Copyright Goodheart-Willcox Co., Inc. 831 Chapter 24 ∣ Track Steering Systems the same speed and in the same direction. The gear set does not contain a ring gear. See Figure 24-24. Three shafts are located inside the steering gear assembly. The outermost shaft is splined (coupled) to the low-speed sun gear, which is the sun gear with the medium-size diameter. A disc is coupled to this shaft and is held stationary when the operator shifts the track into low speed. The middle shaft is splined to the high-speed sun gear, which is the sun gear with the smallest diameter. A disc is coupled to this shaft and is held stationary when the operator shifts the track into high speed. The inside shaft is the output shaft that is driven by the sun gear with the largest diameter, called the pinion drive gear. The output shaft delivers power to the tractor’s final drive, which consists of a pinion bull gear set. The inside shaft (output shaft) also contains a disc that can be held stationary by a steering brake. Any time the transmission is in gear, the planetary carrier is driving forward or in reverse. Either the high-speed disc or the low-speed disc must be held in order for the plan- etary pinion drive gear to propel the output shaft. When either the high- or low-speed disc is held, it causes the carrier to react and drive the pinion drive gear, which propels the output shaft. Hydrostatic Steering (AKA Hydraulic Steering) Dual-Path Hydrostatic Transmission and Steering Differential Steering Purpose of the “hydrostatic steering” system Supply hydraulic power solely for the use of controlling a tractor’s steering and is unrelated to the machine’s propulsion system. A hydraulic pump and a hydraulic motor are used to drive each side of the machine, which controls both the machine’s steering and propulsion. A hydraulic motor provides the steering input into the steering differential. It can be a closed-loop system, or a traditional implement pump that feeds a reversible hydraulic motor. Components • Hydraulic pump • Steering control valve • Steering cylinder • On left side, variable- displacement, reversible hydraulic pump and a reversible hydraulic motor. • On right side, variable- displacement, reversible hydraulic pump and a reversible hydraulic motor. Two inputs (powershift transmission and a hydraulic motor) deliver power into a planetary gear system. The hydraulic motor can be driven by a dedicated closed-loop hydraulic pump or by the implement’s hydraulic system. Propulsion Propulsion is completely unrelated to the hydrostatic steering. The machine can use a mechanical transmission or a hydrostatic drive transmission. Dual-path hydrostatic transmissions. One pump and one motor on each side of the machine drive the tracks or drive wheels providing both propulsion and steering. The powershift transmission is solely responsible for the machine’s propulsion. Machine Examples • Traditional agricultural tractor • Combine • Off-road haul truck • Motor grader • Forklift • Loader backhoe • Wheel loader • Tractor scraper Twin track tractor examples: • Dozers • Track loaders • Tracked skid steers Wheel-type tractor examples: • Skid steers • Ag swathers • Dozers • Agricultural rubber twin track tractors Case 1650K calls their differential steering system “power steering.” Komatsu calls it a “hydrostatic steering system (HSS).” Goodheart-Willcox Publisher Figure 24-23. The term hydrostatic steering can be used by manufacturers to describe three different steering applications in off-highway equipment: traditional hydraulic steering, dual-path hydrostatic drive steering, and differential steering.