TCH.1C-27J-2-29JG
2-2.1
GENERAL (Cont'd)
1.With all types of hydraulic seals, the necessary
pressure to produce a seal even at low hydraulic
pressure is produced by having an interference fit
between the rubber and the metal parts. There is
however, a limit to the amount of squeeze that can be
used because of the increased difficulty of installation,
increased friction at low pressure, and increased
flattening or deformation of the O-rings in its groove.
O-rings normally do not fail in a manner which can
cause complete system failure. O-rings can fail as a
result of spiraling, due to faulty installation. Static rings
can be blown out and moving rings can be torn out
due to excessive clearance when O-rings of incorrect
size are used. O-rings are subject to leaks of the
seepage or drip variety. As pressure is built up or
decreased in a unit the O-ring moves in its groove. If it
moves, it tends to roll because of its shape, causing a
slight pumping action.
2.In cases where instructions read "There shall be no
external leakage", light stain or wetting of the joint,
which does not actually form a measurable drop, is
acceptable. Ordinary units such as landing gear, that
only operate a few times during flight, should be
considered satisfactory if, upon inspection after flight,
only a seepage of oil has been collected. Any
increase in the leakage rate from one flight to the next
one should be viewed with suspicion. The above units
should be allowed some drippage while the aircraft is
parked, for example, a few drops overnight. Any
appreciable increase in leakage over a period of time
would be the signal to replace the unit or the seals.
(Cont'd)
29-00-00
2-6