IVT with Geared Epicyclic Arrangement
At the heart of the Infinitrak TTT is Torotrak's full-toroidal traction drive technology - a new and patented concept for garden tractor and OPE transmissions.
The TTT consists of two toroidal traction drive modules which are combined to form a single integrated IVT unit - leading to the name for this new variable drive unit being the Twin Toroidal Transmission (TTT).
Power from the engine is provided to the TTT via a rubber belt, which runs around a grooved pulley wheel located adjacent to the transmission cooling fan on the transmission input shaft (diagram 1). A simple gear and drive mechanism is used to supply engine power to each of the two independent IVT's housed within the TTT casing.
The input shaft transfers power through a bevel drive to a central drive shaft. This central drive shaft delivers power to the input disc (green) in each of the two variators. Each variator is controlled and operated separately, meaning that that the TTT can provide independent power and torque characteristics to each of the two outputs from the transmission - the left and right driven wheels - giving revolutionary drive and control capabilities.
Key components within each variator include an input disc (green) and an opposing output disc (yellow) (diagram 2). Each disc is formed so that the gap created between the opposing pair of discs is 'doughnut' shaped; that is, the toroidal surfaces on each disc form the toroidal cavity.
Two rollers (blue) are located inside each toroidal cavity (diagram 2) and are positioned so that the outer edge of each roller is in contact with the toroidal surfaces of the input disc and output disc. As the input disc rotates, power is transferred via the rollers to the output disc, which rotates in the opposite direction to the input disc.
The transfer of power through the contacting surfaces of the discs and rollers takes place via the microscopically thin film of specially developed long-molecule traction fluid. This fluid separates the rolling surfaces of the discs and rollers at their contact points (diagram 3) and prevents 'metal to metal' contact, increasing the durability and life of the variator components.
The input and output discs are subjected to an axial clamping force within each variator unit, which applies pressure between the disc and roller surfaces at each of the contact points. The traction fluid in these contact points become highly viscous under this clamping pressure, resulting in an increase in its shear resistance (increasing its 'stickiness') and creating an efficient traction drive mechanism which transfers power between the rotating discs and rollers.
For these reasons, the TTT unit is called a 'traction drive' and is characterised by its ability to transmit power smoothly, efficiently and quietly to the driven wheels.
Geared Neutral in the TTT
The rotating components of the epicyclic gear pack attached to the output to the variator act as a mechanical adding machine, mixing the input speed from the engine with the variator output speed to provide precise control over vehicle speed and direction.
Emerging through the centre of the yellow output disc is a green 'internal sun' gear (diagram 2) - referred to as 'sun gear 1'. This is attached directly to and driven by the central drive shaft within the TTT unit and rotates at the variator input speed.
The yellow output disc acts as the carrier for a specially developed planetary gearset (orange).
The inner planet gears are connected to sun gear 1. The rotation of the inner planet gears is determined by the speed of sun gear 1 and the speed of the output disc (yellow) of the variator.
Attached to the three inner planet gears are three smaller outer planet gears, which connect to a second, centrally mounted sun gear 2 (red) (diagram 2).
This series of sun and planet gears forms the planetary epicyclic arrangement which acts as a mechanical adding machine, mixing the input speed from the engine with the variator output speed to provide precise control over vehicle speed and forwards/reverse direction.
The geared neutral concept can be illustrated by the following equation:
variator output disc speed (planet gears) - input drive shaft speed (sun gear 1) = speed and direction wheels
When the sum of the equation is positive - that is, when the planet gears are travelling at a greater speed than sun gear 1 - the output (and hence the vehicle) will move forward.
When the sum of the power equation is negative - that is, when the planet gears are travelling at a lower speed than sun gear 1 - the output (and hence the vehicle) will reverse.
When the speed from both sources is equal, then the output speed will be zero and the vehicle will be held stationary. This is referred to as the 'geared neutral' condition and is a defining characteristic of the Torotrak full-toroidal traction drive IVT.
Geared neutral means that even when the vehicle is stationary, power from the engine is still supplied to the wheels. This provides high levels of vehicle stability whilst also giving the driver precise and easily modulated control of the vehicle.
The geared neutral arrangement also removes the need for a starting device, such as a clutch or a torque converter, to be fitted between the engine and transmission in the vehicle driveline.