Design and specification
Variator design
The variator is the core of Torotrak’s technology. It overcomes the constraints of stepped ratio automatic transmissions; the ratio range is provided not by a system of gears, as found in a conventional automatic transmission, but by a combination of discs and rollers, separated by a specially developed traction fluid. View the animation which shows how a typical twin cavity variator (suitable for main drive applications) is constructed and how it works.
Inside the variator are two pairs of discs mounted on a shaft; the space between each pair of discs forms a hollow doughnut shape or 'toroid'. Rollers are positioned within each toroidal space; these transmit drive from the outer discs to the inner discs. The speed of the input and output discs determine the variator ratio.
Torotrak’s twin cavity variator
Torotrak's variator can be configured with the outer discs connected to the input, and the inner discs acting as the output or vice versa, depending upon the application.
The variator geometry is arranged such that the size of the rollers conforms to the toroidal tracks of the discs. Consequently, as the roller precesses, the axial separation of the discs remains unchanged. There can be two or three rollers in each cavity; dictated by the requirements of cost and power density. Each roller is supported in a mechanical system that, when connected to a control mechanism, allows the roller to be steered across the disc face. The assembly forms an inherently stable three point system; two of the points are provided at the contacts between each disc and roller while the third is an externally controlled point, constrained to follow a defined path and subject to a controller-defined force.
The rotational speeds of the input and output discs determine the operating ratio of the variator. Each roller adopts a precession angle that conforms to that ratio. Any change in the variator ratio causes a change in roller position that naturally adjusts its precession angle. The mechanism is entirely automatic requiring no overriding action from an external control system. The process is somewhat similar to the automatic steering action of the wheels found on a supermarket shopping trolley.
The variator is controlled by the application of an externally defined force applied at the third point of the roller support mechanism. Variator input and output torques, directly related to the magnitude of the force, arise naturally initiating power transfer between the input and output variator discs. Any resulting speed and therefore ratio change across the variator is naturally followed by an equivalent change in roller precession angle.
The rollers experience similar control forces resulting in an equal distribution of transmitted power between them. Power sharing is a result of the roller control mechanism; it does not rely upon precise or expensive manufacturing processes.
Rotation of the discs and the resulting rolling action of the rollers causes a special oil, termed traction fluid, to be trapped at the point of contact. The surfaces are separated by a hydraulic film that, when subjected to high compressive pressure, becomes viscous and so able to transmit drive across each contact. Power is transmitted without metal to metal contact between disc and roller, avoiding mechanical damage of the running tracks. (More information on the surface engineering which makes this possible can be found on the tribology page.)
Traction fluid trapped between disc and roller
The traction principle requires discs and rollers to be pushed together to cause the oil to increase its viscosity; the force to do this is produced by pushing one of the input discs towards the other. Variator contact loads are directly related to the force applied by the control system. The required contact squeeze load is therefore proportional to the applied load and usually produced by applying roller hydraulic pressure within the endload drum.
Typical sizes
Development experience has brought downsizing of the major components; rollers which were 120mm diameter in 1999 are now only 75mm diameter, for the same power rating (200kW).
Single or twin cavity
In its simplest from, the variator provides a continuously variable transmission (CVT) which is ideal for low-cost variable drives and flywheel-based mechanical hybrids. A single cavity design is possible for low-cost, light-duty applications such as outdoor power equipment (OPE) where cost, size and weight are the main considerations. Twin cavity designs are more common, being better suited to most applications where high power capacity and long life are required.
Materials
The materials used for the discs and rollers are familiar bearing steels such as 1% carbon through hardening steel, finished to the same surface quality as rolling bearings. Future developments include the use of gear steels for lower contact stress applications. For the OPE market, where stresses are lower and service life is shorter, powder forged components give a highly cost-effective solution.