Carrier Aero 39MN Series Installation, Start-Up And Service Instructions Manual page 107

Fig. 46 — Determining Sheave-Shaft Overhang
ALIGNMENT — Make sure that fan shafts and motor shafts
are parallel and level. The most common causes of misalign-
ment are nonparallel shafts and improperly located sheaves.
Where shafts are not parallel, belts on one side are drawn tight-
er and pull more than their share of the load. As a result, these
belts wear out faster, requiring the entire set to be replaced be-
fore 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 dis-
tance 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. 47.
Adjustable sheaves — To check the location of adjust-
able sheave on shaft, make sure that the centerlines of
both sheaves are in line and parallel with the bearing
support channel. See Fig. 47. Adjustable pitch drives
are installed on the motor shaft. Carrier recommends
that adjustable sheaves should only be used for initial
balancing and should 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. Equipment damage may result.
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
progressively.
NOTE: There should be a
mating 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 balanced with fan and
shaft as an assembly. For this reason, substitution of
motor sheave is preferable for final speed adjustment.
A39-376tf
Fig. 47 — Determining Sheave-Shaft Alignment
107
CAUTION
/ -in. to / -in. gap between the
1 1
8 4
A39-137tf