PowerGrip® GT® Belt Drives

Polyflex ® JB ® and Micro-V ® Belts – Engineering
X. Belt Pull and Bearing Loads
Belt pull is the magnitude of force exerted on the
sheaves by the belts. This force is often referred to
as side load. Magnitude of the bearing load in
driveR or driveN machines depends upon both the
side load (shaft load) acting on the shaft and
bearing locations with respect to side load. Side
load is the combined load created by sheave weight
and belt pull. In comparing these force components,
the sheave weight is usually much less than belt
pull and is typically ignored. However, if there is a
critical application that requires exacting load
information. Sheave weights will have to be
included in the calculations. Belt pull is a function
of these variables:
1. Horsepower Transmitted – For the following
given drive, horsepower is directly proportional to
the belt pull; i.e., as the load increases, the belt
pull also increases.
2. Belt Speed – For the same horsepower, higher
belt speed (larger sheave diameters) reduces belt
pull.
3. Arc of Contact – Reduced arc of contact (wrap)
requires more tension to prevent belt slip. This
increases the belt pull for the same horsepower
load.
4. Total Drive Installation Tension – Minimum
tension is required to keep Polyflex JB and
Micro-V belts from slipping. However, if the
installation tension is excessively high, the belt
pull also will be higher than desired.
NOTE: Required belt pull is independent of the number of
belts or ribs used on the drive. The number of belts or ribs only
affects the amount of overhang from the center of belt pull to the
bearings.
After accurately calculating belt pull, the designer
can size the shafts and bearings required for the
driveR and driveN equipment. It is important that the
designer check the capacity of the shafts and
bearings on both the driveR and driveN.
The driveR is usually an electric motor or engine.
For electric motors, the belt pull is limited to an
acceptable amount by either the recommended
sheave diameters listed in Table 37 on Page 73 or
by the minimum recommended diameters specified
by the motor manufacturer.
Many handbooks show belt pull formulas that differ
from formulas and procedures described in this
section. These differences are the result of shortcuts
that either ignore or average factors such as the arc
of contact correction factor. The Gates method
results in accurate calculations of belt pull for drives
operating at design loads and tensions.
Belt tensions are based on a ratio between the
tightside and slackside tensions. The design tension
ratio for Polyflex JB drives is 5:1, and 4:1 for
Micro-V, based upon 180° arc of contact. Design
tension ratio is then corrected for the actual arc of
contact.
The equipment designer should recognize that belts
can be tensioned up to 1.5 times the design tension.
(See Section IV. Belt Tensioning on Page 78 and 79.
This higher tension expedites belt seating, but does
not exist for the life of the drive. Shafts and bearings
must be designed to tolerate these higher tensions
for a reasonable amount of time without sustaining
damage.
The following formulas are correct for Polyflex JB
and Micro-V belt drives. When shaft load
calculations are required, Gates recommends using
the following formulas and procedures:
Belt Pull Calculations
Step 1 Calculate the Drive Tensions.
Belt pull is the vector sum of T T and T S , the
tightside and slackside tensions, respectively.
Calculate these tensions using the formulas below.
Formula 28
T T = 41,250 (DHP) , lb.
GV
Formula 29
T S = 33,000 (1.25 - K) (DHP) , lb.
GV
Formula 30
T T = 44,000 (DHP) , lb.
GV
Formula 31
T S = 33,000 (1.33 - K) (DHP) , lb.
GV
(Polyflex JB)
(Polyflex JB)
(Micro-V)
(Micro-V)
Where: T T = Tightside Tension, pounds
Ts = Slackside Tension, pounds
DHP = Design Horsepower
V = Belt Speed (ft/min) = {(Pitch Diameter,
in.) (rpm)} over 3.82
G = Arc of contact correction factor, (Table
33, Page 72.)
Step 2 Find the Vector Sum of T T and T S .
Calculate the magnitude and direction of the belt
pull by summing the T T and T S vectors. The
simplest method of calculating the belt pull vector is
by graphical addition. After determining the belt pull
vector, calculate true shaft loads by adding belt pull
vectors to sheave weight vectors.
A. If only the magnitude of belt pull is needed,
numerical methods for vector additions are
faster than the graphical approach. If both
magnitude and direction of belt pull are
required, the vector sum of T T and T S can be
calculated by graphical vector addition as
shown in Fig. 32. The T T and T S vectors are
parallel to the tightside and slackside,
respectively, and they should be drawn to a
convenient scale; i.e. 1 in. = 100 lb.
Fig. 32 shows the vector addition for belt pull
on a motor shaft. Use the same procedures to
determine the belt pull on the driveN shaft. This
graphical illustration method should also be
used for drives that have three or more sheaves
or idlers.
For two-sheave drives, belt pull on the driveR and
driveN shafts is equal but opposite in direction. For
drives using idlers, both magnitude and direction
may be different.
Figure 32 – Graphical Addition of T T and T S
The World’s Most Trusted Name in Belts, Hose & Hydraulics.
85