Aerospace Ball Bearings: Specs, Materials and Ordering

Updated October 12, 2026

Bearing types and where they are used

Rolling element bearings on aircraft and spacecraft split by motion and load. Airframe control bearings carry heavy loads through slow oscillation in flight control linkages, bell cranks, pulleys and hinges. Precision bearings run continuously in motors, gyros, actuators, pumps and space mechanisms. The two are not interchangeable: RBC Bearings, one of the main airframe bearing makers, tells designers not to consider its control series bearings for continuous rotation or delicate instrument applications at all.

Spherical plain bearings and plain rod ends are a separate family with their own specs; see spherical bearings and rod ends. Sleeve type parts are covered under bushings.

The specifications that govern airframe bearings

The general standard is SAE AS7949, which replaced MIL-B-7949. The detail dimensions, loads and options live on the individual sheets, which moved from MS numbers to SAE AS numbers in the late 1990s. Parts are still widely ordered by the old MS number, so a PO that says MS27641-6 is common; the current controlling document is the AS sheet.

The DFARS clause is the one buyers miss. Under it, each ball and roller bearing delivered on a covered DoD contract must be made in the United States, its outlying areas or Canada, and the cost of bearing components (rolling elements, retainer, inner and outer race) made there must exceed 50 percent. The clause has to be flowed to subcontracts, has commercial component exceptions, and bearings containing specialty metals must also meet the specialty metals clause. See DFARS specialty metals for that side.

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How the part number is built: MS27641-6 decoded

For the AS27641 family, the dash number tracks the nominal bore in sixteenths of an inch, and letter suffixes select radial play and grease. RBC publishes the cross reference, so a real example can be decoded line by line.

RBC rates the -6 size at a radial limit load of 2,500 lbf and a thrust limit load of 1,100 lbf. The pattern holds through the family: -8 is a 0.500 in bore, -16 is 1.000 in and -20 is 1.250 in. The exception is -3, which fits a 0.190 in (No. 10) bolt. RBC also notes that its KP3AL looks similar but is not an MS27641 part, a good reminder that a manufacturer number and an MS or AS number are only equivalent when the manufacturer's cross reference says so.

The load numbers mean something specific. RBC explains that airframe limit load ratings come from a 1949 joint study by the Bureau of Aeronautics, the Air Force, the National Aircraft Standards Committee and the bearing manufacturers' association, using limit load = K n d squared, where n is the number of balls and d the ball diameter. K is about 10,000 for deep groove bearings and 3,200 for rod end bearings, and the limit load never exceeds two thirds of minimum static fracture strength. Oscillating life is rated separately, at 10,000 cycles of 90 degree oscillation.

Precision classes: ABEC and ISO 492

Precision bearings are bought by tolerance class. ABMA Standard 20 defines ABEC 1, 3, 5, 7 and 9, tightening as the number rises; ISO 492 runs the other way, from Normal through classes 6, 5, 4 and 2.

Two cautions. The classes are general equivalents, not identical tolerance tables, so a drawing should name one system. And an ABEC class controls dimensions and runout only; it says nothing about material cleanliness, ball grade, radial play, preload, retainer material or lubricant. Those belong on the drawing or the manufacturer's part number, and they are usually what separates a gyro bearing from a hardware store bearing of the same size.

Materials, finishes and the trade-offs

Ring and ball material sets the temperature limit and corrosion behavior. Airframe control bearings are mostly 52100 steel with cadmium or zinc nickel plating on exposed surfaces except the bore; 440C stainless is offered where corrosion matters more than load.

Hybrid bearings put silicon nitride balls in steel rings. SKF reports the ceramic is more than twice as hard as bearing steel, that its lower density reduces centrifugal force and contact stress at speed, and that it insulates against stray electrical current. SKF also flags its much lower thermal expansion, which matters at cryogenic temperature where ring shrinkage changes internal clearance.

Plating is the other trade. Cadmium protects steel well and is still on many airframe bearings, but distributor listings carry California Proposition 65 cadmium warnings; RBC offers zinc nickel per OEM specs as the alternative. Never plate or replate a finished bearing locally. For finish background see surface finishing, and for steel and stainless grades see stainless and specialty steels and heat treatment.

Space mechanism bearings and lubrication

In vacuum, the lubricant decides bearing life. Ordinary greases outgas and migrate, so space mechanisms use a short list of low vapor pressure oils and greases, chosen and qualified by life test for each mechanism.

• Perfluoropolyether (PFPE). Braycote 601EF grease uses Braycote 815Z, a PFPE base oil; its data sheet cites extremely low volatility, a service range of -80 to 204 C, and use on the Space Shuttle and satellites.
• Multiply alkylated cyclopentane (MAC), sold as Pennzane. A 2010 ESA tribology paper lists the MAC oil Nye 2001A and the PFPE oil Fomblin Z25 as the two commonly used space oils, and notes Nye 2001 adds phosphate ester boundary additives and antioxidants.
• Quantity and method. A NASA Aerospace Mechanisms Symposium paper on a Mars Science Laboratory tool found a purely grease-plated bearing ran at consistently higher torque, and recommended adding some grease pack for long life bearings.

For a buyer, the practical point is that lubricant type, quantity, and the cleaning that precedes it are part of the bearing's identity. A space bearing ordered without the lubricant specification, fill percentage and cleanliness level is not the same part, even if the dimensions match. Ask for the lubricant lot on the cert.

Cleanliness and packaging

Treat a precision or space bearing as a clean, preserved assembly until the moment it is installed. Specify how it is cleaned, lubricated, sealed in its bag and labeled, and do not let anyone open, wash or relubricate it at receiving.

• Prelubricated, sealed airframe bearings are lubricated for life; washing them removes grease you cannot properly replace.
• Precision bearings should arrive in the manufacturer's sealed package with the part number, lot and date, and stay sealed through stores.
• For space hardware, put the cleanliness level, bagging material and any nonvolatile residue limit on the PO, and route opening to the cleanroom. See precision and oxygen cleaning.
• Pack assemblies so bearings are not vibrated while stationary in transit; crating and packaging covers fixtures and shock.

Ordering checklist for the PO

• Full part number, including suffixes for radial play and grease, and the controlling document with revision (for example AS27641 at its current revision, or the OEM drawing).
• Precision class in one named system (ABEC or ISO 492), plus preload, contact angle, retainer and ball grade where the drawing gives them.
• Material and melt specification for rings and balls, and plating (cadmium, zinc nickel, passivated 440C).
• Lubricant specification and fill, with lubricant lot recorded on the cert; shelf life or cure date where the lubricant has one.
• Manufacturer name and manufacturer lot traceability, a certificate of conformance and, when required, material certs. See certificate of conformance and material certs and traceability.
• DFARS 252.225-7016 and the specialty metals clause when the end item is for DoD.
• Packaging and preservation: manufacturer sealed packaging, no repackaging without approval, cleanliness level for space parts.
• Source restriction: manufacturer or its authorized distributor only, unless your quality team approves otherwise.

Counterfeit and quality risks, and how to receive bearings

Bearings are counterfeited because a fake looks right on a bench: the rings, seals and laser marking can be copied, while the steel, heat treatment and grease cannot be seen. Buy from the manufacturer or its authorized channel, and treat any other source under a counterfeit avoidance plan such as SAE AS6174, which covers authentic and conforming non-electronic materiel.

At receiving, check the package is the manufacturer's and unopened, the label matches the PO part number and suffix, the lot on the box matches the cert, and markings are consistent across the lot. Spin a sample by hand only if the procedure allows it, and never reject self-aligning airframe bearings for tightness when misaligned: RBC states that the balls skid rather than roll in pure misalignment and that a reasonable tightness is intended. Anything from an unauthorized source goes to quarantine and test, as described in counterfeit parts avoidance and receiving inspection.

How we source bearings

We do not stock bearings, publish prices or act as an authorized distributor for any manufacturer. We source them for you with traceability through authorized and franchised distributors or directly from the manufacturer, flow your drawing and PO requirements down to that source, and deliver with the manufacturer's paperwork and our certificate of conformance.

That is most useful when bearings are one line on a larger job: a machined bell crank, a GSE fixture or an actuator assembly where the bearings, bushings and fasteners should arrive with the machined parts on one PO, or be kitted by assembly. When a bearing is out of production, we search for remaining authorized stock first and, failing that, for alternates your engineering approves, under the controls described on our obsolete parts page. Browse the rest of the motion category or the full parts index.

Get bearings sourced with the paperwork

Send the bearing part numbers with suffixes, the controlling spec revision, quantity and any DFARS or space cleanliness requirement. We will come back with a source, lead time and the document package you will receive, and can combine the bearings with the parts they go into. Request a quote for bearings and hardware.

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Related

• Spherical Plain Bearings and Rod Ends for Aerospace
• Aerospace Bushings: Standard Sleeves, Liners and Bronze
• Aerospace Springs: Wire Specs, Processing and Inspection

Sources

• SAE: AS7949 Bearings, Ball, Airframe, Antifriction
• SAE: AS27641 Bearing, Ball, Airframe, Anti-Friction, Intermediate Duty
• EverySpec: MS27640 to MS27646 status and supersession
• RBC Bearings: MS27641 KP-A series data sheet
• RBC Bearings: airframe control ball bearing engineering guide
• Acquisition.gov: DFARS 252.225-7016 restriction on ball and roller bearings
• Engineers Edge: ABEC ball bearing tolerance classes
• NHBB: ball and roller bearing materials reference
• SKF Evolution: evaluating hybrid bearings
• Braycote Micronic 601 EF data sheet
• ESA/ESTL: evaluation of lubricants for space mechanisms using a spiral orbit tribometer (2010)
• NASA NTRS: PFPE lubricant lifetime for Mars applications (2008)