Cub Cadet T65 Series - Manual (Page 110)

T65 Series Vertical Shaft Engines
106
Engine Overspeed
The MTD engine is designed for a maximum speed of 3300 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 is produced by the pistons. As the momentum builds, the connecting rods will start to stretch.
When the connecting rod stretches, it gets weaker. The stress is concentrated at the narrowest part of the con-
necting rod. On the MTD engine this point is at the wrist pin.
The force on the Connecting Rod is greatest when the piston transitions from the upward stroke to the down-
ward 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 bro-
ken 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 create a certain amount of vibration when they are operating within their designed speed range.
Engines are built to withstand this normal level of vibration. When an engine is overspeeding, the amplitude
and frequency of vibration is beyond what the engine is designed to cope with.
When moving parts of an engine are creating vibrations that are outside of the engines design parameters,
other parts can break and fasteners can loosen. Normally stationary parts of the engine start to flex and vibrate
in resonance with the frequencies created by the moving parts.
Fatigue cracks in the blower housing, mufflers and brackets are key indicators of either an overspeed problem
or an imbalance problem.
3. When an engine overspeeds, the moving parts cannot 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 Connecting Rod will usually indicate separate failures. Collectively they would
indicate an overspeed failure.