transmission, attempt to define the nature of the noise. Be aware of the exact conditions
that produce the noise.
Growling.
Low frequency noises such as growling, humming, or a grinding sound are
usually caused by worn, chipped, rough, or cracked gears. Once the symptoms are first
noted, as gears continue to wear, a growling/grinding noise becomes more noticeable,
usually in gear positions that throw the most load on the worn or damaged gears.
Growling can also be caused by improper timing of the transmission during reassembly,
or improper timing due to a gear turning on the counter shaft.
Thumping or knocking.
A
thumping or knocking noise can
begin as growling. As bearings
wear and retainers start to break
up, a thumping or knocking
noise can be produced. This
noise is heard at a low shaft
speeds in any gear position.
Irregularities on gear teeth such
as cracks, bumps, and swells
can also cause thumping or
knocking, Bumps or swells can
be identified as highly polished
spots on the face of the gear
tooth, and these are often
caused by rough handling
during assembly (
Figure 8-1
). In
most housings, the noise
produced will be more
prominent when the gear is
loaded, making it easier to
identify the problem gear. At
high speeds a knocking or
thumping noise can turn into a
howl.
Figure 8-1 -
Bumps or swells on the gear
teeth caused by improper handling.
Rattles.
Metallic rattles within the transmission result from a variety of conditions.
Engine tensional vibrations are transmitted to the transmission through the clutch. In
heavy-duty equipment featuring clutch discs without vibration dampers, rattles may
result that are most noticeable in neutral. In general, engine speeds should be 600 rpm
or above to eliminate rattle and vibrations noticeable during idle. A cylinder misfire could
cause an irregular or lower idle speed, producing a rattle in the transmission. Another
cause of rattles is excessive backlash in the PTO unit mounting.
Vibration.
Drivetrain vibrations can be very difficult to source. Never forget that they
can be produced by any component in the drivetrain, so your troubleshooting must
involve everything from the engine to the drive wheel assemblies. Most driveline
vibrations are produced only at a specific road speed or in a specific gear; these clues
can help to determine whether the cause is in the transmission or elsewhere.
If you have isolated the cause of the vibration to the transmission, first note the speeds
at which it is most prominent. Use what you know of the transmission to make your
diagnosis. Remember that the input shaft rotates at engine speed when the clutch is
engaged, and the output shaft rotates at driveshaft speed. Make a note of the gear the
vibration occurs at, and note whether it occurs only in high range or only in low. Make
8-5
NAVEDTRA
14050A