PowerGrip® GT® Belt Drives

Polyflex ® JB ® and Micro-V ® Belts – Engineering
VI. Idler Usage
– continued
Figure 28 – Equal Arcs
A flat idler pulley (inside or backside) should be
positioned in the span so that the distance between
the idler and the sheave following the idler (in the
direction the belt is traveling) is reasonably large.
See Fig. 29 below. In this position, possible
misalignment due to belt tracking will be minimized
because the increased span length allows the belt to
“realign” itself before entering the adjacent sheave.
This, however, reduces the amount of idler
movement available for belt takeup.
Figure 29 – Locating Flat Idler
In certain applications that have long belt spans and
moderate shock loading, span vibration or belt whip
may occur. If this happens, span vibration can be
minimized by breaking up the long belt spans with
contact idlers.
Tight or slack spans
Figs. 30 and 31 show the idler positioned on the
tightside and slackside spans of the drive.
Figure 30 – Tightside Idler
Figure 31 – Slackside Idler
If possible, the idler should be placed on the
slackside span of the drive (See Fig. 31) rather than
on the tightside.
The span tension directly influences the amount of
stress on the belt. Since the operating tensions are
lower in the slackside span, the belt stresses are
significantly reduced when the idler is placed in this
span. By lower belt stresses, the amount of fatigue
is reduced which results in longer belt life.
Spring-loaded idlers provide belt tension
proportional to the load being transmitted. A useful
paper, “Spring-Loaded Idler Designs for V-Belt
Drives” ASAE Paper No. 75-1524 is available from
Gates Application Engineering upon request.
This text describes proper design and analysis
procedures, as well as a comparison between the
operating characteristics of spring-loaded
(automatic) and manual tensioning systems.
Spring-loaded or weighted idlers should always be
located on the slackside because the spring force,
or weight, can be much less in this position. In
addition, these idlers should not be used on a drive
where the load can be reversed; i.e., where the
slackside can become the tightside.
Contact Gates Application Engineering for
assistance in determining the force which idlers
must impose on belts. The idler force must be such
that the resulting belt tension in the span over the
idler is equal to the slack span operating tension.
Calculate the span operating tensions using the
procedure outlined in Section X. Belt Pull and
Bearing Loads on Page 85.
B. Idler Diameters
Idler diameters should be at least as large as the
recommended sheave diameters in Table 42.
Table 42 – Minimum Recommended
Idler Diameters
Min. O.D. of Min. O.D. Min. O.D. of
Belt of Grooved of Flat Inside outside or Backside
Cross Section Idler (in) Idler (in) Idler (in)
J Micro-V 0.80 0.65 1.25
5M Polyflex JB 1.04 0.90 2.90
C. Grooved or Flat
Inside idlers may be either grooved or flat, and
backside idlers should always be flat.
It is important to remember an outside idler on the
ribbed back of Polyflex JB belts may be noisy. For
additional information, refer to Section I, Part C on
Page 76.
D. Idler Details
Flat idlers should not be crowned. However,
flanging of idlers is good practice. If flanges are
used, the inside bottom corner should not be
rounded because this may cause the belts to climb
off the pulleys.
Brackets for idlers should be rigid. Drive problems
described as “belt stretch,” “belt instability,” “short
belt life,” “belt vibration” and others are frequently
traced to flimsy idler bracketry. This bracketry must
be designed to withstand the forces imposed by the
operating belt tensions.
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