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Pulley Dimension Definitions

Posted by on 8/16/2018 to Guides
Pulley Dimension Definitions
What do all the terms for pulleys, bearings, and belts mean?  Throughbore, offset, flanges, diameters, shoulders... It can be tricky navigating all the technical jargon thrown around when trying to pick out your perfect replacement product. While we have a handy guide on how to measure your pulley, sometimes what is needed is a nice detailed description.

What follows are more than twenty definitions of every dimension used on this site, and a few others for fun. Or, if you want to skip right to the spoilers, reference images for most of our products appear at the bottom of this article.

Pulley Definitions

# of Flanges: Describes the number of flanges on a flat idler or timing pulley. A flange is a name for the piece of product that juts up and away from the flat of the pulley. Most pulleys will have two flanges to keep the belt from slipping, but some operations work best without any: Pulleys with no flanges are typically used on flat belt applications. The rare product with only one flange is recommended for Genuine OEM Replacement only.

# of Set Screw holes: Describes the number of set screw holes on a drive pulley or timing pulley. Once the pulley is in place on the shaft, set screws are tightened so the pulley won't move up or down. Set screws are not necessary in all applications. For example, if the shaft and pulley use a keyway or if the pulley is going on a splined shaft, set screws are considered redundant.

# of Teeth: Describes the number of teeth on the sprocket or timing pulley.

Back Side Offset: The width from the center of the product to the back side edge of the hub or bore adapter. The "back side" is typically thought to be the shorter portion of the throughbore.

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

Belt length: V-Belt lengths are presented here as the measurement from around the outside of the belt. However, A and B type belts are typically measured using the inside. Review the Item description for the A or B type belt measurement length.

eg. A29/4L310: This belt is 1/2" wide. The outside length is 31", the inside length is 29".

Belt Type: The type of V-Belt. Belts with “L” in the name are for fractional horsepower only. It is possible to use A or B belts in fractional horsepower applications, but it is not necessarily recommended for the reverse.

Belt Width: The width of the belt used.

For V-Belts: V-belts take this measurement from the widest point of the trapezoid.

For timing pulleys: For timing pulleys, this is the measurement between the flanges.

Belts that fit: The belts, chains, or rope sizes that will fit into the pulley.

  • For V-groove idlers and drives: A belt is considered to fit into a pulley if both the following are true:
    • The narrowest part of the belt does not touch the bottom of the V-groove.
    • At least 3/4 of the belt fits inside the groove of the pulley. A slight overhang is acceptable.

IMPORTANT: A pulley can work with your belt but still not be appropriate for your system. For more information, check out our blog post, The Best Pulley for your V-Belt or contact us for assistance.

  • For rope and wire pulleys: Reflected as the diameter of the pulley groove. Rope or wires will fit so long as they have a smaller diameter than the groove of the pulley.

  • For sprocket idler pulleys: These are the chains that are found to fit the tooth configuration for the sprocket. This determination is based upon the width of the teeth and the pitch size.

Finish: The finishing material used to prevent rusting in steel or cast iron products. Zinc Plating and powder coat provide the best rust prevention.

Fits with Bearing: The size of the bearing the bore adapter is designed to fit into. Phoenix bore adapters are currently available for 5/8" and 17mm bearings.

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: For flat idler pulleys, the diameter from side to side of the circle of the pulley without including the flanges. 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.

Fractional Horsepower: Any engine or motor that outputs less than 1 "horse" is considered to be fractional.

Frontside Offset: The width from the center of the product to the front edge of the hub, bearing, or bore adapter. Typically the "front side" is considered to be the side that has the longest portion of the throughbore

Genuine OEM Replacement: Any product that is used by the original maker of the equipment being run. For example, this pulley is the exact same part used by John Deere when designing and creating the lawnmower or tractor.

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.

Keyway: The width of the key used to anchor the pulley to a shaft. A negative number in this description means this has an integral keyway, so no separate key will be needed to anchor the pulley. Integral keyways do NOT work on certain applications and so are recommended for OEM replacement only.

Most shafts have the following standard keyway sizes:

Shaft Size/ Inside Diameter

Standard Keyway Size

1/2” (0.50)

1/8” (0.125)

5/8” (0.625)

3/16” (0.187)

3/4” (0.75)

7/8” (0.875)


¼” (0.25)


Material: Describes what the product is made from. Plastic, aluminum, and diecast are softer, lighter, and do not rust. Steel and cast iron are meant for heavy-duty applications but can rust in certain circumstances.

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

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

Pitch Diameter: The diameter of the pitch used in the timing pulley. This is determined by the profile type.

Profile: The name of the combined pitch and tooth configuration in timing pulleys. For example, "3M HTD" has a curvilinear tooth shape and a pitch of 3 millimeters. "Pitch" is the distance between the centers of two teeth

Shoulder Length: The size of the shoulder on the bore adapter. When assembled into a bearing, this is the distance from the edge of the bearing to the edge of the bore adapter. Some bore adapters do not have a shoulder length, these are known as "sleeves" because they match the width of the bearing.

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. Be careful when bolting products with negative stickouts, it will not be appropriate for all applications.

Stock Length: The usable length for timing pulley stock. usable length will only include areas of the pulley stock that contain fully cut teeth.

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 product. In pulleys, this dimension goes across where the belt rides.

Reference Images

bearing dimension diagrambore adapter with dimensions
Drive Pulley Reference Img
v-groove idler dimensionsrope pulley dimension diagram

sprocket dimensions

Anything missing? Let us know in the comments!

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