Quick. You need replacement bearings, but you’re not sure what measurements you require. It’s time to inspect the old bearings and possibly carry out a few measurements.

You may be in luck. If the bearing still has a visible reference number engraved onto it, you’re sorted. Just call SMB Bearings and quote this code. If not, you might be able to dig out the original order documentation with the reference on out of the depths of your filing cabinet. If neither of these are possible, it’s time to get measuring.

There are three main measurements to a bearing. The bore diameter (sometimes referred to as inner diameter or ID), the outer diameter (OD) and the bearing width.

While all three measurements are important, the bore diameter receives particular attention, as this figure is entirely dependent on the shaft size. It should be noted that the shaft diameter does not always equal the bore diameter. In many cases, the shaft diameter is often a tiny bit larger than the bore size, to help the bearing to fit well on the shaft over time. This means induction heaters are required to mount bearings onto the shaft while the material is warm and malleable.

How to measure bearings

To obtain these three measurements, you could attempt to use a ruler.  Many people do this, but it isn’t very accurate.

The best way for most of us to measure bearings is by using digital Vernier calipers. This measurement tool is used to gauge internal and external distances.

A good caliper will have an accuracy of plus or minus 0.02 mm or roughly a ten thousandth of an inch. Suddenly, that ruler you were tempted to use pales in comparison although neither compete with the air gauges that the bearing manufacturers use. These can be accurate to within 0.0002mm or less than 10 millionths of an inch.

Most calipers allow you to switch between millimeters and inches with the touch of a button, ready to measure both imperial and metric bearings.

Ready to measure the bore diameter? Here’s how you do it.

Turn your digital calipers on and zero the data reading in the closed position. Place the bearing onto the inner measurement side of the calipers, roll them out to as far as the bearing will allow and note down the measurement displayed. You should wiggle the calipers inside the bearing inner ring to make sure they sit across the widest part.

Repeat the process using the external measurement side of the caliper to calculate the outer diameter of the bearing. This time, roll the calipers in a closing direction to obtain the outer diameter measurement.

Using the external measurement side once more, measure the width of the bearing, which is done in the same way you would measure the diameter.

With your three measurements to hand, you can now get in touch with SMB Bearings to find out which bearings from our UK stock are a match.


More than bearing size

Even with your diameters and width carefully measured, there are going to be a few more variables to consider:

  • Did the original bearings have any shields or seals?
  • Does it have a flange on the outer ring?
  • Is the inner ring the same width as the outer ring?
  • Does the bearing contain a retainer?
  • What material is the bearing made from?
  • Did the bearing contain specialist lubricant?

To be on the safe side, you could also send us the bearing in question for our team to take a closer look at. You can find our postal address here, but call us up first so we know to look out for it!

If you would like more help, give the SMB Bearings team a call. We have years of experience supplying small and miniature bearings to a range of industries. Call our experts today on +44 (0) 1993 842 555 or e-mail sales@smbbearings.com.

Our determination to remain specialised gives us a high level of product knowledge, providing bearing and lubrication solutions to existing or potential customers, whether individuals or large corporations. We don’t just sell bearings, we help to solve your problems. - Chris Johnson, managing director of SMB Bearings.