Cessna 172 Maintenance Manual page 25

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7. Control of Corrosion on Landing Gear Springs
A. General
(1) The main landing gear springs are made from high strength steel that is shot peened on the
lower surface to increase the fatigue life of the part.
(2) The shot peened layer is between 0.010 and 0.020 inch thick.
(3) If the protective layer of paint is chipped, scratched or worn away the steel may corrode (rust).
(a) If the corrosion pit depth is greater than the thickness of the shot peen layer, the gear spring
fatigue life will be greatly reduced.
(4) Operation from unimproved surfaces increases the likelihood of damage.
B. Corrosion removal and repair.
If damage to the paint finish of the landing gear spring is found, examine the damage area for
(1)
signs of corrosion (red rust).
WARNING: High strength steel parts are very susceptible to hydrogen
(2) Carefully remove any rust by light sanding.
(a) The sanding should blend the damage into the surrounding area in an approximate 20:1
ratio.
EXAMPLE:
(b) Make sure the final sanding marks are along an inboard to outboard direction, or along the
long dimension of the spring.
(3) After the sanding is complete, measure the depth of the damage removal.
(a) Make sure the depth of the damage is not more than 0.010 to 0.012 inch deep and has not
penetrated the shot peen layer.
(4) If the shot peened layer has been penetrated, the gear spring must be removed and sent to an
approved facility to be re-shotpeened.
(a) The shotpeen specification is to be Almen intensity of 0.012 to 0.016 using 330 steel shot.
(5) After the spring is installed, refinish any damaged or removed finish paint.
NOTE:
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Remove residual corrosion by hand sanding.
After removing all corrosion visible through a magnifying glass, fair depression
resulting from rework and finish with 400-grit abrasive paper.
Clean reworked area.
Determine depth of rework area to ensure rework limits are not exceeded.
Prime using rust-inhibitive primer within one hour of rework.
Reapply finish topcoat if required.
embrittlement.
and paint strippers have been found to cause hydrogen
embrittlement. Hydrogen embrittlement is an undetectable,
time delayed process.
failure may occur after the part is returned to service. The
only reliable way to prevent hydrogen embrittlement is not to
use chemical rust removers or paint strippers on landing gear
springs.
An 0.005 inch pit must be blended to a 0.10 inch radius or 0.20 inch
diameter.
Additional information regarding corrosion control can be found in AC-43-4, Chapter
6, or AC43.13-1B Chapter 6.
© Cessna Aircraft Company
Acidic solutions, such as rust removers
Since the process is time delayed,
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Jun 1/2005