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What is an Idler Pulley?

Posted by on 12/18/2018
What is an Idler Pulley?

So you know you have to replace a pulley on your snowblower because of some extensive online research, but you're not sure what kind you have. At first, you thought maybe it was a drive pulley, but after looking at it closely, you can tell it's got some pretty key differences.

The root of the word "idler" has a couple of different meanings - the one most of us use daily refers to people who are lazy or activities that are pointless. I have good news for you, idler pulleys are neither lazy nor pointless, and if you take all of the idleness out of your lawnmower, it will break!

Let's get our bearings together and find out all about idler pulleys!

What is an idler pulley made of?

Similar to drive pulleys, an idler pulley can be made of almost any hard/sturdy material. Steel and plastic are very popular choices.

All idler pulleys must have two components. Those parts are:

  • A round disk, often with a groove and flanges around the circumference. This is the area where a belt will typically ride.
  • A center hole for a bolt to go through and attach to a surface or housing. The center hole can just have a bushing and grease, but most will usually have a radial bearing in the center so that it spins freely and doesn't require as much routine greasing maintenance.
    • A bearing is designed so that the inner ring can remain stationary while the outer ring spins freely (or vice-versa). It uses ball bearings inside of a special housing coated with grease. Tune in soon for more info on bearings!
Idler Pulley Diagrams

You may find yourself wondering what some of the idler dimension terms in the image above refer to. If so, take a look below, we've got some helpful explanations:

Idler Pulley Dimension Terms

Flanges: These are protrusions out from the center of a pulley, designed to keep a belt from slipping off . Most flat idler pulleys will have two flanges, but some operations work best without any flanges.

Back Side Offset: The width from the center of the pulley to the backside edge of the hub or bore adapter.

Bearing ID: The inside diameter of the bearing used in the pulley. This is useful for determining if a bore adapter can be added, and if so, which sizes will fit.

Flange Angle: The angle of the flanges on flat idlers. Presented either as 90 degrees to reference perpendicular flanges, or in degrees away from perpendicular. Generally, wider angled flanges can decrease wear on your belt.

Flat Diameter: The diameter from side to side of the circle of the pulley. This is the diameter where your belt will ride.

Flat Width: The distance from side to side of the flat of the pulley. This is the width where your belt will ride.

Frontside Offset: The width from the center of the pulley to the front edge of the hub, bearing, or bore adapter.

Groove Angle: The angle of the v-groove. This is largely governed by the diameter of the pulley, but can affect how deep in the groove your belt will ride.

Inside Diameter: The distance across the center hole, or bore, of the product. This dimension should match the diameter of your shaft or bolt.

Material Thickness: Refers to the thickness of steel products only. This helps determine the overall sturdiness of the part. For Idler Pulleys, material thickness greater than 0.07” is considered Heavy Duty. For Drive Pulleys, thickness of 0.14” or greater is Heavy Duty.

Outside Diameter: The distance across the circle of the product, from outside edge to outside edge. For flat idler pulleys, this will include the flanges.

Overpin Diameter: An engineering dimension used to represent belt effective diameter. Cylinders or "pins" are placed at the top and bottom of the pulley's v-groove, the measurement is taken from the top of the upper pin to the bottom of the lower pin.

Pin Diameter: Diameter of the cylinder used to determine the belt with the best fit in v-belt drive and idler pulleys. Other belts may work with the pulley, but the fit into the v-groove will be different. The below chart shows the pin that corresponds to various belt sizes.

Pin DiameterBelt Type
0.50004L/A or 5L/B

Stickout: The distance from the edge of the pulley’s flange to the edge of the bore adapter or hub. This is easiest to measure if the pulley is placed hub down on a flat surface. The stickout will be the space between the surface and the outside edge of the pulley. A negative stickout means that the pulley will lie flat on a surface with no gap.

Throughbore: Length of the bore, from one edge to the other. The Frontside Offset and Backside offset together equal the length of the throughbore.

Width: The distance from side to side of the pulley, across the area where the belt rides

Click here if you’d like some tips on how to measure an idler pulley.

Why do idler pulleys have different groove shapes?

Each shape is designed to work with different types of belts, and sometimes even different parts of the same belt. Some examples include

  • 119-8822 is a v-groove idler. Much like v-groove drives, these idler pulleys have been designed for use with v-groove belts.
    • eXmark uses this pulley in thirty different kinds of zero-turn lawnmowers.
    • The belt is meant to seat into the groove and be "pinched" on either side, the tighter the better - so long as the bottom of the belt doesn't touch the bottom of the pulley's "v". Click here for more information on how v-belts fit into grooves
    • This pulley's job is to keep the belt in track and spin when the belt is engaged.
  • 756-04224 is a flat idler with flanges.
    • is also used with v-groove belts, but it's designed to be used against the widest point of a v-belt, instead of the belt fitting tightly in the groove. This is called a "backside idler."
    • MTD and Cub Cadet use this pulley on hundreds of lawn mower models.
  • 756-0178 is a flat idler without flanges.
    • It is used primarily an OEM replacement pulley from MTD, used on more than twenty different models of Snow Thrower.
    • This can be used both as a backside idler or with flat belts.
    • Sometimes this is used as a "tensioner," the pulley is mounted to an arm that moves back and forth to cause the belt to turn.
  • 349000 is a rope/cable pulley. The groove is circular, to complement the ropes or cables it is used on.
    • This is popular for use in zip-linining
    • Unlike in v-groove idlers, the pulley is designed so that the rope/cable rides at the bottom of the groove.
  • 08835300 is a sprocket idler for use with linked chains.
    • Despite their appearance, chains are not much different from belts in functionality, sprocket idlers, too, serve the same kind of purpose as v-groove idlers
    • Sprockets and chains are a conceptual combination between two simple machines: a gear and a pulley system - they provide precise movements with the benefit of going

What does an idler pulley do?

The goal in pulley systems is to move power or items from one place to another. They do this by having a drive pulley attached to a motor, that will cause another pulley, the driven pulley, to spin. Idler pulleys are pulleys in a system that are not connected directly to a power source, and cannot increase speed or torque.

Idler pulleys are also not going to be the "output" for most pulley systems - i.e., the idler pulley's spinning isn't the reason you turned the machine on. If you're having difficulty with some of these terms, check out this post on drive pulleys, it goes into detail about what drive pulleys are used for.

In short, idler pulleys are used to

  • Keep the belt/cable/chain in place
  • Redirect the belt/cable/chain
  • Press against the belt to cause the belt to engage

    There you have it, just about everything you ever wanted to know about idler pulleys, now use code IDLE to receive 10% off your next order! Any questions? Let us know in the comments.

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