WO2007085847A1

WO2007085847A1 - Differential gear casing and components thereof

Info

Publication number: WO2007085847A1
Application number: PCT/GB2007/000272
Authority: WO
Inventors: Roberto Gianone; Marco Bassi; Maurizio Lupi; Clive Harrup; Ugo Costelli
Original assignee: Meritor Heavy Vehicle Systems Cameri Spa
Priority date: 2006-01-27
Filing date: 2007-01-26
Publication date: 2007-08-02

Abstract

A unitary main body (11) of a differential gear assembly (10) of a motor vehicle is bowl-like and has a projecting sleeve (21) on the rotational axis (13) for providing bearing support surfaces, a flange (15) around the mouth for attachment of a crown wheel (17) , and opposite apertures (24) orthogonal to the rotational axis for planetary gear shafts (52, 53) . The mouth is in use closed by a circular cover (12) . A planetary gear shaft (52, 53) and thrust washer (37) are also disclosed.

Description

Differential Gear Casing and Components Thereof

This invention relates to a differential gear assembly of a motor vehicle, and particularly to the differential gear casing components thereof and method of manufacture thereof.

Differential gears are provided in driven axles of vehicles to permit left and right driven wheels to rotate at different speeds whilst transmitting torque; as is well known, such an arrangement is required to permit a vehicle to turn from a straight line.

Many different kinds of differential gear have been proposed, but the most common consists of a casing in which two or four freely rotatable planetary gears are arranged in mesh with opposite pinion gears of the vehicle drive shafts. The casing carries an annular crown wheel which is driven by a pinion of an input shaft which is itself driven from the vehicle engine.

Casings for differential gears have a number of exacting requirements. They must of course have sufficient strength to adequately resist distortion underload. In particular the casing must be stiff enough to ensure that the teeth of the internal gears remain in accurate mesh so as to avoid unnecessary wear and noise. Furthermore the casing must be capable of being accurately machined to ensure that initial placement of components is accurate, and that for example the bearing surfaces are correctly aligned; this latter requirement can be problematic in casings assembled from two major components.

Yet another difficulty is that the casing design must permit ready assembly thereof. For this purpose the casing may include windows to permit insertion of pinions, thrust washers and other components. However such windows inevitably weaken the casing wall, which must be correspondingly stiffened. They also result increased churning losses as the casing rotates in oil. A great variety of constructional arrangements and methods have been proposed, having regard to functional requirements of use, cost of materials, and ease of assembly. Relatively small changes in any of these requirements may have a considerable impact on others, so that it may be difficult to achieve technical superiority in one aspect without causing some adverse effect elsewhere. For this reason it may be difficult to adopt a standard alternative technique without having regard to the disadvantages which may follow in assembly and use.

EP-A-0979960 discloses a differential casing comprising two major components, one each defining a journal of a drive shaft, and each component being in the form of a bowl. To form the casing one bowl is fitted within a flange defined by the other bowl, and each bowl defines a semicircular recess for each planetary gear pinion shaft. The respective semi-circular recesses fit together in register to define circular support surfaces for the pinion shaft(s).

A disadvantage of this arrangement is that extremely accurate machining of the semi-circular recesses is required in order to ensure that, on assembly, the two bowls define circular and concentric bearing surfaces of the required diameter.

According to a first aspect of the present invention there is provided a casing of a differential gear, said casing consisting of a unitary main body comprising a bowl having a rotational axis, a peripheral flange at the outer circumference of the mouth of the bowl, a protruding cylindrical sleeve at the base of the bowl and on said axis, and opposite apertures orthogonal to said axis and adapted to receive a support shaft of planetary gears.

Such an arrangement permits the major control surfaces of the differential casing to be provided on the main body thus ensuring that critical machining can be performed without loss of position, as may for example happen during certain kinds of transfer machining. The sleeve provides the usual inner and outer concentric bearing surfaces associated respectively with one drive shaft and the axle housing. This main body can be closed by a relatively simple annular plug which defines the remaining control region, namely the concentric bearing surfaces for the other drive shaft and the axle housing. Furthermore, these concentric bearing surfaces are also concentric with the outer diameter of said plug, and can be machined by e.g. turning whilst the cover is continuously chucked. The likelihood of misaligned axes is accordingly substantially reduced.

In the preferred embodiment, the main body comprises a circular recess adjacent the mouth thereof and adapted to receive a circular plug as a close fitting insert thereof, the recess also defining a shoulder to limit insertion of said cover to the desired depth. In a preferred embodiment the shoulder is substantially radial and circular, and the periphery of the cover is substantially flush with the edge of said recess on assembly thereof.

The cover may be adapted for permanent attachment to the main body by for example circular laser welding at the interface.

The peripheral flange of the main body is, in the preferred embodiment continuous, substantially radial, and of substantially constant diameter. The side thereof adjacent the base of the bowl preferably includes a circular shoulder adapted to locate an annular crown wheel thereon. The crown wheel may be fixed to the flange by any suitable means, such as by circular laser welding from the mouth side of the main body.

The apertures of both main body and cover are preferably constituted by sleeves defined as circular cylindrical projections so as to define internal bearing surfaces for opposed drive shafts, and external bearing surfaces for the usual rolling element support bearings in the axle housing.

The main body is typically of substantially constant radius and thickness in the circular portion immediately adjacent said flange so as to provide support surface for the planetary gear shafts. Said portion may be locally thickened at the outside in order to provide an adequate wall thickness for these gear shafts, whilst being somewhat thinner in between. Furthermore the portion may be locally thickened at the inside in order to provide radially inwardly projecting pads which in use define thrust faces for the planetary gears. Such pads are adapted for circular machining together, so as to be at a predetermined radius from said axis, and concentric therewith.

The method of connecting a main body to a cover, and to an annular crown wheel, typically may comprises the steps of welding said cover to the main body, and welding said main body to the crown wheel by concentric side by side circular welds at the radially inner and outer edges of said flange.

Such a method permits welding from one side, generally perpendicular to plane of the flange. The concentric welds may be made simultaneously and are for example electron beam or laser welds. The arrangement ensures that position and concentricity of components can be maintained during the welding operation.

The most common kind of differential gear comprises a casing in which two opposite drive pinions are in mesh with two or four freely rotatable planetary gears. The casing is driven by a large input gear, and is itself supported for rotation within the vehicle axle casing.

The planetary gears are generally mounted on a respective rotatable shaft, and both the gear and shaft are hardened to resist wear thereof. Each planetary gear usually has a thrust washer to resist radially outward forces. The respective pins and thrust washers must be anchored against rotation relative to the relatively soft casing so as to prevent rapid wear thereof.

Furthermore the planetary gear shafts must be anchored against radially outward movement as the casing rotates in use, and in one prior art method, suitable pins or screws are provided to engage the head of each shaft after assembly into the casing. This kind of retention solution requires additional machining of the casing and of the respective shafts, and may reduce the strength thereof. The number of separate components is also increased by the additional pins or screws. What is required is an inexpensive solution which both retains the pinion shafts in axial position and against rotational movement.

According to a second aspect of the invention there is provided a differential gear assembly including a casing having a rotational axis and a shaft in a through hole of the casing and orthogonal to said axis, said shaft being adapted to support a planetary pinion for rotation thereon, and said shaft being substantially flush with the exterior of said casing, having a peripheral chamfer at the radially outer end thereof, a land immediately inboard of said chamfer, and a recess inboard of said land, wherein said land provides staking material adapted to be permanently deformed radially outwardly of the shaft to engage said casing and thereby retain said shaft against movement radially outwardly of the casing.

Such an arrangement permits an adequate retention force to be obtained without the requirement for additional retention means, such as pins or screws.

In one embodiment, four equispaced portions of said land are staked so as to permanently deform shaft material against the corresponding wall of said through hole. Preferably said recess is circular and continuous, and may comprise a dished end of the shaft.

Said chamfer is preferably straight at e.g. 30-60°, most preferably 45°, and of constant width.

On one embodiment said land comprises a circular flat face extending generally radially of said shaft, and of substantially constant width. Preferably the width of said land and of said chamfer is substantially equal.

In a preferred embodiment said dished end is symmetrical and of substantially constant curvature; the maximum depth of the dish is preferably greater than the axial extent of said chamfer. According to a third aspect of the invention, there is provided a staking tool for planetary gear shaft of a differential gear assembly, said shaft having a peripheral chamfer at one end, a land immediately inboard of said chamfer, and a recess inboard of said land, and said staking tool comprising a generally cylindrical body having a plurality of ball bearings mounted on one end thereof in a common plane orthogonal to the longitudinal axis of the body and substantially equispaced on a pitch circle substantially equivalent to a diameter of said land.

Such a tool can provide an adequate retention force against movement of said shaft axially thereof. Ball bearings are relatively hard, inexpensive, and thus suitable for use as the working elements.

Preferably four equispaced and identical ball bearings are provided, each ball bearing having a diameter greater than the width of said land, and preferably at least twice the width of said land. Preferably the pivot circle of said ball bearing is centred on said land and on the axis of said shaft.

A particular requirement in differential gear assemblies is to provide a thrust washer for each of the planetary gears, so as to prevent the hardened planetary gear from wearing away the relatively soft surface of the differential casing. Such wear is particularly important to avoid, since it alters the radial position of the respective gear(s) and hence affects correct meshing with the gear neighbours. Incorrect meshing may result in further wear, and excessive noise.

The bearing surface of the differential gears may be flat or part-spherical, and corresponding thrust surfaces are required in the differential casing. Spherical and flat thrust surfaces are however difficult to machine directly in the casing. The most easily manufactured thrust surface for the casing is cylindrical, but such a surface does not provide a matching face for either kind of planetary gear thrust face. Hitherto, thrust washers for interposition between the respective planetary gears and differential casing have been formed from flat sheet metal, and for example spherically pressed to match corresponding surfaces of the gear and casing.

A further difficulty with such thrust washers is that rotation thereof with respect to the casing is preferably avoided since such rotation may result in wear of the casing thrust surface. Most advantageously this relative rotation should be between the respective thrust washer and planetary gear. In this way the thrust washer can be a relatively inexpensive service item which can be replaced if wear occurs.

The present invention aims to overcome these problems by providing an improved thrust washer, and differential gear assembly.

According to a fourth aspect of the invention there is provided an annular thrust washer of a planetary gear of a differential gear assembly, the thrust washer having a non-circular periphery adapted to engage a corresponding shoulder of a differential casing.

The non-circular periphery provides an asymmetry which can assure against relative rotation if in abutment with the corresponding shoulder. Preferably the asymmetry comprises a radially inwardly directed portion, such as a flat. A plurality of flats, may be provided in order to give multiple installation positions.

According to a fifth aspect of the invention there is provided an annular thrust washer of a planetary gear of a differential gear assembly, the thrust washer having a projection on one thrust face thereof adapted to engage a corresponding recess of a differential casing.

Such a thrust washer can also be latched against rotation relative to the casing. A suitable casing recess may be machined, and into which the projection is dogged. The projection may comprise two identical radial ribs aligned on a diameter of the thrust washer so as to give multiple installation positions. The ribs preferably extend over the full annular width of the thrust washer.

According to a sixth aspect of the invention there is provided a differential gear assembly having one of the aforesaid thrust washers in engagement with a corresponding shoulder or recess thereof.

In a preferred embodiment lubrication pathways are provided on the differential pinion side so as to permit oil to pass to the face of the thrust washer which is subjected to the relative rotation.

Preferably the thrust washer is sintered. This arrangement permits the two faces of the thrust washer to have different profiles formed in a single manufacturing step, with or without lubrication pathways. The differential casing side is preferably part-cylindrical, whereas the planetary gear side is flat.

In a preferred embodiment the thrust washer has four flat edges symmetrically arranged at right angles so as to have a 'square' appearance. The edges are preferably radiused at the corners of the washer. Such a washer is not sensitive to a single installation orientation. Preferably four lubrication pathways are provided extending radially through the respective corners of the washers.

Other features of the invention will be apparent from the following description of a preferred embodiment shown by way of example only in the accompanying drawings in which:-

Fig. 1 is an axial cross-section through a differential gear assembly incorporating the casing of the present invention.

Fig. 2 is an isometric view of the main body of the casing of the invention from one side. Fig. 3 corresponds to Fig. 2, and is a view from the other side.

Fig. 4 corresponds to Fig. 1 and shows an alternative means of retaining a pinion shaft.

Fig. 5 is a schematic part cross-section through a differential gear assembly according to the invention, and including a staking tool.

Fig. 6 is a scrap section showing interlocking of pinion shafts.

Fig. 7 is an elevation of the inner end of a short pinion shaft.

Fig. 8 is an end view of the inner plate of a staking tool.

Fig. 9 is an end view of the assembled staking tool of Fig. 8.

Fig. 10 is a part sectional view on line 6-6 of Fig. 8; and

Fig. 11 is an enlarged end view of a pinion shaft when staked.

Fig. 12 is an isometric view of one side of a thrust washer according to the invention.

Fig. 13 corresponds to Fig. 12 and shows the other side.

Fig. 14 is a transverse section through the washer of Fig. 12 on line 3-3.

Fig. 15 is an isometric view of an alternative thrust washer according to the invention. With reference to the drawings, a differential gear assembly 10 has a casing comprising a main body 11 and a cover 12.

The main body 11 comprises a unitary symmetrical bowl-like casting having a major axis of rotation 13.

The main body has at its mouth 14 a generally radial continuous flange 15, which on the side opposite the mouth 14 defines a circular shoulder 16 adapted to receive a close-fitting, bevel gear crown wheel 17. As illustrated in Fig. 1, the flange has a circular recess 18 for a corresponding circular projection of the crown wheel, and a circular projection 19 for engagement in a corresponding recess of the crown wheel. On the side adjacent the mouth 14, the flange 15 is substantially plain (Fig. 3).

The flange 15 and projection 19 are adapted to support the crown wheel at substantially the thrust centreline thereof.

At the base, the main body 11 defines a circular cylindrical projection 21 concentric with the axis 13 and having an inner bearing surface 22 for one drive shaft, and an outer bearing surface 23 for the necessary rolling element bearing provided to mount the differential assembly in the axle housing.

The side wall of the main body 11 is of substantially constant radius adjacent the flange 15 and provides four equispaced holes 24 in a common plane at right angles to the axis 13. These holes 24 support the shafts on which the usual differential planetary gears rotate, a common shaft and two opposed gears being illustrated in Fig. 1. The other pair of opposed gears have respective shafts which may bear upon a respective flat pads on either side of the common shaft, to prevent rotation thereof.

At the periphery of each hole 24, the main body is thickened at the outside, and at the inside. Thickening 25 at the outside provides a locally greater depth of support for the planetary gear shafts. Thickening at the inside also increases the depth of support whilst providing a pad 26 which supports each planetary gear against radially outward thrust. It will be appreciated that such pads may be machined together concentric about axis 13, thus ensuring accuracy of manufacture.

Oil drain apertures 27 are provided around the main body 11 in the circular portion between the projection 21 and the pads 26.

At the inside of the mouth, a circular recess 28 is provided having a generally radial shoulder 29 outward of the pads 26. This recess 28 and shoulder 29 co-operate to provide axial location of a close fitting cover 31, which has a cylindrical projection 32 corresponding to projection 21 and for the opposite drive shaft/support bearing.

Fig. 1 illustrates stepped stop pins 33 located in apertures parallel to axis 13 and which engage the respective end of the planetary gear shafts to prevent them moving radially outwardly. The pins may each comprise socket head screw-threaded inserts. Fig. 1 also illustrates the usual thrust washers 34 provided between the planetary gears and the pads 26.

Fig. 4 illustrates an alternative to the threaded stop pins 33. The pinion shafts 40 include a circular groove 41 adjacent the outer end, and an axial through hole 42 is formed in the casing 11 at the base of the recess 28 on the shoulder 29. A pin 43 may be dropped into the hole 42, and is of sufficient length to engage the groove 41 whilst being retained in place by the cover 12. If desired the pin 43 may be a push fit, for example a roll pin.

It will be readily appreciated that the majority of the major machined surface of differential gear are provided on the main body. These surfaces comprise inner and outer diameters of the projection 21; the holes 24 for the planetary gear shafts; the thrust pads 26; the shoulder 18 and projection 19; the recess 28 and shoulder 29. As a consequence these surfaces are more likely to be in the correct relative position with each other, and most importantly concentric about the major axis 13. Concentricity of the recess 28 can be assured with ease during machining, and the corresponding diameter of the cover 31 is also easy to machine concentrically to size. Accordingly the cover 31 will be easily made concentric so that major axis 13 is straight after assembly.

The main body 11, cover 31 and crown wheel 17 may be permanently attached by welding. The arrangement of the invention is particularly advantageous because a double ring of concentric welds can be applied simultaneously from the same side, at locations identified by arrows A & B in Fig. 1. Weld A is axial, whereas weld B is oblique.

In practice the main body 11 is of relatively soft material, whereas the ring gear 17 and cover 12 are harder. Thus a thrust washer 37 is provided for the right half-shaft gear 38 whereas a thrust washer is not necessary for the left half shaft gear 39. A suitable circular axially inwardly directed thrust face 40 is provided on the cover 12. The thrust washer 37 has a suitable anti-rotation feature (not shown) for engagement with the housing 11, for example a radially outwardly extending finger.

The casing 11 of the invention is adapted for automatic assembly from the mouth side, whereby all gear components may be progressively introduced and retained prior to press fitting and axial welding of the cover 12.

With reference to Figs. 5-11, a differential gear assembly comprises a half-casing 111 rotationally symmetrical about an axis 112 and defining bearing surfaces 113, 114 associated respectively with an output shaft and an axle housing. The casing has a radially extending circular flange 115 for mounting of a crown wheel drive gear. The casing is adapted to four equispaced planetary gears (not shown) which are rotatable about orthogonal axes in a common plane, and about respective pinion shafts 116, 117, 118. As described thus far, the assembly is of a conventional kind. It will be well understood that a single pinion shaft 117 may be provided for one opposite pair of planetary gears, but two separate shafts 116, 118 are necessary for the other pair of gears because the shafts have crossing axes in a common plane.

One of the short shafts 116 will now be described in detail. As will become apparent the other short shaft 118 is identical, and the long shaft 117 comprises two short shaft portions with a central linking region 119.

The shaft 116 is rotationally symmetrical and has a mid-region 121 adapted to support a planetary gear pinion for rotation thereon. A radially outer portion 122 is a close fit in a corresponding aperture of the casing 111. The radially inner portion 123 bears on one side of the long pinion shaft 117, and the opposite identical shaft 118 bears on the other side as illustrated.

At its mid-portion 119, the long shaft 117 has opposite flats 124 machined thereon to support the respective ends of the short shafts 116, 118. Rotation of the long shaft 117 in the casing 112 is thereby prevented.

Each short shaft 116, 118 is also reduced in diameter at the sides, as illustrated in Figs. 6 and 7 so as to form shoulders 125 to fit against the sides of the flats 124, and this allows the long shaft to lock with the short shafts to prevent rotation of the short shafts.

The shafts 116, 117, 118 must be radially retained against the relatively small centripetal force which occurs when the casing is rotating in the axle. For this purpose, the outer end of each shaft has a circular dish 126 at the centre, and a circular chamfer 127 at the periphery. Between the dish and chamfer is provided a generally flat circular land 128 having a width of about 20% of the radius of the shaft.

A staking tool 130 is provided to deform regions of the land and chamfer so as to cause a locking force radially of the respective shaft which is sufficient to prevent shaft movement radially with respect to the axis 112. The staking tool comprises a generally cylindrical body 131 having a head 132 attached thereto by countersunk screw 133. The head comprises an inner plate 134 and an outer plate 135. The inner plate 134 defines four circumferentially spaced recesses 136 for respective ball bearings 137, which are retained in place and protrude slightly through four respective apertures 138 of the outer plate 135. The recesses 136 are not strictly necessary since sufficient location and retention for the ball bearings 137 can be provided by the outer plate 135; however the recesses are an aid to assembly and can isolate the outer plate 135 from transverse forces. If required both the inner plate 134 and outer plate 135 can be dogged against relative rotation with each other and with the body 131, but the clamping force of the screw may be sufficient. One side of the tool 130 has a flat 139, which ensures clearance with the closely adjacent flange 115.

In use the shafts are supported 116, 117, 118 and the staking tool 130 pressed down on the land 128 to deform it into the region of the chamfer 127, and thereby induce a permanent deformation against the corresponding wall of the casing.

An example view of a deformed shaft 116, 117, 118 is illustrated in Fig. 11, the land 128 being indented in four locations 140, and material being squeezed into the region of the chamfer 127 in order to radially lock the shaft to the casing 111 by radial force.

The size of the ball bearing 137, the position and protrusion thereof, and the staking force can be selected empirically to give a desired indentation sufficient to lock the shafts in place.

On one example, the shafts 116, 117, 118 have an approximate diameter of 35mm, the other dimensions being in relative proportion, as illustrated in Fig. 5. Typically four equispaced ball bearings of 7mm diameter are used for staking, and generate axial retention loads according to the following table:

Such forces are more than adequate to resist radially outward movement of the shafts in use.

With reference to Fig. 1, a typical differential gear assembly includes drive pinions 38,39 for connection to respective drive shafts, and planetary gears 51,52 therebetween. Two or four identical planetary gears may be provided, according to the required capabilities of the assembly.

The pinions 38,39 and gears 51,52 are supported for rotation in a main body 11, the gears rotating on respective shafts 53,54. The gears 51,52 have respective thrust washers 55 to resist the inevitable radially outward thrust.

A thrust washer 221 according to this aspect of the invention is illustrated in Figs. 12-14.

The washer 221 is formed of an annulus of sintered material and has a part cylindrical casing face 222, and a flat gear face 223. The four edges 224 are substantially straight, generally at right angles, and merge at somewhat rounded corners 225. Lubrication grooves 226 extend radially from the central aperture 227 to the rounded corners 225 as illustrated; these grooves are in straight line pairs at an included angle to 60°.

A sintered washer permits the faces 222,223 to be of different shapes so as to permit convenient and easy machining of the casing thrust face as part cylindrical surfaces wherein the gear thrust faces may be flat, or in the alternative part spherical. In the latter case, the washer face 223 could also be part-spherical, but flat faces are preferred.

The cylindrical thrust face 222 has the effect of giving two opposite thin edges 224, and two opposite thick edges 224, as clearly illustrated in Figs. 12-14. In use the main body of the differential casing is provided with a shoulder or other suitable projection which can closely engage a side of the washer 221 so as to prevent rotation thereof by virtue of the greater radial dimension at the corners 225.

It will be understood that the edges of the thrust washer need not be straight, so long as the radial dimension at the corner 225 is largest. Furthermore the shoulder 219 may be a corresponding straight edge or some other suitable shape. Straight edges are however preferred.

Fig. 15 illustrates an alternative embodiment in which a circular thrust washer 231 has a flat face 232 for contact with a planetary gear, and a part cylindrical face 233 for contact with a differential casing. Suitable oil grooves (not shown) are provided in the flat face in the manner illustrated in Fig. 2. The washer 231 is preferably sintered to form opposite faces of different shape in one manufacturing step.

The part cylindrical face 233 has two protruding radial ribs 234 arranged on a diameter and extending across the full width of the annulus. Each rib is of generally rectangular section and adapted to engage a part-circular square-sided recess of the differential casing. Such a recess can be machined relatively easily, e.g. by a wheel-type milling cutter.

Engagement of the recess and ribs 234 ensures anti-rotation. Furthermore the ribs 234 tend to self-locate, thus facilitating assembly of the differential gear.

Other embodiments are possible within the scope of this disclosure and/or within the scope of the appended claims.

Claims

1. A casing of a differential gear, said casing consisting of a unitary main body comprising a bowl having a rotational axis, a peripheral flange at the outer circumference of the mouth of the bowl, a protruding cylindrical sleeve at the base of the bowl and on said axis, and opposite apertures orthogonal to said axis and adapted to receive a support shaft of planetary gears.

2. A casing according to claim 1 wherein said main body comprises a circular recess adjacent the mouth thereof and adapted to receive a circular plug as a close fitting insert thereof, the recess also defining a shoulder to limit insertion of said cover to the desired depth.

3. A casing according to claim 2 wherein said shoulder is substantially radial and circular.

4. A casing according to any preceding claim wherein said peripheral flange is of substantially constant diameter, the side thereof adjacent the base of the bowl including a circular shoulder on said axis and protruding axially, said shoulder being adapted to locate an annular crown wheel thereon.

5. A casing according to claim 4 wherein said flange tapers to the periphery from the mouth side of said bowl.

6. A casing according to any preceding claim wherein said main body is of substantially constant radius and thickness in the circular portion immediately adjacent said flange.

7. A casing according to claim 6 wherein said portion is locally thickened at the outside in the region of said apertures.

8. A casing according to claim 6 or claim 7 wherein said portion is locally thickened at the inside in the region of said apertures to define radially inwardly projecting thrust pads for respective planetary gears.

9. A casing according to any of claims 1-3 wherein said flange is turned towards the base of the bowl at the outer periphery thereof.

10. A casing according to any of claims 1-3 and further including a circular plug in said recess, said plug and casing being radially flush at the outer side.