W638-2-5-ZZ bearing: complete specifications & equivalents
The W638-2-5-ZZ is a ISO 15 radial bearing sized 2.5 x 6 x 2.6 mm (bore x OD x width) with ZZ sealing. Verify dimensions against the official catalog before ordering.
Specification table
| Parameter | Value |
|---|---|
| Brand | SKF |
| Bore diameter d | 2.5 mm |
| Outer diameter D | 6 mm |
| Width B | 2.6 mm |
| Material | SS440C |
| Seal / shield | ZZ |
| Internal clearance | CN |
| Tolerance class | P0 |
| Cage | steel, stamped ribbon (J-type) |
| Basic dynamic load rating C | 0.117 kN |
| Basic static load rating C0 | 0.036 kN |
| Fatigue load limit Pu | 0.002 kN |
| Reference speed (thermal) | 170000 rpm |
| Limiting speed (mechanical) | 85000 rpm |
| Operating temperature | -40 to +120 °C |
| Mass | 0.31 g |
| Manufacturer designation | W 638/2.5-2Z |
| Standard | ISO 15 |
| Catalog version | SKF PUB BU/P1 17000/1 EN (Oct 2018) |
| Last verified | 2026-09-27 |
Source: catalog reference (SKF PUB BU/P1 17000/1 EN (Oct 2018)) · extracted 2026-09-27
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.
Dimensions in inches & 63800-series range
| Bore d | 0.098 in |
|---|---|
| Outer diameter D | 0.236 in |
| Width B | 0.102 in |
| ISO bore code | — |
| Radial section height (D−d)/2 | 1.8 mm |
| Smallest bore in 63800-series | 1.5 mm |
| Largest outer diameter in 63800-series | 28 mm |
| Width range across 63800-series | 2–9 mm |
| Bore position in 63800-series (smallest first) | 3 of 8 |
| Bore above smallest 63800-series size | 1 mm |
Cross references & equivalents
| Equivalent model | Brand | Match level | Note |
|---|---|---|---|
| W639-2-ZZ | SKF | partial Main dimensions match; tolerances, seals or internal clearance differ. | Same 2.5×6×2.6 mm envelope as SKF W 639/2-2Z, but catalogue C/C0 differ (0.117/0.036 kN vs 0.19/0.051 kN) — envelope match only after a load check. |
| W637-3-ZZ | SKF | full Dimensions and internal design equivalent — standard interchange. | Envelope-identical (2.5×6×2.6 mm) SKF deep-groove bearing W 637/3-2Z with ratings within 10 % — interchangeable after checking the execution details on both pages. |
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 W638-2-5-ZZ model number is decoded
The W prefix denotes SKF stainless steel deep-groove bearings (X108CrMo17 / AISI 440C class rings). In the 63800-series, the bore is written directly after the slash as 2.5 mm (bores below 10 mm use the slash spelling). Envelope 2.5×6×2.6 mm (0.098×0.236×0.102 in), C 0.117 kN, C0 0.036 kN, Pu 0.002 kN; limiting speed 85000 r/min and mass 0.31 g. SKF spells the double shield W 638/2.5-2Z (suffix 2Z); the shields are non-contact, retain grease and block coarse debris but are not liquid-tight. Stainless is chosen for corrosion and washdown duty, not for extra load — the same-envelope chrome-steel bearing rates higher.
Typical equipment & failure modes
Common applications
- medical and laboratory devices
- small pumps handling corrosive media
- bicycle and outdoor equipment
- appliances exposed to moisture
Known failure patterns
- seal and grease failure before the rings corrode in sustained washdown service
- surface staining and pitting when even the stainless steel is left unlubricated in salty or acidic duty
- fine-particle abrasion when non-contact shields let dust through in a dirty environment
- grease aging inside the shielded cavity with no relubrication path
Selection pitfalls — read before you substitute
- Temperatures above roughly 300 °C temper the stainless races; even standard greases fail far earlier — keep W638-2-5-ZZ inside the quoted grease temperature range.
- W-prefix stainless rings (X108CrMo17 / AISI 440C class) resist moisture and weak chemicals but will still rust in chloride-rich or salt-spray duty without protection — W638-2-5-ZZ is not marine-grade stainless.
- The stainless steel is martensitic and MAGNETIC; do not reject W638-2-5-ZZ over magnetism — only austenitic grades are non-magnetic, and those are too soft for bearing raceways.
- Stainless bearings carry lower load ratings than chrome steel of the same envelope: W638-2-5-ZZ is quoted at C 0.117 kN versus the 2.6 mm-wide 52100 version — do not swap on dimensions alone.
- Do not preload W638-2-5-ZZ to silence it: a deep-groove ball bearing is not angular-contact, and axial preload only raises temperature and shortens grease life.
- Assign interference to the rotating ring: on a conventional shaft-mounted 2.5 mm position the inner ring rotates, so use a k5/m6 shaft seat with an H7 housing — reverse the logic if the outer ring rotates.
Frequently asked questions
What are the dimensions of the stainless W638-2-5-ZZ?
SKF W 638/2.5-2Z measures 2.5×6×2.6 mm (0.098×0.236×0.102 in), about 0.31 g, C 0.117 kN.
What does the W in W 638/2.5-2Z mean?
W is the SKF prefix for stainless steel deep-groove bearings (rings in X108CrMo17 / AISI 440C class); the rest of the designation follows the normal series and bore rules.
Where is W638-2-5-ZZ typically used?
Food and packaging machines, pharmaceutical equipment, washdown conveyors, chemical and marine-atmosphere drives and appliances exposed to moisture.
What grease is supplied in W638-2-5-ZZ?
Sealed units are pre-greased for life; for food or pharmaceutical duty order (or re-grease before service) with an NSF H1 registered food-grade grease.
Reviewed & verified by the MPN Base engineering desk against source catalog SKF PUB BU/P1 17000/1 EN (Oct 2018) on 2026-09-27. Read our editorial & correction policy.