PowerGrip® GT® Belt Drives

Polyflex ® JB ® and Micro-V ® Belt Drive Selection Procedures
How to Design Polyflex JB and Micro-V Belt Drives
Note: The upcoming drive selection and
engineering sections provide information for
Polyflex JB and Micro-V belts and sheaves (or
pulleys). Where we refer to sheaves (for Polyflex JB
belts), you can substitute pulleys for Micro-V belts.
If you need additional information, contact Gates
Application Engineering, Denver.
This section describes how to design standard twosheave
Polyflex JB and Micro-V drives.
To select either a Gates Polyflex JB and Micro-V
Belt drive, you need to know these five facts:
1. Horsepower or kilowatt rating of the driveR
machine
2. rpm of the driveR machine
3. rpm of the driveN machine
4. Approximate center distance required
5. Hours per day operation.
Follow these seven steps to design a drive:
Step 1 Find the Design Horsepower (Design kw)
Design Horsepower =
(Service Factor) x (Horsepower Requirement)
A. Select the proper service factor from Table 36
on Page 73. If your driveN machine is not
listed, find a machine with comparable starting,
running and shock load characteristics.
B. The horsepower requirement of the drive usually
is taken as the nameplate rating of the driveR.
The actual requirement of the driveN machine
may be used as the horsepower requirement, if
it is known. This type of load approximation is
used in those applications where a small
auxiliary machine is being driven from a large
motor or engine.
Step 2 Find the Speed Ratio
Use Formula 8 below to calculate the desired speed
ratio for your drive. The speed ratio can be
determined from either the shaft speeds or the
sheave pitch diameters. To determine the pitch
diameter of a sheave, measure the outside diameter
and add the appropriate add-on factor from Table
32.
Formula 8
Speed =
RPM of Faster Shaft Pitch Dia. of Larger Sheave
Ratio RPM of Slower Shaft = Pitch Dia. of Smaller Sheave
Step 3 Choose the Sheave Diameters
Standard sheave diameters for Polyflex JB and
Micro-V belts are listed in Tables 27 and 31 on
Page 68 and 70.
A. The drive can be designed around a known
sheave diameter. For example, if you have one
sheave available, or if a minimum or maximum
sheave diameter is known, use Formula 8 in
conjunction with Table 27 or Table 31 to
determine the other sheave diameter. To
calculate the pitch diameter of the larger sheave,
multiply the pitch diameter of the smaller
sheave by the speed ratio, or divide the pitch
diameter of the smaller sheave. To convert the
pitch diameter to outside diameters, subtract the
values in Table 32 from the pitch diameters.
Table 32 – Factors to Calculate Polyflex JB
and Micro-V Pitch Diameters
O.D. to P.D. Value
Cross Section (in) (mm)
5M Polyflex JB +0.050 +1.27
J Micro-V +0.030 +0.76
NOTE: Pitch diameters are always used in speed ratio
calculations. If your drive uses an electric motor, the minimum
selected sheave O.D. should be at least as large as the sheave
outside diameters specified in Table 37 on Page 73.
B. Calculate the sheave rim speed for your drive
using Formula 9 below:
Formula 9
Sheave Rim (O.D. of either sheave) x (rpm of same sheave)
=
Speed (ft/min) 3.82*
*This constant is derived from 12/π.
For standard Polyflex JB and Micro-V sheaves the
rim speeds should not exceed 6,500 feet per
minute. If the rim speeds exceed this figure, special
sheave materials and dynamic balancing are usually
required. If possible, redesign the drive using
smaller diameter sheaves so that the rim speed is
between 4,000 and 6,000 fpm.
C. When designing a drive, consider these factors:
1. Advantages of Larger Diameter Sheaves:
Cost Considerations:
Since sheave diameter and rated horsepower per
belt or rib are proportional, larger diameter
drives will usually require fewer belts or ribs to
transmit a specific load. This generally results in
a more cost effective drive.
Space Limitations:
In addition to being more cost effective, large
diameter sheaves will result in a “narrower”
drive. Therefore, to minimize the sheave face
width, select the largest diameter drive from the
group of drives being considered.
However, if there are space restrictions and
larger diameter sheaves cannot be used,
consider using a smaller diameter drive from the
group of drives being considered. If the driveR is
an electric motor, the smallest sheave must be at
least as large as the Minimum Recommended
Sheave O.D. specified in Table 37 on Page 73.
Component Life:
As sheave diameters are increased, the required
drive tensions and shaft pulls are decreased. By
lowering the forces on the bearings, belts and
sheaves, component wear is reduced and
component service life is extended.
NOTE: On some lightly-loaded drives, the cost
of the larger sheave (to obtain belt speed
between 4,000 and 6,000 feet per minute) may
result in a less economical drive. For high
horsepower drives, it is usually best to check
several designs for economics before making a
final choice.
D. If the drive requires an idler, refer to Table 42
on Page 81 for recommended idler diameters.
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