RBCB358DDSealed Airframe Control Bearing to MS27646-38, 5/8″ Bore x 1-1/16″ OD, Extended Inner Ring 0.281″ / Outer Ring 0.250″, PTFE Seals Both Sides

Product Description

What this part is

This is a sealed, ground precision single-row deep groove ball bearing built to military specification MS27646-38, part of the B500DD / MB500DD anti-friction extra light duty airframe control bearing series. Airframe control bearings are a distinct family from ordinary industrial ball bearings. They are designed for the pivots in aircraft flight control systems — bellcranks, control-rod ends, pulley and cable idlers, torque tubes, flap and aileron linkages — where the bearing spends most of its life oscillating through small angles under high static load rather than spinning continuously. The design priorities are therefore high static capacity, extremely low and repeatable starting torque, positive sealing against dirt and moisture, and tightly controlled internal clearance. To be explicit about the grade: this is a ground precision bearing, not an unground or semi-ground commercial part.

Correcting the source data

This item was listed as B358DD. No bearing with that number exists in any manufacturer or distributor catalog we could find. The correct designation is B538DD — a simple digit transposition, 358 for 538. The proof is dimensional rather than a guess: B538DD’s published dimensions (0.6250 in bore, 1.0625 in outside diameter, 0.281 in inner ring width, 0.250 in outer ring width) match the dimensions given in the original listing exactly, including the unusual two-width figure. A wrong number that produces four exact dimensional matches is not a coincidence. We have corrected the title to lead with B538DD and deliberately kept B358DD as a cross-reference and known misspelling so that anyone searching the transposed number still finds the right part.

Why an RBC-branded bearing carries a Fafnir number

This listing shows the brand as RBC while the B538DD number reads as a Fafnir designation, and buyers routinely flag that as an error. It is not one. B500DD / MS27646 is an industry-standard airframe control bearing series defined by a military specification, not a proprietary number belonging to one manufacturer. Multiple qualified producers build to it: RBC (through its Nice and Heim divisions), Timken/Fafnir, Kaydon, NHBB and Boca among them. Any of them can legitimately supply a part marked with the standard number. So the accurate way to describe this item is “B538DD airframe control bearing to MS27646-38”, with the manufacturer as a separate fact rather than as part of the identity. If your paperwork requires a specific manufacturer of record or a specific certification package, ask us to confirm the ring and box markings before ordering.

Resolving the two-width question — 9/32″ and 1/4″ are not conflicting specs:
This is the most common question on this bearing, and the answer is straightforward once you know it. The two figures describe two different rings:

Inner ring width (Bi): 0.281 in / 9/32 inOuter ring width (Bo): 0.250 in / 1/4 in
Neither number is wrong and neither supersedes the other. Across the B500DD series the inner ring is deliberately extended — made wider than the outer ring by design. There are two reasons for it. First, the extra inner-ring width gives the two seals room to sit and seal properly without protruding past the outer ring faces. Second, and more importantly for installation, the wide inner ring gives you something to clamp. The 0.031 in of extra inner ring width (about 0.0155 in protruding at each face) means the inner ring stands proud of the outer ring on both sides.

That protrusion is the correct clamping surface. Install this bearing by clamping the wide inner ring between shoulders, or between a shoulder and a spacer or nut, so the clamping force passes entirely through the inner ring. The outer ring then remains free to be a light or transition fit in its housing or rod end and rotates or oscillates without restraint. If instead you clamp the outer ring — by torquing a housing cap down onto the outer faces, for example — you will squeeze the outer ring axially against a bearing that is not sitting on it, distort the raceway, destroy the carefully controlled internal clearance and drive the starting torque up. In a flight control linkage, a bearing whose starting torque has gone up is a bearing that has introduced friction and hysteresis into the control path. Clamp the inner ring.

Published dimensions and tolerances

Bore: 0.6250 in, tolerance ±0.0007 inOutside diameter: 1.0625 in, tolerance −0.001 / 0 inInner ring width Bi: 0.281 in (9/32 in)Outer ring width Bo: 0.250 in (1/4 in)Ball complement: 21 balls of 1/8 in diameterInner ring shoulder diameter: 0.785 inOuter ring shoulder diameter: 0.898 inChamfer: 0.015 in eachRadial internal clearance: 0.0008–0.0018 in (radial play code P818)Weight: 0.03 lb / 0.014 kg
The shoulder diameters matter when you are designing or checking the mounting. The 0.785 in inner shoulder tells you the maximum diameter your clamping spacer or nut face can bear against without touching the seal, and the 0.898 in outer shoulder tells you the minimum housing shoulder you can use without contacting the inner ring. Note the bore tolerance is a symmetric ±0.0007 in while the O.D. tolerance is one-sided −0.001 / 0 in — the outside diameter is never oversize, which is what allows a repeatable slip or light-transition fit into a rod end.

Load, torque and clearance ratings

Static radial capacity: 3,280 lbStatic thrust capacity: 1,500 lbDynamic radial capacity, inner ring rotating: 1,990 lbDynamic radial capacity, outer ring rotating: 1,820 lbMaximum starting torque: 2 oz-in
Three of these deserve explanation. The static radial figure of 3,280 lb against a dynamic figure under 2,000 lb is the signature of an oscillating-duty bearing: it is built to hold a large load without brinelling while barely turning, which is exactly what a control linkage pivot does. The two different dynamic ratings for inner-versus-outer-ring rotation are not a redundancy — when the outer ring rotates, the load zone sweeps around the outer raceway while staying fixed on the inner, which stresses the material differently and yields the slightly lower 1,820 lb figure. Use the number that matches how your joint actually moves. The 2 oz-in maximum starting torque is arguably the most important spec on the page for a flight control application: it is the guarantee that the bearing will break away under a very small force, which is what keeps the control system free of stiction and preserves pilot feel and servo response. It is also a useful acceptance check — a used or contaminated bearing from this series that needs noticeably more than 2 oz-in to start is telling you something.

The radial internal clearance band of 0.0008–0.0018 in (play code P818) is tight and controlled. Clearance in this range keeps the joint from developing perceptible free play, which in a control linkage translates directly to lost motion, while still leaving enough room for the fit-up and thermal effects of installation. This is another reason clamping the outer ring is destructive: it is easy to consume the entire clearance band with axial distortion.

Seals, materials and precision grade

The DD suffix denotes two PTFE (Teflon) seals, one on each side. PTFE was chosen for this series rather than a nitrile or Buna lip seal because it tolerates a wide temperature range, resists aircraft fluids, and generates very little drag — which is what allows the 2 oz-in starting torque figure to be met with a fully sealed bearing. Rings and balls are 52100 chrome steel. Standard precision is ABEC 1, with ABEC 3, 5, 7 and 9 available on order from the series — so if your application specifies a higher ABEC grade, confirm the grade of the specific unit rather than assuming, because the base part number does not encode it.

Fitting notes

Clamp the inner ring, never the outer, as explained above. Respect the 0.785 in inner and 0.898 in outer shoulder diameters when sizing spacers and housing shoulders. Do not attempt to remove the PTFE seals to inspect or add grease — the bearing is a sealed unit, the seals are part of the torque and contamination specification, and disturbing them ends the bearing’s service life. Press the bearing into a rod end or housing using a tool that contacts only the outer ring, and onto a shaft using a tool that contacts only the inner ring; never let installation force pass through the balls from one ring to the other.

Interchange guidance

Search and cross-reference forms include B538DD, B538-DD, B538 DD, B538, MS27646-38, MS27646, and the series designations B500DD and MB500DD. The transposed form B358DD is retained here because that is how the part was originally listed. Verify the MS27646 dash number against your parts manual rather than matching on dimensions alone — the series contains many dash numbers with closely spaced bore and O.D. combinations.