BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a bearing with a preload arrangement, and a method of preloading a bearing, especially a variable pitch propeller bearing. 
     2. Description of the Related Art 
     The increase in the power handling characteristics and diameters of propeller systems makes it increasingly necessary to preload the bearings within the propeller hub which allow the propeller blades to change pitch. The preload serves many purposes, the primary one of which is to ensure that all of the bearing elements are kept stable during all flight conditions. Preloading also serves to align the blade root within the propeller hub and, in applications having removable blades, also provides a means for locking the blades into the hub. 
     A known propeller blade root assembly typically comprises a split upper inner race which faces radially outward of the propeller blade and is inclined with respect to the axis of the blade so as to face slightly towards the propeller root. An outer bearing race is disposed generally radially outwards of the inner bearing race and has an inwardly facing bearing surface which is slightly inclined so as to be generally parallel to the bearing surface of the inner bearing race. Motion of the outer bearing race along the axis of the propeller varies the distance between the bearing surfaces of the inner and outer bearing races and this feature is exploited to preload the bearings. The outer bearing race has an outwardly facing screw thread located thereon which cooperates with a preload nut which is wound down to bear against the hub, either directly or indirectly (to prevent fretting). The winding down of the nut causes the outer bearing race to be moved axially of the propeller blade and to preload the propeller blade bearings. The preload is applied via a spanner acting on the nut and considerable effort may be required. The friction between the nut, the thread, and the hub makes it difficult to determine the torque required to obtain a desired preload. This often necessitates the performing of tests in order to determine what preload had been achieved. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, there is provided a bearing having a preload arrangement for preloading the bearing, comprising a first member having a first bearing surface movable with respect to a second member having a second bearing surface so as to bear against at least one bearing located between the first and second bearing surfaces, a fluid operated actuator for urging the first member to move with respect to the second member so as to apply a load to the bearing, and mechanical locking means for locking the first member against return movement. 
     It is thus possible to apply a preload to a bearing without relying solely on the force applied by rotation of a preload nut. The preload force may be more easily applied and accurately controlled than is the case with the known arrangement described hereinbefore. 
     Once the preload has been applied using the fluid actuator, the mechanical locking means is operated so as to lock the first member in position such that the bearings remain preloaded when fluid pressure is removed from the actuator. 
     The fluid operated actuator may advantageously be used to apply a force to preload the bearing during assembly or maintenance of a device incorporating the bearing. 
     Advantageously, the first member is moved in a first direction to apply a load to a bearing and further comprises, or cooperates with, the locking means for preventing motion of the first member in a return direction opposing the first direction. The locking means may comprise a threaded portion located on the first member which, in use, cooperates with a retaining nut which bears directly or indirectly against a fixed surface. 
     Advantageously, the first member cooperates with a third member to define a chamber and the first member has a third surface which forms a movable wall of the chamber, such that fluid pressure within the chamber exerts a force against the third wall and thereby urges the first member to move along the first direction. Advantageously, the fixed surface is a surface of the third member. Thus, the fluid operated actuator is formed between the first member and the third member which are moved apart by fluid pressure applied therebetween, and between which the locking means cooperates to hold them apart. 
     Preferably, the bearing is part of a bearing arrangement having third and fourth bearing surfaces arranged to engage bearings therebetween, and one of the third and fourth surfaces is movable in response to relative movement of the first member such that the bearings between these surfaces also become preloaded when the actuator is operated. 
     The third member is associated with one of the third and fourth surfaces and serves to store a preload reaction force between the bearings of the first and second surfaces and the third and fourth surfaces. 
     Preferably, the bearing arrangement is provided at the fixing region of a propeller blade. 
     Preferably, the first bearing surface is coaxial with and outwardly radially disposed of the second bearing surface, the first and second bearing surfaces being substantially parallel with one another and inclined with respect to their common axis (i.e. the surfaces are portions on a conic surface having the same or substantially the same cone angles) such that longitudinal motion of the first member with respect to the second member changes the clearance between the first and second bearing surfaces. The fixing region of the propeller blade also has a third bearing surface which faces radially outwardly and away from the base of the propeller blade root and cooperates with an opposing fourth bearing surface to retain bearings therebetween. Operation of the actuator preloads the bearings between the first and second bearing surfaces, and in so doing, causes the blade root to move so as to preload the bearings between the third and fourth bearing surfaces, and also places the blade root portion intermediate the bearings under tension. 
     Thus, when the fluid pressure is released from the actuator, the tension stored in the blade root maintains the preload on all the bearings. 
     Preferably, the first member is substantially cylindrical and is arranged to fit within a corresponding recess within a housing. 
     Preferably, the first member is arranged to engage within a corresponding recess within a propeller hub so as to hold the propeller against wobble about its own axis. 
     According to a second aspect of the present invention, there is provided a preload arrangement for a propeller blade, comprising a member movable in response to fluid pressure acting on a surface thereof so as to preload the bearings of the propeller blade. 
     According to a third aspect of the present invention, there is provided a method of preloading a bearing having a preload arrangement, the preload arrangement comprising a first member having a first bearing surface movable with respect to a second member having a second bearing surface so as to bear against at least one bearing between the first and second bearing surfaces, a fluid operated actuator for urging the first member to move with respect to the second member so as to apply a load to the bearing, and mechanical locking means for locking the first member against return movement, the method comprising the steps of: applying fluid pressure to the fluid operated actuator; and operating the locking means to lock the first member against return movement. 
     Advantageously, the fluid pressure is removed after the locking means has been operated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 A cross-sectional view of a propeller blade root constituting an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will further be described, by way of example, with reference to the accompanying drawing which is a cross-section through a propeller blade root constituting an embodiment of the present invention. 
     The accompanying figure shows a propeller blade root and bearing assembly in cross-section. The rotational axis of the propeller blade is represented by chain dot line  1 . A propeller blade root  2  has a generally cylindrical form which tapers towards a region of increased diameter  4  located at the end thereof. The propeller blade root  2  carries a blade root outer sleeve  6  which can, when viewed as shown in the FIGURE, be regarded as being divided into upper and lower portions. The upper portion of the blade root outer sleeve  6  has a radially outward facing surface  8  bounded at an upper edge thereof by an outwardly extending flange  10 . The surface  8  and flange  10  cooperate to define a seat for a split upper inner bearing race  12 . The bearing race  12  has an outwardly facing bearing surface  14  which is inclined with respect to the axis of the propeller such that the radius at the uppermost edge of the bearing surface  14  is greater than the radius at the lower-most edge. The flange  10  carries an outwardly facing “U” shaped recess which carries as blade root seal  16 . 
     The upper inner bearing race  12  is held in place by a split spacer  18  having a shallow “U” shaped cross-section positioned around the blade root such that a first rim  20  of the spacer  18  bears against the bearing race  12  and a second rim  22  of the spacer  18  bears against a lower split inner bearing race  24 . 
     Upper roller bearings  30  are located between the surface  14  and an upper outer bearing race  32  which is carried on a sleeve  34 . The upper outer bearing race  32  is parallel to the upper inner bearing race  14 , and the sleeve  34  is axially slidable with respect to the propeller so as to vary the distance between the opposing surfaces of the upper bearing races. 
     The sleeve  34  carries, at an upper end thereof, a threaded portion  36  which cooperates with a locking nut  38 . 
     The sleeve  34  fits within a recess formed in the propeller hub  40 . The sleeve  34  and the walls of the recess within the propeller hub  40  cooperate to define an annular chamber  42 . The sleeve  34  and propeller hub  40  are profiled such that the sleeve  34  effectively forms a piston within a cylinder defined in the propeller hub  40  such that the introduction of fluid pressure into the chamber  42  urges the sleeve  34  to move outwardly of the propeller hub  40 , i.e. in an upward direction as shown in the figure. 
     The sleeve  34  has a radially outwardly extending lip  46  which, in use, bears against the walls of the recess and prevents wobbling of the sleeve within the recess. Ring seals  48  and  50  are provided above and below the chamber  42  so as to prevent leakage of fluid therefrom. A lower end of the sleeve  34  carries pins  52 . A ball rake  54  engages the pins  52  and extends towards the split lower inner bearing race  24 . In use, ball bearings  60  are held between the inner lower bearing surface  24  and an outer lower bearing surface  62  which is attached to the propeller hub  40 . A closeable delivery hole  64  provides a route through which the ball bearings  60  may be inserted into the space between the races  24  and  62 . 
     A fluid delivery passage  66  extends from the periphery of the propeller hub  40  to the annular chamber  42 . The delivery passage  66  has a threaded end portion such that a delivery hose or similar may be attached thereto in order to deliver fluid at pressure into the chamber  42 . 
     In order to assemble the arrangement, the split upper bearing race  12  is fitted on to the blade root outer sleeve  6 . The blade root seal  16  is then fitted into its holding groove on the blade root outer sleeve. The split spacer  18  is positioned so as to retain the upper inner bearing race  12 , and then the upper rollers  30  are positioned on the race  12 . Following this, the sleeve  34  is fitted over the rollers  30  so as to keep them in position. The ball rake  54  is then clipped on to the sleeve  34  via the pins  52 . The lower split inner race  24  is then located into its recess on the blade root outer sleeve  18 . The split spacer  6  also serves to retain the lower race  24  in position. The propeller blade root assembly is now complete. 
     In order to position the propeller blade within the propeller hub  40 , the root assembly is passed through a blade port (i.e. the open top of the recess) within the propeller hub until the blade root outer sleeve  6  rests against a secondary retention ring  70 . The ball bearings  60  are then passed through the delivery hole  64  into the space between the lower outer race  62  and the split lower inner race  24 . Once the ball bearings  60  are in position, the delivery port  64  is closed and fluid pressure, for example, from a pneumatic source, is applied to the chamber  42  via the delivery passage  66 . The fluid pressure urges the sleeve  34  to move upwardly and thereby to preload the bearings  30  and  60 . The locking nut  38  is then tightened to a nominal torque so as to bear against the upper end of the hub  40  and thereby to prevent motion of the sleeve  34  in a downwardly direction as shown in the figure. The chamber  42  is then depressurised and the delivery passage  66  is closed to prevent the ingress of dirt. The bearing assembly is now preloaded. 
     It is thus possible to provide a bearing assembly, suited for use with variable pitch propellers, which allows the bearing preload to be easily implemented and relatively well controlled.
A bearing arrangement, for example, for a variable pitch propeller, has a fluid operated actuator ( 42, 34, 40) for causing relative movement between bearing surfaces of races ( 12, 32) so as to preload bearings ( 30) held there between. Mechanical locking means ( 34, 36, 38) is provided to maintain the bearings under preload when the fluid pressure is released from the actuator.