How to Measure Plastic Plain Bearings Correctly

January 15, 2021 Lars Butenschön

Measuring a plain bearing is as simple as using a caliper, right? This method may work well with plain bearings made of steel, but plastic bearings come with a different set of challenges. So, what should be considered? And which measuring methods are better suited for measuring plastic plain bearings? Let's take a closer look at that.

Problem 1: Plastic Is "Soft"

If you place a vernier caliper or outside micrometer on a plastic plain bearing, you will notice that you can press on it for a relatively long time until the value on the display (or, in the case of older models, the position of the lines relative to one another) no longer changes. Unsurprisingly, this is because the steel of the measuring instrument is pressed into plastic.

Problem 2: Plastic Plain Bearings Are Pressed In

In order for the press-in to work, plastic plain bearings often have a press-in oversize. This means that the actual outside diameter can differ significantly from the nominal size. It is not uncommon for the outside diameter to not even be tolerated. This in turn is due to the fact that the necessary press-fit oversize can vary from plastic to plastic. 

The reason: Different plastics expand to different degrees under the influence of temperature fluctuations and humidity in the air. Accordingly, the required allowance for the press fit is often based on the manufacturer's experience.

Another difficulty arises from the press fitting. The dimension of the inside diameter changes with the pressing. The press fit is based on the idea that the bearing is pressed into the bore. It narrows and is stuck. The inside diameter also changes accordingly.

Many "common" measuring methods do not do justice to the application profile of plastic plain bearings

In practice it looks like this: The newly ordered plain bearings are taken out of the bag and measured with the vernier caliper or on a 3D measuring machine. The dimensions do not match what the drawings indicate, so they are returned. Then later the AHA moment: When pressed in, the plain bearings are perfectly within the tolerance.  

Tip 1: Only Measure Slide Bearings After They Have Been Pressed In

Pressing in a plain bearing

Use a press-fit sleeve - or a copy of the component in which the bearing will later be mounted - that meets the requirements described on the drawing or in the product documentation. The correct tolerance of the mounting hole and the absence of burrs or other foreign particles are important. These can influence the dimensions of the pressed-in plain bearing.

Tip 2: Use Test Pins

The use of special test pins is recommended for checking bore fits. The fit of the plain bearing can be measured most sensibly through their fine tolerance.

In addition, the pressed-in bearings can be measured in several planes on the inside diameter to check the uniformity of the wall thickness. Measurement using a slide probe is ideal for this purpose.

Do you need support with the correct design and measurement of plain bearings? Contact an igus® expert here or call 800-521-2747. 

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