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