Most of the troubles listed in figure 3-23
require the replacement of an antifriction bear-
ing. The cause of damage must be determined and
eliminated so that similar damage to the replace-
ment bearing may be prevented.
Dirty bearings may be made serviceable with
a proper cleaning, providing other damage does
not exist. In some cases, races abraded on the
external surfaces can be made serviceable, but it
is generally advisable to replace abraded bearings.
Dirty frictionless bearings must be thoroughly
cleaned before being rotated or inspected.
AUXILIARY DRIVE MECHANISMS
Auxiliary drive mechanisms are used in inter-
nal combustion engines to maintain a fixed and
definite relationship between the rotation of the
crankshaft and the camshaft. This is necessary in
order that the sequence of events necessary for
the correct operation of the engine may be car-
ried out in perfect unison. Timing and the rota-
tion of various auxiliaries (blowers, governor, fuel
and lubricating oil pumps, circulating water
pumps, overspeed trips, etc.) are accomplished by
a gear or chain drive mechanism from the
crankshaft. (Some small engine auxiliaries may
be belt-driven.)
GEAR MECHANISMS
The principal type of power transmission for
timing and accessory drives in most diesel engines
is a system of gears similar to those shown in
figure 3-24. In some of the larger engines, there
may be two separate gear trains, one for driving
the camshaft and the other for driving certain
accessories.
The type of gear employed for a particular
drive depends upon the function it is to perform.
Most gear trains use single helical spur gears, while
governor drives are usually of the bevel type;
reverse and reduction gear units employ double
helical gears to balance fore and aft components
of tooth pressure.
Small gears are usually made from a single
forging, while larger ones are quite often built up
in split sections. (See the crankshaft gear in figure
Chapter 3—ENGINE MAINTENANCE
121.8
Figure 3-24.—Relative arrangement of the gears in an auxili-
ary drive mechanism.
3-24.) Most gears are made of steel, although cast
iron, bronze, or fiber are sometimes used.
The timing gear train shown in figure 3-24 is
used on some two-stroke cycle diesel engines. The
camshafts rotate at the same speed as the
crankshaft. Note that two idler gears are necessary
to transfer crankshaft rotation to the camshaft
gears. The idler gears are used because the cam-
shafts and crankshaft are displaced a considerable
distance. If idler gears were not used, the
crankshaft and camshaft gears would have to be
considerably larger.
A similar timing gear train may be found in
some four-stroke cycle engines, except that the
camshaft gear or gears will have twice as many
teeth as the crankshaft gear to permit the cam-
shaft to rotate at one-half the crankshaft speed.
3-37
