Abstract:
A decoupler for coupling/decoupling drive between a power train output and road wheels or marine drive means in an amphibious vehicle, is integrated with a constant velocity joint, saving space, weight, and cost for low production volumes, and simplifying mounting arrangements. Power train output shaft enters casing through aperture, and terminates in a flange with splines. Baulk ring and synchrocone provide synchromesh action between drive ring and driven CV joint cap. Rod is attached to selector arm, located in slot, allowing coupling and decoupling by an external control, which may be assisted by a pneumatic or hydraulic cylinder. The synchromesh parts may be sourced from truck gearboxes. CV joint is mounted in bearings, and may be a Rzeppa type. Groove allows fitment of a conventional dust gaiter.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an amphibious vehicle having a decoupler for coupling/decoupling a drive shaft of the vehicle, and more particularly to an amphibious vehicle having a drive-shaft decoupler for engaging and disengaging a drive shaft which driveably connects an output from a power train of the vehicle with the wheels or marine propulsion system of the vehicle. 
     It has been found convenient to drive the marine propulsion systems of an amphibious vehicle through the transmission by which the wheels are also driven. With this arrangement it is necessary to disengage the drive to the wheels while the drive to the marine propulsion is engaged when the amphibious vehicle is in water mode. It is also desirable to be able to decouple and couple the drive to the wheels and marine propulsion system independently of one another as the vehicle makes the transition between land and marine modes of travel. 
     In the case of a transmission which incorporates a final-drive and differential unit as an integrated part of the whole transmission, it would only be possible to incorporate a decoupler into the transmission by designing a new internal arrangement. For amphibious vehicles, which are used in specialized applications and are produced in relatively low volumes, a dedicated transmission would be prohibitively expensive. 
     Therefore, on an amphibious vehicle in which the engine drives the road wheels and the marine propulsion system through an integrated transmission/differential, it has been found necessary to provide external drive shaft decouplers to disconnect the drive between the differential and the driven wheels, and between the transmission and the marine propulsion system. Typically a decoupler is provided in the driveline between the transmission and each driven wheel. However, it is possible to use a decoupler in the driveline between the transmission and only one of the driven wheels, since disconnecting drive to one of the driven wheels will effectively disengage drive to both wheels due to the effects of the differential. 
     A problem with known external drive shaft decouplers is the amount of extra space they require. This is a particular problem in amphibious vehicles in which the wheels are designed to retract upwardly and inboard of the vehicle for use of the vehicle in water. In such vehicles the provision of wheel retraction systems and specialized suspension systems reduces the available space for external drive shaft decouplers. 
     A further problem which arises is the need to synchronise the speeds of the input and output means of the decoupler when drive is being coupled. This problem arises, for example, when the vehicle is preparing to leave the water with the wheels deployed. In these circumstances it is necessary for drive to be maintained to the marine propulsion system, in order to push the vehicle towards the shore, whilst drive to the road wheels is coupled. This enables the vehicle to propel itself out of the water using a combination of drive from the marine propulsion system and the road wheels. It is necessary, therefore, for the decoupler to have a clutch means to progressively engage the drive between the differential output of the transmission (which may be spinning at 1000 RPM), and a rotating assembly consisting of the drive shaft, brake disc, hub and wheel (which initially will be stationary in the water) representing the inertia to be overcome. 
     A similar problem arises when the vehicle enters the water, when it is desirable to couple drive to the marine propulsion system whilst drive to the road wheels is maintained. This enables a smooth transition from land to waterborne use of the vehicle but requires a stationary marine propulsion system to be coupled to a rotating power take-off shaft of the transmission. 
     Furthermore, where the decoupler is to be used in the driveline between the transmission and a drive shaft for a wheel of an amphibious vehicle, the decoupler must be capable of handling the high torque loads which are experienced by the drive shaft. For example very high torque loads are experience in such drive shafts when drive to the wheels is engaged, wherein engine torque (say 250 Nm) is multiplied by a first gear ratio (say 4:1) times a final drive ratio (say 3.5:1):
 
250×4×3.5=3500 Nm
 
     When shock-loads from wheel/ground-contact torque reactions are factored in, it is common to allow for 10-12,000 Nm peak torque loads for the drive shafts of an average sized road vehicle. 
     It is an object of the invention to provide an amphibious vehicle having a decoupler which is capable of meeting the above requirements and which requires less space than known decouplers. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention there is provided an amphibious vehicle comprising a decoupler for coupling/decoupling drive between an output from a the vehicle power train and a component to be driven, characterised in that the decoupler comprises a synchroniser adapted to synchronise the speed of an input means and an output means of the decoupler when drive is being coupled, and in that the decoupler forms part of an integrated unit also comprising a constant velocity joint. 
     By integrating a decoupler with synchroniser and a constant velocity joint into a single unit, the integrated unit can be positioned in the space usually taken up by a conventional constant velocity joint. Thus, the integrated unit requires less space than would be the case if a separate decoupler and constant velocity joint were to be used. Furthermore, there is a reduction in the number of components required and the mounting arrangements are simplified. This reduces the weight of the vehicle, the manufacturing and assembly costs of the vehicle and improves reliability. 
     In a particularly preferred embodiment the synchroniser is a synchromesh device comprising a baulk-ring and a cone, the baulk-ring and cone of the synchroniser providing a graduated drive engagement means. 
     This arrangement has the added advantage that a known synchromesh device from a conventional gearbox can be adopted for use in the decoupler. This enables the synchroniser device of the decoupler to be manufactured using commercially available components at much lower cost than would be the case if the components were to be purpose designed and manufactured. 
     Whilst known synchromesh devices, as found in a conventional manual gearbox, perform the tasks of clutching and coupling/decoupling in a combined operation as an integrated mechanism, such devices have not previously been employed for other than their designed purpose of manual gear selection inside the conventional gearbox. In this respect it should be noted that the inertia loads residual in gearbox internal shafting are very much lower than those of a drive shaft and wheel submerged in water. Also drive torque in a gearbox is not subject to final-drive multiplication and extreme wheel-to-ground shock-loading. 
     However, extensive analysis and tests have indicated that the clutching requirement of a drive shaft decoupler for an amphibious vehicle could be sustained by a synchromesh comprising a baulk-ring and synchro-cone, provided it is subjected to a greatly reduced number of application cycles than that normally experienced in the life of a gearbox. 
     Subsequent extensive testing based on the precise requirements of a drive shaft decoupler for an amphibious vehicle with regard to inertia loadings, road-load inputs, torsional vibration frequencies, and amphibious duty-cycles, have proved that a heavy goods vehicle gearbox synchromesh is capable of performing these unaccustomed tasks of high-inertia clutching and high-torque coupling, while fitting within the cost and space specifications when designed into a dedicated decoupler casing as shown herewith. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An embodiment of the present invention will now be described by way of example only with reference to the accompanying drawings in which: 
         FIG. 1  is a section through a decoupler of an amphibious vehicle in accordance with the invention, the decoupler is shown in the position in which drive is disengaged or decoupled; 
         FIG. 2  is view similar to that of  FIG. 1  but showing the decoupler in the position in which drive is engaged or coupled; and 
         FIG. 3  is a schematic plan view in section of an amphibious vehicle in accordance with the invention, showing a drive train including an engine, gearbox and three decouplers of the type shown in FIGS.  1  and  2 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring firstly to  FIG. 1 , a decoupler incorporating a constant velocity CV joint is indicated generally at  10 . The decoupler  10  is of the form of an integrated unit, which is housed in a casing  12 . A driving shaft  14 , which may be from the output stage of a gearbox or from a differential (not shown), enters the casing  12  through a circular aperture  16  to the left hand side of the casing  12  (as viewed), and is free to rotate within the casing. An oil seal  18  seals between the driving shaft  14  and the aperture  16 . The driving shaft  14  comprises an input of the decoupler  10 . 
     The driving shaft  14  terminates inside the casing  12  in a flange  20 , the periphery of which is splined  22 . A drive ring  24 , which is correspondingly internally splined, is in permanent driving engagement with the spline  22  and rotates with the driving shaft  14 . A circumferential slot  26  is provided in the periphery of the drive ring  24 , and a selector arm  28  locates in the slot  26 . A rod  30 , which is mounted for reciprocating movement, indicated by arrow A, in a bore  32  of the casing  12 , mounts the selector arm  28  at one end. A linkage (not shown) is provided to enable an operator of an associated vehicle ( 80 ,  FIG. 3 ) to selectively slide the rod  30  and selector arm  28  to a required position. Alternatively the movement of the rod  30  can be remotely controlled by means of a pneumatic or hydraulic cylinder (not shown). 
     A CV joint indicated generally at  34  is rotatably mounted in ball bearings  36  in the right hand side of the casing  12  (as viewed). An oil seal  38  seals between the casing  12  and the CV joint  34 , with the roller bearings  36  sealed in a protected position inside the casing  12 . The oil seal  38  and the roller bearings  36  are positionally aligned and supported in the casing  12  by a pair of circlips  40 . 
     The end of the CV joint  34  facing the flange  20  of the driving shaft  14  is stepped at  42 . A stepped cap  44  is rigidly mounted on the stepped end  42 . The periphery of the outer step of the cap  44  is splined at  46 , and the periphery of the inner step is provided with a synchrocone  48 . The spline  46  has the same form as the spline  22 . 
     A baulk ring  50  formed as a truncated cone extending into a flange, is located between the stepped cap  44  of the CV joint  34 , and the flange  20  of the driving shaft  14 . The periphery of the flange of the baulk ring  50  is splined at  52 , and the spline  52  also has the same form as the splines  22  and  46 . The flange  20 , the baulk ring  50  and the outer step of the cap  44  are of the same diameter and are concentric. 
     The CV joint  34  is of the “RZEPPA” type, and comprises splines  54 , a plurality of roller balls  56 , and a driven shaft  58  which comprises an output of the decoupler. Typically there are three or four roller balls  56  mounted in equally spaced arrangement about the periphery of the end of the drive shaft  58 . Tie CV joint  34  is capable of an articulation of up to 45° away from the axis of the driving shaft  14  and includes a conventional dust gaiter (not shown) which extends between the drive shaft  58  and a groove  59  provided in the body of the CV joint  54 . 
     Although in the preferred embodiment the CV joint is of the Rzeppa type, it should be understood that any suitable type of CV joint could be used. For example, the CV joint could be any of the following: Tracta, Weiss, Tripode, AC, VL, UF, UFC, GI, GE, GIC, ARR or Triplan type. 
     The operation of the decoupler  10  will now be described with reference also to FIG.  2 . In  FIG. 1  the decoupler  10  is shown uncoupled and coupling is effected by movement of the rod  30  to the right as viewed. As the rod  30  is moved, for example, by a hydraulic cylinder, (not shown) the selector arm  28 , which is engaged in the circumferential slot  26  of the drive ring  24 , moves the drive ring  24  in the spline  22  towards the CV joint  34 . The internal spline of the drive ring  24  engages the spline  52  of the baulk ring  50 , and pushes the baulk ring  50  onto the synchro-cone  48 . The baulk ring  50  rotates at the speed of the driving shaft  14  and the frictional contact between the conical part of the baulk ring  50  and the synchro-cone  48 , synchronises the speed of the stepped cap  44  and the CV joint  34  with the speed of the driving shaft  14 . The arrangement of the baulk ring  50  synchro-cone  48  and drive ring  24  is commonly known as a synchromesh device. 
     Further movement of the drive ring  24  by the selector arm  28  causes the drive ring  24  to slide to a position engaging both the spline  46  of the cap  44  and the spline  22  of the driving shaft flange  20 , as shown in FIG.  2 . The full torque of the driving shaft  14  can then be passed through the drive ring  24  to the driven shaft  58  of the CV joint  34 . 
     Referring now to  FIG. 3 , an amphibious vehicle, indicated generally at  80 , has a transverse power train  60 . The power train  60  comprises an engine  62 , an in-line gear box  64 , a differential unit  66  driven from the gearbox and a transfer gearbox  67  driven from the differential. Drive is provided from the differential to a pair of drive shafts  68  which drive the rear wheels  69  of the vehicle, and from the transfer gearbox  67  to a third shaft  70  which drives a marine propulsion system in the form of a water jet  71 . A decoupler  10 , of the kind described above in relation to  FIGS. 1 and 2 , is provided between each of the rear wheel drive shafts  68  and the differential  66  and between the drive shaft  70  and the transfer gearbox  67 . The decouplers  10  allow drive to be selectively and independently connected between the differential  66  and each of the rear wheel drive shafts  68  and between the transfer gearbox and the water jet drive shaft  70 . In the arrangement shown, the drive shafts  68 ,  70  comprise the same part as the drive shaft referenced  58  in  FIGS. 1 and 2 . 
     In the embodiment shown in  FIG. 3 , a decoupler  10  is provided for each of the rear wheel drive shafts  68 . However, in an alternative embodiment, a decoupler  10  may be provided for only one of the rear wheel drive shafts  68 , the other drive shaft  68  being provided with a conventional CV joint or the like. Those skilled-in the art will readily understand that decoupling drive to one of the rear wheels will effectively disengage drive to both of the rear wheels, due to the effects of the differential  66 . 
     Similarly, it may not be considered essential to provide a decoupler in the marine propulsion system drive, as it is feasible to allow the marine propulsion unit to freewheel when the amphibious vehicle is driven in road mode. This entails a small loss of power, which is undesirable; but allows simplification of the marine drive train. 
     Some amphibious vehicle power trains do not require any road wheel decouplers. These include power trains where the marine power takeoff is upstream of the road wheel transmission. For example, a sandwich power takeoff may be used between the engine and transmission, as shown in  FIG. 1  of our co-pending application no. GB0020887.6. In this case, road wheel decouplers are not required, because drive to the road wheels can be decoupled simply by placing the gearbox in neutral gear. 
     Road wheel decouplers are also not required where the marine power takeoff is from the timing end of the crankshaft, for example according to  FIG. 2  of our co-pending application no. GB0021007.0. 
     It will be understood from the above that although  FIG. 3  shows an amphibious vehicle with a transverse engined power train, the decoupler  10  is equally suitable for use with a longitudinally engined power train or indeed any power train arrangement suitable for use in an amphibious vehicle. 
     The decoupler  10  is especially suitable for use in an amphibious vehicle because of the limited space around the engine and drive shafts, and the need to keep the weight of the vehicle to a minimum. 
     It is not intended that the decoupler  10  be used to couple drive between the transmission and the wheels when the vehicle is on land when the torques to be coupled would be high. Rather, it is intended that the decoupler  10  will be used to couple drive to the wheels when the vehicle is afloat in water and the wheels (not shown) are able to spin almost unimpeded. By the time that the wheels reach land, the driving ring  24  of the decoupler  10  is fully engaged with the spline  46  of the CV joint. Furthermore, coupling of the drive shaft  70  and of the wheel drive shafts  68  will occur when the gearbox is in low gear, with the engine running at low speed. This reduces the inertia to be overcome in synchronising speeds of the drive shafts  68 ,  70  with the driving shaft  14 .