Material choices for durable bearings

CORROSION-RESISTANT bearings, as the name suggests, can be used in applications where corrodible materials wouldn’t suffice — think food processing, chemical manufacturing, pharmaceutical production and bearings for marine applications. 

Corrosion isn’t just visually alarming, it can also significantly impact a business’s finances. According to the impact study conducted by NACE International, the world’s leading corrosion control organisation, it has been estimated that 15-35% of annual corrosion could have been saved if optimum corrosion management practices were followed. This equates to savings between $375 and $875 billion annually on a global basis. 
When selecting a bearing material there are many factors to consider including operating temperature, magnetism, sealing lubrication and corrosion resistance. While all these factors should be weighed up in equal measure, there are three materials you should consider for corrosion resistance.

Stainless steel bearings

Stainless steel is the most obvious choice for corrosion resistance. In fact, bearings made of 316 grade stainless steel are often referred to as ‘marine grade’ bearings for this exact reason. These bearings are highly resistant to seawater, salt spray and many aggressive chemicals. However, 316 is not the only grade that provides resistance to corrosion.

440 grade stainless steel also provides good corrosion resistance in damp environments and these are often used in the food and beverage industry where bearings are frequently washed down. Unlike 316 stainless steel however, 440 grade stainless steel has poor resistance to salt water and many stronger chemicals such as caustic cleaning fluids. 

When choosing a stainless steel bearing, 316 is certainly the most effective grade for corrosion resistance. That said, 316 stainless steel bearings are only effective if temporarily submerged. The material is much less effective when permanently submerged, unless there is a regular high-rate flow of water over the bearing. 316 grade steel cannot be hardened by heat treatment. As a result, it has a much lower load capacity than the standard 440 grade stainless steel. As 316 grade steel is more difficult to grind to very close tolerances, marine grade 316 bearings are semi-precision. 316 stainless steel has a very wide temperature range.

Another option for enhanced corrosion resistance is a corrosion-resistant coated bearing. By opting for a coating such as nickel on the outer and inner rings to enhance durability, this will increase the corrosion protection of your stainless steel bearing further.

Plastic bearings

Plastic bearings provide excellent corrosion resistance, and most are also chemically resistant. Plastic bearings are also much lighter than steel, offering a lightweight and cost-effective alternative in certain applications. These are often made from acetal resin (POM) but other materials are available for stronger acids and alkalis such as polyether ether ketone (PEEK), polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (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. Plastic bearings cannot be manufactured to tight tolerances, so are only suitable for low precision applications.

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. 

But – wait – shouldn’t you be able to use all corrosion-resistant bearings underwater? 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. 

Ceramic bearings

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. Opting for the correct lubricant matters. In an environment where superficial corrosion may occur on the outside of the bearing, it should not be allowed to occur on the inside. 
Sealed bearings with waterproof greases that contain corrosion inhibitors protect the internal surfaces of the bearing and can be matched to the specific application environment the bearing is operating in. Full ceramic bearings are mostly specified without lubrication but can be lubricated with waterproof grease for extended life.  

Clearly, "corrosion-resistant" is a broad specification when looking for a bearing. Even in the most gruelling applications — 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.

Chris Johnson is managing director SMB Bearings

http://www.smbbearings.com/ 
01993 842555