PowerGrip® GT® Belt Drives

Polyflex ® JB ® and Micro-V ® Belts – Engineering
I. Operating Characteristics
A. High Speed Drives
One major advantage of Gates Polyflex JB and
Micro-V belts is the ability to operate smoothly and
efficiently at high belt and shaft speeds. For
example, shaft speeds of 30,000 rpm and higher
have been achieved with smaller Polyflex JB cross
sections. Polyflex JB and Micro-V belts both have
the ability to transmit power to shafts turning at high
speed mainly because the self-imposed centrifugal
force acting on it is low.
Centrifugal tension occurs in a belt because of
centrifugal force – the belt is rotating around the
drive. Two things govern the magnitude of
centrifugal tension acting in a belt drive:
(1.) The size of the belt (cross-sectional mass).
(2.) The speed the belt is traveling.
The relationship of these items is expressed in the
following formula:
Tc = MV 2
Where: Tc = Centrifugal tension, (lb)
M = A constant depending on mass of belt
V = Belt speed, (ft/min)
The formula shows, as the size (mass) of the belt
increases, centrifugal tension increases. But, more
influential is speed, which increases exponentially.
Because a V-belt is basically a tension carrying
member, its horsepower rating increases with speed
until a maximum value is reached, as illustrated in
Fig. 21. As belt speed increases, the increase in
centrifugal tension creates a need to increase the
installation tension so the desired running tension
is obtained. Beyond this point, centrifugal tension
increases to such a level it starts to reduce the
available working tension, causing the horsepower
rating to drop off. (See Fig. 21).
Figure 21
Polyflex JB belts have proportionally higher
horsepower ratings than any other conventional type
V-belt. This is due mainly to the belt cross-sectional
shape and special polyurethane material, as
previously described. For high speed drives, this
higher horsepower rating is further extended
because the rotating mass of Polyflex JB belts is
reduced ? smaller cross section, lighter material.
Because of these unique characteristics, Polyflex JB
belts may be applied on high speed drives where
other belt types can not be used.
B.Smooth Running
Gates Polyflex JB and Micro-V belts are the
smoothest running V-belt drives available. For most
applications, conventional V-belts transmit power
from the driveR to driveN shafts very smoothly, with
a minimum of vibration. Some high speed, high
precision equipment may demand better
performance than a conventional V-belt which is
designed mainly for heavy duty, industrial power
transmission.
Smooth running characteristics of a belt drive are
determined by measuring the variation in center
distance as the belt slowly rotates around sheaves
on a special measuring machine. The machine uses
one shaft on a movable base and holds the belt at
constant measuring tension. There are no industry
standards prescribing levels of smooth running
performance. When smooth running is specified as
a special requirement, most manufacturers will only
supply these belts on a made-to-order basis.
Conventional V-belts are usually manufactured by
plying up layers of various rubber stocks, fabric and
tensile cord. In most applications, the layers and
overlaps do not disturb the drive operation. But, as
speeds increase and sheave diameters decrease, or
the machinery supporting the drive becomes more
sensitive or compliant, the small discontinuities in
the layers, coupled with overlaps in fabric, become
much more apparent with respect to smooth
running.
Due to the special patented manufacturing process,
Polyflex JB belts have exceptionally smooth
operating characteristics. Belts are cast in
polyurethane and precision ground to assure the
best possible cross sectional uniformity. The result
is low center distance variation and minimal
vibration.
Gates Micro-V belts also have excellent smooth
running characteristics. Micro-V and Polyflex JB
drives have been used extensively on applications
such as precision grinders, medical equipment,
office equipment, business machines, etc. where
smooth running is an essential performance
requirement.
C. Noise
Polyflex JB and Micro-V drives generate minimal
noise levels when installed and operating properly.
They can, however, generate noise in adverse
conditions such as misalignment, improper tension,
under design, etc.
Since Polyflex JB belts have a polyurethane body
with a high coefficient of friction, they may react on
a steel surface causing noise, particularly on a
misaligned drive. When a drive is misaligned the V-
belt will not enter the sheave groove properly. It will
stick and rub heavily on the side on the sheave
causing noise. (See Fig. 22). Low belt tension can
also be a source of noise problems. (See Section III.
Drive Alignment and Section IV. Belt Tensioning,
page 78.)
Figure 22
Ribs molded laterally across the top of Polyflex JB
belts help to reduce bending stresses and increase
torsional stiffness. Because of the ribbed overcord, a
backside idler may generate objectionable noise.
The sound level produced is usually inversely
proportional to the pulley diameter. So, the larger
the pulley diameter, the lower the noise level.
Backside idlers should be as large as possible. See
Table 42, Page 81, Minimum Sheave Diameter for
more information on backside idler
recommendations.
76 The World’s Most Trusted Name in Belts, Hose & Hydraulics.