MOTOR TRUCK SERVICE MANUAL
Fig . 5-Fatigue Fracture-Bevel Drive Pinion
The fatigue or progressive fracture develops over a
area is worn and pitted away the remaining bearing area,
period of time, gradually working through the metal of the
decreased in size, must carry the entire load. Unit
tooth section until a point is reached where there is not
pressures are increased in an inverse ratio.
sufficient strength remaining to carry the load applied.
concentration of stresses starts fatigue resulting in final
Final failure then occurs and a portion of the tooth is torn
and complete failure.
Momentary excessive stresses applied through the
Fig. 5 above is a typical fatigue fracture of a bevel drive
gears, such as "bucking" to start an overload, or
pinion tooth. The clear cut wavy area identifies the
"frogging" to pull out of a bad spot, will frequently crack
fatigue. It will be found in all failures of this type, making
tooth surfaces. This crack becomes a focal point for the
them easily and quickly recognizable.
stress and fatigue fracture is the result. Abnormal axid
abusive operations most frequently are the direct causes
Continued operation of pitted gears, as described under
of this kind of trouble.
Fig. 4 will result in fatigue fractures. When part of the