Abstract:
An axle assembly having two hubs, each for connection to a wheel, a housing for differential gears and a mounting arrangement for a braking system for braking a connected wheel. The housing has a depression formed therein. The depression is configured so as to accommodate:
       (a) when the mounting arrangement has a drum brake mounted thereon, movement of a slack adjuster of the drum brake in a direction away from one of the hubs nearest the drum brake so as to allow detachment of the slack adjuster; and/or   (b) when the mounting arrangement has a disc brake mounted thereon, an air chamber of the braking system, wherein the air chamber is capable of being partially accommodated within the depression.

Description:
TECHNICAL FIELD 
     The present invention relates to an axle assembly. In particular, an axle assembly for a heavy vehicle. 
     BACKGROUND 
     A drive axle of a heavy vehicle generally includes a pair of axle shafts housed within a live or rigid axle housing for driving wheels secured to wheel hubs. When a vehicle is driven along a straight path, the wheel hubs will be turning at approximately the same speed, and the drive torque will be equally split between both wheels. When the vehicle negotiates a turn, the outer wheel has to travel over a greater distance than the inner wheel. A differential assembly allows the inner wheel to turn more slowly than the outer wheel as the vehicle negotiates a turn. 
     Many different kinds of differential gear have been proposed, but the most common consist of a casing in which two or four freely rotatable planetary gears are arranged in mesh with 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. 
     An example differential assembly is described in EP 1591694. 
     Two main types of braking systems are used on heavy vehicles; drum brakes and disc brakes. 
     A drum brake generally has a pair of brake shoes pivotably mounted within the drum and moveable between an applied position and a released position. The applied position causing a wheel connected to the drum brake to brake, and the released position permitting free rotation of a wheel connected to the drum brake. The brake is actuated via an actuation shaft and an actuation device, to cause the brake shoes to move to the applied position. As the brake shoes wear, a slack adjuster adjusts the actuation device to account for the wear of the brake shoes. The slack adjusters typically need maintenance through the life of a brake. Current axle assemblies require removal of multiple components of the drum brake and movement of the actuation shaft, to remove the slack adjuster for maintenance. This is an inconvenient and time consuming process. 
     A disc brake generally has a brake disc located between two opposed friction elements. To brake a connected wheel, the friction elements are moveable to an applied position, i.e. into contact with the brake disc. On heavy vehicles, the brake disc is generally actuated to an applied position using a pneumatic system. Due to the number of components, including for example vehicle suspension, that need to be assembled to the chassis of a heavy vehicle, it is preferable to arrange components of an axle assembly in a compact manner. 
     There is therefore a need for an axle assembly that permits easy assembly/disassembly of the slack adjusters of a drum brake and/or has a compact arrangement. 
     SUMMARY 
     In a first aspect the present invention provides an axle assembly having:
         two hubs, each for connection to a wheel;   a housing for differential gears;   a mounting arrangement for a braking system for braking a connected wheel; and   characterised in that the housing has a depression formed therein, the depression being configured so as to accommodate:   (a) when the mounting arrangement has a drum brake mounted thereon, movement of a slack adjuster of the drum brake in a direction away from one of the hubs nearest said drum brake so as to allow detachment of the slack adjuster; and/or   (b) when the mounting arrangement has a disc brake mounted thereon, an air chamber of the braking system, wherein the air chamber is capable of being partially accommodated within the depression.       

     Advantageously, accommodation of feature (a) eases the assembly and disassembly of the slack adjuster. In axle assemblies of the prior art to assemble/disassemble the slack adjuster it is necessary for a user to remove the wheel, and components of the braking system including drum and brake shoes and then move an adjuster shaft of the braking system away from the housing so as to create enough room to remove the slack adjuster. Providing a housing with accommodation for feature (a) permits the slack adjuster to be removed/replaced without the need to remove the wheel, the drum or the brake shoes, and there is no need to move the adjuster shaft. 
     The accommodation of feature (b) enables the assembly to be more compact compared to axles of the prior art, because the depression enables an air chamber of a braking system to be angled towards the housing. 
     Advantageously the housing of the present invention can be used with either a disc brake or a drum brake. 
     The axle assembly may comprise two half axle shafts, each connected to one wheel hub. 
     The axle assembly may comprise an actuation shaft mount for an actuation shaft of a drum brake system, and the depression being positioned adjacent said actuation shaft mount. 
     The actuation shaft may be mounted to the actuation shaft mount via a bracket. 
     The housing may comprise an air cylinder mount for mounting an air cylinder of a drum brake system, and the depression may be positioned adjacent said air cylinder mount. 
     The air cylinder may be mounted to the air cylinder mount via a bracket. 
     The axle assembly may comprise two depressions; one depression being positioned either side of the air cylinder mount. 
     In the present application reference to direction refers to the direction when the axle assembly is in conventional attachment to a heavy vehicle, and is for reference purposes only, and not intended to limit the invention to such orientation of attachment to a vehicle. 
     Reference in the present application to fore and aft direction, and the axis thereof refers to the fore and aft of a vehicle the axle assembly may be attached to, and the axis extends between these opposing directions. Reference to upward and downward direction and the axis thereof refers to the direction towards the main body of the vehicle and away from the main body of the vehicle, respectively, and the axis extending between these opposing directions. Reference to right and left refers to the right and left of the vehicle when travelling in a forwards direction, and the axis thereof extends between the two. The right and left axis is parallel to a longitudinal axis defined by the axle assembly. 
     The housing may have an aft face opposing a fore face, and an upward face opposing a downward face. Each depression may be positioned on the aft face of the housing. An air cylinder mount may be positioned on the aft face. 
     For example, the depressions may be positioned either side of the air cylinder mount. In some embodiments the depressions may be positioned towards a downward side of the aft face. Each depression may have a concave transition from a base of the depression to an aft-most region of the aft face in an upward-downward direction, and/or in right-left direction along a longitudinal axis of the axle assembly. 
     The housing may comprise a coupling on the fore face for a connector shaft to transfer drive from a prime mover to the axle assembly via a further axle assembly. 
     The housing may have an aft face opposing a fore face, and an upward face opposing a downward face, and each depression may be positioned on the aft face of the housing. An air cylinder mount may be positioned on the upward face. 
     This positioning of the air cylinder is a safe position away from any obstructions e.g. on the road. 
     The housing may comprise a coupling for an input drive shaft from a prime mover on a fore face and a coupling for a connector shaft to provide the drive for a further axle assembly on an aft face. 
     For example, one depression may be positioned on the left most side of the aft face, and one depression may be positioned on the right most side of the aft face. In some embodiments the depression on the left most side of the aft face may extend the full extent of the housing in the upward-downward direction. The right most depression may extend across only a portion of the fore face in the upward-downward direction. The right most depression may extend to the position of the coupling for the connector shaft. Each depression may have a concave transition from a base of the depression to an aft-most region of the aft face in a right-left direction along a longitudinal axis of the axle assembly. The left most depression may have a concave transition from a base of the depression to an aft-most region of the aft face in an upward-downward direction along a longitudinal axis of the axle assembly. 
     The axle assembly may comprise:
         two drum brakes each for braking one of the wheels, each drum brake having a brake shoe moveable between an applied position and a released position with an associated brake drum, an actuation shaft rotatable about a shaft axis and an actuation device for transferring rotational movement of the actuation shaft into movement of the brake shoe from the released position to the applied position;   a slack adjuster connected to the actuation shaft for moving the actuation device so as to account for wear thereof; and   differential gears within the housing for permitting the wheels to turn at different speeds when the vehicle is negotiating a corner.       

     A tandem axle assembly may comprise an axle assembly of an embodiment of the first aspect connected via a connector shaft to another axle assembly of an embodiment of the first aspect. 
     The depression formed in the housing may be configured so as to permit the slack adjuster to slide off the end of the actuation shaft without movement of the actuation shaft in the direction of a longitudinal axis of the axle assembly. 
     The axle assembly may comprise:
         at least two disc brakes each for braking one of the wheels;   a pneumatic actuation system for actuating each disc brake, the actuation system including two air chambers; and   differential gears within the housing for permitting the wheels to turn at different speeds when the vehicle is negotiating a corner.       

     Each air cylinder may be mounted to a component of the disc brakes and angled towards the housing. 
     The component of the braking system may be a brake caliper of the disc brake. 
     The air chamber may be positioned between the housing and an airbag of a suspension arrangement of the vehicle. 
     The suspension arrangement may comprise a suspension arm to which the airbag can attach, and the air chamber may be positioned above the arm when the axle is mounted to a vehicle. 
     The two air chambers may both be positioned on the same side of the axle assembly. 
     The suspension arm of the suspension arrangement may be positioned transverse to the longitudinal axis of the axle assembly. In some embodiments the arm is positioned substantially perpendicular to the longitudinal axis of the axle assemble, i.e. in the fore aft direction. The suspension arrangement may comprise four suspension arms and four airbags, each airbag being attached to one arm. Two airbags may be positioned on one side of the axle assembly and the other two airbags may be positioned on another side of the axle assembly. The airbags may be positioned so as to oppose each other. That is, two airbags may be positioned on the fore side, and two on the aft side of the axle assembly, and on each fore or aft side one airbag is positioned towards a right position of the axle assembly and the other airbag is positioned towards a left position of the axle assembly. 
     The bearing cap may contain at least one bearing and attaches the differential to the housing, and the bearing cap may comprises a depression positioned to accommodate a portion of a depression formed in the housing. 
     In a second aspect the present invention provides a vehicle having an axle assembly according to the first aspect. 
     In a third aspect the present invention provides an axle assembly comprising:
         an axle for connection to at least two wheels;   two drum brakes each for braking one of the wheels, each drum brake having a brake shoe moveable between an applied position and a released position with an associated brake drum, an actuation shaft rotatable about a shaft axis and an actuation device for transferring rotational movement of the actuation shaft into movement of the brake shoe from the released position to the applied position;   two slack adjusters one connected to each actuation shaft for moving the actuation device so as to account for wear thereof; and   a housing containing differential gears for permitting the wheels to turn at different speeds when the vehicle is negotiating a corner; and   wherein a depression is formed in the housing and is configured so as to accommodate movement of the slack adjuster in a direction away from the respective actuation shaft so as to allow detachment of the slack adjuster.       

     In a fourth aspect the present invention provides an axle assembly comprising:
         an axle for connection to at least two wheels;   two disc brakes each for braking one of the two wheels;   a pneumatic actuation system for actuating each disc brake, the actuation system including an air chamber; and   a housing containing differential gears for permitting the wheels to turn at different speeds when the vehicle is negotiating a corner; and   wherein the housing has a depression formed therein, the depression being configured so as to accommodate the air chamber, the air chamber being angled towards the housing.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described with reference to the accompanying drawings, in which: 
         FIG. 1  is a rear view of a fore axle assembly of the prior art; 
         FIG. 2  is a rear view of an aft axle assembly of the prior art; 
         FIG. 3  is a rear view of a fore axle assembly according to an embodiment of the present invention; 
         FIG. 4  is a rear view of an aft axle assembly according to an embodiment of the present invention; 
         FIG. 5  is a schematic profile plot of an aft face of a housing of the axle assembly of  FIG. 3 ; 
         FIG. 6  is a schematic profile plot of an aft face of a housing of the axle assembly of  FIG. 4 ; 
         FIG. 7  is a rear view of the fore axle assembly of  FIG. 3  comprising two drum brakes; 
         FIG. 8  is a rear view of the fore axle assembly of  FIG. 7 ; 
         FIG. 9  is a view of an aft and upward side of the axle assembly of  FIG. 7 ; 
         FIG. 10  is a view of an aft and upward side of the axle assembly of  FIG. 7 ; 
         FIG. 11  is a view from a downward direction of the axle assembly of  FIG. 7 ; 
         FIG. 12  is a rear view of the aft axle assembly of  FIG. 4  comprising two drum brakes; 
         FIG. 13  is a rear view of the aft axle assembly of  FIG. 12 ; 
         FIG. 14  is a view of an aft side and a downward side of the aft axle assembly of  FIG. 12 ; 
         FIG. 15  is a rear view of an aft axle assembly of  FIG. 12 ; 
         FIG. 16  is a view from a downward direction of  FIG. 12 ; 
         FIG. 17  is a perspective view of the fore axle assembly of  FIG. 3  when the axle assembly comprises two disc brakes; 
         FIG. 18  is a plan view of the fore axle assembly of  FIG. 17 ; 
         FIG. 19  is a view of an air chamber and air bag of the fore axle assembly of  FIG. 17 ; 
         FIG. 20  is a view of an air chamber and air bag of the fore axle assembly of  FIG. 17 ; 
         FIG. 21  is a plan view of the fore axle assembly of  FIG. 17  in a worn condition; 
         FIG. 22  is a view of an air chamber and air bag of the fore axle assembly of  FIG. 21 ; 
         FIG. 23  is a view of an air chamber and air bag of the fore axle assembly of  FIG. 21 ; 
         FIG. 24  is a perspective view of an aft axle assembly of  FIG. 4  when the axle assembly comprises two disc brakes; 
         FIG. 25  is a plan view of the aft axle assembly of  FIG. 24 ; 
         FIG. 26  is a view of an air chamber and air bag of the aft axle assembly of  FIG. 24 ; 
         FIG. 27  is a view of an air chamber and air bag of the aft axle assembly of  FIG. 24 ; 
         FIG. 28  is a plan view of the aft axle assembly of  FIG. 24  in a worn condition; 
         FIG. 29  is a view of an air chamber and air bag of the aft axle assembly of  FIG. 28 ; 
         FIG. 30  is a view of an air chamber and air bag of the aft axle assembly of  FIG. 28 ; 
         FIG. 31  is a schematic of the arrangement of axle assemblies on a heavy vehicle; 
         FIG. 32  is a schematic side view of the heavy vehicle; 
         FIG. 33  is a schematic comparison of the positioning of an air chamber and air bag of the prior art compared to the axle assembly of  FIGS. 17 to 30 . 
         FIG. 34  shows a view through the centre of housing of an aft axle assembly of the prior art; 
         FIG. 35  shows a view through the centre of a housing of an aft axle assembly according to an embodiment of the invention; 
         FIG. 36  shows a view through the centre of a housing of a fore axle assembly of the prior art; and 
         FIG. 37  shows a view through the centre of a housing of a fore axle assembly according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. 
     Referring to  FIGS. 31 and 32 , a heavy vehicle is indicated generally at  2 . Heavy vehicles often have a prime mover  9 , for example an engine, at the front of the vehicle above a steered axle  4  that may or may not be driven by the prime mover. In addition to the axle assembly at the front of the vehicle, many heavy vehicles have two axle assemblies towards the rear of the vehicle, i.e. a rear tandem axle assembly. The following description will describe in more detail the rear tandem axle assembly and refer to the axle assembly of the tandem axle assembly, nearest the prime mover  9  as the fore axle assembly  6 , and the axle furthest from the prime mover as the aft axle assembly  8 . A drive shaft  7  is connected between the prime mover  9  and the fore axle assembly  6 , and a connector shaft  5  is connected between the fore axle assembly  6  and the aft axle assembly  8 . Reference to the positioning relative to the prime mover  9  is specific to certain embodiments and is not intended to limit the invention to this specific arrangement, and in other embodiments the prime mover may be in a different position. Additionally, the present invention may be applicable to a rear single axle assembly. 
     In the following description the fore direction F is the direction towards the front of the heavy vehicle  2  in the usual direction of travel and the aft direction A is the direction towards the rear of the heavy vehicle  2 . The right direction R is the direction towards the right of the vehicle and the left direction L is the direction towards the left of the vehicle. The upward direction U is the direction from the axle assemblies towards a body  3  of the vehicle and the downward direction D is the direction from the axle away from the body of the vehicle (e.g. when travelling on the ground, the direction towards the ground). 
     Axle assemblies of the present invention will be described further below, and in each case the directions are described as though the axle assembly were positioned on the vehicle  2 . 
     A fore axle assembly of the prior art is indicated generally at  110   b  in  FIG. 1 , and an aft axle assembly of the prior art is indicated generally at  110   a  in  FIG. 2 . Provided on the axis is a housing  118   a ,  118   b  within which is positioned differential gears (not shown), the housing being enlarged with respect to other components of the axle assembly. The housing  110   b  of the fore axle assembly has a coupling (not shown in the figures) for receiving a drive shaft from a prime mover (e.g. the prime mover  9  of  FIGS. 31 and 32 ). The housing  110   b  of the fore axle assembly also has a coupling for a connector shaft that extends between the housing  110   b  of the fore axle assembly and a coupling (not shown) of the housing  110   a  of the aft axle assembly. The connector shaft transfers the drive from the drive shaft to the aft axle assembly. A half shaft  112   a ,  112   b  is positioned to the right R and the left L side of the housing  118   a ,  118   b , and are surrounded in part by an axle casing  114   a ,  114   b . The drive shaft provides the drive to rotate the half shafts  112   a ,  112   b  and as such the wheels of the vehicle when attached to the axle assembly are caused to rotate. The two half shafts within the axle casing define an axis  116   a ,  116   b  which extends in the longitudinal direction of the axle, i.e. along the axis of right R to left L. 
     Two housings for differential gears, each according to embodiments of the present invention, are shown in  FIGS. 3 and 4 . One of the housings is indicated generally at  10   b  in  FIG. 3 , and is intended for use on a fore axle assembly and the other housing is indicated generally at  10   a  in  FIG. 4 , and is intended for use on an aft axle assembly. The numbering of features in  FIGS. 3 and 4  is similar, with a suffix of “a” indicating a feature of a housing intended for an aft axle assembly and a suffix of “b” indicting a feature of a housing intended for a fore axle assembly. 
     Referring to  FIGS. 3 and 4  the housing  18   a ,  18   b  is a cast component and has four principal faces with rounded edges connecting between the faces in a circumferential direction. Two of the faces are positioned along the fore-aft axis, and extend along the upward-downward axis and the right-left axis. One face is positioned to the fore of the housing (the fore face) and one face is positioned to the aft of the housing (the aft face). A further two faces of the housing are positioned along the upward-downward axis and extend in the direction of the fore-aft axis and right-left axis. One of the faces is positioned on the upward side of the housing (the upward face) and the other face is positioned on the downward side of the housing (the downward face). 
     A longitudinal axis  16   b ,  16   a  defined by the half axle shafts of the axle assembly  10   b ,  10   a  extends longitudinally through the housing  18   a ,  18   b.    
     Now referring to  FIG. 3 , the housing  18   b  of the fore axle assembly  10   b  will be described in more detail. The fore face of the housing  18   b  has a coupling (not shown in  FIG. 3 , but indicated at  19   b  in  FIG. 17 ) for connection of the axle assembly to a drive shaft from a prime mover. The aft face of the housing  18   b  has a drive coupling  20   b  for connecting a connector shaft between the fore axle assembly and the aft axle assembly (similar to that described for the prior art) so as to transfer drive from the drive shaft to the aft axle assembly. In this embodiment, the coupling  20   b  is positioned in a region of the fore face that is towards the upward side of the face and towards the right side of the face. 
     The housing  18   b  reduces in diameter at the left-most and right-most extents of the housing. At the left-most and right-most extent of the housing and on the same side of the axle assembly as the fore face of the housing are positioned two actuation shaft mounts  22   b . The actuation shaft mounts  22   b  may be positioned on the housing or on another component of the axle assembly, e.g. an axle casing. In this embodiment, the actuation shaft mounts are also on the same side of the axle assembly as the downward side of the aft face of the housing. In use, an actuation shaft may be mounted to the actuation shaft mount via a bracket (described in more detail later). 
     An air cylinder mount  24   b  is positioned on the upward-face of the housing. In use, one or more air cylinders are mounted to the housing at the air cylinder mount via a bracket (described in more detail later). 
     Contrary to housings of the prior art, the housing  10   b  of the present invention has two depressions  26   b ,  28   b  formed in the aft face. One of the depressions  26   b  is towards the left-most side of the aft face, near one of the actuation shaft mounts, and the other depression  28   b  is towards the right-most side of the aft face, near the other actuation shaft mount. 
       FIG. 5  shows a general profile (not to scale) of the profile of the aft face of the housing from left to right, directly through the centre of the depressions  26   b  and  28   b . The F-A axis of  FIG. 5  indicates the deviation of the profile in the fore-aft direction. Referring to  FIGS. 3 and 5 , it can be seen that the fore face is profiled such that the diameter is narrowest at the left-most (i) and right-most (ix) sides of the aft face. When viewed from left to right, the diameter gradually increases (ii) from the left-most region (i) of the aft face to the base (iii) of the depression  26   b . The diameter then again increases (iv), with a steeper gradient, to the aft-most surface (v) of the aft face. Then at a position towards the right-most side of the aft face, the diameter decreases (vi) to a base (vii) of depression  28   b . The diameter then further decreases (viii) to connect with the right-most (ix) side of the aft face. 
     It can be seen from  FIGS. 3 and 5  that the progression from the base (iii) of the depression  26   b  to the aft-most region (iv) of the aft face is concave, and also from the base (vii) of the depression  28   b  to the aft face to the aft-most portion (iv) of the aft face is concave. 
     Referring to  FIG. 3 , it can be seen that the depression  26   b  at the left-most side of the aft face extends across the aft face in the upward-downward direction to the extremities of the aft face. The depression  28   b  at the right-most side of the aft face also extends in the upward-downward direction, but only extends across a portion of the aft face. The progression from the aft-most region of the aft face to the base of the depression  28   b  is concave in the upward-downward direction as well as in the left-right direction. The depression  28   b  is further extended in the upward direction, with a gradually increasing diameter from the base of the depression to a position where the depression  28   b  meets or is near to the coupling  20   b.    
     Now referring to  FIG. 4 , the housing  18   a  of the aft axle assembly  10   a  will be described in more detail. The fore face of the housing  18   a  has a coupling (not shown in  FIG. 4 , but shown in  FIG. 28  at  19   a ) for receiving a connector shaft from the fore axle assembly (similar to that described for the prior art). 
     The aft face of the housing  18   a  has an air cylinder mount  24   a  positioned centrally along the left-right axis of the housing and near to the downward side of the aft face. The air cylinder mount  24   a , in use, mounts a bracket for connecting one or more air cylinders to the housing. 
     The housing  18   a  reduces in diameter at the left-most and right-most extents of the housing. At the left-most and right-most extent of the housing and on the aft face are positioned two actuation shaft mounts  22   a . In this embodiment, the actuation shaft mounts are on the upward side of the aft face. In use, an actuation shaft is mounted to the actuation shaft mount via a bracket (described in more detail later). 
     Contrary to housings of the prior art, the housing  18   a  of the present invention has two depressions  26   a ,  28   a  formed in the aft face. One of the depressions  26   a  is towards the left-most side of the aft face, near one of the actuation shaft mounts, and the other depression  28   a  is towards the right-most side of the aft face, near the other actuation shaft mount. 
       FIG. 6  shows a general profile (not to scale) of the profile of the aft face of the housing  18   a  from left to right, directly through the centre of the depressions  26   a  and  28   a . The F-A axis in  FIG. 6  indicates the deviation of the profile in the fore-aft direction. Referring to  FIGS. 4 and 6 , it can be seen that the aft face is profiled such that the diameter is narrowest at the left-most (xi) and right-most (xix) side of the aft face. When viewed from left to right, the diameter gradually increases (xii) from the left-most (xi) region of the aft face to the base (xiii) of the depression  26   a . The diameter then again increases (xiv), with a steeper gradient, to an aft-most surface (xv) of the aft face. Then at a position towards the right-most side of the aft face, the diameter decreases (xvi) to a base (xvii) of depression  28   a . The diameter then further decreases (xviii) to connect with the right-most (xix) side of the aft face. 
     It can be seen from  FIGS. 4 and 6  that the progression from the base (xiii) of the fore face to the fore-most surface (xiv) of the fore face is concave, and also from the base (xvii) of the depression  28   a  to the fore-most surface (xiv) of the fore face is concave. 
     Referring to  FIG. 4 , it can be seen that the depression  26   a  at the left-most side of the aft face extends across approximately half of the aft face in the upward-downward direction, and extends from the downward-most side of the aft face towards the position of the axis  16   a  of the housing. Similarly, the depression  28   a  at the right-most side of the aft face extends across approximately half of the aft face in the upward-downward direction, and extends from the downward-most side of the aft face towards the position of the axis  16   a  of the housing. 
     The progression from an aft-most surface of the aft face to the base of each depression  28   a  and  28   b  is concave in the upward-downward direction as well as in the left-right direction. 
     The housing  18   b  in a fore axle assembly is shown in  FIGS. 7 to 11 . In this embodiment, the fore axle assembly comprises two drum brakes  230   b  at the left most end and right most end of the axle assembly mounted to the axle assembly via a hub at each end of the axle assembly (hidden in the Figures due to the presence of the drum brakes). The housing  18   b  is positioned centrally on the axle assembly and connected to two half axle shafts one on each side. The axle half shafts are positioned within an axle casing  232 . 
     The drum brake  230   b  in many embodiments has a pair of brake shoes moveable between an applied position and a released position. The applied position causing a wheel connected to the drum brake to brake, and the released position permitting free rotation of a wheel connected to the drum brake. An actuation shaft  234   b  is rotatable about an axis along the longitudinal direction of the actuation shaft. An actuation device, for example an s-cam, transfers rotational movement of the actuation shaft  234   b  into movement of the brake shoe from the released position to the applied position. A slack adjuster  236   b  is connected to the actuation shaft  234   b  via a splined connection  238   b , and is operable to account for wear of the brake shoes in a known manner. 
     Two air cylinders  240   b  are mounted to the housing  18   b  at the air cylinder mount  24   b  via a bracket  242   b . A rod  244   b  extends from each air cylinder. In normal use, i.e. not during maintenance, each rod  244   b  attaches to a free end of one of the slack adjusters  236   b . The air cylinders are operable to transfer movement via the rod  244   b  and the slack adjuster  236   b  to the actuation shaft  234   b  so as to actuate the brake shoe to an applied position. 
     In the present embodiment, the slack adjuster  236   b  is cranked and connects between the rod  244   b  and the actuation shaft  234   b . The crank accounting for the fact that the rod  244   b  is further towards the centre of the housing, along the left-right axis, than the actuation shaft  234   b . In alternative embodiments, the slack adjuster may not be cranked. The slack adjuster  236   b  connects to the rod  244   b  via a c-shaped connector  246   b  at the end of the rod. A pin (not shown) goes through the c-shaped connector and the slack adjuster  236   b  to secure the connection. The slack adjusters  236   b  are positioned such that they are within a space created by the depressions  26   b ,  28   b  formed in the housing  18   b.    
     The slack adjuster  236   b  may need replacing or removing so as to permit maintenance. To disconnect the slack adjuster  236   b  from the axle assembly  210   b  the pin is removed from the c-shaped connector  246   b  and slack adjuster  236   b . The slack adjuster  236   b  is then moved out of engagement with the c-shaped connector  246   b . Once the slack adjuster  236   b  is disengaged from the c-shaped connector, the slack adjuster  236   b  can be slid along the splined connection  238   b  of the actuation shaft to an extent that the slack adjuster  236   b  disconnects with the actuation shaft  234   b . That is, the slack adjuster  236   b  on the left side of the housing  18   b  is moved towards the right side of the housing, and the slack adjuster on the right side of housing  18   b  is moved towards the left side of the housing. Once disconnected from the actuation shaft the slack adjuster can be simply removed. The axle assembly with the slack adjuster in a disconnected position is shown in  FIGS. 9 to 11 . 
     To reconnect a slack adjuster  236   b  or connect a new slack adjuster, the above procedure is followed in reverse. That is, the slack adjuster is slid on to the splined connection  238   b  of the actuation shaft so as to connect to the actuation shaft  234   b . The slack adjuster is then moved to engagement with the c-shaped connector of the rod, and the pin is positioned between the slack adjuster and c-shaped connector so as to connect the rod and the slack adjuster. 
     The depressions  26   b  and  28   b  permit the slack adjuster  236   b  to be disconnected from the actuation shaft  234   b  without the need to remove any other components (except the pin that connects the slack adjuster to the rod) of the axle assembly  210   b . This is advantageous over housings of the prior art. In housings of the prior art it is necessary to remove the wheel, the drum, the brake shoes and move the actuation shaft so as to assemble/disassemble the slack adjuster. This is time consuming and therefore inconvenient for a user. 
     The housing  18   a  in an aft axle assembly  210   a  is shown in  FIGS. 12 to 16 . In this embodiment, the aft axle assembly comprises two drum brakes  230   a  at the left most end and right most end of the axle assembly. The aft axle assembly has many features in common with the fore-axle assembly  210   b . The common features will not be described further. 
     In this embodiment, two air cylinders  240   a  are attached to the aft face of the housing  18   a  at the air cylinder mount  24   a  via a bracket  242   a . The air cylinders are arranged such that the longitudinal axis  250   a  of each air cylinder is substantially parallel to the upward-downward axis. 
     As with the housing  18   b  of the fore axle assembly  210   b , the axle assembly has slack adjusters  236   a  positioned within the depressions  26   a  and  28   a  of the housing, and connected between an actuation shaft  234   a  and a rod  244   a  that connects to the air cylinders  240   a.    
     The slack adjuster  236   a  is removed and reconnected in a similar way to that described for the fore axle assembly and the same advantages in terms of ease of assembly and disassembly of the slack adjuster are achieved. 
     The housing  18   b  in a fore axle assembly is shown in  FIGS. 17 to 23 . In this embodiment, the fore axle assembly  310   b  comprises two disc brakes  330   b  at the left most end and right most end of the axle assembly instead of the drum brakes shown previously. The housing  18   b  is connected centrally to the axle assembly  310   b , such that it connects to both half shafts of the axle assembly (similar to the half shafts discussed for the prior art axle assembly). 
     Associated with each disc brake is an air chamber  340   b . The air chambers are both positioned on the aft side of the axle assembly, with one being positioned near the left side of the housing (i.e. close to one of the disc brakes) and the other being positioned near the right side of the housing (i.e. close to the other disc brake). Each air chamber is mounted to a caliper  356   b  of the disc brake which is in turn mounted on a carrier  357   b  that is secured to the axle casing  232   b.    
     In this embodiment, a suspension system of the axle assembly is also illustrated. The previously described embodiments also utilised a suspension system, but it was not illustrated as it is not pertinent to the function of the invention. The suspension system comprises four suspension arms  352   b . A pair of suspension arms is connected to the axle casing to the left of the housing. One of the arms of the pair extends from the position of the axle casing substantially in the aft direction and one substantially in the fore direction. The other pair of suspension arms are connected to the axle casing to the right of the housing. One arm of each pair extends from the position of the axle casing substantially in the aft direction and one substantially in the fore direction. Each pair of suspension arms are connected to the axle casing using two u-shaped brackets. The brackets extend around the axle casing and bolt to a plate that is positioned on the downward side of the pair of suspension arms. 
     Connected to the upward side of each suspension arm is a cylindrically shaped air bag  354   b . The air bags  354   b  are positioned and designed so as to absorb undulations in the road and minimise transfer of such undulations to an user of the vehicle. The pneumatic system of the vehicle may be arranged such that a compressor and reservoir of air supplies both the air chambers and the air bags with the required air supply. 
     The housing  18   b  of the present embodiment permits the air chambers  340   b  to be angled towards the longitudinal axis of the axle assembly, that extends in the right-left direction, i.e. the air chambers  340   b  are arranged at a negative angle with respect to the longitudinal axis of the axle assembly. 
     The angled position of the air chamber is illustrated at  60  in  FIG. 33 . For comparison, the air chamber and airbag arrangement of an assembly of the prior art is illustrated at  161  in  FIG. 33 . 
     In axle assemblies of the prior art, the air chamber  140   d  is substantially parallel to the longitudinal axis  62  of the axle assembly. 
     The air chambers  340   a, b  of the axle assemblies of embodiments of the present invention are angled towards the housing, such that a longitudinal axis  66  extending through the longitudinal length of an air chamber forms an acute angle with the longitudinal axis  62  when measured in a clockwise direction from the longitudinal axis. The described negative angling of the air chamber means that a tangent  68  to the air bag  354   a, b  of an embodiment of the present invention is closer to the longitudinal axis than a tangent  69  to an air bag  154  of the prior art  161  by a distance Z. Thus the axle assembly is more compact than axle assemblies of the prior art in a fore-aft direction. 
     In use, as friction material and a brake disc of the disc brake wear, the brake caliper moves to account for the wear. Accordingly, the air chambers  340   b  also move. The movement of the brake caliper and the air chambers is towards the housing  18   b  of the axle assembly. 
     The depressions  26   b ,  28   b  provide a space into which the air chambers can travel as the disc brakes wear, which avoids impact with the housing, and therefore permits the air chambers to be positioned at a negative angle. The axle assembly in a worn position is shown in  FIGS. 21 to 23 . Thus, the housing of the present invention advantageously permits the axle assembly to be more compact than axle assemblies of the prior art. 
     An aft axle assembly  310   a  having a housing  18   a  of the present invention is shown in  FIGS. 24 to 30 . The main features and advantages of this axle assembly are similar to those of the fore axle assembly  310   b , so will not be described in detail again.  FIGS. 28 to 30  show the axle assembly and position of the air chambers when the friction material and/or brake disc of the disc brake are worn. 
     To enable the housing  18   a ,  18   b  to be modified in the way described above it is necessary to alter the shape of components within the housing  18   a ,  18   b .  FIGS. 35 and 37  illustrate these alterations.  FIGS. 34 and 36  show differentials of the prior art and are provided for comparison purposes. 
     Referring to  FIG. 34  a housing  118   a  of an aft axle assembly of the prior art is shown and referring to  FIG. 35  a housing  18   a  of an aft axle assembly according to an embodiment of the invention is shown. The common features will now be discussed. The housing  118   a ,  18   a , is provided in two parts which are connected together via a flange  170   a ,  70   a . A carrier  172   a ,  72   a  of the differential is within the housing  118   a ,  18   a . A bearing cap  174   a ,  74   a ,  176   a ,  76   a  is positioned at the right and left side of the carrier  172   a ,  72   a . The bearing caps  174   a ,  74   a ,  176   a ,  76   a  are used to retain bearings and attach the differential to the axles housing. 
     Referring to  FIG. 35 , the width  78   a  of the carrier in the upwards-downwards direction is shorter than the comparable width  178   a  of the carrier of the prior art (shown in  FIG. 34 ). Further, the upper left corner  80   a  of the differential has a similar inner arc of a radius comparable to an inner arc of corner  180   a  of the prior art, but has an outer arc of reduced radius compared to a comparable outer arc of corner  180   a  of the prior art. 
     The bearing cap  74   a  positioned on the left side of the carrier comprises a concave depression  82   a . To accommodate the depression  82   a , a screw  83   a  that secures the bearing in the bearing cap is positioned offset from a central axis in the left-right direction. In this embodiment, a centre of the depression  82   a  is offset in the downward direction to a central axis in the left-right direction and the screw  83   a  is offset in an upward direction to a central axis in the left-right direction. 
     Referring to  FIG. 36  a housing  118   b  of a fore axle assembly of the prior art is shown and referring to  FIG. 37  a housing  18   b  of a fore axle assembly according to an embodiment of the invention is shown. Features in common with the housings  118   a ,  18   a  of the aft axle assemblies shown in  FIGS. 34 and 35  will not be described again here. 
     Referring to  FIG. 37 , the profile on an inner and outer perimeter of the housing  18   b  of an upper left hand corner  80   b  is chamfered to a greater extent than a comparable corner  180   b  of the prior art. In this embodiment, an arched concave formation  81   b  is provided on an inner side of the corner  80   b.    
     The bearing cap  74   b  on the left hand side of the carrier comprises a concave depression  82   b  across the thickness of the bearing cap and positioned substantially central to the bearing cap in an upwards-downwards direction. The bearing cap  76   b  on the right hand side of the carrier comprises a planar depression  86   b . The depression  86   b  is positioned on the right-most side of the bearing cap and extends across just under half the length of the bearing cap in the right-left direction. The planar depression has a substantially flat profile and extends in a upward-downwards direction. Due to the presence of the depressions  82   b ,  86   b  in the bearing caps  74   b ,  76   b , screws  85   b  (only one shown) connecting bearings to the bearing caps are positioned offset from a central axis in a right-left direction, whereas in the prior art the comparable screws  183   b ,  185   b  are coincident with a central axis in the right-left direction. 
     The bearing caps and housing undergo substantial loading during use. However, the present inventors have found using finite element modelling techniques that the removal of material from the bearing caps and changing the shape of the housing in the manner described does not unduly affect the load bearing performance of the bearing caps or housing. 
     While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.