Fig . 5-Fatigue Fracture-Bevel Drive Pinion

MOTOR TRUCK SERVICE MANUAL

AXLE-REAR

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.

The

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.

out.

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