In this third of four installments we will be detailing what makes a drive pulley different from an idler pulley and what each typically look like.
To begin, what is a drive pulley? According to thefreedictionary.com a drive pulley is one “that drives a conveyor belt”. Could it be that simple? Not entirely. A drive pulley is a pulley that applies force to the belt, cable, chain, etc. to drive the system it’s attached to. A shaft connects the pulley to an engine or motor. A drive pulley drives the system it’s attached to.
A drive pulley spins
with its shaft
What is an idler pulley? Merriam Webster simply calls an idler pulley “a guide or tightening pulley for a belt or chain”. A more complex definition, describing more of the function of an idler comes from audioenglish.org. It’s “a simple machine consisting of a wheel with a groove in which a rope can run to change the direction or point of application of a force applied to the rope”. An interesting point is that an idler can provide tension to either side of the belt depending on the assembly and application. Typical applications have the drive pulley on the front side of the belt and the idler on the flat backside of the belt.
An idler pulley does not
spin with its shaft
Put simply, a drive pulley spins with its shaft and an idler does not.
There are instances where a drive (below right) and idler (below left) look very much the same.
Typically they look very different. From the definitions above do you know which of the below pictures are drives and which are idlers?
Key differences include the presence of a static shaft at the center of a drive pulley (upper set of pulleys above) and a bearing or special bushing at the center of an idler (lower set of pulleys above). The idler must spin freely so it can be driven where the drive pulley must be able to transfer and apply force generated by the engine or motor it’s attached to.
Another large difference you may have already noticed is the space allowed for a belt. Typical drive pulleys have a narrow groove, also known as a V groove, which the belt sits in. Tension applied to the belt snugs it into the V groove, allowing the groove to pinch and grip it. Idler pulleys have a larger flat groove that allows a belt to freely move along its surface.
What if a pulley is a drive pulley but isn’t attached to the main source of force, the engine or motor? The pulley is being driven by another source (typically a drive pulley on the engine or motor) but is also driving more pulleys or components of an assembly. It’s both driven and driving. That would simply be a driven pulley.
A driven pulley both
drives and is driven
To recap; a drive pulley spins with its shaft. An idler pulley does not spin with its shaft. A driven pulley spins with its shaft but is being driven by a source other than the engine or motor, typically a drive pulley.
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