Typically radial internal clearance is designated by a clearance range. Organizations such as the ABMA (American Bearing Manufacturers Association) and ISO (International Standards Organization) have established standards for five radial clearance ranges. The five ranges are designated with the following codes:

Within each of the five clearance ranges (C2 to C5) above, additional ranges are further defined based on the bore size of the bearing. In other words, to determine the clearance range for a bearing you need to know the clearance code and the bore size.

The table below illustrates for a given bore size, what the clearance range would be for each Clearance Code.

Unit: µm

The Normal or Standard range of radial internal clearance was established to provide a proper operating clearance once the bearing is mounted when using “normal” mounting fits and operating conditions.

Most bearing manufacturers use the clearance codes as a suffix to a bearing part number. The two most common clearance ranges used are Normal or C3 clearance.

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What is a Static Load Rating?
by Mike Mortensen - Director of Engineering RBI-USA

As radial load is applied to a bearing, elastic deformation occurs between the rolling element and raceway. As radial load is increased, the stress on the rolling element and raceway increase. As stress level increases, non-elastic or permanent deformation to the rolling element and raceway will occur. Non-elastic deformation increases in area and depth as the load increases, and when the load exceeds a certain limit, the smooth running of the bearing is affected.

The basic static radial load rating is defined in accordance with ISO and ABMA standards as the static radial load which corresponds to a calculated contact stress at the center of the most heavily loaded rolling element/raceway contact. For the following bearing types, the listed contact stress level will cause a total permanent deformation of approximately 0.0001 of the rolling element diameter.

(MPa or MegaPascal or 10 6 Pascal is a unit of stress. 1 Pa or Pascal equals 1 N/m 2. 1 N or Newton is a unit of force)

This small amount of non-elastic (or plastic) deformation for standard wide-purpose bearings will not have any substantial influence on the bearing performances (vibration, noise, stiffness, friction moment, etc.)

For stainless steel bearings, static load ratings are approximately 75% of the load ratings for chrome steel bearings.

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What is a Basic Dynamic Load Rating?
by Mike Mortensen - Director of Engineering RBI-USA

A Basic Dynamic Load Rating is a theoretical, statistically based value of load that a bearing can carry for 1,000,000 revolutions with 90% reliability. For a radial bearing, this rating was based on the amount of pure radial load that a rotating inner ring could tolerate for 500 hours at a 33-1/3 RPM.

Bearing manufacturers typically display these values in their catalogs as “Cr” for radial bearings and “Ca” for thrust bearings.

Most bearing companies base their Basic Dynamic Load Ratings using this method. In the US, you may sometimes encounter bearings using a C90 designation for their dynamic load rating. This load rating method is based on a 90 million revolution statistical model. A C90 rating for a bearing has a value about ¼ of the ABMA or ISO rating. It is possible to convert ratings based on the 1 million or 90 million revolution method to the other using the following formula.

C1 = C90 x 3.857

For further information on how Basic Dynamic Load Ratings are determined please refer to ABMA Standards 9 and 11 or ISO 281:1990.

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How do we calculate bearing life?
by Mike Mortensen - Director of Engineering RBI-USA

Bearing life refers to the amount of time any bearing will perform in a specified operation before failure. Bearing life is commonly defined in terms of L-10 life, which is sometimes referred to as B-10. This is the life which 90% of identical bearings subjected to identical usage applications and environments will attain (or surpass) before bearing material fails from fatigue. The bearing’s calculated L-10 life is primarily a function of the load supported by (and/or applied to) the bearing and its operating speed.

There are many other factors which will have an effect on the actual life of the bearing; temperature, lubrication and improper care in mounting. As a result of these factors, an estimated 95% of all failures are classified as premature bearing failures. Once you know the bearing you are interested in and its Basic Dynamic Load Rating, you still need to know the speed and load to be applied to the bearing. Click here for the formula used to calculate L10 bearing life in hours.

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Is this bearing really stainless steel?
by Mike Mortensen - Director of Engineering RBI-USA

Some bearing customers will use a magnet to determine if a bearing is made of stainless steel. Their understanding is based on the belief that magnets are attracted to stainless steel. When they used this test on a stainless steel bearing and find the magnet is not attracted to the bearing they suspect the bearing is not made of stainless steel. It is at this point I need to explain that this is a valid test but only for certain stainless steels.

Stainless steels can be divided into three basic groups based on their metallurgical structure: Austenitic, Ferritic, and Martensitic (and precipitation hardenable stainless steel).

All Austenitic stainless steels are nonmagnetic although cold working of these steels can result in some magnetic properties.

Examples of austenitic stainless steels are Type 302, 304, and 316.

Ferritic, Martensitic and most precipitation hardenable stainless steels are magnetic.

Examples of these stainless steels are Type 430F, 446, 420 and 440C.

RBI stainless steel bearings use 440C stainless steel (magnetic) for the inner rings, outer rings and balls. Cages and shields typically use 302 or 304 stainless steel (non-magnetic).

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What are ABEC precision grades?
by Mike Mortensen - Director of Engineering RBI-USA

Various grades of precision bearings have been established by the American Bearing Manufacturers Association (ABMA), previously known as the Anti-Friction Bearing Manufacturers Association (AFBMA). The Annular Bearing Engineers Committee (ABEC) committee established several classifications of increasing precision levels similar to the class or grades used by the International Standards Organization (ISO). 

ABEC classes               ISO classes

ABEC 1                        ISO class 0/Normal

ABEC 3                        ISO class 6

ABEC 5                        ISO class 5

ABEC 7                        ISO class 4

ABEC 9                        ISO class 2

 As the ABEC number increases so does the precision of the bearing and its cost. Although ABEC grades bearings will be precision ground, as you increase in precision, super finishing and remarkable grinding precision are required. The main difference between the precision classes is the tolerances on the bearing bore diameter and outer diameter and the radial and axial run out of the inner and outer rings. Higher precision classes allow for higher speeds and extreme accuracy.  

Typically, bearings made to higher precision grades such as ABEC 7 or 9 are used in specialized applications such as high precision machine tool spindles, superchargers, jet engines, etc.

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Common causes for noisy bearings
by Mike Mortensen - Director of Engineering RBI-USA

The cause of noise in bearings can be a challenge to determine. Many times bearing noise is blamed on manufacturing defects within the bearing. The probability of manufacturing defects from most major bearing manufacturers is very low. Most manufacturers typically have noise or vibration testing built into their manufacturing process to eliminate defects. Knowing some of the more common causes for noise can help with the detective work needed to determine the cause(s).

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What steel do we use?
by Mike Mortensen - Director of Engineering RBI-USA

The standard steel used in our ball bearings is 52100 steel. 52100 is a high chromium steel (about 1.5% chromium) with a high carbon content (roughly 1% carbon). The composition of this steel makes it very suitable for through hardening resulting in a material hardness range between 58 – 64on the Rockwell “C” scale. This steel provides both excellent wear resistance and rolling contact fatigue resistance. It is the most common bearing steel used worldwide and can be found in almost every bearing type.

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Grease and oil are both common lubricants used in bearings. The main difference between grease and oil is that grease consists of an oil and a thickener. The thickener acts like a sponge that retains the oil. Grease can also contain various additives such as rust inhibitors, EP (extreme pressure) additives, oxidation preventatives, etc. Typically greases will have a semi-solid to solid consistency. This consistency allows grease to stay in place longer than oil. For many applications, the ability to stay in place and slowly release oil gives grease an advantage over oil alone.

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What material are bearing seals made of?
by Mike Mortensen - Director of Engineering RBI-USA

Three materials are commonly used in bearing seals. By far the most common material used in bearing seals is nitrile rubber, a shortened version of acrylonitrile rubber. Sometimes this material is referred to as Buna N rubber. It has a high temperature capability between 100°C – 120°C.

Another slightly more expensive seal material that is sometimes used is poly acrylic rubber. It offers a higher temperature capability of 160 – 170°C but its low temperature performance is not as good as nitrile rubber.

The third most common seal material is fluorocarbon rubber also referred to as Viton. This is most expensive of the three seal materials and offers the highest temperature capability and chemical resistance. Fluorocarbon seals can endure temperatures over 200°C.

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How do we specify grease consistency?
by Mike Mortensen - Director of Engineering RBI-USA

An important characteristic of grease is its thickness or consistency. Grease consistency can vary from a solid to a semi-fluid consistency. The National Lubricating Grease Institute (NLGI) has a system to classify greases. A standardized ASTM test using a cone of a specific weight allowed to sink into the grease for 5 seconds at 25 °C is used to measure consistency. The table below shows the various grades of grease that have been established.

The most common NLGI grades used in ball bearings are 1, 2 or 3. Of these three common grades, grade 2 greases are the most popular in general bearing applications.

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Through, Case and Zone hardening
by Mike Mortensen - Director of Engineering RBI-USA

All three of these processes are used to provide a hard wear surface on a bearing. Through hardening is a process where a bearing made of higher carbon steel such as 52100 is hardened uniformly throughout to a very high hardness.

Case hardening is used typically on lower carbon steels. When heat treated in a special carbon rich atmosphere, the carbon penetrates into the surface of the bearing. The result is a bearing with a high surface hardness and a softer, more ductile center.

Zone hardening is a process where flame or induction heating is used to harden a specific zone or area on a bearing such a raceway or seal riding surface leaving the remainder of the bearing softer and more malleable.

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Common greases used in RBI bearings
by Mike Mortensen - Director of Engineering RBI-USA

The standard grease used in our radial ball bearings is Chevron SRI #2. Chevron SRI #2 offers a wide application range, excellent corrosion stability, and excellent rust protection. It is one of the most popular bearing greases and is supplied by bearing manufacturers worldwide. Another popular grease RBI uses is Mobil Polyrex EM. Mobil Polyrex EM offers outstanding grease life, excellent corrosion resistance, and low noise properties. RBI primarily uses Polyrex EM grease in our electric motor quality bearings where low noise and vibration characteristics are important. Table 1 displays some key characteristics of these two greases.

Table 1 . Characteristics of Chevron SRI #2 and Mobil Polyrex EM greases.

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Seals or Shields - which is better?
by Mike Mortensen - Director of Engineering RBI-USA

“Seals or shields, which is better?” is a common question. The answer often involves a tradeoff between the level of advantage or disadvantage each option offers.

Shields offer some protection against contamination but will not stop grease purging. Shields can resist some chemicals better than seals and endure temperature extremes. The fact that shields do not contact the inner ring means they do not generate frictional heat.

Seals generally offer better protection against contamination and purging than shields. Nitrile rubber seals have problems if exposed to certain chemicals or excessive temperatures. Changing the seal material from nitrile to poly acrylic or Viton material can increase chemical resistance and temperature capability. Seals generally contact the inner ring which can limit the rotational speed of the bearing due to heat generation. The drag from the seals contacting the inner ring generally requires more energy or torque to rotate the bearing. Seals can address some of their performance limitations with design modifications. Changing the lip design can change the torque or heat generated by modifying the seal from a non-contact to a light contact to a heavy contact seal. Often changing the seal material or lip design can increase the cost or availability of the seal.

The table below provides some relative comparison of seals and shields.

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How are precision balls made?
by Mike Mortensen - Director of Engineering RBI-USA

Balls are cut from wire (smaller balls) or rod stock (larger balls) to lengths that are approximately that of the desired ball diameter. They are taken to a heading machine where they are cold stamped between two hemispherical dies into the general shape of a ball with a ring of excess material around it (flashing).

1. The balls undergo a grinding process in order to remove the flashing. During this grinding process, the balls travel within grooves between two plates rotating at different speeds and direction (usually one stationary).
2. After the grinding process, the balls undergo a heat treatment process in order to improve their strength and hardness, thus improving their wear resistance.
3. Once the heat treatment is completed, a series of grinding operations are used progressing from rough to fine grinding. Each grinding step removes less material than the preceding one, resulting in improved ball roundness.
4. The balls then undergo a lapping process to achieve a highly refined dimensional accuracy as well as a polished appearance.
5. Finally, the balls are cleaned, inspected, and readied for assembly.

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What materials are used in precision ball bearings?
by Mike Mortensen - Director of Engineering RBI-USA

Several materials can be used to make bearing balls. When considering which material to use, it is important to factor in the cost, hardness, and strength. The most common ball material is 52100 chrome alloy steel. 52100 chrome alloy is versatile in regards to its lower cost, high hardness and strength.

Another commonly used material is stainless steel. When compared to 52100 chrome steel, stainless steel lacks some hardness and strength, but has the added benefit of being resistant to corrosion. Still another ball material, which is growing in use, is ceramic. Ceramics vary widely, but compared to 52100, they are much harder, stronger, lighter, and more resistant to corrosion. The main drawback in using ceramic is its higher cost.

The ball material used depends upon on the application. Most applications would only require 52100 chrome steel. In environments where corrosion is an issue, stainless steel may need to be considered. For the most demanding applications (e.g. high temperature, high stress, high speed, etc.) only a ceramic ball material may suffice.

Some various material properties can be found in Table 1.

Table 1 . Properties of various precision ball materials.

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What are the factors that determine ball grade?
by Mike Mortensen - Director of Engineering RBI-USA

Multiple factors determine the grade of a precision bearing ball. These are specified using dimensional form and surface condition tolerances. Tables 1 and 2 below are adapted from ABMA Std 10 and highlight some of the different tolerances for various ball grades. The tolerance definitions listed below can also be found from ABMA Std 10.

Ball Diameter Variation
The difference between the largest and the smallest actual single diameters of one ball.

Deviation from Spherical Form
The greatest radial distance in any radial plane between a sphere circumscribed around the ball surface and any point on the ball surface.

Lot Diameter Variation
The arithmetic mean of the mean diameter of the largest ball and that of the smallest ball in the lot.

Surface Roughness
Consists of all those irregularities which form surface relief and which are conventionally defined within the area where deviations of form and waviness are eliminated.

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Recommended torque for RBI set screws
by Mike Mortensen - Director of Engineering RBI-USA

It is important that the set screw be tightened to a specified torque for RBI bearing inserts. If the set screw is not tightened enough, the bearing may loosen from the shaft. If the set screw is tightened too much, the bearing or shaft could be damaged. RBI uses a knurled cup point set screw for all of our bearing inserts.

Table 1 shows recommended tightening torques for several set screw sizes used in various bearing insert series that RBI offers. Be aware that different bearing manufacturers use different set screw types and sizes, requiring different tightening torques.

Table 1 . Set screw usage and recommended tightening torque.