PowerGrip® GT® Belt Drives

Polyflex ® JB ® and Micro-V ® Belts – Engineering
IV.Belt Tensioning – continued
Formula 15
(PD) (RPM)
V =
19100
, m/s
Table 39 – Factor m, Y and Minimum
Tst (Metric)
Cross Section m Y *Minimum T st (Newtons)
5M Polyflex JB 0.009 2.2 35.0
J Micro-V 0.006 1.03 12.5
Formula 19 – Metric Units
( )
t
T st +
Deflection Force per strand, Min. = Le
25
Formula 20 – Metric Units
Y
( )
t
1.5 T st + Y
Deflection Force per strand, Max. = Le , N
25
, N
Step 3. Minimum and maximum deflection
forces are calculated on a per
strand or rib basis. The values in Step
2 should be multiplied by the number of
strands in the belt which is being
deflected. For example, if a 3-strand
Polyflex JB belt is to be deflected, the
minimum and maximum deflection forces
should be multiplied by 3.
*If the value of T st , calculated in Step 1, is less than the Minimum
Tst in Table 38 or 39, use the Minimum T st value from the table for
T st to calculate Minimum and Maximum deflection forces in Step
2. The minimum value must be used on lightly loaded drives due
to belt stiffness so the belt will properly conform to the sheave.
Step 2. Calculate the minimum and
maximum recommended deflection
forces per strand.
A. Measure the span length (t) on the drive or
calculate span length (t) using the formula
below:
Formula 16
2
[ ( ) ]
t = CD 1 – 0.125
D–d
CD
Where: t = Span length, inches
CD = Center distance, inches
D = Large sheave or pulley diameter, inches
d = Small sheave diameter, inches
B. If the drive uses only a single, 2 or 3-strand
Polyflex JB belt, calculate the minimum and
maximum recommended deflection force using
these formulas.
Formula 17 – English Units
Deflection Force per strand, Min. =
( )
t
T st +
Le
16
Y
, lb.
Formula 18 – English Units
t
1.5 T st + ( ) Y
Deflection Force per strand, Max. =
Le , lb.
16
Where: Tst =Static tension per strand from Step 1.
Y = Constant from Table 38.
t = Span length, inches
Le = Effective Belt length, inches
Where: T st =Static tension per strand from Step 1.
Y = Constant from Table 39.
t = Span length, millimeters
Le = Effective Belt length, millimeters
C. If the drive uses two or more, 2 or 3-strand
Polyflex JB belts (should be matched sets),
calculate the minimum and maximum
recommended deflection forces using the
formula below:
Formula 21 – English Units
Deflection Force per strand, Min. =
T st + Y
16
, lb.
Formula 22 – English Units
Deflection Force per strand, Max. =
1.5 T st + Y
16
Where: Tst =Static tension per strand from Step 1.
Y = Constant from Table 38.
Formula 23 – Metric Units
T st + Y
Deflection Force per strand, Min. =
25
, N
, lb.
Formula 24 – Metric Units
1.5 T st + Y
Deflection Force per strand, Max. =
25
, N
Where: Tst =Static tension per strand from Step 1.
Y = Constant from Table 39.
Step 4. Calculate the deflection distance.
The force deflection procedure is based on the
following belt span deflection distances:
Formula 25
t
t
Deflection distance =
64
, inches; or = , mm
100
Where: t = Span lengths
Step 5. Apply the tension.
A. Midway between the belt contact points on the
two sheaves, apply a force perpendicular to the
belt large enough to deflect it from its normal
position by the amount calculated in Step 4. Be
sure the force is distributed evenly across the
belt top width. When pushing on a multiple
strand belt, for example, it may be necessary to
use a bar under the gauge to distribute the force
equally on all strands. (This force can be
applied by pushing the belt in or pulling it out.)
At least one sheave should be free to rotate.
B. Compare this deflection force with the range of
forces calculated in Step 3.
1. If this force is less than the recommended
minimum, the belt should be tightened.
2. If this force is more than the recommended
maximum, the belt should be loosened.
Ideally, the maximum deflection force should be
used when installing the belts and they should be
retensioned when the deflection force falls below the
minimum value calculated in Step 2.
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