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HAT IS A TOROIDAL DRIVE

30th July 2009, Page 47
30th July 2009
Page 47
Page 47, 30th July 2009 — HAT IS A TOROIDAL DRIVE
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An infinite number of ratios There are two main elements in a single cavity drive unit. The pair of toroids can be described by imagining a mould for a ring doughnut, split into two halves around the circumference. The two halves are moved apart and mounted on two shafts, one driven by the engine and one driving to the wheels. The two halves are joined by a number of discs which fit inside the hollows of the facing toroids and which pivot so that they move across the hollow. In automotive applications, the pivoting Fs provided by hydraulic pressure on the pivot shafts.

The varying circumferences of the contact point changes the ratio between the two shafts from low to high. The unit has an infinite number of ratios within its designed range, which, depending on the shape of the toroid, can span from 6:1 to 11:1.

For almost all automotive applications, Torotrak uses two cavities, each with three rollers. We mentioned the contact point above, but there is no actual contact between the parts. They are separated by a film of traction fluid just 0.1 of a micron thick. Traction fluid is a special long-polymer oil that — under extreme pressure — grips tightly enough to transmit drive between two surfaces, and was initially developed to allow submarines to operate in silence. The traction fluid has a virtually unlimited service life, only degrading slightly from contamination from moving part wear, and the complete transmission could last the whole truck life with no maintenance. The size of the transmission is expressed by the diameter of the rollers.

The largest current unit has a 140mm roller diameter to allow a torque capacity of around 2,5001\1m, but the design can be upscaled as far as needed, a doubling of roller diameter giving around a threefold increase in torque capacity. Much smaller units are being developed for on-demand ancillary drives, including a Kinetic Energy Recovery System (KERS) using energy stored in a flywheel, said to be twice as efficient as an electrical hybrid drive. And that's the basic toroidal drive.

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