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58 CLCH-IM-16A
As shown in the example of
Table 47
, the correct
tension (pounds force) is 9.6 pounds, at 1/2-inch
deflection. Deflection is determined by dividing the
belt span distance by 64, as shown in
Figure 48
.
To measure belt tension, use a belt tensiometer as
shown in
Figure 49
. Determine actual deflection by
depressing one belt with the belt tensiometer and
measuring the deflection relative to the other belts or
to belt line. Adjust the belt tension to the correct
pounds force and tighten all set screws to the proper
torque. (See
Table 47
.)
For belt cross-section types not given in
Table 47
,
use the following equations to calculate correct belt
tension.
F = (T + K)/16
where
F = force measured in pounds at specific deflection.
K = constant determined by belt cross-section type.
T =24,750 (fan hp per belt)/ belt speed
Belt speed = ((fan pitch diameter)/12) X 3.1416 X
fan rpm
For example, given the following:
Motor sheave pitch diameter: 5.7 inches, eight groove
Fan sheave pitch diameter: 10.0 inches, eight groove
Fan horsepower: 2.4 bhp
Fan rpm: 1000 rpm
Belt type: C
Sheave span: 32 inches
Belt speed = (10/12) X 3.14 X 1000 = 2618
T = (24,750 X (24 bhp/2 belts)/2618 = 113.4 lb
F = (13.4 + 40)/16 = 9.6 lb.
Also, D =(Belt span (inches))/64 = 32/64 =.50 in.
Therefore, the belt tensiometer should read 11.5
pounds force at 15/16-inch deflection. This will yield
159.4 pounds force belt tension.
Belt tensions determined by using
Table 46
are
minimum values. The correct operating tension for a
V-belt drive is the lowest tension at which the belts
will not slip under the peak load conditions. It may be
necessary, however, to increase the tension of some
drives to reduce excessive belt flopping or to reduce
excessive start-up squealing.
NOTE: THERE IS A LABEL LOCATED ON THE
BEARING SUPPORT ON THE DRIVE SIDE OF THE
UNIT THAT LISTS ALL DRIVE PARTS AND THE
PROPER BELT TENSION FOR THE SPECIFIC DRIVE.
CAUTION
Do not over-tension the belts.
Excessive tension will reduce fan
and motor bearing life, accelerate
belt wear and possibly cause shaft
failure.