Ball Screw Support Unit Interchange: BK, BF, EK, EF, FK and FF

October 2026 8 min read Selection Guide

This is the strongest interchange story on the whole site. BK, BF, EK, EF, FK and FF are shared industry designations rather than one company’s trademark, used by essentially every maker of screw support housings. If your machine takes a BK15, a BK15 is what you ask for — from us or from anyone.

The caveat is narrower here than elsewhere but still real: the designation fixes the bearing bore and the broad housing form, not the bolt-hole centres or the overall height. Those can differ by a millimetre or two between makers, so the drawing check is short, not optional.

Our range at a glance

The table below states, for every RFU Motion family, whether the designation itself is shared across the industry or is our own house code. That single distinction decides how the rest of the page applies to you.

What carries over between makers — and what does not

The 7-point check before you substitute

Three of these settle most cases: bearing bore, bolt-hole centres and overall height. Send us those and we will confirm the part — or machine a housing to the drawing if a standard size will not do.

1. Bearing bore — equal to the screw journal at that end; the code fixes this, but verify against the drawing.
2. Bolt-hole centre distance and hole diameter — the dimension that most often differs by a millimetre between makers.
3. Overall height from mounting face to shaft centreline — this sets the screw centre height relative to your bearing blocks and motor.
4. Fixed end versus supported end — the fixed unit carries an angular-contact pair, the supported unit a deep-groove bearing — they are not interchangeable roles.
5. Screw end type — EK / FK / EF / FF expect a finished journal; BK / BF clamp a general-purpose end. Ordering one for the other gives a housing that will not clamp.
6. Bearing axial capacity — the fixed end resists the thrust of your application; check rating against axial load and duty.
7. Housing material, sealing and lubrication — cast versus machined bodies, seal types and grease provisions vary subtly between makers.

When you should not substitute

• BK where the drawing calls for EK (or FK, EF, FF): the shaft end preparations are different and the housing will not clamp or seat correctly.
• Fixed and supported roles swapped. A supported-end unit has no thrust-bearing pair and cannot take axial load.
• Mixing makers within one screw without checking centre height: a millimetre of height mismatch puts permanent bending load into the screw.

Which code belongs with which screw

The catalogue distinction worth repeating because it prevents the most common ordering mistake: EK, FK, EF and FF are intended for screws with finished shaft ends, where the journal is already stepped down to the bearing size. BK and BF are for general-purpose screws, where the housing clamps the screw at or near nominal diameter. The wrong pairing is the single most frequent support-unit error we see.

Back to the cross reference overview

HIWIN, THK, PMI, TBI MOTION, ABBA, CPC, NSK, IKO, NB, SBC and Bosch Rexroth are trademarks of their respective owners. Brand names on this page identify dimension families only and do not imply endorsement, affiliation or authorised replacement status.