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Miniature Bearing Information

Technical Article Title


Bearing Selection

To ensure optimal speed and load carrying capacity, several factors must be considered when choosing the proper bearing for your application. These factors include the ring material, design, shields & seals, cage, ABEC grade, radial play, and lubricant.


The design of a bearing is critical in determining its load-carrying capability and maximum operating speed & factors which directly impact the bearing's operating life. Various types of bearings have been designed to meet the operating parameters of your application.

The radial or conrad bearing (also referred to as deep groove) is the most popular type due to its ability to handle radial and thrust loads in either direction. This type is offered with various seal or shield options.

The angular contact bearing is designed with a relieved shoulder to allow for a greater number of balls, thereby increasing its load-carrying capability. The angular contact design also allows for the use of a full section cage which is desirable for high speed applications. This type of bearing can handle thrust loads in one direction only.


Miniature and instrument bearings are normally made of either stainless steel or chrome alloy steel. National Precision Bearing offers stainless steel for applications that require corrosion resistance, and 52100 chrome steel for maximum fatigue life. These materials are heat-treated to achieve optimum hardness and dimensional stability, and are suitable for most applications.

Shields and Seals

Shields and seals are used in ball bearings to retain lubricants and prevent particulate contamination from reaching the critical surfaces. Shields are popular for most applications; seals are used where minimal clearance to light contact is required. Seals offer greater deterrence to particulate contamination, but increase torque and limit operating speed.


The cage, also referred to as the retainer or separator, is the component that separates and positions the balls at approximately equal intervals around the bearing raceway. Proper selection of a bearing cage is critical for meeting the load, speed and temperature requirements of your applications.

ABEC Tolerance

ABEC Grade

When choosing the ABEC grade, the factors to be considered are: radial and axial runout requirements, bore and O.D. fits, and audible noise level. The table below shows the bore and O.D. size tolerances and the radial runout limits for each ABEC grade. Grades 3 and 7 are preferred for most standard applications.

Normally, race finish and race geometry are superior in ABEC 5 and higher. National Precision Bearing recommends these grades for precision assemblies where low noise (mechanical or audible), minimal runout and long life are important considerations.

Radial Play

Radial Play is the free internal radial looseness between the balls and the races with no load applied to the bearing in any direction. Radial play is necessary to accommodate differential thermal expansions, the effects of interference fits, and to control axial play and deflection. The chart on page 7-3 of the Engineering Section shows the suggested radial play for some typical applications.


There are literally hundreds of lubricants available for ball bearings; selecting the optimal one is critical. Each has a particular characteristic which makes it suitable for a specific application. Unless torque is a problem, grease is preferred for prelubrication since it is less susceptible to migration and leakage. Grease can increase bearing torque by a factor of 1.2 to 5.0 depending on the grease type and quantity used.

O.D. SIZERadial
Mean Diam.
1P0 - 18mm.00040.00060+.0000+.0000
(0 - .7086 in.)–.0003–.0003
over 18 - 30mm.00040.00060+.0000+.0000
(over .7086 - 1.1811 in.)–.0004–.0004
3P0 - 30mm.00020.00040+.0000+.0000
(0 - 1.1811 in.)–.0002–.0003
5P0 - 30mm.00015.00020+.0000+.0000
(0 - 1.1811 in.)–.0002–.0002
7P0 - 30mm.00010.00015+.0000+.0000
(0 - 1.1811 in.)–.0002–.0002
9P0 - 18mm.00005.00005+.0000+.0000
(0 - .7086 in.)–.0001–.0001
over 18 - 30mm.00010.00010+.0000+.0000
(over .7086 - 1.1811 in.)–.0001–.00015

* ABEC 1 miniature and instrument bearings of both the metric and inch configurations meet the tolerances of ABMA Standard 20 for ABEC 1 metric series bearings.


When a ball bearing is running under a load, force is transmitted from one bearing ring to the other through the balls. Since the contact area between each ball and the rings is relatively small, moderate loads can produce stresses of tens, even hundreds of thousands of pounds per square inch. These internal stresses have a significant impact on bearing life and performance. Thus the internal geometry of a bearing–its radial play, raceway curvature and contact angle–must be carefully chosen so loads can be distributed for optimal performance.

Most ball bearings are assembled in such a way that a slight amount of looseness exists between the balls and the raceways. This looseness is referred to as radial play and axial play. Radial play is the maximum distance that one bearing ring can be displaced with respect to the other, in a direction perpendicular to the bearing axis when the bearing is in an unmounted state.

Axial play, or end play, is the maximum relative displacement, in a direction parallel to the bearing axis, between the two rings of an unmounted ball bearing.

Since radial play and axial play are both consequences of the same degree of looseness between the components, they bear a mutual dependence. Yet their values are usually quite different in magnitude.

Radial play can often vary between .0002 and .0020, while axial play may range from .001 to .010. The suggested radial play ranges for typical applications should always be consulted when a device is in the initial design phase. In most ball bearing applications, radial play is functionally more critical than axial play. While radial play has become the standard purchasing specification, you may also specify axial play requirements. Keep in mind, however, the values of radial play and axial play for any given bearing design are mathematically interdependent, and that radial play is affected by any interference fit between the shaft and bearing I.D. or between the housing and bearing O.D.

See charts, below.

Typical ApplicationsSuggested Radial Play*
Small Precision High Speed Electric Motors.0005 to .0008
Tape Guides, Belt Guides, Low Speed.0002 to .0005
Tape Guides, Belt Guides, High Speed.0005 to .0008
Gyro Gimbals, Horizontal Axis.0002 to .0005
Gyro Gimbals, Vertical Axis.0005 to .0008
Precision Gear Trains, Low Speed Electric Motors, Synchros and Servos.0002 to .0005
Gyro Spin Bearings, Ultra-High Speed Turbines and SpindlesConsult NPB

* Measurement in inches.


DescriptionRadial Play Range*NPB Code
Tight.0001 to .0003P13
Normal.0002 to .0005P25
Loose.0005 to .0008P58
Extra Loose.0008 to .0011P811

* Measurement in inches. Non-standard ranges may be specified.


Shields and seals are protective closures which retain lubricants and assist in preventing contaminants from reaching internal surfaces. In torque-sensitive applications, it may be advantageous to use shields rather than seals because there are no contacting surfaces to create drag.

The following chart illustrates the more common type of shields and seals.

Consideration should be given to the compatibility of cage and shield type designs to allow for appropriate clearance. Consult with NPB for availability.

DescriptionTypeDesignMaterialOperatingTemp. Max.
Shield - Removable with snap wire. Minimal clearance. Most PopularZStainless Steel
300 Series
Seal-Removable with snap wire. Minimal clearance to light contact. Low torque.LGlass-reinforced PTFE400F
Seal - Excellent sealing characteristics.DBuna-N Bonded to Steel Insert250F
Seal - Excellent sealing characteristics.
High cost.
D1Viton® Bonded to Steel Insert400F

Bore Tolerance Limits+.0000+.0000+.0000+.0000+.0000
Bore 2 pt. out of Roundness.0001.0001.00005
Bore Taper.0001.0001.00005
Radial Runout.0004.0002†.00015.0001.00005
Width Variation.0002.0001.00005
Bore Runout with Face.0003.0001.00005
Race Runout with Face.0003.0001.00005

* Measument in inches.
† Add .001 to the tolerance if bore is over 10mm (.3937 inch)

Miniature and Instrument Ball Bearings Inner Ring *
The ABEC tolerances listed are current at this catalog’s printing. These tolerances are reviewed regularly and updated as required. The ABMA standards may be obtained by contacting:, 1101 Connecticut Ave. N.W., Suite 700, Washington, DC 20036.


CharisteristicConfigurationABEC 1ABEC 3PABEC 5PABEC 7PABEC 9P
WidthSingle Bearing+.000+.000+.000+.000+.000
Duplex Pair+.000+.000+.000

*Measurement in inches unless otherwise indicated.

CharisteristicConfigurationSize RangeABEC 1ABEC 3PABEC 5PABEC 7PABEC 9P
Mean OD
Tolerance Limits
All0 - 18mm+.0000+.0000+.0000+.0000+.0000
(0 - .7086)–.0003–.0003–.0002–.0002–.0001
Allover 18 - 30mm+.0000+.0000+.0000+.0000+.0000
(.7086 - 1.1181)–.0004–.0003–.0002–.0002–.00015
Maximum OD
Tolerance Limits
Open0 - 18mm+.0001+.0001+.0000+.0000+.0000
(0- .7086)–.0004–.0004–.0002–.0002–.0001
over 18 - 30mm+.0001+.0001+.0000+.0000+.0000
(.7086 - 1.1181)–.0005–.0004–.0002–.0002–.00015
Shielded0 - 18mm+.0002+.0002+.00004+.00004
(0- .7086)–.0005–.0005–.00024–.00024
over 18 - 30mm+.0002+.0002+.00004+.00004
(.7086 - 1.1181)–.0006–.0005–.00024–.00024
OD 2 pt. Out
of Roundness
Open0 - 18mm.0001.0001.00005
Openover 18 - 30mm.0001.0001.0008
Shielded0 - 30mm.0002.0002
OD TaperAll0-18mm.0001.0001.0005
Allover 18 - 30mm.0001.0001.00008
Shielded0 - 30mm.0002.0002
Radial RunoutAll0 - 18mm.0006.0004.0002.00015.00005
Allover 18 - 30mm.0006.0004.0002.00015.0001
Width VariationAll0 - 30mm.0002.0001.00005
OD Runout
with Face
All0 - 30mm.0003.00015.00005
Race Runout
with Face
Plain0 - 18mm.0003.0002.00005
Plainover 18 - 30mm.0003.0002.0001
Flanged0 - 30mm.0003.0003
Flange Width
Tolerance Limits
Flange Diameter
Tolerance Limits

*Measurement in inches unless otherwise indicated


We have included a table of Commercial and Military Bearing Lubrication and their recommended uses. When ball bearings are ordered without a specified lubricant, it is the policy of National Precision Bearing to lubricate with MIL-L-6085 oil.

Standard bearing lubricants are Windsor L-245X oil and Royco 27 grease. The standard quantity of oil varies with bearing size, but is approximately one drop (3 - 6mg) per bearing up to R-2 size and two drops (6 - 12mg) for larger sizes. The standard quantity of grease is 30% ±5% of the bearing’s internal free volume.

See also NPB state-of-the-art Cleaning & Lube Facility

Brand NameBasic TypeOperating
Temp. F
Windsor L-245X(Mil-L-6085)Synthetic Oil–65 to +300Light general purpose oil. Low torque.
Krytox 143 ACFluorinated Oil–30 to +550High temperature. Hostile chemical
environments and low pressure.
Beacon 325Synthetic Grease–65 to +250General purpose grease. Low torque.
Royco 27(Mil-G-23827)Synthetic Grease–100 to +275Corrosion resistant.
Heavy loads and high speed.
Rheotemp 500Synthetic (Non-silicone) Grease–65 to +350Oxidation inhibitor.
High temperature and high speed.
Mobil 28(Mil-G-81322)Synthetic Hydrocarbon Grease–65 to +350Low torque. Low speed oscillation.
SRI-2Mineral Grease–20 to +350Long life. Water resistant.
High speed and high temperature.
Krytox 240ACFluorinated Grease–30 to +550High temperature.
Miltitemp SRLSynthetic Grease–40 to +300Very quiet. Low torque.
Braycote Micronic 601EFPerfluorinated Polyether Grease–112 to +400Low pressure/vacuum environments.
Hostile chemical environments.
Polyrex EMChanneling Mineral Grease–20 to +350Heavy loads. Minimum migration.
Nig Ace WSDiurea Synthetic Hydrocarbon–50 to +300High speed, rapid oscillation.

Proper bearing lubrication is vitally important to the optimal operation and life of miniature bearings. There are several common lubricants and hundreds of specialty lubricants that have been specially formulated to meet design requirements and the demands of various applications. The many available oils and greases are designed to lubricate the sliding contact between bearing components, protect the finished surfaces of the bearing from corrosion, provide a sealing barrier against contamination and in some cases, carry away heat. Miniatures bearing rolling contact surfaces are manufactured out of alloy steels to tolerances and geometries nearing one millionth of an inch. Normal wear and damage due to sliding contact, corrosion, contamination, and heat will accelerate in the absence of proper lubrication reducing bearing life significantly.

The following information can be used as a reference for lubricant selection and specification in general applications that fall well within bearing and lubricant ratings. For lubricant selection on bearings and applications with special characteristics or requirements please contact NPB for design and engineering support.

Windsor L-245X, Fuchs Lubricant Co., (MIL-L-6085 and MIL-PRF-6085)
Windsor L-245X is synthetic light Ester oil with an extremely low base viscosity (11.8 cSt at 40C). This lubricant is used in a wide variety of applications from aerospace and medical instruments to hobby applications. It is most commonly specified where minimal starting and running torque are requirements and low heat or loads can be expected. The pour point of Windsor L-245X is -65C with a flash point of 227C. This oil is specified for many applications required to meet MIL-L-6085 and MIL-PRF-6085. It is best suited for applications that fall within a temperature range for -57 to 177C. Windsor L-245X also contains a corrosion inhibitor and is often used as a protective coating for chrome steel bearings.

Krytox 143 AC, DuPont
Krytox 143 AC is a synthetic, relatively thick, Fluorinated oil with a high base viscosity (243 cSt at 40C). This oil is clear, colorless, non-reactive and non-flammable. Krytox 143 AC is best suited for use in high temperature applications and can be used in hostile chemical environments. It is also suitable for some low pressure environments and space applications. The temperature range for Krytox 143 AC is wider than some bearing metals -34 to 288C.

Beacon 325, Exxon Mobil
Beacon 325 is a Lithium grease with a light Ester oil base that smooth and buttery in texture and beige in color. Beacon 325 is a non-channeling grease with an NLGI grade of 2. This general purpose grease can be used in a wide variety of applications, but is generally considered advantageous where low torque is a consideration even at low temperatures. Beacon 325 also has excellent oxidation stability for the extended grease life needed in lube for life applications. The operating temperature range for Beacon 325 is -50 to 120C.

Royco 27, Anderol (MIL-G-23827 and MIL-PRF-23827)
Royco 27 is a Lithium grease with a diester oil base that is smooth and buttery in texture and tan in color. Royco 27 is a non-channeling grease with an NLGI grade of 2. This lubricant is intended for use in high load as well as high speed applications. Royco 27 has a number of additives for oxidation and corrosion resistance as well as wear protection. This oil is specified for many applications required to meet MIL-G-23827 and MIL-PRF-23827C. The operating temperature range for Royco 27 is -73 to 135C.

Rheotemp 500, Nye Lubricants
Rheotemp 500 is a Sodium complex grease with a synthetic Ester oil base that is smooth in texture and bluish-black in color. Rheotemp 500 is a non-channeling grease with an NLGI grade of 1.5. This lubricant is designed for use in high speed applications where wide ranges of operating temperatures are expected. Rheotemp 500 contains anti-oxidation additives. The operating temperature range for Rheotemp 500 is -65 to 350C

Mobil 28, Mobil (MIL-PRF-81322)
Mobil 28 is a combination of synthetic Hydrocarbon fluid and clay thickener that is smooth and buttery in texture and dark red in color. Mobil 28 is a non-channeling grease with an NLGI grade of 1.5. This grease was designed for aerospace applications where high speed oscillation, low wear, high friction reduction, and low lubricant friction losses are required. Common applications include, airframe control bearings, motor applications, and high-speed instruments. Because of the clay thickener Mobil 28 is not recommended for instruments or motors where extremely low noise or extremely smooth operation are required. This grease is specified for many applications required to meet MIL-PRF-81322. The operating temperature for Mobil 28 is -54 to 177C.

SRI-2, Chevron
Chevron SRI-2 is a Polyurea grease with a Petroleum base oil that is smooth and buttery in texture and dark green in color. Chevron SRI-2 is a non-channeling grease with a NLGI grade of 2. This lubricant is often used in high speed applications where water washout is a consideration, such as medical and dental instruments. Light fills of this grease, or application using a dilution and plating process can facilitate higher speeds and lower starting torque where required. The operating temperature for Chevron SRI-2 is -29 to 177C.

Krytox 240 AC, DuPont (MIL-G-27617C)
Krytox 240 AC is a Fluorotelomer grease with a Fluorinated oil base that is smooth and creamy in texture and white in color. Krytox 240 AC is a non-channeling grease with and NLGI grade of 1.5. This grease is specified for many high temperature applications and in applications where lubricants are required to meet MIL-G-27617C. The operating temperature for Krytox 240 AC is -35 to 288C.

Kyodo Multemp SRL, Kyodo Yushi
Kyodo Multemp SRL is a lithium thickened grease with a polyol ester and diester oil base. The lubricant is smooth and creamy in texture and slightly transparent off-white in color. Kyodo SRL has an NLGI grade of 2.5. This lubricant is a high-grade high-performance low-noise grease that also provides excellent low-temperature starting torque and provides long life over a wide working temperature range. Kyodo SRL is used in applications ranging from hard disk drive spindles to medical pumps.
The operating temperature range fro Kyod Multemp SRL is -50 to 150C.

Bracote Micronic 601EF, Castrol
Braycote® 601 EF is a non-toxic, nonflammable, Perfluorinated Polyether grease that is smooth and buttery in texture and off-white in color. Braycote® 601 EF has an NLGI grade of 2. Castrol Braycote® 601 EF was designed to operate in the presence of rocket fuels, oxidizers, and high vacuum. Additional applications include high load-carrying capabilities, hostile chemical or extreme environmental conditions. Additional benefits of this grease include increased shelf life, high chemical resistance, and extremely low volatility. Braycote® 601 EF has an operating temperature range of -80°C to 204°C.

Polyrex EM, Exxon Mobil
Polyrex EM is a mineral Polyurea grease that is blue in color smooth and tacky in texture. Polyrex EM is a channeling grease with an NLGI grade of 2. This grease is designed for use in electric-motor bearings where heavy loads and minimum migration are desired. It is considered a replacement for Andoc C lubricant in many industrial applications. Polyrex EM has an operating temperature range of -20 to 350C.

Nig Ace WS, Nippon Grease Co.
Nig Ace WS is a Diurea synthetic Hydrocarbon grease that is smooth and semi-solid in texture and light brown in color. Nig Ace WS is a channeling grease with an NLGI grade of 2. Nig Ace WS is the lead-reduced, RoHS complaint version of Nig Ace W grease. This grease is commonly used in stepper motor applications as well as encoders and motors where low noise and torque are factors. The operating temperature range of Nig Ace WS is -20 to 150C.

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