51256-M bearing: complete specifications & equivalents

Specification table

Source: catalog reference (SKF PUB BU/P1 17000/1 EN (Oct 2018)) · extracted 2026-09-28

Parameters extracted from the official SKF Rolling Bearings catalogue (SKF PUB BU/P1 17000/1 EN (Oct 2018)); SKF® and SKF Explorer® are trademarks of AB SKF, used here for identification only — mpnbase is not an authorized SKF distributor. Boundary dimensions are ISO 15 interchangeable; current production values may differ, verify at skf.com.

Cross references & equivalents

Interchange levels: full standard interchange · partial verify parameters · verify manual confirmation required. Links resolve to the equivalent model's specification page where available.

How the 51256-M model number is decoded

51256 M is SKF's 280 mm-bore single-direction thrust ball bearing in the 51200 series to the ISO 104 dimension plan, with a flat housing seating surface, measuring 280×380×80 mm. The separable set comprises a shaft washer (ground bore, tight fit on the shaft), a loose-bore housing washer and one ball-cage assembly, reacting axial force in one direction and carrying no radial load. in the 51xxx designation the first 1 marks a single-direction bearing with a flat housing seating surface; the third digit 1–4 grades the dimension series from the light 511 section through the heavy 514 section. the final two digits 56 encode the bore as 280 mm (code 56 × 5), the standard SKF bore-code rule shared with radial bearings. suffix M identifies a one-piece machined brass cage, ball-centred: the large-size and heavy-section option for higher speeds, shock loads and thin-film lubrication where a pressed cage would distort. Envelope 280×380×80 mm (11.024×14.961×3.15 in), 50.0 mm radial section, C 488 kN, C0 2320 kN, Pu 47.5 kN. reference speed 750 r/min (thermal) and limiting speed 1,000 r/min (mechanical), catalogue mass 26500 g. Speed-ratio note for this exact unit: the reference speed is 750 r/min against a 1,000 r/min limiting speed, a reference-to-limit ratio of 0.75. Imperial cross-check for procurement drawings: 11.024 in bore, 14.961 in OD, 3.15 in height, and the 26500 g catalogue mass converts to 26.500 kg (934.7 oz). Rating split for documentation: dynamic 488 kN (L10 life), static 2320 kN (safety factor s0), fatigue limit 47.5 kN (adjusted life) — three different uses of three different numbers for 51256 M, none interchangeable with another. Speed planning separates the two ceilings mechanically: the pressed brass cage and balls tolerate 1,000 r/min while thermal balance under catalogue load sets the 750 r/min reference; at dm ≈ 330.0 mm the corresponding dn is approximately 330k mm·r/min. Capacity density: 488 kN C from 26.500 kg of bearing steel gives about 18.4 kN per kilogram; static density is correspondingly 87.5 kN/kg at C0 2320 kN. Machined-cage order-number note: the M brass cage is a one-piece ball-centred window cage, chosen at the heavy end of the section for speed and shock duty; it is heavier and stiffer than the pressed-steel execution of the smaller sizes and tolerates interrupted lubrication far better, which is why the catalogue restricts the M suffix to the large bores. The M cage of 51256 M adds a small amount of mass and cage strength without changing the 280×380×80 mm ISO 104 envelope, so the pressed-cage execution drops into the same recess but should not be assumed equal under the 1,000 r/min ceiling.

Typical equipment & failure modes

Selection pitfalls — read before you substitute

• The third digit grades the section: 511 light, then 512, 513 and 514 progressively taller and heavier at the same bore — select from 488/2320 kN and the available 380 mm OD, never substitute a lighter series for a heavier one without rechecking C and C0.
• Keep the applied thrust above Fam ≈ A·(n·dm/1000)² with the printed factor A 28; for 51256 M a spring preload or the rotor weight is part of the selection at light load and high speed, because unloaded balls skid rather than roll.
• Identify the washers before fitting: the shaft washer has the ground, smaller bore and takes an interference fit on the 280 mm shaft, while the housing washer bore is larger and runs loose; swapping them locks the ball set and burns the tracks.
• On vertical shafts retain the loose housing washer positively with a cover or ring and verify the grease fill does not drain out of the 80 mm stack; thrust-ball bearings on horizontal shafts should have the shaft washer (the tight one) against the rotating element.
• Ordering discipline for 51256: the catalogue page quotes 51256 M and its 488/2320 kN ratings as one exact article; bare-number substitutions from another series or brand need a fresh dimension and rating check, not a like-for-like assumption.