29
Fig. 28 — Determining Sheave-Shaft Overhang
ALIGNMENT
Make sure that fan shafts and motor shafts are parallel and level.
The most common causes of mis-alignment 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 will enter and leave the
grooves at an angle, causing excessive belt cover 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 will be 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 lined up the string will touch them at the
points indicated by the arrows in Fig. 29.
Adjustable sheave — 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. 29. Adjust-
able pitch drives are installed on the motor shaft.
3. Rotating each sheave a half revolution will determine
whether the sheave is wobbly or the drive shaft is bent. Cor-
rect any misalignment.
4. With sheaves aligned, tighten cap screws evenly and
progressively.
NOTE: There should be a 1/8-in. to 1/4-in. gap between the
mating part hub and the bushing flange. If gap is closed, the
bushing is probably the wrong size.
Fig. 29 — Sheave Alignment
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 the 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.
CAUTION
With adjustable sheave, do not exceed maximum fan rpm.