Chapter 11: Failure Analysis
105
Engine Overspeed
The MTD engine is designed for a maximum speed of
3600 rpm. When the governor is unable to control the
engine rpm the engine can accelerate past the safe
maximum speed.
When an engine runs beyond its designed speed a few
things happen:
1. As the piston moves up and down in the cylinder
it builds momentum. The higher the rpm’s the
more momentum produced by the pistons. As
the momentum builds, the connecting rods will
start to stretch. When the connecting rods
stretch, they get weaker. Generally speaking this
is at the narrowest part of the connecting rods.
On most engine that would be about an inch
below the wrist pin, but on the MTD engine it is
at the wrist pin.
The force on the connecting rod is greatest
when the piston transitions from the upward
stroke to the downward stroke. Because of this,
most overspeed connecting rod failures will
occur with the piston at top dead center.
When a connecting rod fails, the piston stops
moving but the crankshaft is still moving. This
will allow the broken connecting rod to get
knocked around in the cylinder causing more
damage to it. Usually the connecting rod will be
in several pieces after it breaks making it hard to
find where the first failure was.
2. All engines have vibrations and are designed to
handle those vibrations, but in overspeed the
vibrations change resonance. Parts that can not
handle the new resonance will crack. This may
result in parts flying off of the engine which is an
unsafe condition such as when a flywheel shat-
ters and pieces of it fly off of the engine.
The vibration can also lead to fasteners loosen-
ing up. Evidence of this could be elongated
mounting holes. The area around the mounting
holes may be polished due to the two surfaces
rubbing against each other. See Figure 11.10.
NOTE: The vibration can also be caused be a
loose implement or blade.
3. When an engine overspeeds, the moving parts
can not pull the oil in between them. This allows
metal to metal contact. Because of this, signs of
inadequate lubrication will show.
4. When trying to diagnose an overspeed failure,
look at all the pieces. Individually the lack of
lubrication, piston position and condition of the
connection rod will usually indicate separate fail-
ures. Collectively they would indicate an over-
speed failure.
Figure 11.10
Elongated mounting hole
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