Whether the pumps and motors of hydraulic transmission 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. For more information concerning hydraulic pumps and motors, read section 2 of NSTM, Chapter 556, Hydraulic Equipment (Power Transmission and Control). Neoprene is the most commonly used 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, shaft stuffing box packing is of the square-braided pure asbestos type. This packing is easily removed, but you must take care to be sure that it is not replaced too tightly. If properly installed, this packing makes a tight joint when you apply light pressure. If packing wears quickly, the shaft should be inspected for roughness. If a lathe is available, you may be able to eliminate the roughness from the 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 being replaced, make certain there is a uniform thickness around the shaft. An excess of packing on one side of the shaft will cause breakage. Stuffing boxes should be packed loosely and the packing gland set up lightly to allow adequate leakage for cooling and lubrication. See NSTM, Chapter 078, Gaskets, Packings, and Seals, for more detailed discussion of O-rings and other types of seals for hydraulic system equipment.

There is 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 were properly set and secured when the connecting units were installed. However, when a casualty occurs or a unit is replaced, it is possible for the unit to become misaligned enough to cause severe stress and strain on the coupling and connected parts. Excessive misalignment should be eliminated as soon as possible by replacing 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.

Pipings and Fittings

If properly installed, the piping and valves of a hydraulic system are seldom a source of a trouble, except for leakage. Some leaks, however, can be serious enough to cause a reduction in the efficiency of the unit. You should make frequent inspections for leakage and take steps to eliminate any leakage found. Guidance and requirements for the installation, inspection, and maintenance of piping and associated fittings are contained in NSTM, Chapter 505, Piping Systems, If leaks occur at a flanged joint in the line of a hydraulic system, tighten the flange bolts evenly, but not excessively. If the leaks persist, use the auxiliary gear while the leaking flange is being refitted with copper asbestos or O-ring packing. Be sure the flange surfaces are cleaned carefully before the packing is applied.

CAUTION

Exposure to asbestos fibers is a recognized health hazard. Refer to NSTM, Chapter 635, Thermal, Fire, and Acoustic Insulation, for safety requirements applicable to handling asbestos packing and gaskets.

If certain measures are taken, operation of hydraulic equipment may be continued while leakage repairs arc being made in some parts of the system. When the lines in an auxiliary system leak, they should be valved off from the main line connection to prevent leakage between the two systems. If leaks occur in the pumping connections to the three-way valves of a steering gear installation, the pump can be cut out with the valve, and another pump cut in. If the three-way valves fail to cut out the leaking unit, and it becomes necessary to cut out both pumps of a steering gear installation, the valves may be closed at the ram cylinder. Hydraulic systems will work without pressure control. So by closing the valves in the lines where they join the main piping, leaking pressure control pipes or cylinders can be cut out of the system for repairs.

Expansion lines and replenishment lines in hydraulic systems of older ships are seldom a source of leakage or breakage, since they are not under any appreciable pressure. However, all hydraulic line connections must be maintained intact. in more recent installations, however, replenishing lines are under pressure as much as 300 psi. In these modern installations, the hydraulic systems should not be operated during the repair of these lines.

Relief valves and shuttle valves of a hydraulic system may also be a source of trouble. The seats of relief valves that are leaking should be reground. Loss of power is a symptom of a leaking relief valve. Shuttle valves may stick and fail to cut off. This condition is evidenced either by the escape of oil from the high-pressure side of the line into the expansion tank or by the failure of the pressure control. When a shuttle valve fails to operate, the stop valves should be closed and the defective valve removed and repaired.

Incorrectly adjusted needle valves can be another source of trouble. Needle valves that are adjusted too fine may cause the device operated by the valve to stop short of its intended stopping point. This may happen because the valve adjustment allows more fluid to pass through leakage points in the system than through the valve. NSTM, chapter 556, provides a good source of general information concerning different types of valves used in hydraulic systems and their maintenance.