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. This is a ground precision bearing, not an unground or semi-ground commercial 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. 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. The accurate way to describe this item is therefore “B538DD airframe control bearing to MS27646-38”, with the manufacturer as a separate fact rather than as part of the identity.

The two-width question: 9/32 in and 1/4 in are both correct:
The two figures describe two different rings.

Inner ring width Bi: 0.281 in (9/32 in).

Outer ring width Bo: 0.250 in (1/4 in).

Across the B500DD series the inner ring is deliberately extended, made wider than the outer ring by design. There are two reasons. First, the extra inner-ring width gives the two seals room to sit and seal properly without protruding past the outer ring faces. Second, the wide inner ring provides the clamping surface. 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, and that protrusion is the correct clamping surface.

Clamping force passing through the inner ring leaves the outer ring free as a light or transition fit in its housing or rod end, so it oscillates without restraint. Clamping force applied to the outer ring instead squeezes it axially, distorts the raceway, consumes the controlled internal clearance and raises starting torque. In a flight control linkage, raised starting torque means friction and hysteresis in the control path.

Published dimensions and tolerances

Bore: 0.6250 in, tolerance ±0.0007 in.

Outside diameter: 1.0625 in, tolerance −0.001 / 0 in.

Inner 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 diameter.

Inner ring shoulder diameter: 0.785 in.

Outer ring shoulder diameter: 0.898 in.

Chamfer: 0.015 in each.

Radial internal clearance: 0.0008 to 0.0018 in (radial play code P818).

Weight: 0.03 lb / 0.014 kg.

The shoulder diameters govern the mounting. The 0.785 in inner shoulder is the maximum diameter a clamping spacer or nut face can bear against without touching the seal; the 0.898 in outer shoulder is the minimum housing shoulder that clears the inner ring. The bore tolerance is a symmetric ±0.0007 in while the outside diameter tolerance is one-sided −0.001 / 0 in, so 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 lb.

Static thrust capacity: 1,500 lb.

Dynamic radial capacity, inner ring rotating: 1,990 lb.

Dynamic radial capacity, outer ring rotating: 1,820 lb.

Maximum 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 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. The applicable number is the one matching how the joint actually moves. The 2 oz-in maximum starting torque is the most consequential spec for a flight control application: it guarantees the bearing breaks away under a very small force, which keeps the control system free of stiction and preserves pilot feel and servo response. It also serves as an acceptance check, since a used or contaminated bearing from this series needing noticeably more than 2 oz-in to start is showing wear.

The radial internal clearance band of 0.0008 to 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 room for the fit-up and thermal effects of installation.

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; the base part number does not encode the grade.

Quantity and configuration

One bearing per unit, sealed both sides with PTFE seals, extended inner ring.

Mounting data

The extended inner ring is the clamping surface: it stands about 0.0155 in proud of the outer ring at each face. Inner ring shoulder diameter is 0.785 in, which sets the maximum bearing diameter for a clamping spacer or nut face. Outer ring shoulder diameter is 0.898 in, which sets the minimum housing shoulder. Chamfer is 0.015 in at each corner. The bearing is a sealed unit; the PTFE seals are part of the torque and contamination specification.

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. The series contains many dash numbers with closely spaced bore and outside diameter combinations, so the MS27646 dash number is the definitive identifier rather than the dimensions alone.