Introduction
In a conventional Torotrak full toroidal variator consisting of two cavities, each of the six rollers is connected via a mechanical linkage to its own hydraulically controlled piston. The six mechanical linkages comprise of a carriage to support the roller, a spherical joint to allow the necessary degrees of freedom to articulate the roller, a pivoted lever to alter the applied force direction and a piston and stem to apply the force.
The fulcrum pins for each pivoted lever are housed in a fulcrum plate above the hydraulic block. In turn, this block houses the six pistons that are subjected to hydraulic pressure used to apply force to the rollers.
As each roller is independently controlled, each one shares the load equally since the geometrical differences and tolerances between roller mechanisms simply result in a different piston position that the hydraulic system is insensitive to.
(For a full introduction on how IVT works please press here).
ERC
The ERC concept simplifies the conventional arrangement by linking the six traction rollers together with a single mechanism and hydraulic piston. The basic structure of the mechanism takes the form of an epicyclic gear-train comprising of a sun gear, an annulus gear and planet roller assemblies. Each roller is mounted on and steered by a planet roller; the teeth of the sun and annulus gears mesh with the teeth of each planet roller to position them around the toroid.
Steering input to the rollers is achieved by rotating the planets via controlled differential rotation of the meshing sun and annulus gears. This control input is achieved by a fourth planet element which is connected by a sliding linkage to a single hydraulic ram located external to the variator. The movement of the ram creates the necessary displacement of the control planet, which is then replicated at each roller by the resulting rotation of the sun and annulus.
The roller is mounted to the planet element on combined spherical and needle bearings. The planet gear teeth extend over a narrow arc, which is sufficient to permit the limited deflection required to steer the roller. The two geared segments and the centre of the spherical bearing form the body of the planet. The rollers are free to comply with the variator disc geometry by a combination of axial and radial movement permitted by the epicyclic spur gears and the needle roller bearing.
As the rollers are connected by one linkage, equal distribution of power flow is assured by accurately fixing the relative rotational position of each planet assembly. The inherent accuracy of conventional involute gearing using DIN 6 quality gears (already used in automatic transmission) provides four times the accuracy necessary for successful variator operation. The epicyclic gear-train is therefore an ideal platform for providing a simple, well understood and cost effective variator control mechanism.
Although each variator track requires an epicyclic gear set to support its three rollers, only one of these two epicyclic arrangements is connected directly to the control ram and cylinder. This epicyclic is located in the ‘open’ track, which facilitates its connection to the control cylinder mounted on the transmission casing. The ‘enclosed’ second track, located within the output drum, receives its control demand and reaction forces via torque-tube connections to its sun and annulus gears. Since the sun and annulus gear rotations in each of the two variator track are identical, all six rollers operate together.
When used in a coaxial arrangement (typically found in a rear wheel drive vehicle application) the variator can be connected to the transmission drive train via the output drum in a manner which removes the need for a further epicyclic gear set that would normally be required with the conventional control system.
Benefits
In summary, the use of ERC improves efficiency whilst improving the overall cost, weight and package of the transmission.
The ERC concept reduces the parts count within the variator by 30% and this translates into a typical variator cost reduction of around 20%.
Replacement of the individual hydraulic roller control system produces significant package and weight savings when compared to a conventional control mechanism. When ERC is incorporated into a rear wheel drive IVT the following benefits are noted:
- Transmission length is reduced by 5%
- Overall transmission efficiency improvement of around 10%.
