Torotrak JV Develops New Epicyclic Drive Technology That Replaces Gears with Spheres to Enhance Refinement, Durability, Cost, Weight and Packaging

7 March 2010

Infinitrak’s epicyclic traction drive eliminates gears and integrates the thrust bearing function to save cost, reduce noise, improve durability and increase the range of ratios available within a given package space. Click to enlarge.

Infinitrak, the US-based joint venture company owned by transmission innovation specialist Torotrak and outdoor power equipment (OPE) market leader MTD (earlier post), has developed a new epicyclic drive that replaces gears with traction spheres running in prescribed tracks, combining the functionality of a thrust bearing and an epicyclic drive stage. The first application of the traction drive epicyclic will be in a compact and affordable Infinitely Variable Transmission, developed by Infinitrak for the OPE market.

The new spherical traction drive elements transfer torque through traction fluid using the same mechanism proven by Torotrak for its full-toroidal variator technologies. Eliminating meshing teeth ensures very low noise, while the use of fewer moving parts combined with materials and manufacturing technologies already proven in established ball bearing applications reduces cost and weight as well as providing increased durability compared with conventional epicyclic systems.

Epicyclic drives play a fundamental role in Infinitely Variable Transmissions (IVTs). Without an epicyclic drive, a full-toroidal variator based on Torotrak’s traction drive technology works as a continuously variable transmission (CVT) that can change speed steplessly over a wide range of ratios. To allow the direction of drive to be changed within the transmission, giving it an in-built ‘forwards-through-neutral-to-reverse’ capability, an epicyclic arrangement is added to manage the input to and output from the variator, transforming the CVT into an IVT.

Animation of Infinitrak’s Epicyclic Traction Drive Technology. Click to launch.

A conventional epicyclic gear train consists of a central gear (the sun gear) around which several planet gears are mounted on a rotating carrier. These gears also mesh with an outer, internally toothed ring (the annulus). By connecting the variator input to the sun and the variator output to the annulus, the planet carrier rotates at the speed difference between the two which can be positive, negative or zero. The planet carrier, effectively the output from the transmission, is connected to the vehicle’s axle allowing the machine to move seamlessly from forward, through geared neutral (a condition where the engine and elements of the transmission are turning but the wheels are not), to reverse.

In the new traction drive epicyclic, the planet gears are replaced with steel spheres running in prescribed tracks in the rear face of the variator output disc and the end plate, which replaces the conventional annulus.

Engineers at OPE transmission specialist Infinitrak have also been successful in integrating the thrust bearing function (which is required to provide a reaction for the clamping forces within a single cavity variator transmission) within the epicyclic arrangement. This significantly reduces the number of moving parts, cutting weight, reducing packaging requirements and providing increased power density.

The first production application of Infinitrak’s new traction epicyclic technology will be in a new transmission being finalized for launch in the outdoor power equipment (OPE) market.

Torotrak is a developer of full-toroidal traction drive technology. The company designs highly efficient continuously variable transmissions (CVT) and infinitely variable transmissions (IVT). Torotrak develops main drive transmissions as well as variable ratio drive units for application in flywheel-based mechanical hybrid systems and for use as auxiliary drives.

Torotrak operates in the automotive, truck, bus, off-highway and agricultural markets, in motor sport and in the outdoor power equipment (OPE) market through its joint venture business, Infinitrak.