6391-K-6320 bearing: complete specifications & equivalents
The 6391-K-6320 is a ISO 355 radial bearing sized 59.987 x 135.755 x 53.975 mm (bore x OD x width). Verify dimensions against the official catalog before ordering.
Specification table
| Parameter | Value |
|---|---|
| Brand | SKF |
| Bore diameter d | 59.987 mm |
| Outer diameter D | 135.755 mm |
| Width B | 53.975 mm |
| Material | GCr15 |
| Seal / shield | open |
| Internal clearance | CN |
| Tolerance class | P0 |
| Cage | steel, stamped ribbon (J-type) |
| Basic dynamic load rating C | 353 kN |
| Basic static load rating C0 | 400 kN |
| Fatigue load limit Pu | 45.5 kN |
| Reference speed (thermal) | 4000 rpm |
| Limiting speed (mechanical) | 5000 rpm |
| Operating temperature | -40 to +120 °C |
| Mass | 3950 g |
| Manufacturer designation | SKF 6391/K-6320 |
| Standard | ISO 355 |
| Catalog version | SKF PUB BU/P1 17000/1 EN (Oct 2018), ch. 8.2 inch single-row TRB |
| Last verified | 2026-09-28 |
Source: catalog reference (SKF PUB BU/P1 17000/1 EN (Oct 2018), ch. 8.2 inch single-row TRB) · extracted 2026-09-28
Parameters extracted from the official SKF Rolling Bearings catalogue (SKF PUB BU/P1 17000/1 EN (Oct 2018), ch. 8.2 inch single-row TRB); 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.
Dimensions in inches & 6300-series range
| Bore d | 2.362 in |
|---|---|
| Outer diameter D | 5.345 in |
| Width B | 2.125 in |
| ISO bore code | 91 |
| Radial section height (D−d)/2 | 37.9 mm |
| Smallest bore in 6300-series | 59.987 mm |
| Largest outer diameter in 6300-series | 135.755 mm |
| Width range across 6300-series | 53.975–53.975 mm |
| Bore position in 6300-series (smallest first) | 1 of 2 |
| Bore above smallest 6300-series size | 0 mm |
Cross references & equivalents
| Equivalent model | Brand | Match level | Note |
|---|---|---|---|
| 6391-K-6320 | Timken | full Dimensions and internal design equivalent — standard interchange. | Same inch-series cone/cup boundary envelope (d/D/T); load ratings and speed limits are brand-specific and may differ. |
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 6391-K-6320 model number is decoded
On the SKF nameplate, 6391 denotes a matched cone-and-cup pair: the designation is not a single number but a cone reference and cup reference joined by a slash, with the 6300-series marking the dimensional series. SKF lists the plain number as the basic execution; letter suffixes such as B denote a steeper contact angle and are different order numbers. The 37.9 mm radial section height ((135.755−59.987)/2) and 3950 g catalogue mass frame a 59.987 mm bore bearing rated at 353 kN C — roughly 5.88 kN of dynamic capacity per millimetre of bore before application factors. Note the minimum-axial-load requirement: at light radial load and high speed the rollers of 6391 must still be held into the rib contact, which is another reason the catalogue recommends paired mounting with controlled setting. Operating context for the 59.987 mm bore: a shaft of this diameter in the large-bore band typically runs between 47990 and 71984 r/min in real machinery, and the 135.755 mm outside diameter places the housing shoulder at 144 mm outer abutment; the 53.975 mm width and 3950 g mass together set the shaft critical speed and the housing resonance check for this 63900-series envelope. Housing and abutment design flows from the cup: in 6391 the K-6320 cup fixes the 135.755 mm OD, the outer-ring face position and the rib shoulder that the rollers contact under thrust, while the 6391 cone determines the 59.987 mm shaft interface; the 53.975 mm width is the assembled pair value.In SKF catalogue PUB BU/P1 17000, 6391 appears as SKF 6391/K-6320: an open, single-row tapered roller bearing in the 6300-series, supplied with a pressed-steel cage as standard.
Typical equipment & failure modes
Common applications
- large process pump and compressor shafts
- rolling-mill auxiliary drives
- crane sheave and hoist gearboxes
- crane sheave and hoist gearboxes
Known failure patterns
- cup or cone misalignment when the separable rings are not seated square during mounting
- loose cup rotation in the housing when the specified press fit is not achieved
- thermal preload runaway in an O-arrangement pair when inner-ring heat expands the setting
- preload loss as the mounted pair relaxes, letting rollers skid and smear the raceways
- pressed-steel cage wear or fracture after prolonged running above the limiting speed
- grease degradation after long shelf storage without rotation or re-greasing
Selection pitfalls — read before you substitute
- Mount the 59.987 mm bore 6391 by induction heating of the cone (max 120 °C — the pressed-steel cage sets the ceiling, not the steel); driving it home through the cup or rollers brinells the tapered raceway.
- The 6300-series inch series at 59.987 mm bore (135.755×53.975 mm) follows the Timken cone/cup convention; substitutions must match both the cone and cup reference exactly — interchanging with another series or a metric 3xxxx bearing of the same bore changes OD, width and contact angle, so re-run the pair setting and shoulder-diameter checks before swapping.
- Setting is a design value, not an assembly feel: excessive preload in the 6391 pair raises operating temperature and can trigger thermal runaway, while too little lets the rollers skid and smear the raceway under light load.
- O (back-to-back) pairs are stiffer against moments, X (face-to-face) pairs tolerate misalignment and thermal growth better; choosing the arrangement from the 2D drawing rather than the actual shaft thermal expansion is a common 6391 mounting error.
- Catalogue reference speed and limiting speed are different limits: the 4,000 r/min reference is a thermal criterion under load, while the 5,000 r/min limiting value is the mechanical ceiling — observe both.
- The 353/400 kN ratings are SKF catalogue 17000 values for the basic execution; SKF can revise internal geometry without changing the order number, so keep the catalogue version (PUB BU/P1 17000/1 EN, 2018) with the specification and re-verify for critical drives.
- SKF lists several executions of 6391 (basic, B steeper angle, Q/J2 clearance variants); they are not interchangeable order numbers — quote the full SKF 6391/K-6320 designation when purchasing and match the execution to the mounting arrangement.
Frequently asked questions
How does the 6391 inch-series designation work?
Inch-series tapered roller bearings use a Timken-style cone/cup convention: the number before the slash identifies the cone (inner ring with roller and cage assembly), the number after the slash identifies the cup (outer ring), and the 6300-series code marks the dimensional series. The metric dimensions 59.987×135.755×53.975 mm are the converted equivalents of the original inch boundary dimensions.
What do the cone and cup numbers in 6391 mean?
The cone number (before the slash) identifies the inner ring, rollers and cage; the cup number (after the slash) identifies the outer ring. Both are quoted together because a single-row tapered roller bearing is a matched separable pair — the cone and cup are ground to work together and are not interchangeable with arbitrary other parts. The series prefix (6300-series) groups sizes that share the same boundary-dimension plan.
Can a single 6391 take axial load in both directions?
No — one single-row tapered roller bearing reacts thrust in one direction only, and under radial load its contact angle induces an axial force. Use two bearings as a matched pair: O (back-to-back) for a rigid, moment-resisting arrangement, X (face-to-face) where thermal expansion must be tolerated, or tandem where one-direction thrust is shared.
How does 6391 compare with a ball bearing of the same bore?
A tapered roller bearing has line contact and a steep contact angle: much higher combined axial-plus-radial capacity and shock resistance than a ball bearing of the same bore, at the cost of speed, one-direction thrust, mandatory pairing for reversing loads and a minimum axial load requirement. A ball bearing cannot be dropped into a combined-load 6391 position and vice versa — verify load direction, Fa/Fr ratio and speed before substituting.
What shaft and housing fits suit 6391?
For a rotating shaft under normal load, seat the cone on a k5 shaft fit; the cup is usually a loose-to-transition fit in the housing so it can creep slightly and distribute wear. Chamfer and shoulder diameters must clear the bearing rings so the 53.975 mm-wide cup seats square; the mounting setting (end play or preload) is set by the axial position of the cup relative to the cone.
Reviewed & verified by the MPN Base engineering desk against source catalog SKF PUB BU/P1 17000/1 EN (Oct 2018), ch. 8.2 inch single-row TRB on 2026-09-28. Read our editorial & correction policy.