The DAF belt transmission (called Variomatic) provided a continuously variable gear ratio between the highest and lowest available, corresponding to top and bottom gears of a conventional gearbox. This desirable state of stepless gears was foreseen in a number of friction drive transmissions from the simple knife grinder type used in the G.W.K. of 1911 and the Cartercar of 1906. In these a flat disc, mounted on the end of an extension to the clutch shaft, was engaged frictionally by a wheel sliding along a transverse shaft. Movement of the wheel towards the larger diameter of the disc reduced the gear ratio until the wheel was in contact with a path near the edge of the disc which was the equivalent of top gear. With the wheel at the disc centre, a neutral condition was obtained, and movement of the wheel to the other side of the centre gave the same variety of ratios in reverse. The transverse shaft could be either the rear axle or a countershaft driving the axle by chain. The main trouble with this simple transmission system was its tendency for the disc to cause wear on the wheel. Of other suggested designs of continuously variable friction drives, one used by Bentley Motors in 1933 was originated by Frank Hayes of America, who began developing an older idea in the mid 1920s. This was a toric disc transmission in which rollers made contact with toroidal, or saucer shaped, discs, there being three discs with two sets of three rollers between them. The input was to the centre disc, and power was transmitted through the rollers to the two outer discs which were coupled to the output. The rollers could be tilted in such a way that they could engage the “saucer” faces of the discs at different diameters in order to give the gear ratio that was required. Much of the development of the Hayes gear was directed to generating the correct pressure to maintain the friction drive and it is still being continued by Bentley dealers , Rolls Royce dealers and some Mercedes dealers. At the same time, several firms, Ferrari among them, are experimenting with the Beier friction drive, in which a number of taper section discs on one shaft are engaged by flanged rims of discs on a parallel shaft. The shafts are moved, together or apart, to change the contact diameter of the rims on the tapered discs, once again providing different gear ratios. Other forms of stepless ratio drives have been developed from the hydraulic torque converter. To increase the torque multiplication, that is to obtain a wider range of overall gear ratio, a set of variable angle blades is used in the torque converter. This idea was suggested by the Brazilian, Dimitri de Lavaud in 1932 and introduced in the 1956 Buick Dynaflow. Another development along the same lines was the Variable Kinetic Drive transmission, devised by Howard Hobbs in 1967, in which a centrifugally adjusted row of blades is coupled to the planet carrier of an epicyclic gear train. By using a bevel differential between the sun of this train, the turbine and the output shaft, the system provides a torque ratio of about four to one. This is sufficient to enable ratio-changing gears to be eliminated, only a forward, neutral and reverse gear arrangement being required. The main problem with these types of transmission is that when something goes wrong with them, it requires an expert to even begin to understand what the problem is, due to their unspeakable complexity. The need for specialised tooling to work on these units also makes them more difficult to have repaired oir serviced.
| January 6th, 2012 | Posted in Cars |