MOTOR TRUCK SERVICE MANUAL
Fig. 14-Shock Fracture
Excessive clutching--commonly known as "bucking" or
axle shafts or cracked bevel gear teeth. If the truck
"frogging"--will very frequently cause differential side
continued in operation after the differential broke up,
gears and side pinions to fail in the manner shown in Fig.
damaged cases, battered idlers, and ruined bearings
14. The fractures are of the shock type--produced by
would most likely be found.
stresses greater than the maximum strength of the parts.
The break occurs at once when the. high abnormal
During the winter months when trucks are operating on
pressures are applied. For this reason, the failure is
icy pavements, trouble of this nature is not unusual.
known as a shock fracture.
Wheels spinning under load and suddenly breaking
through ice to stop against high traction pavement,
In the above illustration, the grainy structure
subject differential gears to fracture creating shocks.
characteristic of all shock fractures is clear. Note how
Severe operations such as construction work and
teeth from the side gear were broken out about 900 apart
logging are apt to develop numerous failures of this kind
where each of the side pinions meshed. One of the side
when ground or working conditions are poor. Lightened
pinions shown received sufficient shock load to cause it
loads are often all that is necessary to eliminate them
With failures of this kind, it is not at all unusual to find
that secondary failures also occurred, such as broken