MOTOR TRUCK SERVICE MANUAL
Fig . 2-Scuffed Gear Teeth--Coast Sides Only .
Fig. 2 illustrates how the contact surfaces on the coast
pressures are greater than can be carried by the oil film.
sides of the teeth are severely scored and how the drive
When this happens, the friction of metal on metal
sides are undamaged. During operation, extreme
creates heat and scoring occurs.
pressures sufficient to break down the oil film were
applied to this gear.
The pinion was similar in
The other cause of such failures is abusive driving. This
appearance. Bevel gears in this condition are easily
generally comes about when vehicles are operated at
detected, for though they run quietly under load on the
high speeds on down grades and snubbing the truck with
pull, they are noisy on the coast or in reverse.
the clutch to brake the speed. Such operation is
particularly detrimental if the transmission is in one of the
Two separate and entirely different circumstances cause
lower ratios. The best preventive for trouble of this kind
failures of this type. One is worn pinion bearings,
is the immediate replacement of worn pinion bearings
allowing end play of the pinion resulting in improper
and good driving procedure.
contact between the gear and pinion teeth on the coast
sides. The contact areas are localized, and the unit