oil to both servo cylinders and keeps the swash plate tilted. The swash plate will remain in position until the speed control lever is moved again by the operator.

With the swash plate tilted forward and the pump drive shaft and cylinder block rotating clockwise, the ports reverse and the inlet port becomes the outlet and the outlet port becomes the inlet. As the pump cylinder block rotates past the pump inlet port, a check valve opens and oil is forced by the charge pump into the piston bores that align with the inlet port of the pump. As rotation continues, the oil is pressurized and forced out of the outlet port of the pump by each of the pistons, as they align with the outlet port. This action forces oil to flow to the motor, and as high-pressure oil from the pump enters the inlet port of the motor, the pistons are pushed against the swash plate. The pistons slide down the inclined surface of the swash plate, rotating the cylinder block. This action rotates the drive shaft counterclockwise, driving the piece of equipment in reverse. As the motor cylinder block continues to rotate, oil is forced out the outlet port at low pressure and returns to the pump.

NOTE

The PUMP DRIVE SHAFT and cylinder block always rotate clockwise, but the MOTOR DRIVE SHAFT and cylinder block rotate in clockwise and counterclockwise directions, depending on the direction of the oil entering the pump.

Maintenance of Hydrostatic Drives

As with any hydraulic system, the hydrostatic drive system is fairly easy to' maintain. The fluid provides a lubricant and protects against overload. Like any other mechanism, it must be operated properly; too much speed, too much heat, too much pressure, or too much contamination will cause damage.

Before removing any part of the system, ensure that the area is clean. Use steam-cleaning equipment if available; however, do NOT let any water into the system. Ensure that all hose and line connections are tight. If steam cleaning is not possible, diesel fuel or a suitable solvent may be used. Be certain to remove all loose dirt and foreign matter that may contaminate the system. Impurities, such as dirt, lint, and chaff, cause more damage than any one thing. Always seal openings when doing work to prevent foreign matter from entering the system.

Clean the work bench or table before disassembling any hydrostatic system component for servicing. Be sure that all tools are clean and free of dirt and grease.

NOTE

NEVER perform internal service work on the shop floor or ground or where there is a danger of dust or dirt being blown into the parts.

Before disassemble of any system component for internal service, certain items must be available. These items include the following:

Clean plastic plugs of various sizes to seal the openings when removing hydraulic hoses and lines.

Clean plastic bags to place over the ends of the lines and hoses. Secure the bags to the line and hoses with rubber bands.

A container of solvent to clean internal parts. Ensure that all parts are clean before replacing them. Compressed air may be used to dry the parts after cleaning.

A container of hydraulic fluid to lubricate the internal parts as they are reassembled.

A container of petroleum jelly to lubricate surfaces where noted by the manufacturer during reassembly.

Anytime the components are serviced and reassembled, always install new O rings, seals, and gaskets. This provides tight seals for mating parts and eliminates leakage.

NOTE

For instructions on the disassembly and reassembly of hydrostatic components, refer to the manufacturer's service manual.

Never operate the hydraulic system empty. Always check the fluid supply after servicing the system. If fluid is to be added to the system. use ONLY the fluid recommended in the service manual.

REVIEW 1 QUESTIONS

Q1. A power shift transmission has what total number offorward and reverse speeds?