How to Measure Bearings

April 15, 2021

Shizu Yamaguchi

Whether you are looking to replace an old bearing or seeking out extras to have on hand, knowing how to measure your bearings beforehand is important and can save you some hassle. This article will describe how to measure both metal ball bearings as well as plastic plain bearings, both of which are to be approached differently. Let’s start with plastic plain bearings.

How to Measure Plastic Plain Bearings

Too often, a newly ordered plastic bearing is measured right out of the box using calipers, a measuring instrument made of two long, thin metal pieces adjoined together. When the dimensions of the bearing do not match catalog values, the bearings are typically returned. However, the bearing may not have been the wrong size – plastic plain bearings are simply measured differently than their metal counterparts.

A few considerations need to be taken into account when measuring plastic bearings:

First, plastic plain bearings do not lend themselves well to measuring with calipers. The jaws can press themselves into the material, compress it and create more than one measure since plastic is soft.

Second, plastic plain bearings are made slightly oversized because they are designed to be press fit. Only after being arbor pressed into its final shape will a plastic bearing be ready to be measured properly. Click here for installation instructions.

A pin-gauge test is what igus® uses to measure its bearings. During the test, a pin that parallels the size of the intended shaft is inserted through the top of the press fit bearing. If it falls through the bearing by virtue of its own weight, it’s a “go”. If the pin sticks or fails to fall through the bearing, it’s a “no-go.”

What matters most with a plastic shaft is whether the shaft can fit through the bearing. Minor variations in the width of the bearing (i.e. the peaks and valleys of the inner diameter of the bearing) are relatively unimportant because these will eventually smooth out due to the gradual release of embedded lubrication.

How to Measure Metal Plain Bearings

The best way to find out the size of a given metal ball bearing is to find a reference number on the bearing itself. Usually this number is engraved or laser stamped into the bearing.

However, if the number has been rubbed off, measuring a metal bearing can be done with vernier calipers. Though less accurate, a steel ruler can be used if calipers are not available.(Photo, right: “Digital calipers” by Dvortygirl, licensed under CC BY-SA 3.0)

The three lengths measured to determine a metal bearing’s dimensions are the inner diameter (ID), outer diameter (OD) and width (W), as shown in the image below. (Fig. 1 below: “bearing_measure_guide.jpb”; Bearingboys, 17 February 2021, https://www.bearingboys.co.uk/Measuring-Bearings-15-a; photograph)

To measure the ID, place the outer anvils of the caliper against the inside bore. The fit does not need to be tight to be accurate. Read the value from the caliper.

To measure the OD, put the jaws of the caliper around the outside of the bearing. Ensure a good fit and read the value.

To measure the W, repeat the steps above for the OD.

Note that bearings can be in imperial or metric sizes. Usually metric sizes are in full millimeters, but if you see a measurement that is a half millimeter, it’s likely that the bearing is actually in inch sizing.

Additional Metal Bearing Info

If you see additional markings on a metal bearing, these will further specify qualities that your replacement bearings should have.

“ZZ or 2Z = 2 Metal Shields

2RS1 or 2RSR or DDU= 2 Rubber Seals” (Bearingboys).

Additional markings will indicate a clearance value, which is the room between bearing races for expansion, adding an amount of play.

Here are the markings to reference for this information:

“C2 = Clearance is less than international standard

No Markings = Standard Clearance

C3 = Clearance is greater than international standard

C4 = Clearance is greater than C3” (Bearingboys).

See a full guide to bearing suffixes.

There you have it. Now, you know that plastic and metal bearings require different measuring techniques. Follow the simple steps above and you'll get their measurements right every time -- and you won't have to worry about returning your plastic plain bearings again.

igus® makes plain plastic bearings among other engineered wear-resistant plastic products. To find out more about whether plastic bearings will work for your application, visit our webpage.

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