How do drive pulleys and idler pulleys function differently?

While they are both called "pulleys", one of them does a whole lot more "pulling." In fact, in many pulley common systems, idler pulleys are not present at all!

These "two pulleys" systems only require a few different components:

A motor with a shaft

a drive pulley that attaches to the motor shaft

a driven pulley connected to an output, such as a fan or pump

a belt

Idler pulleys will come into play if:

There are multiple driven pulleys because the belt will have to go to more places (the direction of the belt will need to change)

The belt needs to have tension added in order to cause the output to spin

The belt is traveling across long distance and needs to stay in place

Some more ways to think about the differences between idlers and drives include:

Because drive pulleys are directly connected to motors, they turn at whatever RPM (revolutions per minute) the motor turns. Idler pulleys instead are spun by the belt connected to the drive pulley.

If you change the size of a drive pulley, you can change the speed of the belt and the driven pulley. If you change the size of an idler pulley, the speed of the belt and the speed of the output are not changed

If your drive pulley is spinning freely without the motor shaft turning, you have a serious problem. Conversely, if your idler pulley doesn't spin with minimal effort, you will probably need to order a replacement.

How can you tell an idler pulley from a drive pulley?

Sometimes, it can be a little confusing which is which.

These two pulleys, for instance:

Look very similar, but if you pick out the wrong one, you will not be able to do what you planned.

If you're staring at the owner's manual on your John Deere lawnmower and not sure what kind smudged-up pulley you're looking out, ask the following:

Is this the engine page? You probably going to be looking at a drive pulley

Is this the mower deck pack? You are either looking at a bunch of idler pulleys or blade adapters

Blade adapters are part of the "output" of the lawnmower, they connect directly to the mower blades.

Does the pulley seem to have a flat face? You are probably looking at a flat idler pulley

Does the description of the item say "idler" or "drive"? Well, then that would be your best bet.

Okay, now say you've got a pulley sitting in front of you on a table, how can you tell what kind of pulley it is?

Does the pulley have a bearing in the center? If so, then you definitely have an idler pulley

If it fits, put your finger through the hole and try to spin the outside of the pulley. Does your finger twist or does the pulley spin freely while your finger remains stationary? If it spins freely, you definitely have a bearing

Is there a keyway in the center hole? A keyway is a rectangular cut-out along the outside edge of a hub. It is not required for a drive pulley, but if there is a keyway, you are looking at a drive pulley

Here are some other common shapes you can see in the center hole (also known as a bore or hub) of a drive pulley:

Note in the image the furthest to the right, that this drive pulley has a plain circular bore, with no keyway or splining. In cases such as this, the drive pulley hub is married tightly to the shaft size, so much that it cannot spin freely. Usually, these are further tightened down with set screws.