Bearing Material

The bearing industry uses different bearing material for the production of the various bearing components. The materials are processed to achieve desirable properties to maximize bearing performance and life. The bearing material described here are the most commonly used. Additional information can be found in the Technical Information Sheets for Balls, Closures, and Retainers.

STEELS

SAE52100 Chrome Steel (no prefix)

• Higher hardness so longer lfe ratings
• Lower cost
• Good for temperatures up to 150C
• Poor corrosion resistance

This is the standard steel for most ball bearings. It is harder than stainless steel and gives 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 so not suitable for corrosive environments or for dry (no lubricant) bearings as chrome bearings require an oil coating on the exterior surfaces to protect against corrosion which can contaminate the inside of the dry bearing. Chrome steel can tolerate continuous temperatures of up to 120C. Above this temperature, chrome steel undergoes greater dimensional change and the hardness is affected, reducing load capacity. It can withstand up to 150C intermittently but above this temperature, bearing life is significantly reduced.

Designation Country C% Carbon Si% Silicon Mn% Manganese P% Phosphorous Cr% Chromium Mo% Molybdenum Ni% Nickel S% Sulfur
AISI 52100 USA .95-1.1 .15-.35 .5 max .012 max 1.3-1.6 .08 max .25 max 0.25 max
100CR6 Germany .95-1.1 .15-.35 .25-.45 .03 max 1.35-1.65 .1 max 0.2 max
SUJ2 Japan .95-1.1 .15-.35 .5 max .025 max 1.3-1.6 .08 max .25 max 0.25 max
GCR15 China .95-1.05 .15-.35 .25-.45 .027 max 1.4-1.65 .1 max .23 max 0.2 max
440 Grade Martensitic Stainless Steel (prefix “S”)

• Good corrosion resistance to water and many weak chemicals
• Corrosion resistance can be improved by passivation.
• Good for temperatures up to 300C
• Slightly softer than chrome steel so lower load ratings
• Will corrode in salt water or salt spray and poor resistance to acids/alkalis
• More expensive than chrome steel

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 hardenable and gives a good combination of strength and corrosion resistance. It is magnetic unlike some 300 grades.

AISI440C grade The load capacity of AISI440 grade is approximately 20 percent 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. The corrosion resistance also depends on the surface finish. Iron particles and other impurities left on the surface during maching can lead to premature localised corrosion while surface irregularities or poorly finished surfaces also increase the likelihood of corrosion.

The corrosion resistance of stainless steel can be increased by passivation (see section below). 400 grade stainless steel will also withstand higher temperatures than chrome steel, coping with up to 250C constant and up to 300C intermittent with reduced load capacity. Above 300C, bearing life can be considerably shortened.

Passivation is a process by which free iron particles and other impurities are removed from the surface of stainless steel by immersion in nitric or citric acid, thus regenerating the passive film. This reduces the likelihood of surface discolouration so making it a useful process in some corrosive environments. Passivation does not increase the resistance of stainless steel to pitting corrosion. This means that where a bearing has incidental contact with, say, salt spray, passivation may be beneficial but it will not offer long term protection in harsher applications.

Designation Country C% Carbon Si% Silicon Mn% Manganese P% Phosphorous Cr% Chromium Mo% Molybdenum Ni% Nickel S% Sulfur
AISI 440C USA .95-1.2 1.0 max 1.0 max .04 max 16-18 .75 max .25 max 0.3 max
SUS440C Japan .95-1.2 1.0 max 1.0 max .04 max 16-18 .75 max .25 max 0.3 max
9Cr18 China .90-1.05 .75 max .75 max .035 max 16-19 .75 max .23 max 0.3 max
Enhanced 440 Stainless Steel – ACD34
Designation Country C% Carbon Si% Silicon Mn% Manganese P% Phosphorous Cr% Chromium Mo% Molybdenum Ni% Nickel S% Sulfur
KS440 Japan .6-.7 1.0 max 1.0 max .03 max 12-13.5 .25 max 0.1 max
GCR15 China .6-.7 1.0 max 1.0 max .04 max 12-14 .75 max 0.3 max
AISI316 Austenitic Stainless Steel (prefix “S316”)

• Excellent corrosion resistance to water, salt water and chemicals
• Non magnetic
• Expensive due to low production quantities
• Only semi-precision grade is possible
• Suitable for very low load and low speed only

Used for greater corrosion resistance or where bearings must be non-magnetic, bearings made from this material are semi-precision and fine for applications such as marine pulleys but not suitable for precision instrument use. The smoothness and close tolerances possible with 52100 or 440 grade steel cannot be achieved with 316 grade bearings. Also 316 grade stainless steel is non hardenable so as a softer steel, will only support low loads and low speeds. The dynamic load rating of a 316 grade bearing may only be 10% of the 440 grade equivalent whereas the maximum speed may be 5% or less of the 440 stainless steel version. 316 grade stainless steel exhibits good corrosion resistance in sea atmosphere and may perform well submerged in seawater. However, as 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 under washers or o-rings) the steel may be prone to pitting or crevice corrosion although 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. Due to the difficulty of using 316 grade for the cage, 304 grade stainless steel is normally used for metallic cages and nylon for non-metallic cages. Please remember that, as 316 grade bearings are far less popular, minimum quantities may apply and some smaller instrument bearings may not be available.

PLASTICS

Acetal resin (prefix “AC”)

• Excellent corrosion resistance to water, salt water and weak chemicals
• Non magnetic
• Only semi-precision grade is possible
• Maximum temperature 110C
• Suitable for very low load and low speed only

Polypropylene (prefix “PP”)

• Excellent corrosion resistance to water, salt water and many chemicals
• Non magnetic
• Only semi-precision grade is possible
• Maximum temperature 80C
• Suitable for very low load and low speed only

PTFE (prefix “PTFE”)

• Excellent corrosion resistance to water, salt water and most chemicals
• Good high temperature performance.
• Non magnetic
• Only semi-precision grade is possible
• Maximum temperature 260C
• Suitable for low load and low speed.

PEEK (prefix “PEEK”)

• Excellent corrosion resistance to water, salt water and most chemicals
• Good high temperature performance
• Non magnetic
• Only semi-precision grade is possible
• Maximum temperature 260C
• Greater strength so suitable for higher load and speed than other plastics

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

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 and PEEK 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 110C and polypropylene should only used up to 100C but other materials have good high temperature resistance such as PVDF (polyvinylidene) which can be used at up to 150C and PTFE or PEEK which are both suitable for temperatures of up to 260C. Generally.plastic bearings are not recommended for vacuum applications. PEEK is the exception with very good outgassing characterisitics.

CERAMICS (prefix “CB”hybrid or “CC” all-ceramic)

Zirconia (suffix “ZrO2”)

• Good corrosion resistance to cold water and many chemicals
• Good high temperature performance up to 300C without cage
• Non magnetic and electrically insualting
• Lower speed and load than steel bearings
• Not suitable for low noise applications
• Higher fracture toughness than other ceramics so better for small shock loads
• Expansion similar to steel so not a problem to use with steel shaft at high temperature.

Silicon Nitride (suffix “Si3N4”)

• Very good corrosion resistance to water, salt water and most chemicals.
• Good high temperature performance up to 1000C without cage
• Non magnetic and electrically insulating
• Lower speed and load than steel bearings but Si3N4 balls are used in high speed hybrid bearings.
• Not suitable for low noise applications
• Much lighter than steel or Zirconia
• Very low expansion at high temperature

Silicon Carbide (suffix “SiC”)

• Best corrosion resistance of the ceramics
• Best high temperature performance up to 1600C without cage
• Non magnetic
• Electrically conductive
• Much lighter than steel or Zirconia
• Very low expansion at high temperature

Bearings can be supplied with steel rings and ceramic balls (hybrid) or “all ceramic” bearings with ceramic rings and balls. These all-ceramic bearings may have PTFE or PEEK retainers or be supplied as full complement type.

Hybrid Bearings have steel rings and ceramic balls. Silicon nitride is the most popular for the balls as it has only 40 percent of the density of bearing steel but is much harder giving greater wear resistance. Zirconia is heavier with 75 percent of the density of steel so is less suitable for hybrid bearings.. Hybrid bearings are also capable of higher speeds. Sometimes, excessive claims are made about the high speed capabilities of hybrid bearings. They can run faster than all steel bearings due to the lower centrifugal force generated by the ceramic balls but this is partially counteracted by the lower elasticity of the balls. As the balls are harder, the contact area between the balls and the raceway is smaller which causes a higher contact pressure which can can cause the raceways to wear faster. The speed increase for hybrid bearings is approximately 30-40 percent with adequate lubrication. Hybrid bearings can also operate better with limited lubrication but running speed should be reduced. Hybrid bearings are also less subject to ball skidding under inital acceleration.

All-ceramic bearings have good to excellent corrosion resistance, are non-magnetic and, apart from silicon carbide, are electrically insulating. All-ceramic bearings can be used in high to very high temperatures if supplied without a cage (full complement).

Electrical resistance: Si3N4 best, then ZrO2, then SIC which is conductive.
High temp resistance: SiC best (1600C), then Si3N4 (1000C), then ZrO2 (300C)
Corrosion resistance: SIC (excellent), then Si3N4 (very good) and ZrO2 (good)
Load capacity: SIC highest then Si3N4, then ZrO2
Fracture toughness: ZrO2 best, then Si3N4, then SiC

The limiting speeds for all-ceramic bearings are lower due to the lower precision and roundness of the rings and the loads are lower because the bearing material is more brittle. Under heavy loads and particularly heavy shock loads, there is a risk of cracking. For the same reason, great care should be taken with interference fits. Zirconia is the least brittle so it will handle shock loads and very small interference fits better than the other ceramic materials with silicon carbide being the most brittle but generally, shock loads should be avoided. Using ceramic bearings on steel shafts at high temperature can cause bearing damage due to the large difference in expansion coefficient. For more information see Shaft/Housing Fit.

Hybrid bearings can offer very low frictional torque but the bearing rings must have very good roundness and a high quality raceway finish while the balls must also have very good roundness and surface finish. A lower quality hybrid bearing can have much higher friction levels and a lower maximum speed than a good quality all-steel bearing.

If there is a for several different bearing material issue you would like us to write about, just let us knowNPB® Bearings provides “Value beyond the Bearing.”