Hydraulic Leakage

 
By 5 February 2018
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Any hydraulic system will have a certain amount of leakage. Any leakage will reduce efficiency and cause power loss.
Some leakage is built in (planned), some is not. Leakage may be internal, external, or both.

Internal. This type of leakage (nonpositive) must be built into hydraulic components to lubricate valve spools, shafts, pistons, bearings, pumping mechanisms, and other moving parts. In some hydraulic valves and pump and motor compensator controls, leakage paths are built in to provide precise control and to avoid hunting (oscillation) of spools and pistons. Oil is not lost in internal leakage; it returns to a reservoir through return lines or specially provided drain passages.

Too much internal leakage will slow down actuators. The power loss is accompanied by the heat generated at a leakage path. In some instances, excess leakage in a valve could cause a cylinder to drift or even creep when a valve is supposedly in neutral. In the case of flow or pressure-control valves, leakage can often reduce effective control or even cause control
to be lost.

Normal wear increases internal leakage, which provides larger flow paths for the leaking oil. An oil that is low in viscosity leaks more readily than a heavy oil. Therefore an oil’s viscosity and viscosity index are important considerations in providing or preventing internal leakage. Internal leakage also increases with pressure, just as higher pressure causes a greater flow through an orifice. Operating above the recommended pressures adds the danger of excessive internal leakage and heat generation to other possible harmful effects.

A blown or ruptured internal seal can open a large enough leakage path to divert all of a pump’s delivery. When this happens, everything except the oil flow and heat generation at a leakage point can stop.

External. External leakage can be hazardous, expensive, and unsightly. Faulty installation and poor maintenance are the prime causes of external leakage. Joints may leak because they were not put together properly or because shock and vibration in the lines shook them loose. Adding supports to the lines prevents this. If assembled and installed correctly, components seldom leak. However, failure to connect drain lines, excessive pressures, or contamination can cause seals to blow or be damaged, resulting in external leakage from the components.

Prevention. Proper installation, control of operating conditions, and proper maintenance help prevent leakage.

Installation. Installing piping and tubing according to a manufacturer’s recommendations will promote long life of external seals. Vibration or stresses that result from improper installation can shake loose connections and create puddles. Avoid pinching, cocking, or incorrectly installing seals when assembling the units. Use any special tools that the manufacturer recommends for installing the seals.

Operating Conditions. To ensure correct seal life, you must control the operating conditions of the equipment. A shaft seal or piston-rod seal exposed to moisture, salt, dirt, or any other abrasive contaminate will have a shortened life span. Also, operators should always try to keep their loads within the recommended limits to prevent leakage caused by excessive pressures.

Maintenance. Regular filter and oil changes, using a high-quality hydraulic oil, add to seal life. Using inferior oil could wear on a seal and interfere with desirable oil properties. Proper maintenance prevents impurity deposits and circulating ingredients that could wear on a dynamic seal.

Never use additives without approval from the equipment and oil suppliers. Lubrication can be critical to a seal’s life in dynamic applications. Synthetics do not absorb much oil and must be lubricated quickly or they will rub. Leather and fiber do absorb oil. Manufacturers recommend soaking a seal overnight in oil before installing it. Do not install a seal dry. Always coat it in clean hydraulic oil before installing it.