12. Use of Idlers
Use of idlers should be restricted to those cases in which they are functionally
necessary. Idlers are often used as a means of applying tension
when the center distance is not adjustable.
Idlers should be located on the slack side span of the belt drive. General
size recommendations are listed for inside grooved, inside flat, and backside
idlers. In some cases, such as high capacity drives utilizing large
sprockets, idlers as large as the smallest loaded sprocket in the system
may be more appropriate.
Idler Size Recommendations
Outside or backside idlers should be flat and uncrowned; flanges may or
may not be necessary. Drives with flat inside idlers should be tested, as
noise and belt wear may occur.
Idler arc of contact should be held to a minimum. All idlers should be rigidly
mounted in place to minimize movement or deflection during drive startup
and operation.
14. Self-Generated Tension
All synchronous belt drives exhibit a self-generating or self-tightening characteristic when transmitting a load. Laboratory testing has shown this
characteristic to be similar with all tooth profiles. The designer/user should
be aware that self-tensioning can result in increased bearing and shaft
loads and reduced drive performance; i.e., short belt life. This can be
avoided by following proper tensioning procedures.
While belt overtensioning can impose higher bearing and shaft loads and
lead to reduced belt life, undertensioning can result in self-tensioning.
Properly designed and tensioned drives will not be significantly affected by
self-generated tension.
When a belt is too loose for the design load, the self-tensioning characteristic
results in the belt teeth climbing out of the sprocket grooves, leading
to increased stresses on the belt teeth, accelerated tooth wear and
reduced belt life. When a belt is severely undertensioned, this self-tensioning
characteristic can result in the belt ratcheting (jumping teeth). When
this occurs, significant shaft separation forces are instantaneously developed
in the drive, resulting in damage to bearings, shafts, and other drive
components including the belt.
NOTE: This is true for all synchronous belts.
Maximum drive performance and belt life are achieved when the belt is
properly tensioned for the design load and maintained.
