51208 bearing: complete specifications & equivalents
The 51208 is a ISO 104 radial bearing sized 40 x 68 x 19 mm (bore x OD x width). Verify dimensions against the official catalog before ordering.
Specification table
| Parameter | Value |
|---|---|
| Brand | NTN |
| Bore diameter d | 40 mm |
| Outer diameter D | 68 mm |
| Width B | 19 mm |
| Material | GCr15 |
| Seal / shield | open |
| Internal clearance | CN |
| Tolerance class | P0 |
| Basic dynamic load rating C | 47 kN |
| Basic static load rating C0 | 98.5 kN |
| Fatigue load limit Pu | 4.45 kN |
| Limiting speed (mechanical) | 2700 rpm |
| Operating temperature | -40 to +120 °C |
| Mass | 276 g |
| Manufacturer designation | NTN 51208 |
| Standard | ISO 104 |
| Catalog version | NTN Ball and Roller Bearings CAT.No.2203E |
| Last verified | 2026-10-01 |
Source: catalog reference (NTN Ball and Roller Bearings CAT.No.2203E) · extracted 2026-10-01
Parameters extracted from the official NTN Ball and Roller Bearings catalogue (NTN Ball and Roller Bearings CAT.No.2203E); NTN® is a trademark of NTN Corporation, used here for identification only — mpnbase is not an authorized NTN distributor. Boundary dimensions are ISO 15 interchangeable; current production values may differ, verify at ntnglobal.com.
Dimensions in inches & 51200-series range
| Bore d | 1.575 in |
|---|---|
| Outer diameter D | 2.677 in |
| Width B | 0.748 in |
| ISO bore code | 08 |
| Radial section height (D−d)/2 | 14 mm |
| Smallest bore in 51200-series | 10 mm |
| Largest outer diameter in 51200-series | 520 mm |
| Width range across 51200-series | 11–112 mm |
| Bore position in 51200-series (smallest first) | 9 of 39 |
| Bore above smallest 51200-series size | 30 mm |
Cross references & equivalents
| Equivalent model | Brand | Match level | Note |
|---|---|---|---|
| 51208-SKF | SKF | full Dimensions and internal design equivalent — standard interchange. | Identical ISO 15 envelope, seal and ratings band — standard interchange; re-verify precision class and clearance per supplier. |
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 51208 model number is decoded
From the medium-section 51200 thrust family, 51208 measures 40 mm through the shaft washer, 68 mm over the outside and 19 mm in assembled height. Two fields make up the code — 512 is the dimension series (medium-section 51200 thrust family) and 08 encodes the 40 mm bore (bore code x 5). Catalogue axial dynamic rating Ca is 47 kN against C0a 98.5 kN — a static/dynamic ratio of 2.10 that is the signature of a thrust raceway. For a 40 mm shaft with combined radial-plus-axial load, pair this bearing with a separate radial bearing — the thrust ball bearing must never be asked to carry the radial share. The housing washer needs radial freedom in its seat so the bearing can centre itself; clamping it radially defeats the self-centring the loose fit is designed to give.
Typical equipment & failure modes
Common applications
- winch and capstan vertical shafts
- press and punch-machine thrust seats
- conveyor vertical-drive thrust positions
- vertical motor and pump thrust shoes
Known failure patterns
- cage pocket wear and ball skid polish after long running below the minimum axial load, the balls sliding instead of rolling on the washers
- housing-washer rocking on an out-of-square support face, seen as uneven wear around the raceway circle and rising running torque
- rapid flaking after radial load was allowed onto the bearing — a single-direction thrust ball bearing carries axial load only
- washer face corrosion under long standstill in humid service, later printed into the raceway as evenly spaced pitting
- cage breakage after repeated shock reversal banged the ball set against the pocket bridges
Selection pitfalls — read before you substitute
- In this size class the frequent miss is washer identification: keep the catalogued shaft/housing washer pairing of the 40 mm size together from unpacking to assembly.
- The 512 medium section is the default choice: balanced height and capacity for general vertical-shaft duty at 40 mm bore.
- Single-direction means exactly that: thrust on 51208 in the wrong direction separates the washers and drops the ball set — if reversal is possible even during coast-down, engineer against it.
- Radial load is the silent killer here: a belt pull or gear force on the same shaft must land on a radial bearing, because 51208 has no radial capacity at all.
- Swapping the shaft and housing washers is an easy assembly error on the 40 mm size — the bores differ only slightly, but the wrong pairing walks off-centre under load.
- The Ca 47/C0a 98.5 kN ratings and 4.45 kN Pu here are NTN Ball and Roller Bearings CAT.No.2203E figures for 51208; raceway geometry legitimately differs between manufacturers, so re-verify ratings and washer identification before substituting another make.
Frequently asked questions
What are the dimensions and catalogue ratings of NTN 51208?
51208 is a single-direction thrust ball bearing to ISO 104: 40 mm shaft-washer bore, 68 mm outside diameter, 19 mm stack height (1.575 x 2.677 x 0.748 in). From NTN Ball and Roller Bearings CAT.No.2203E: axial Ca 47 kN, C0a 98.5 kN and a 4.45 kN fatigue load limit Pu; the grease limiting speed is 2,700 r/min and the oil rating 3,900 r/min; mass about 276 g.
How is the 51208 designation structured?
512 is the dimension series (medium-section 51200 thrust family); 08 encodes the 40 mm bore (bore code x 5). The 68 mm OD and 19 mm height follow the ISO 104 boundary plan for that series.
Can 51208 carry radial load or reversing thrust?
No. The raceways contact the balls at 90 deg to the shaft, so the rating is purely axial and single-direction. Radial load must be carried by a separate radial bearing, and reversing thrust needs a second thrust bearing or a double-direction execution.
Why does NTN specify a minimum axial load for 51208?
At the 2,700 r/min class speed, centrifugal and gyroscopic effects make an under-loaded ball set skid and smear instead of roll. The catalogue minimum-load formula gives the axial preload — from the machine weight, springs or a dead-weight — that keeps the contact rolling.
What should be checked before substituting another make for 51208?
The 40/68/19 mm envelope is ISO 104 interchangeable, but washer bore codes, raceway osculation and ratings are maker-specific. Re-verify Ca 47 kN / C0a 98.5 kN, the speed figures and the shaft/housing washer identification against the donor catalogue.
Reviewed & verified by the MPN Base engineering desk against source catalog NTN Ball and Roller Bearings CAT.No.2203E on 2026-10-01. Read our editorial & correction policy.