Epicyclic Traction Drive Technology
Infinitrak™ has developed a traction drive alternative to conventional toothed gear arrangements for use in the epicyclic mixing stages of IVTs for the OPE market. The new type of epicyclic arrangement, which uses ball bearings running in prescribed tracks and transferring torque through conventional traction fluid, offers lower noise, increased durability and fewer moving parts than conventional geared alternatives.
The new patented technology has created a new standard in transmission technology for the OPE market. It combines the functionality of a thrust bearing and epicyclic drive stage, and eliminates the requirement for meshing gear teeth in the transmission epicyclic arrangement, leading to exceptional refinement and durability.
The epicyclic drive stage plays a fundamental role in Infinitrak™ IVT architectures. Used alone, 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. To give the transmission an in-built 'forwards-through-neutral-to-reverse' capability, it is customary to add an epicyclic gear train to the CVT layout to create an Infinitely Variable Transmission (IVT).
At the heart of a typical Infinitrak™ transmission, a single cavity full-toroidal variator is used; this comprises a pair of opposing discs with traction rollers positioned between them within the toroidal cavity. Torque is transmitted from the input disc to the output disc via traction fluid, which also separates the rollers from the discs and eliminates metal-to-metal contact.
Unlike a conventional IVT where the input to and output from the variator are connected to an epicyclic gear train, Infinitrak™ have developed an alternative to this geared epicyclic arrangement by replacing the meshing gears in this part of the overall drivetrain with spherical bearings. These act as the planet elements within the epicyclic drive and run in prescribed tracks transferring torque through traction fluid using precisely the same mechanism used in the variator itself and proven by Torotrak over many years.
In addition to increased refinement and durability, a further benefit of the new approach is to reduce overall parts count and hence both cost and weight. A typical Infinitrak™ single cavity variator employs a single thrust bearing to react the clamping forces within the unit; however, the innovative new design of the epicyclic traction drive combines the thrust bearing function with that of the epicyclic pack, significantly reducing the number of moving parts and providing increased power density.
The concept of the epicyclic traction drive is simple and is analogous to a large angular contact ball bearing; the traction spheres correspond to the balls in a conventional race and act as planet gears, whilst the carrier plate corresponds to the cage in a ball bearing and replaces the conventional epicyclic's planet carrier. The annulus corresponds to the outer race of the bearing and is connected to the variator input shaft, while the inner race is formed as a track on the rear of the variator output disc and acts as the sun gear. Instead of using meshing teeth to transfer power, shearing traction fluid provides a silent and efficient drive from the two inputs (sun and annulus) though the spheres to the output – the carrier plate.
The first production application of Infinitrak™ new epicyclic traction drive technology will be the company's forthcoming Single Toroidal Transmission (STT) being finalised for launch in the outdoor power product (OPE) market, where demand for easy control, refined operation, low noise and durability make it the ideal transmission solution for this sector.
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