Whether the pumps and motors of hydraulic transmissions are of the axial or radial piston type, the maintenance procedures, as well as the operating principles, are relatively the same. In general, maintenance information on other types of pumps also applies to hydraulic pumps and motors.

Neoprene is utilized as a seal around the shafts of most modern hydraulic pumps and motors, but other types of shaft packing are also used. On some modern hydraulic transmissions, the SHAFT STUFFING BOX PACKING is of the square-braided pure asbestos type. This packing is easily removed, but care must be taken to ensure that it is not replaced too tightly. If prop-erly installed, this packing makes a tight joint when you apply light pressure. If packing wears out quickly, the shaft should be inspected for roughness. If a lathe is available, roughness may be eliminated from a shaft by a finishing cut to smooth the surface. If a lathe is not available, it may be necessary to replace the shaft. Packing should be renewed at prescribed intervals to eliminate the possibility of the packing becoming hard and scoring the shaft. When packing is be-ing replaced, make certain that there is a uniform thickness around the shaft. An excess of packing on one side of the shaft will cause shaft deflec-tion and may cause breakage. Stuffing boxes should be packed loosely and the packing gland set up lightly to allow adequate leakage for cool-ing and lubrication.

There will be very little likelihood of poor alignment between the driving and driven members of a hydraulic transmission if the wedges, shims, jacking screws, or adjusting setscrews are properly set and secured when con-necting units are installed. However, when a casualty occurs or a unit is replaced, it is possi-ble for units to get out of alignment sufficiently to cause severe stress and strain on the coupling and connected parts. Excessive misalignment should be eliminated as soon as possible by replac-ing any defective parts and by readjusting the aligning devices. If this is not done, pins, bushings, and bearings will wear out too fast and will have to be replaced frequently. Since there is no end play to either the pump shaft or the motor shaft, flexible couplings are generally used in hydraulic transmissions. Such couplings permit satisfactory operation with a slight misalignment, without requiring frequent renewal of parts.