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Step 8 — Check Fan Sheave and Belt Alignment
Factory-supplied drives are pre-aligned and tensioned, however,
Carrier recommends checking the belt tension and alignment be-
fore starting the unit. Always check the drive alignment after ad-
justing belt tension.
To install sheaves on the fan or motor shaft, remove any rust-pre-
ventive coating on the shaft. Make sure the shaft is clean and free
of burrs. Add grease or lubricant to bore of sheave before install-
ing. Mount sheave on the shaft; to prevent bearing damage, do not
use excessive force (i.e., a hammer). Place sheaves for minimum
overhang (see Fig. 18).
Fig. 18 — Determining Sheave-Shaft Overhang
Each factory-assembled fan, shaft, and drive sheave assembly is
precision aligned and balanced. If excessive unit vibration occurs
after field replacement of sheaves, the unit should be rebalanced.
To change the drive ratio, reselect and replace the motor sheave,
not the fan sheave.
After 24 hours of unit operation, the drive belts may stretch.
Check the belt tension after 24 hours of operation and adjust if
necessary. Periodically check belt tension throughout the run-in
period, which is normally the initial 72 hours of operation.
ALIGNMENT
Make sure that fan shafts and motor shafts are parallel and level.
The most common causes of misalignment are nonparallel shafts
and improperly located sheaves. Where shafts are not parallel,
belts on one side are drawn tighter and pull more than their share
of the load. As a result, these belts wear out faster, requiring the
entire set to be replaced before it has given maximum service. If
misalignment is in the sheave, belts enter and leave the grooves at
an angle, causing excessive belt and sheave wear.
1.
Shaft alignment can be checked by measuring the distance
between the shafts at 3 or more locations. If the distances are
equal, then the shafts are parallel.
2.
Sheave Alignment:
Fixed sheaves — To check the location of the fixed
sheaves on the shafts, a straightedge or a piece of string
can be used. If the sheaves are properly aligned, the string
will touch them at the points indicated by the arrows in
Fig. 19.
Adjustable sheaves — To check the location of adjustable
sheave on shaft, make sure that the centerlines of both
sheaves are in line and parallel with the bearing support
channel. See Fig. 19. Adjustable pitch drives are installed
on the motor shaft. Carrier recommends that adjustable
sheaves should only be used for initial balancing and be
replaced with fixed pitch sheaves by the air balancer prior
to the final system air balance.
Do not exceed maximum fan speed rpm with adjustable
sheave or unit damage could occur.
3.
Rotate each sheave one-half revolution to determine whether
the sheave is wobbly or the drive shaft is bent. Correct any
misalignment.
4.
With sheaves aligned, tighten cap screws evenly and
gressively.
NOTE: There should be a
part hub and the bushing flange. If gap is closed, the bushing is
probably the wrong size.
5.
With taper-lock bushed hubs, be sure the bushing bolts are
tightened evenly to prevent side-to-side pulley wobble.
Check by rotating sheaves and rechecking sheave alignment.
When substituting field-supplied sheaves for factory-supplied
sheaves, consider that fan shaft sheave has been factory bal-
anced with fan and shaft as an assembly. For this reason, sub-
stitution of motor sheave is preferable for final speed
adjustment.
V-BELTS
When installing or replacing belts, always use a complete set of
new belts. Mixing old and new belts will result in the premature
wear or breakage of the newer belts.
19
CAUTION
/
-in. to
/
-in. gap between the mating
1
1
8
4
pro-
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