Aircraft Alternator
Maintenance Manual
ES1031
24-30-01
Rev. B: 20 Apr. 2022
Page: 3-9
© 2022 - Hartzell Engine Technologies - All rights reserved
3.4 Continued
E. Unusual belt condition, low belt tension, belt contamination, and belt vibration are other sources of belt noise.
A screech or squeal that occurs when starting or increasing engine RPM rapidly typically indicates a lack
of tension so check belt tension. A tapping or grinding sound caused by debris imbedded in the drive side
of the belt can be the cause and should be checked. Grinding noise can result from worn or damaged
bearings which must be replaced (overhaul or replace alternator). This will eliminate the bearing noise
and possible damage to the alternator. Mechanical noise from vibration can develop over time as drive
components such as pulleys and spring tensioners (if used) wear beyond tolerance, as bearings wear, or as
belts wear and stretch allowing brackets and attachments to loosen.
F. Specific belt noises may give clues to misalignment conditions. Noises can occur in both single V type and
Poly-V type belts. The belt is less likely to generate misalignment noise when in new condition. As a belt
wears, it will develop a smooth, glossy surface which will increase the possibility of noise. A wear condition
will be accelerated if misalignment exists between any of the pulleys in the drive system.
G. A drive misalignment will make a “Chirp” noise in both the common
V belt and the flat Poly-V belt
. This is
caused by entry of the belt contact surface into the pulley as the V(s) seat into the pulley groove there sill be
a chirp that is not repeated as the belt exits. The angle between belt surface and pulley is the critical factor
responsible for causing the “chirp” from a misalignment. Misalignment angle can result from many different
combinations of
pulley positions. An in or out “positioning” error and/or an angular “tilting” error are two typical
examples.
H. Misalignment noise is most likely to occur on the shortest span in a drive system, such as between two
adjacent accessory pulleys. Proper pulley alignment is particularly critical in these locations. The common
V belt pulley has a generally large groove while the nearly flat multi-groove pulley has very small grooves
guiding the belt both can have considerable misalignment and create the same noises
.
I. Misalignment noise is generally loudest at idle speed and diminishes with increasing engine rpm, often
vanishing higher RPM’s. The presence of high humidity (or a wet belt) often increases the likelihood for
misalignment noise to occur.
J. Whenever the pilot notes belt noise, it is important to determine the type of noise and under what
circumstances it occurs. Repairing a drive misalignment (chirp) is not likely to resolve a slip noise (squeal)
that may be caused by insufficient tension.
(1) Check if the problem is more noticeable when starting and while the engine is cool (cool wet belt).
(2) Is the noise loudest at idle speed, or when accelerating the engine RPM (rapid changes in RPM speed
can cause a belt to slip).
(3) Try to recreate the problem in the maintenance run area. If the noise is heard, use a spray bottle or
other source of misting water and mist the belt lightly. If the noise level recedes or goes away for
several seconds, then returns, the problem is most likely misalignment.
(4) If the noise immediately increases after the belt is sprayed, slipping is likely.
(5) If the water spray test is inconclusive and the noise remains unchanged, the problem is not likely related
to drive alignment.
