Steel bearings may be the most common, but they’re not always appropriate

Non-metallic bearings may not be suitable for every application, but in some, they are ideal. One of these is in submerged or saltwater environments where metal alternatives are prone to corrosion.

Plastic bearings provide excellent corrosion resistance and most are also chemically resistant. These are often made from acetal resin (POM) but other materials are available for stronger acids and alkalis such as PEEK, PTFE and PVDF. These plastics also have good resistance to chlorine and are often used in swimming pool equipment. However, these should only be used in low load and low precision applications. Equally, bearings used across the food and beverage industry need to be able to handle regular wash-downs and steam cleaning as well as potentially corrosive fluids and materials, yet they must also meet the industry’s hygiene standards.

Plastic can even operate effectively in water and salt water. What’s more, unlike stainless steel varieties, the bearings performance is unaffected, even when completely submerged. However, if used with PA66 cages — a type of reinforced nylon — the cage will absorb water after lengthy exposure, causing a loss of tensile strength for the bearing.

There is a common misconception that all corrosion resistant bearings are suitable for underwater use. However, this is not the case. It is important to consider every aspect of the bearing before declaring it safe to use in wet environments. For plastic bearings, there are several alternative materials for rings, cages and balls, when PA66 cages will not suffice. However, the most appropriate choice would depend on the required application of the bearing.

Full ceramic bearings, made from zirconia or silicon nitride with PEEK cages and seals are not affected by sea water and can therefore be used in marine environments, even when fully submerged. However, most ‘ceramic bearings’ are hybrid bearings — the inner and outer rings of the bearings are made of steel, but the rolling elements of the bearing are ceramic.

When using a hybrid bearing, it is important to remember that the stainless steel elements will still rust and corrode if used in submerged environments.

Ceramic bearings made from zirconia or silicon nitride can operate when fully submerged and are the best option if looking for a complete ‘underwater bearing’. These bearings are unaffected by seawater and operate seamlessly when permanently underwater. For complete underwater corrosion resistance however, the bearing may require relubrication to a more suitable oil or grease.

Clearly, corrosion resistant is a broad specification when looking for a bearing. Whether the application is for food processing, chemical manufacturing or marine applications, choosing the correct bearing is no easy feat. In fact, the material, cage and lubrication all depends on the exact application of the bearing.

“Our range of plastic bearings are ideal for these types of applications,” Chris Johnson, managing director at SMB Bearings said. “Plastic bearings are non-corrosive and are usually unlubricated which makes them ideal for commercial food equipment. Acetal resin is the standard material but alternatives are available for greater corrosion resistance in aggressive environments such as the chemical industry. The standard range of plastic bearings are available with glass or 316 stainless steel balls, the latter being more popular in food applications as the balls are easily detected in the event of bearing failure.

“While plastic bearings are suitable for a wide array of applications, they do have limitations,” Johnson explains. “As the material is much softer than steel, plastic bearings should only be used in low load applications. However, many manufacturers don’t realise that the bearings can be paired together to increase their overall load capacity.”

The range of plastic bearings offered by SMB Bearings is ideal for lower precision applications, such as conveyor rollers and trolleys. They can also be supplied with flanges for easier location in a housing.

In a recent experiment at the company’s dry-tech test lab in Cologne Germany, igus compared the resilience of its xiros flanged ball bearing and a conventional metallic bearing in salt water. Results prove that the use of lubrication-free and maintenance-free tribo-plastics enables the xiros bearing to remain completely unharmed.

For the experiment, the igus engineers filled a container with saltwater from the sea and heated it to 80°C. Then two bearings were put inside for 120 hours: a classic 2-hole flange bearing made of metal and a xiros flange bearing made of xirodur B180. Both bearings were not completely covered in the test, but exposed to air in order to trigger the corrosion effect.

“The test result spoke clearly in the end,” says Robert Dumayne, dry-tech director at igus. “After just a few hours, the metal bearing began to corrode.” At the end of the experiment, significant traces of rust were visible on all the metal bearing components. The xiros flange ball bearing remained untouched, even after 120 hours in the heated salt water - being corrosion-free is a clear advantage for machine builders designing equipment for use in cleanrooms and food and medical technology, where rust poses a hygiene risk.

The xiros ball bearings usually consist of four components: the inner and outer rings as well as the cages made of plastic and the balls made of stainless steel or glass. Unlike metal bearings, the wear-resistant polymer bearings enable a very smooth and hygienic dry operation without a single drop of lubricating oil and are maintenance-free. Their long service life can be easily calculated online.

In addition, the plastic bearings are electrically insulating, temperature-resistant from -40 to 80°C, non-magnetic and 60% lighter and up to 40% more cost-effective than comparable metal bearings. They are suitable for absorbing medium loads and, due to their reliability, are widely used in conveyor belts, labelling, handling and packaging machines as well as in filling machines.