Plain bearings under water: What you should bear in mind

Lars Butenschön | 26. October 2021

Underwater use presents some special challenges for machines in general and for bearing arrangements in particular. The main problem is obvious. The water – but also other liquids – affects the bearing points in various ways. In this blog post you will find out what these are and what you need to bear in mind when using plain bearings under water or other liquids.

corrosion

In addition to moisture, the corrosion process also requires oxygen. It would now seem logical to conclude that components permanently used under water with accordingly less contact with air are more protected against corrosion. In fact, however, the corrosion process is at best slowed down. All the used components should also be protected against corrosion if they are to be used permanently under water. Corrosion-protected steels or corrosion coatings are suitable for this purpose. But what about the bearings themselves?

The corrosion protection of bearings – whether ball bearings, rolling bearings or plain bearings – usually consists of grease. It not only decouples the contact surfaces from each other, but also protects them from moisture. Plain bearings made of plastic or textile fabric do not require this protection as they do not corrode.

Solid lubricants

The so-called “dry-operating” plain bearings do not require grease. Instead, the so-called solid lubricants ensure low friction. The advantage is that no grease has to be retained in the bearing position by means of complex seals. The disadvantage is that some solid lubricants must first be transferred to the surface of the counterpart i.e. the shaft, during operation. Water can hinder this effect. The result is increased wear. When selecting the plain bearing, make sure that the manufacturer recommends it for operation in water or an appropriate medium.

Special features of non-metallic plain bearings – swelling

Plastics and artificial fabrics absorb moisture and thus increase in volume. This circumstance affects the function as a bearing, since dimensional changes to the bearing affect the clearance of the bearing position. In the worst case, the bearing jams on the shaft and blocks. However, you can minimise or eliminate this risk by increasing the bearing clearance or by choosing an appropriate plastic. Plastics for use as plain bearings underwater, offer water or moisture absorption of approx. 0.1% by weight.

Pay attention to resistance & certifications

Is it just water? Salt water already requires corresponding resistance here. There are also materials that are resistant to chemicals such as fuels or acids and bases. Most manufacturers offer resistance tables for their materials. Note, however, that these tables usually say nothing about how good the wear behaviour of the materials is in the various media, but often only say something about whether the mechanical resistance remains unchanged over a certain period of time.

We would be happy to assist you!

Do you need assistance with the design of your plain bearing for underwater use? We will advise you without obligation on site, by telephone or virtually and we can advise on the selection of the ideal material. We will then support you in the development and choice of the most suitable manufacturing process up to the production and delivery of the parts.