PowerGrip® GT® Belt Drives

Polyflex ® JB ® and Micro-V ® Belts – Engineering
III. Drive Alignment
The “high performance” features (high HP capacity,
smooth running, etc.) of Gates Polyflex JB and
Micro-V belt drives may be realized on drives where
proper sheave alignment is maintained. Polyflex JB
and Micro-V belts should not be used on twisted or
turned drives, or where shaft misalignment can
exceed the amount specified in the following text.
Sheave misalignment can cause belt instability, belt
and sheave wear, strand separation, noise or
excessive machine vibration.
There are two types of misalignment:
■ Parallel
■ Angular
Parallel misalignment occurs when the driveR and
driveN shafts are parallel, but the sheave faces are
in different planes. When the two shafts are not
parallel the drive is angularly misaligned. Fig. 23
illustrates both parallel and angular misalignment.
The fleeting angle is the angle at which the belt
enters and exists the sheave and is the sum of
parallel and angular misalignment.
Any degree of sheave misalignment will result in a
reduction in belt life which is not accounted for in
the normal drive design procedure. Misalignment
of Polyflex JB and Micro-V belt drives should not
exceed 1/4° or about 1/16 inch offset per foot of
linear distance.
One easy way to check alignment is to use a
straightedge checking alignment in both directions.
In other words, lay the straightedge across the face
of the driveN sheave and check driveR alignment.
Then lay the straightedge across the driveR and
check the positioning of the driveN sheave. This
procedure will check for both parallel and angular
misalignment. (Make sure the groove location from
the sheave face is equal for both sheaves. If they are
not equal, allow for the difference when measuring
alignment.)
Figure 23 – Types of Misalignment
IV. Belt Tensioning
In order to obtain full benefit of all the performance
advantages offered by using Polyflex JB and Micro-
V drives on various applications, special attention is
required for belt tensioning. Excessive belt slip can
result from low belt tension or improper drive
design. If a Polyflex JB belt slips, enough heat may
be generated to cause catastrophic belt failure.
Tensioning Polyflex JB and Micro-V drives by “feel”
may result in performance problems unless the
installer has extensive experience with the particular
drive assembly. The relatively small belts, coupled
with proportionally higher horsepower capacity
makes the belts “feel” tighter than they actually are.
For example, 50 pounds tension in a large industrial
V-belt may not remove the sag, whereas 50 pounds
in a 5M or J section belt makes them feel quite
snug.
It is common practice to measure installation
tension. Numerical methods for measuring tension
have several advantages. For example, they prevent
over or under tensioning a drive, thus preventing
possible bearing or belt damage.
The procedures given below explain how to properly
pre-tension a drive when it is stopped (static
tension) so belt tension will be correct when the
drive is operating. Static tension (sometimes
referred to as installation tension) can be measured
by the force-deflection method. The amount of force
needed to deflect a belt a known amount (see Fig.
24) is measured and compared to the recommended
force. Adjustments can then be made as needed.
Figure 24
Step 1. Calculate the required static tension
per strand.
Formula 12 – English Units
*T st (Static Tension) = 15 ( B G
- G )
Where: B = Belt type (5M Polyflex JB = 2.50, J
Micro-V = 2.67)
G = Arc correction factor from Table 33 on
Page 72
Nb = Number of Belts (This is the total
number of strands or ribs on the drive.)
V = Belt speed (ft/min; Formula 11)
m = Constant from Table 38.
Tst = Static tension (lb), per strand.
PD = Pitch Diameter (in) (See Table 32,
Page 71.)
Formula 13 – English Units
V =
(Design HP) (10 3 )
(N b ) (V)
Table 38 – Factor m, Y and Minimum
*See note under Table Tst 31. (English)
Cross Section m Y *Minimum T st (lb)
5M Polyflex JB 0.050 1.20 8.0
J Micro-V 0.035 0.56 2.8
*See note under Table 31.
(PD) (rpm)
3.82
, ft/min
Formula 14 – Metric Units
*T st (Static Tension) = 450 ( B G
- G )
(Design kW)
(N b ) (V)
+ mV2
10 6 , lb.
+ (mV 2 ) , N
Where: B = Belt type (5M Polyflex JB = 2.50,
J Micro-V = 2.67)
G = Arc correction factor from Table 33 on
Page 72
Nb = Number of Belts (This is the total
number of strands or ribs on the drive.)
V = Belt speed (m/s; Formula 8)
m = Constant from Table 39, Page 79
Tst = Static tension in Newtons, per strand
PD = Pitch Diameter (mm) (See Table 32,
Page 71.)
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