Caterpillar Challenger MT-845
The Challenger was donated to PSU by Caterpillar Corporation and Martin Tractor. It is controlled by a powershift transmission and differential steer. Differential steer tractors use 3 planetary gear sets connected through a series of common sun gears and uses two inputs.
When the transmission is the only input to the planetaries, the tractor is moving just forward or just reverse. When the hydraulic steering motor is the only input to the planetaries, the tractor is steering right or left (actually counter rotating). When the transmission and the steer motor are both inputting power flow to the planetaries, the tractor is moving forward or reverse and steering simultaneously.