Continuously variable transmission
The present invention relates to a continuously-variable-ratio transmission, which will be referred to as a CVT. It relates especially to CVT&#39;s on which the variator, that is to say the ratio-varying component, it is of the toroidal-race, rolling-traction type, and in which the ratio range of the CVT is enhanced by providing it with the facility to operate in more than one &#34;regime&#34;. 
     Multi-regime CVT&#39;s of the toroidal-race, rolling-traction type are now well known in the art: three different examples are described and illustrated in patent specifications GB-A-1078791, GB-C-2023753 and US-A-4718299. The typical basic components of a multi-regime CVT are the variator and a gearing sequence consisting of two or more interconnected epicyclic gear sets, the output (or &#34;final drive&#34;) of the transmission being provided by one of the components of the final gear set of the sequence. Typically the gearing sequence receives inputs both from the output of the variator and also from the variator input--that is to say, from the prime mover of the vehicle. At least one of the gearing components within the sequence is associated with a clutch or brake, which can either be engaged to hold the component against rotation or released to allow it to rotate. Engaging or releasing such a component or components changes the ratio presented by the gearing sequence as a whole. For each different arrangement of the engagement of the clutch or clutches within the gearing sequence, a progressive change in the setting of the variator from one extreme of its movement to the other will cause the final drive to progress through a different range of speed values, and in the course of progressing through one of those speed ranges the rotation of the final drive also changes direction in some known designs, so that the full ratio range of the CVT enables the vehicle to operate in both forward and reverse drive. It is also well known in the art so to arrange ratios within the variator and gearing sequence that it is possible to change from one clutch arrangement to another--and so from one regime to another--with no instantaneous change of overall ratio. The range of speed values provided by the new regime then follows on smoothly and without a break from that provided by the previous regime. Such a change is known in the art as a &#34;synchronous change&#34;. 
     The invention also applies especially to CVT&#39;s which include two variators and which are thus specially suitable for use in a tracked or other vehicle which has first and second laterally-displaced rotary driving members, and which is steered by the differential rotation of those members. 
     Such a CVT is described in patent specification US-A-4718299, and an object of the present invention is to provide an efficient yet simpler two-variator CVT. The invention is defined by the claims, the disclosure of which is to be read as included within the disclosure of this specification, and the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which: 
    
    
     FIG. 1 is a schematic illustration of the operation of a two-regime CVT; 
     FIG. 2 shows an arrangement of epicyclic gear sets in such a CVT; 
     FIG. 3 shows a novel variator unit for use in such a CVT, and 
     FIG. 4 shows an alternative to the unit of FIG. 3. 
    
    
     FIG. 1 shows a CVT for a tracked vehicle having an input 31 from a prime mover 32 and outputs 1 and 2 to drive the tracks of the vehicle 1&#39;, 2&#39;, respectively. The transmission input drives respective inputs of first and second variators 4 and 3 and also one input of an epicyclic gear set 5, referred to elsewhere in this specification as the third epicyclic gear set. A second input of the epicyclic 5 is connected to a brake 6 operable to engage Low Regime, that is to say a first of the two regimes. The output 14 of the variator 3 is connected to an input 9 of an epicyclic gear set 7, referred to elsewhere in this specification as the second epicyclic gear set, and is also connectable by a clutch 13 directly to the opposite one of the outputs of the transmission (2). The output 15 of the variator 4 is connected to a first input 10 of an epicyclic gear set 8, referred to elsewhere in this specification as the first epicyclic gear set. Second inputs 11, 12 of the epicyclics 7 and 8 are connected to each other and to the output of the epicyclic 5. 
     In operation, in low regime, the clutch 13 is disengaged and the brake 6 is engaged. When an input is applied to the transmission at 31 the variators 3 and 4 will drive the first inputs 9 and 10 of the epicyclics 7 and 8 respectively. Because the brake 6 is engaged the epicyclic 5 will drive the second inputs 11 and 12 of the epicyclics 7 and 8. These epicyclics will therefore drive their respective outputs 1 and 2, and it will therefore be seen that in low regime the speeds of outputs 1 and 2 will be dependent upon the output speeds of their related variators 3 and 4. 
     In high regime the brake 6 is disengaged and the clutch 13 engaged. Thus variator 3 now drives output 2 directly. It also continues to drive input 9 of epicyclic 7. Meanwhile variator 4 also continues to drive input 10 of epicyclic 8. It will be seen, however, that epicyclic 8 is now effectively receiving inputs both from input 10 and also from the transmission output 2. Thus it will itself generate an output at 12, driving the input 11 of the epicyclic 7 independently of input 31 because brake 6 is disengaged. Thus epicyclic 7 is now receiving two inputs and will generate an output at 1, driving the associated track as required. In high regime the speed of output 2 will be dependent on the speed of variator 3 while the speed of output 1 will be dependent on the speeds of both of the variators. 
     FIG. 2 shows a layout of epicyclic gear sets which achieves the transmission arrangement of FIG. 1. In FIG. 2 the same numerals are used to denote the same integers as in FIG. 1. In this FIGURE the variators 3 and 4 are shown as being mounted with their input and output axes all coaxial (16). This can be achieved by the use of variators of the toroidal-race, rolling traction type. As is already customary with variators of this type, the outputs of variators 4, 3 are connected to the inputs 10, 9 of epicyclic gear sets 8, 7 by bell-shaped members 15, 14 which are coaxial with the variators themselves. 
     FIG. 3 shows a novel combination of such variators, suitable for use with the transmission of the preent invention. Transmission input 20 drives first and second opposing input discs 21, 22 connected to be driven together by a shaft 23. Sets of rollers 24, 25 transmit drive to first and second output discs 26 and 27. Unlike the input discs and unlike the corresponding components in known toroidal-race variators, the two output discs are not connected to be driven together but are separated by a thrust bearing 19 able to rotate one at a different speed to the other. The speed of each disc may be varied independently of the other by varying the angle of the relevant set of rollers. In this way coaxial input and first and second outputs can be achieved. It should be noted that while input disc 21 is fixed to shaft 23, there is a keyed connection 18 between the shaft and disc 22 so that the two parts always rotate together but are capable of limited relative movement axially in response to an end load which creates the appropriate contacting forces between the discs and the rollers. The magnitude of the end load and the appropriate means to generate it are well understood in the art, and the means are indicated schematically at 17. 
     In the alternative toroidal race variator of FIG. 4 a single input disc 40 is driven by the same transmission input 20 and rotates with clearance about an output shaft 41 to which a first output disc 42 is fixed and a second output disc 43 is keyed so that it has limited freedom of axial movement relative to the shaft, and end load is applied at 44. A thrust bearing 45 separates disc 43 from a shoulder 46 on shaft 41. 
     The invention thus provides a CVT in which steering is effected by differential setting of the two variators 4 and 3, and in which epicyclic gear sets 8 and 7 may therefore be considered as &#34;steering&#34; epicyclics, and the third gear set 5 as the &#34;regime change&#34; epicyclic. Potential advantages of the CVT include great economy in the number of components used, compactness and neatness inherent in the coincidence of the operating axes of the variators 4, 3 and epicyclics 8, 7 and 5 as shown in FIG. 2, identical stear performance in high and low regimes, and no problem for the engineer skilled in the art to choose ratio ranges for variators 4 and 3, and epicyclic ratios for gear sets 8 and 7, so as to permit synchronous change between high and low regimes. 
     Numerical values for one design of CVT according to the invention will now be stated and discussed with reference to the variator shown in FIG. 3 of the drawings. If in that figure the speeds of rotation of input shaft 23, output disc 26 and output disc 27 are N 1 , N 2  and N 3  respectively, then for a value of N 1  equal 1000 rpm, the CVT was designed so that the quantity (N 2  +N 3 )/2 ranged in value between -500 rpm to -2000 rpm, and the value of the function (N 2  -N 3 )/2 lay between +200 rpm at maximum stear to one hand, and -200 rpm at the opposite steering extreme. The thrust bearing 19 therefore perceives only a maximum relative speed of 400 rpm between the discs 26, 27 to either side of it: such a difference requires a bearing of only modest size compared with other thrust bearings known in this art. In summary, the above figures indicate a variator with a propulsion range of from -0.5 to -2.0, and a total ratio range (allowing for the fact that full steer may be added to full propulsion) of from -0.3 to -2.2. Maximum reverse speed can be as much as 50% of maximum foward speed, and at full steer track speed difference (i.e. variator 4 at one extreme ratio setting, and variator 3 at the opposite extreme) a forward speed equal to 20% of the maximum forward speed is still possible.