Copyright Goodheart-Willcox Co., Inc. Heavy Equipment Power Trains and Systems 830 steering differential has the left and right tracks tied together through a gear setup. When the operator commands the differential-steered tractor to steer, the differential simulta- neously increases the speed of one track and decreases the speed of the other track. Late- model hydrostatic-propelled dozers appear to operate similarly to the differential steer tractors, but since they use two separate hydrostatic transmissions, they require an elec- tronic control module to simultaneously increase the speed of one track while slowing the speed of the other track. The differential-steered tractor also uses a powershift transmis- sion, and the operator will feel increases and decreases in machine speed as the transmis- sion is upshifted and downshifted. Differential Steering Advantages Some machine owners prefer the powershift transmission over a hydrostatic transmission, and a tractor equipped with differential steering gives owners that option. Differential- steered tractors provide excellent maneuverability. In a differential steering system, the hydraulic steering motor is not operating at a high pressure when the machine is driving in a straight direction. The hydraulic steering motor only requires higher pressures when the tractor is steering. The differential steering system also has a longer service life compared to steering clutches and brakes that wear over time. Hydrostatic Steering Terminology This chapter has discussed different types of hydraulic controls as applied to track steer- ing systems, specifically dual-path hydrostatic-propelled tractors and differential-steered tractors. The term hydrostatic steering could be used to describe both of these examples. In addition, manufacturers of off-highway equipment also use the term hydrostatic steering to describe a traditional tractor’s hydraulic steering system. The three different usages of the phrase “hydrostatic steering” can be easily confused. Figure 24-23 shows a comparison of hydrostatic steering terminologies based on three different steering applications. Two-Speed Planetary Steering Systems In 1947, International Harvester manufactured a machine with the first two-speed planetary steering system, the TD24 dozer. In 1982, Dresser purchased the construction machinery division of International Harvester. Today, the two-speed planetary dozer is manufactured by Dressta, which is a company formed in a venture with Komatsu and Dresser (KDC–Komatsu Dresser Company). Examples of Dressta dozers equipped with the two-speed planetary steering system include the following models: TD-14M, TD-15M, TD-20M, TD-25M, and TD-40E. Two-Speed Planetary Steering Operation In a two-speed planetary steering system, the transmission delivers power to the bevel pinion and ring gear set. The bevel gear set delivers power to the right and left steering system. The operator steers the tractor by changing each track’s speed with the tractor’s steering system controls. Each track can be placed in one of the following four steering gear configurations: • High speed. • Low speed. • Neutral. • Brake. Each steering system contains a combination planetary gear set. The bevel gear set drives a carrier assembly. The carrier assembly contains a set of cluster gears. Each cluster gear is a single gear assembly that has different-sized pinions that rotate all at