Bearing Materials

Steel

SAE52100 Chrome Steel

 

This is the standard steel for most ball bearings. It is harder than stainless steel meaning greater life ratings. It also has superior low noise qualities to standard 440 grade stainless steel. Chrome steel actually has a low chromium content and is not corrosion resistant. Chrome steel can tolerate continuous temperatures of up to 120°C. Above this temperature, it undergoes greater dimensional change and the hardness is affected, reducing load capacity. It can withstand up to 150°C intermittently but above this temperature, bearing life is significantly reduced.

440 Grade Martensitic Stainless Steel (prefix “S”)

 

More resistant to corrosion due to the greater chromium content and the addition of nickel, 440 grade stainless steel is the most commonly used for corrosion resistant ball bearings. The chromium reacts with oxygen in the air to form a chromium oxide layer, known as the passive film, on the surface of the steel. It is hardened by heat treatment and gives a good combination of strength and corrosion resistance. This steel is magnetic unlike 300 grade austenitic steel.

The load capacity of AISI440 grade is approximately 20% less than chrome steel so life ratings will be slightly reduced. This grade exhibits good corrosion resistant when exposed to fresh water and some weaker chemicals but will corrode in seawater environments or in contact with many aggressive chemicals. KS440/ACD34/X65Cr13 grade stainless steel with a lower carbon content is used by EZO Japan and has greater corrosion resistance, greater load capacity (approx 10% less than chrome steel) and superior low noise qualities to the standard AISI440C grade. 440 grade stainless steel will also withstand higher temperatures than chrome steel, coping with up to 250°C constant and up to 300°C intermittent with reduced load capacity. Above 300°C, bearing life can be considerably shortened.

AISI316 Austenitic Stainless Steel (prefix “S316”)

 

316 grade stainless steel bearings are used for greater corrosion resistance to seawater, salt spray and some acids/alkalis. They are suitable for very high temperature applications as the steel is useful in temperatures of up to 500°C. They can also be used in cryogenic applications as the steel retains its toughness down to -250°C. Unlike 440 grade bearings, 316 stainless steel bearings are classed as non-magnetic due to their negligible response to a magnetic field although 316 stainless steel may become more magnetic after being cold worked.

316 grade stainless steel cannot be hardened by heat treatment and will only support low loads and low speeds. The load and speed ratings of 316 stainless steel ball bearings are significantly less than the equivalent 440 grade bearings. 316 grade stainless steel exhibits good corrosion resistance in marine environments when used above the waterline or when temporarily submerged if washed down with clean water. It is less suitable when permanently submerged unless there is a regular high rate flow of water over the bearing. This is because the passive film on the surface of stainless steel relies on the presence of oxygen to regenerate itself. In a low oxygen underwater marine environment (e.g stagnant seawater or under mud/silt) the steel may be prone to pitting or crevice corrosion. 316 stainless steel is less resistant to warm seawater. Pitting corrosion is a risk in seawater over 30°C whereas crevice corrosion can occur in as little as 10-15°C. 316 grade is still much more resistant to corrosion than 440 grade.

Bearings made from 316 grade stainless steel can be used at high temperatures provided a suitable cage material is used or the bearings are full complement. Polyethylene, PEEK or PTFE are often used for retainers in 316 stainless steel bearings.

 

Plastics

Acetal resin / POM-C (AC)

 

PEEK (PK)

 

Polyethylene (PE)

 

PTFE (PT)

 

PVDF (PV)

 

Our standard plastic corrosion resistant bearings have acetal resin (POM-C) rings, nylon (PA66) cages and balls made from 316 stainless steel or glass. They are also suitable for food applications. They will however, corrode in the presence of certain chemicals and PA66 cages will absorb water after long exposure causing loss of tensile strength. A number of alternative materials for rings, cages and balls are available such as polypropylene, PTFE, PEEK or PVDF.

All plastic bearings are semi precision and like 316 stainless steel bearings, should not be used for precision applications. Due to the softer material, they are not suitable for anything other than low loads and low speeds although PEEK has better load bearing capabilities. Corrosion resistance varies between the materials with PTFE, PEEK and PVDF giving the best all round chemical resistance.

Care should be taken to choose the correct material when using plastic bearings at elevated temperatures. Acetal bearings should not be used in temperatures of greater than 110°C and polypropylene should only used up to 80°C but other materials have good high temperature resistance, particularly PTFE and PEEK which are both suitable for temperatures of up to 250°C although PTFE has lower load ratings. Generally, plastic bearings are not recommended for vacuum applications. PEEK is the exception with very good outgassing characteristics.

 

Ceramics

Zirconia / ZrO2 (prefix “CCZR”)

 

Silicon Nitride / Si3N4 (prefix “CCSI”)

 

Silicon Carbide / SiC (prefix “CCSC”)

 

Full ceramic bearings are much more expensive than steel bearings so are normally used in environments that are too hostile for steel bearings. They have good to excellent corrosion resistance depending on the material and the chemicals encountered and are normally supplied without lubrication. They are non-magnetic and, apart from silicon carbide, are electrically insulating. Full ceramic bearings may have PTFE or PEEK retainers or be supplied as full complement type i.e. without a retainer. They can be used in very high temperatures if supplied as full complement.

As ceramics are much harder than steel, they are more brittle. Steel can tolerate large impacts through plastic deformation whereas ceramics are more likely to crack. For this reason, full ceramic bearings, particularly silicon nitride and silicon carbide, are not recommended where heavy shock loads are likely. Full ceramic bearings will accept approximately 65% to 75% of the load of a steel bearing due to the greater brittleness. The limiting speed of a full ceramic bearing is only about 25% of the speed of the same steel bearing because of the inferior roundness of the rings and greater risk of sudden failure due to the lower flexural strength compared to steel.

Using silicon nitride or silicon carbide bearings with steel shafts or housings in high temperature applications can cause fitting problems due to the large difference in expansion coefficient. Bearing damage can occur if allowance is not made for the the greater expansion of a steel shaft in a ceramic inner ring at high temperature. There is less of a problem with Zirconia as the coefficient of expansion is much more similar to steel. For more information see the section on Shaft/Housing Fit.

Hybrid bearings (prefix “CB” or “SCB”): Silicon nitride is the most popular for the balls in hybrid bearings as it has only 40% of the density of bearing steel but is much harder giving greater wear resistance. Hybrid bearings are also capable of higher speeds due to the lower centrifugal force generated by the ceramic balls. However, due to the lower elasticity of the balls, the contact area between the balls and the raceway is smaller which causes a higher contact pressure. This can cause the raceways to wear faster. The speed increase for hybrid bearings is approximately 30-40% with adequate lubrication. Hybrid bearings can also operate better with limited lubrication but running speed should be reduced. They are also less subject to ball skidding under high acceleration with a low load.

For a huge database of detailed and useful information on all kinds of different materials including steels, ceramics and plastics, see the AZoM website by clicking below:
http://www.azom.com/materials.asp.