Copyright Goodheart-Willcox Co., Inc. 825 Chapter 24 ∣ Track Steering Systems When the powershift transmission delivers the only input into the steering differen- tial, the tractor drives forward or in reverse in a straight path. When the hydraulic steering motor delivers the only input into the steering differential, the tractor counter rotates to the left or to the right. If the steering differential receives input from both the transmission and the hydraulic steering motor, the tractor moves forward or in reverse and steers either to the left or right. AGCO and Caterpillar Differential Steering The differential steering systems found in AGCO Challenger tractors and Caterpillar doz- ers are very similar. The steering differential contains three planetary gear sets that are connected with a sun gear shaft, also known as the inner axle shaft. See Figure 24-17. One outer planetary gear set, known as the equalizing planetary gear set, uses a fixed ring gear. The other outer planetary ring gear, known as the steering planetary gear set, is either held or driven by the hydraulic steering motor. The middle planetary gear set, also called the drive planetary gear set, has the planetary carrier driven by the powershift transmission. The middle planetary ring gear is connected to the steering gear set’s planetary carrier. All three of the planetary gear sets are connected through the sun gear shaft. When the machine travels in a straight path, the hydraulic steering motor holds the steering ring gear stationary. As the powershift transmission drives the middle gear set, power is distributed equally to the left and right axle shafts and on to both drive sprockets. If the operator chooses to steer the tractor when it is driving forward or in reverse, the hydraulic steering motor rotates, either clockwise or counterclockwise. The rotation of the steering motor causes one axle shaft to increase speed and the other axle shaft to proportionately decrease speed. The tractor begins to turn in the direction of the track that is moving slower. If the operator chooses to steer the tractor while the transmission is in neutral, one axle shaft spins in one direction and the other axle shaft spins in the opposite direction, which causes the tractor to counter rotate. Large Caterpillar dozers equipped with differential steering have the double-reduc- tion final drives shown in Figure 24-17. The Challenger twin rubber track tractors, how- ever, have a single-reduction inboard planetary final drive. Goodheart-Willcox Publisher Figure 24-17. A differential steering system used on large Caterpillar dozers. The D7R, D8T, and D9T use double-reduction final drives as explained in Chapter 17, Final Drives. Hydraulic steering input Powershift transmission input Double-reduction final drive Axle shaft Planetary carrier Drive sprocket Steering planetary gear set Steering bevel gear Drive planetary gear set Transmission bevel gear Equalizing planetary gear set (fixed ring gear) Drive sprocket Double-reduction final drive Axle shaft Sun gear shaft