Power transmission apparatus
This invention relates to gearing for power transmission apparatus provided with friction devices, i.e. at least one clutch and at least one brake, for holding and freeing parts of the apparatus for changing the driving ratio. An example of such apparatus is described in patent specification No. 1,199,521 with reference to FIG. 13 thereof and No. 1,347,045. 
     A difficulty associated with gearing of this kind results from the very high actuating loads often required for engaging the friction elements of the devices. 
     Certain types of peripheral loading devices such as band brakes may have a self loading effect but axially engaging devices (i.e. cone and plate brakes and clutches) do not have self loading characteristics but are often more suitable for other reasons. 
     A primary object of the present invention is to provide a required degree of self loading in either one or both directions of operation of axially engaging friction devices. 
     A further object of the invention is to achieve the primary object with a simple and effective construction. 
    
    
     The invention will be further described by way of example with reference to the accompanying diagrammatic drawings, wherein: 
     FIG. 1 is a sectional view of a frictional device made in accordance with the invention; and 
     FIG. 2 shows cam teeth. 
    
    
     FIG. 1 shows a gearing having a torque converter which is similar to that described in the specification of British Pat. No. 1,347,045. This includes an input member 10 connected to an impeller 11, a circulatory turbine 12 connected by pinions 13 and ring gear 14 to the input member 10. An output turbine 16 is geared by sets of pinions 17 to a double sunwheel 19 and to a reaction sunwheel 20. The double sunwheel 19 is also meshed with the pinions 13. The reaction sunwheel 20 is connected to a reaction member 22 which surrounds a reaction hub 23 splined on to a reaction sleeve 24. Between the member 22 and 23 are friction elements providing a unidirectional clutch. The sleeve 24 is fixed by bolts 25 to the fixed casing 26 of the apparatus. The impeller is connected to a pump driving member 30 which drives a pump 31. The pump 31 feeds fluid pressure into a bore 32 which is connected to various channels such as 33, 34 leading to clutches and brakes to be described. The spindles of the pinions 17 are mounted in a cage 37 which is splined to an output shaft 38 which forms an imput shaft to a toothed gearing forming the subject of the present invention and now to be described. 
     Splined on to the shaft 38 is an input member 40 which, carries a power take-off gear 42 on one side and a cylinder member 43 on the other side. 
     The member 40 has an outer flange which with the member 43 forms a cylinder containing an annular piston 46. The member 40 also has an inner flange 48 which with the member 43 forms a cylinder containing a piston 49 that forms a drum. Fluid can be introduced at 50 between the member 40 and the piston 46 and fluid can be introduced at 51 between the member 40 and the piston or drum 49. 
     The drum 49 carries the outer friction plates 53 of a first clutch the friction surfaces of which are engageable with the friction surfaces of inner friction plates 54 when fluid pressure is introduced into the space 51. Diaphragm type springs 52 serve to disengage the plates of the first clutch. 
     The drum has cam projections in the form of cam teeth 55 engageable with similar teeth 56 (FIG. 2) on the member 40, the teeth being shaped to provide an axial separating force between the member 40 and drum 49 which force increases with increasing torque load occurring only in one direction of rotation which occurs when the first clutch is engaged to effect positive drive and down change of transmission ratio. 
     The piston 46 carries one set of friction plates 58 of a second clutch engageable with plates 59 on introduction of fluid pressure into the space 50. The second clutch is not provided with cam projections such as 55. Springs 57 serve to disengage the plates of the second clutch. 
     The plates 54 of the first clutch are carried by an annular member 60 which is splined on to a sungear 61 which has two sets of teeth 62, 63. 
     The plates 59 of the second clutch are carried by an annular member 65 that is carried by a plate 66 that forms a cage in which are mounted the spindles 67 of a set of pinions 68 which mesh with the teeth 62. A second set of pinions 70 have their spindles 71 mounted in a second cage 72. The second cage is carried on an output shaft 69. The pinions 70 mesh with the teeth 63. The cage 71 carries a ring gear 73 meshing with the pinions 68. A second ring gear 74 is carried by a support plate 75 and meshes with the pinions 70. The plate 75 is mounted on a bearing 72A. The ring gear 74 carries friction plates 76, the friction surfaces of which are engageable with a set of friction plates 77 carried by a drum 78 which is shaped as an annular cylinder housing a piston 79 which is fixed by bolts 80 to the fixed casing of the apparatus. Fluid pressure introduced into a space 81 urges the drum axially to engage the brake and thereby hold the ring gear 74 stationary. Springs 82 serve to disengage the brake plates. The drum 78 has cam projections 83 engaging cam projections on the piston 79 these projections being similar to 55, 56 and provide axial engagement loading in one direction only of torque which occurs during down change of transmission ratio, i.e. reaction or negative torque. 
     To provide reverse drive a piston member 90 is fixed by bolts 91 to plates 92, 93 the former of which is fixed to the casing of the apparatus. The piston 90 is housed in a cylinder formed by drum 95. The drum 95 carries friction plates 96 of a reversing brake. The plates 96 are engageable with plates 97 carried by an annular member 99 which is an extension of the member 65 and cage 66. Engagement is effected by introducing fluid pressure into a space 98 between the piston 90 and the drum 95. The plates are normally held disengaged by springs 100. 
     A fixed plate 101 serves as an abutment plate for the friction plates and spring of both brakes. 
     1st gear 
     This is produced by engaging the first clutch 53, 54 and brake 76, 77. The second clutch is disengaged. The brake 76, 77 is engaged. Drive occurs through 38, 40, 53, 54, 60, 61, 63, 72, 69. Torque occurs on the first clutch, which is self loaded by cam projections 55 acting in the driving direction. 
     2nd gear 
     The second clutch 58, 59 is engaged. The brake 76, 77 remains engaged and the first clutch 53, 54 is disengaged. The drive occurs through 38, 40; 65, 66, 68; 73, 72, 69. 
     3rd gear (top or 1:1 gear) 
     The first clutch 53, 54 is re-engaged. The second clutch remains engaged. The brake is released. The gears cease to rotate relatively to each other and solid drive ensues. In making this change the torque is in the non-self loading direction -- it is running faster than the input so that this up change is not self loading. Torque is low. 
     In changing back 3rd to 2nd gear 
     The first clutch is released. The second clutch remains engaged. The forward drive brake engages and is self timing. 
     In changing back 2nd to 1st gear 
     The second clutch is released and the first clutch is engaged in the self loading direction and provides self timing. 
     Reverse gear 
     The first clutch 53, 54 is engaged. The reversing brake 96, 97 is engaged. The brake 76, 77 is disengaged. The pinions 68 now drive in reverse and drive the cage 72 and output member 69. 
     It will be seen that in the first clutch there is a drum 49, a first apparatus part 40, the parts 49, 40 being formed as a cooperating piston and cylinder device, said drum carrying one set of friction surfaces 53. A second apparatus part 60 carries the other set of friction surfaces. The friction surfaces, drum and second apparatus are rotatably displaceable about a common axis. A first actuating means (pump 31) serves to urge the piston and cylinder in one direction to engage the friction surfaces. A second actuating means (springs 52) serves to urge the piston and cylinder in the opposite direction to disengage the friction surfaces. Cam projections 55, 56 on the drum and first apparatus part 40 provide the self loading effect on the friction surfaces. This clutch is self loading in first gear but when second gear is engaged the first clutch runs faster than the input and automatically unloads for disengagement. 
     The brake 76, 77 has a similar drum and first apparatus part (in this case the fixed casing) and second apparatus part (ring gear 74) and similar cam projections 83. The drum in this case does not rotate but is rotatably displaceable to the small extent permitted by the cam projections 83. The projections 83 provide self loading only in the direction of reaction. This brake is always self timing when about to provide driving reaction. It loads when required and unloads when not required. 
     The reverse brake is similar with self loading to carry the high reverse reaction. 
     In addition to enabling high torques to be more easily carried by the friction clutches and brakes, self loading action is very desirable when changing down i.e. to a ratio providing an increase in input speeds. Input speed can be allowed to increase during the change and the clutch or brake will automatically begin to engage when speeds synchronise and torque begins to act in the appropriate direction thereby eliminating shock but self loading action is undesirable for up changes i.e. to a ratio providing a reduced input speed as torque acts as soon as the change is commenced and self loading action can produce high engaging loads when speeds are not synchronised thereby producing shock. 
     For a given angle on the cam projections and coefficient of friction a number of plates can be selected to give an appropriate maximum self engaging action.