The principal requirements necessary to keep a hydraulic transmission in satisfactory operating condition are regular operation, proper lubrica-tion, and the maintenance of all the units and the fluid in the required state of cleanliness. Regular operation of hydraulic equipment prevents the accumulation of sludge and the freezing of adja-cent parts, and aids in preventing corrosion. The necessity of proper lubrication and cleanliness cannot be too strongly emphasized.

Detailed instructions on the maintenance of a specific unit should be obtained from the appropriate manufacturers technical manual, but the following general information will also be useful.

The Fluid System

If an inspection of an oil sample drawn from a hydraulic system reveals evidence of water, sludge, or acidity, the system must be DRAINED, then CLEANED with the prescribed acid-free cleaning fluid (flushing oil), and FILLED with clean hydraulic oil. A hydraulic system may be drained and cleaned as follows:

1. Remove permanent filters and wash them in flushing oil. Then use low pressure air for dry-ing purposes. If filters have replaceable elements, install new elements.

2. Drain the system of old hydraulic oil as completely as possible.

3. Close all connections and fill the system with acid-free cleaning fluid.

4. Start and operate the unit under idling con-ditions in order to fill the system thoroughly with the cleaning fluid.

5. Secure the unit and allow it to stand idle for the prescribed period (usually about an hour). This period of idleness permits the cleaning fluid to dissolve any sludge.

6. Start and operate the unit with a light load for a short interval of time (3 to 5 minutes, unless otherwise specified). Allow the equipment to stand idle for about 15 minutes, then repeat the whole cleaning process. Do this two or three times. Never operate a hydraulic unit with a full load when it is filled with cleaning fluid. Keep the operating pressure as low as possible. After each short operating period, turn the cleaning handles of edge type filters (if installed) and drain from the filter an amount of cleaning fluid equal to its volume.

7. If time permits, allow the system to stand idle for an additional hour following the series of short operating periods.

8. Drain the system of cleaning fluid. Reclean permanent filters, if necessary; install new replaceable filters. Close the system, and fill it with the proper hydraulic oil.

As the system is filled, the hydraulic oil should be strained through a fine wire screen of 180 or 200 mesh. If oil is not clean, it should be run through a centrifuge. Adequate protection should be provided against dust and moisture. Moisture should be expelled from the oil before it is poured into a system; oil with noticeable water content should be rejected or centrifuged.

When a hydraulic system is being filled, suf-ficient hydraulic fluid should be used to com-pletely fill the active parts of the mechanism, leaving no air pockets. Air valves should be opened during the filling process, so that air can escape to the oil expansion box. Be sure the valves are closed tightly after the system has been filled.