotation of the gearbox, but this is done by turning the whole package 180 degrees, as mentioned above in fig. 4.

Fig. 6 shows a detailed cross-section of one embodiment of the invention (without oil pump) that clearly illustrates the key features of the design: - the light weight planet wheels 25 mounted in roller element bearings 26 and supported radially by

- the support ring 26 (here shown as a separate, hardened ring connected to the housing 31 by a shrink fit),

- the flexible part 24 of the conveyer shaft 23 to enable self-alignment of the

gear meshes, and

- the attachment of the ring gears 27 and 28 to conveyer shaft 23, and housing 31 , resp. with screws, thus enabling a simple swap of the ring gears 27 and 28 to change the gearbox characteristic from moving the conveyer slower than the bowl to moving it faster, or vice versa.

Claims

1. A gearbox for a centrifuge, such as a decanter centrifuge, comprises a rotatable bowl (1) connected to a housing (31) of the gearbox drivable at a first speed by a first external driving means (11) and a helical conveyer (2) coaxially arranged for rotating therein at a second rotational speed, wherein the gearbox comprises a compound planet epicyclic gear train, and compound planet wheels (29) of the epicyclic gear train are radially supported by one or more rings (36) to counteract the centrifugal forces acting on them.

2. A gearbox according to claim 1 , wherein at least one ring gear(s) (27) is/are connected to their load member by a flexible connection (24).

3. A gearbox according to claim 1 or claim 2, wherein the compound planet wheels (29) have the same number and shape of teeth for both meshes.

4. A gearbox according to claims 1 to 3, wherein the compound planet wheels (29) are made hollow to save weight.

5. A gearbox according to claims 1 to 4, comprising simple means for exchanging

the two internal gears (27, 28) to provide a change of direction of movement of

output shaft (23) in relation to gear housing (31).

6. A gearbox according to claims 1 to 5, comprising a centripetal pump (38)

arranged in relation to a stationary part eg of the decanter centrifuge, inside gear housing (31), the gear housing providing holding space for the lubricant in such a way as to keep the gear mesh free of the relative oil level inside the gear housing (31) when the gear housing (31) is rotating at its operating speed.

7. A gearbox according to claims 1 to 5, wherein the gearbox comprises one or more gear pumps (42, 43) mounted in a housing (44) onto the carrier, the driven pump gear being connected to a planet shaft (29) to provide pumping action for lubricant supply without connection to the exterior of the gearbox.

8. A gearbox according to claim 7, comprising one or more gear pumps (42, 43) mounted in the housing (44) onto the carrier in such a way that it can be adapted to a change in direction of rotation of the gear housing (31) by being dismantled, turned 180 degrees and re-mounted onto the carrier.

9. A gearbox according to claims 1 to 5, comprising a centrally mounted gerotor pump (52, 53, 54), the inner rotor of which (53) being connected to the carrier (34), the outer rotor or rotors of which are mounted in one or two opposite eccentric cavities in a housing (54) to provide pumping action for lubricant supply without connection to the exterior of the gearbox.

10. A gearbox according to claim 9, comprising one centrally mounted gerotor pump in a housing (54) onto the gear housing (31) in such a way that it can be adapted to a change in direction of rotation of the gear housing (31) by being dismantled, turned 180 degrees and re-mounted onto the carrier.