Caused by Thermal Overstress
Failure caused by wide variations of temperature and pressure within the combustion chamber
Caused by Mechanical
Failure caused by mechanical origin. This includes wear and breakage, which have nothing to do with the combustion chamber environment.
Result: The valve head breaks, along a chord of its circle, by the under head radius (the fracture starts with a fissure in the radius) Cause: Very high pressures and temperatures in the combustion chamber. (These problems are mainly associated with the exhaust valves) Contributing Factors:
a) Use of inadequate valve material
b) Engine overspeed. Valve float
c) Weak valve springs
d) High seating speed
e) Abnormal combustion
f) Incorrect fuel *
Result: Breakage in the middle of the underhead radius with total valve head detachment Cause: Excessive engine load and accumulated heat. Contributing Factors:
a) Engine overspeed
b) Excessive accumulated heat in the valve neck.
c) Weak springs. Valve float.
d) Seating speed too high due to excessive lash.
Result: A radial crack inward from the margin. If the fissure advances, the head will break. Cause: Thermal fatigue due to high temperatures and unequal temperatures in different zones of the valve head Contributing Factors: a) Thermal shock
b) Engine overload
c) Excessive combustion temperature and pressure
d) Weak valve springs
e) Too high seating speed
Result: Valve head to stem breakage Cause: Repeated stem stresses Contributing Factors:
a) Weak springs. Valve float
b) Engine overspeed.
c) Seating speed too high due to excessive lash.
d) Incorrect valve timing.
e) Sticking valve stem.
f) Guide/seat misalipnment.
Result: Valve stem breakage in the keeper groove area Cause: Material fatigue due to mechanical hardening on the surface. It occurs on the contact zone between grooves and keepers Contributing Factors:
a) Defective keeper assembly
b) Keeper groove damaged
c) Valve float
d) Excessive valve train clearance
e) Engine overspeed, especially on valves with rectangular keeper grooves.
Characteristic appearance of an impact failure
Note: Breakage by impact is a sudden failure (Example: Piston/Valve collision)
Note: Breakage by material fatigue happens after thousands of cycles, starting in a small defect and advancing until breakage occurs.