PowerGrip® GT® Belt Drives

Polyflex ® JB ® and Micro-V ® Belts – Engineering
V. Installation and
Takeup Allowances
Like any power transmission belt drive, installation
allowance is necessary for Polyflex JB and Micro-V
drives to assure the belt can be installed without
damage and then tensioned correctly. The standard
installation allowance is the minimum decrease in
center distance required to install the belt on the
sheaves (See Fig. 25).
NOTE: A belt should never be rolled onto a
sheave. This can damage tensile cords in the belt
resulting in premature failure. The damage may be
invisible and difficult to identify as the cause of
failure. A rolled on belt, because of cord damage,
may also elongate rapidly and run out of takeup.
Figure 25 – Installation and Takeup
Takeup allowance is the increase in center distance
which permits retensioning the belt due to wear and
elongation. Takeup values lower than standard
practice may not permit full belt life and could be a
cause for excessive slipping.
Installation and takeup allowances are listed in
Tables 40 and 41.
Table 40 – 5M Polyflex JB Installation
and takeup Allowances
Center Distance Allowance for
Standard Installation Takeup
Effective Length (in) (mm) (in) (mm)
280 - 300 0.4 10 0.2 5
307 - 710 0.6 15 0.6 15
730 - 1090 0.9 25 1.1 30
1120 - 1500 1.1 30 1.4 35
Table 41 – J Micro-V Installation and
Takeup Allowances
Center Distance Allowance for
Standard Installation Takeup
Effective Length (in) (mm) (in) (mm)
Up through 20.0 0.4 1.02 0.3 0.76
20.1 through 40.0 0.5 1.27 0.5 1.27
40.1 through 60.0 0.6 1.52 0.7 1.78
60.1 through 80.0 0.7 1.78 0.9 2.29
80.1 through 100.0 0.8 2.03 1.1 2.79
VI. Idler Usage
Idlers are either grooved sheaves or flat pulleys
which do not transmit power. They are used in
Polyflex JB and Micro-V drives to perform these
functions:
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Provide takeup for fixed center drives.
Maintain belt tension; i.e., when the idler is
spring-loaded or weighted.
Increase arc of contact on critically-loaded
sheaves.
Break up long spans where span vibration or belt
whip may be a problem.
Clear obstructions.
NOTE: An idler can solve the design problems
above. Since an idler always imposes additional
bending stresses on the belt, it usually is best to
design the drive without idlers if possible. Also,
from an economic standpoint, it is generally more
cost effective to provide takeup by movement of
either the driveR or driveN shaft rather than by
inserting an idler.
When using idlers, certain principles should be
followed to obtain the best possible drive and to
minimize common idler problems. The important
design considerations are:
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Placement in the drive (inside vs. outside).
Placement on tight or slack sides.
Idler location in span.
Diameter.
Grooved or flat.
Center distance, belt length, installation and
takeup.
Inside or Outside:
Idlers may be placed either inside or outside the
drive, as shown in Fig. 26 and Fig. 27.
An inside idler decreases the arc of contact on the
adjacent sheaves; whereas, a backside idler
increases the arc of contact on the adjacent sheaves.
Either may be used, but a backside idler must be
larger (See Idler Diameters on Page 81). If a
backside idler is used for belt takeup, the idler
penetration (amount of takeup) is limited by the belt
span on the opposite side of the drive.
Figure 26 – Inside Idler
Figure 27 – Outside or Backside Idler
Backside idlers are always flat pulleys, since they
contact the top of the Polyflex JB or Micro-V Belt.
Inside idlers can be either flat or grooved. If they are
grooved, the idler sheave must be machined
correctly for the specific belt cross section. (See
Sections VIII. Polyflex JB Sheave Groove
Specifications and IX., Micro-V Pulley Groove
Specifications on pages 83 and 84.
A. Idler Placement In the Span
Idler position in the span with respect to
other sheaves.
A grooved inside idler may be located at any point
in the span, but preferably so that it results in nearly
equal arcs of contact on the two adjacent sheaves.
See Fig. 28 on Page 81.
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