Plain bearings and humidity, you should pay attention to this

Lars Butenschön | 1. October 2020

Plain bearings are often exposed to moisture. The effects of moisture are often underestimated. The result: bearing damage, the cause of which can often not be adequately traced in retrospect. If you know the mechanisms and effects of increased moisture in plain bearing applications, you can easily identify potential problems and avoid them in advance by choosing the right material. So let’s take a look at them.

How does moisture affect plain bearings?

In the case of plain bearings made of metal, corrosion is of course the first and foremost issue. This problem does not occur with bearings made of plastic. This is where another factor comes into play. The intake or absorption of moisture.

Plastics – depending on the “type” – absorb moisture in different amounts and at different rates. You can think of it more or less like a sponge that slowly soaks up. The test of “maximum moisture absorption (at 23° C/73° F, 50 % r.  h.)” is a testing method according to DIN 53495 and provides information on the amount (namely the increase in volume of the component) of moisture absorbed from the air. The information in brackets describes the air temperature and humidity, whereas “maximum water absorption” means the change in volume due to the moisture absorbed in direct contact with water. This involves the bearing being immersed in water and taken out again after 24 hours.

You can avoid this problem quite easily in practice. Either by design measures – i.e. defining the bearing clearance in such a way that the plain bearing can still expand sufficiently in the bearing position – or by choosing a plastic with lower moisture absorption.

Also ask yourself how and in what way the bearing is exposed to moisture. How long is the exposure time? How high is the humidity? Unlike a sponge, plastic does not “soak up” immediately. In fact, components reach the first 5-10% of total saturation after a few minutes, the next 20% in a few hours. The next 50% in days. Can the component release the moisture into the air occasionally? Then the plastic with higher moisture absorption also does it.

Corrosion protection – even for plastic plain bearings?

I mentioned earlier that corrosion is primarily an issue with metal bearings. Of course – plastic does not rust. But the bearing bolt or the shaft would. Polymer plain bearings often operate dry. They therefore do not require lubrication. If this lubrication is omitted in contact with moisture, the plain bearing will not rust, but the shaft will (unless it is made of stainless steel or has an appropriate coating). The housing hole should also be protected against corrosion.

Summary: this is what you should bear in mind when dealing with plain bearings and moisture

• Can the shaft, housing hole and bearing be damaged by corrosion?
• How long is the bearing exposed to moisture?
• Can the moisture be released again?
• Is the bearing clearance designed large enough?

Depending on these questions, you can either take appropriate countermeasures or choose a material with lower moisture absorption and additional corrosion protection.

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