Copyright Goodheart-Willcox Co., Inc. Heavy Equipment Power Trains and Systems 818 Dual-Path Hydrostatic Drive Advantages Hydrostatic-propelled track-type tractors offer the following advantages: • A simpler power train (two pumps and two motors) compared to a steering clutch and brake tractor power train that normally contains a torque converter, driveshaft, powershift transmission, a bevel gear axle assembly, axle shafts, steering clutches, and brakes. • Design simplicity for engineers to locate the pump and motor in separate locations. The pumps can be directly coupled to the engine, and the motors can be individually placed at each track, typically splining to a final drive. • Independent control of each track. • Infinitely variable track speed controls for the operator to precisely manage propulsion and steering. • Capable of counter rotation. • Capable of maintaining a straight forward motion when the machine is digging with just one side of the dozer blade. Note When a dozer blade is cutting on just one side, the dozer normally pulls to the same side because of a loss of traction on that side. With hydrostatic steering, the operator can easily steer the tractor to offset the pulling motion and maintain a straight forward motion. A dozer equipped with a steering clutch and brake system does not have the same ability. • Limited machine space used for hydraulic pumps and motors. • No gear shifting. • Hydrostatic braking. As explained in Chapter 13, Hydrostatic Drives, when the propulsion levers are reversed, the motor acts like a pump and the pump acts like a motor, resulting in engine braking slowing the machine. Dual-Path Hydrostatic Drive Disadvantages As with any type of system, dual-path hydrostatic transmissions also have some disadvan- tages. First, compared to the other track-type steering systems, hydrostatic propulsion gen- erates more noise, which may cause problems for operators. Second, any time the machine is moving or steering it requires drive pressure, which can be very high pressure depending on the tractor’s load. As a machine’s service hours accumulate, the hydraulic drive lines and hoses are more susceptible to leaks due to the constant high pressure. Lastly, and perhaps the costliest to machine owners, is that hydrostatic transmissions are very sensitive to fluid contamination and must be maintained with extreme cleanliness in order for the transmis- sion to have a long service life. Although steering clutch and brake systems must also be kept clean, they are not as sensitive to contamination as hydrostatic transmissions. Examples of Dual-Path Hydrostatic Track-Type Tractors The following machines are examples of hydrostatic-propelled track-type tractors: • Case dozers (650L, 750M, 850M, 1150M, 1650M, 2050M). • Caterpillar small dozers (D3K2, D4K2, D5K2, D6K2). • Caterpillar track loaders (953K, 963K, 973D). • John Deere dozers (450K, 550K, 650K, 700K, 750K, 850K, 950K, 1050K). • John Deere track loaders (605K, 655K, 755K). • Komatsu dozers (D37EXi-23, D37PXi-23, D61EXi-23, D61PXi-23, D61PXi-24). • Liebherr dozers (PR 716, PR 726, PR 736, PR 746, PR 756, PR 766, PR 776). • Liebherr track loaders (LR 626 and LR 636).