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the capacity to maintain the necessary accuracy.
4. Pulleys for fixed center systems should be manufactured with a process that is capable of
producing the required O.D. tolerances accurately enough.
5. The performance capabilities of the drive system should be verified by testing belts produced
over their full length tolerance range on drive systems representing the full potential center-
distance variation.
SECTION 13 IDLER USAGE
Idlers in synchronous belt drives are commonly used to take up belt slack, apply installation
tension or to clear obstructions within a system. While idlers cause additional belt bending,
resulting in fatigue, this effect is generally not significant as long as proper design procedures are
followed. Synchronous belts elongate very little over time, making them relatively maintenance
free. All idlers should be capable of being locked down after being adjusted and should require
little additional attention. Specific guidelines and recommendations are given below.
13.1 Inside/Outside
Inside idlers are generally preferred over backside idlers from a belt fatigue standpoint. Both
are commonly used with good success. Inside idlers should be pulleys, but can be flat, if the O.D.
is equivalent to the pitch diameter of a 40-groove pulley. Backside idlers should be flat and
uncrowned.
13.2 Tight Side/Slack Side
Idlers should be placed on the slack (or nonload-carrying) side, if possible. Their effect on belt
fatigue is less on the slack side than on the tight (or load-carrying) side. If spring-loaded idlers are
used, they should never be placed on the tight side (see Spring-Loaded Idlers). Also, note that
drive direction reversals cause the tight and slack spans to reverse, potentially placing the idler on
the tight side.
13.3 Idler Placement
In synchronous belt drives, idlers can be placed nearly anywhere they are needed. Synchronous
drives are much less sensitive to idler placement and belt wrap angles than V-belt drives. The
designer should make sure that at least 6 belt teeth are in mesh on load-carrying pulleys. For
every tooth in mesh less than this (with a minimum of 2), 20% of the belt torque rating must be
subtracted. In order to minimize the potential for belt ratcheting, each loaded pulley in the system
should also have a wrap angle of at least 60°. If a loaded pulley has less than 6 teeth in mesh and
60° of wrap, idlers can often be used to improve this condition. Nonloaded idler pulleys do not
have tooth meshing or wrap angle restriction.
13.4 Spring-Loaded Idlers
Using a spring to apply a predetermined force against a tensioning idler to obtain proper belt
installation tension is acceptable as long as the idler can be locked down after belt installation.
Dynamic spring-loaded idlers are generally not recommended for synchronous belt drives. If
used, spring-loaded belt idlers should never be used on the tight (or load-carrying) side. Tight side
tensions vary with the magnitude and type of load carried by the system. High tight side tensions
can overcome the idler spring force allowing the belt to ratchet. In order to prevent this from
occurring, an excessively high spring force is required. This high spring force can result in high
shaft/bearing loads and accelerated belt wear.
If dynamic spring-loaded idlers are to be used, they should be used on the slack (or nonload-
carrying) side of the drive. Potential drive loading variations in the system will have the least
possible impact on idler movement due to spring compression with the idler placed in this way. Be
sure to note that the tight and slack spans shift as the direction of drive rotation reverses. This
could place the spring-loaded idler on the tight side. In some cases, drive vibration and harmonic
problems may also be encountered with the use of spring-loaded idlers.
