51209 bearing: complete specifications & equivalents
The 51209 is a ISO 104 radial bearing sized 45 x 73 x 20 mm (bore x OD x width). Verify dimensions against the official catalog before ordering.
Specification table
| Parameter | Value |
|---|---|
| Brand | NTN |
| Bore diameter d | 45 mm |
| Outer diameter D | 73 mm |
| Width B | 20 mm |
| Material | GCr15 |
| Seal / shield | open |
| Internal clearance | CN |
| Tolerance class | P0 |
| Basic dynamic load rating C | 47.5 kN |
| Basic static load rating C0 | 105 kN |
| Fatigue load limit Pu | 4.75 kN |
| Limiting speed (mechanical) | 2600 rpm |
| Operating temperature | -40 to +120 °C |
| Mass | 317 g |
| Manufacturer designation | NTN 51209 |
| 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 | 1.772 in |
|---|---|
| Outer diameter D | 2.874 in |
| Width B | 0.787 in |
| ISO bore code | 09 |
| 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) | 10 of 39 |
| Bore above smallest 51200-series size | 35 mm |
Cross references & equivalents
| Equivalent model | Brand | Match level | Note |
|---|---|---|---|
| 51209-SKF | SKF | partial Main dimensions match; tolerances, seals or internal clearance differ. | Same 45×73×20 mm envelope per ISO 15; catalogue load ratings differ by >10% — re-rate the application before substituting. |
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 51209 model number is decoded
51209 is an NTN single-direction thrust ball bearing of the medium-section 51200 thrust family: 45 mm bore, 73 mm outside diameter, 20 mm overall height in NTN Ball and Roller Bearings CAT.No.2203E. Decoding 51209: 512 is the dimension series (medium-section 51200 thrust family), with 09 encodes the 45 mm bore (bore code x 5). Flat-seat washers make the bearing tolerant of simple housings but demand square faces: the whole 47.5 kN rating assumes the load arrives truly axial. Against a thrust roller bearing of the same 45 mm bore, this bearing is cheaper, quieter and happier at speed, but carries far less load per mm of section. Preload or working clearance must guarantee the minimum axial load at the catalogue speed; a spring or dead-weight arrangement is the usual answer on high-speed vertical shafts.
Typical equipment & failure modes
Common applications
- conveyor vertical-drive thrust positions
- vertical motor and pump thrust shoes
- machine-tool tailstock and steady-rest screws
- medium crane hook blocks and slewing jibs
Known failure patterns
- ball and raceway smearing from gyroscopic spin when the bearing runs fast under too little axial load — the minimum-load rule exists exactly for this
- fretting between shaft washer and shaft when the seat is cut loose — the washer creeps, the centre line shifts and the balls start running off-track
- early fatigue from thrust reversal hammering the unloaded raceway, because the bearing has no contact surfaces for the opposite direction
- edge loading from a shaft washer mounted off-centre, cutting the contact ellipse short at one side of the track
- washer face corrosion under long standstill in humid service, later printed into the raceway as evenly spaced pitting
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 45 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 45 mm bore.
- Below the minimum axial load the balls skid; the catalogue Ca of 47.5 kN never saves a bearing that is running too lightly loaded at 2,600 r/min.
- Below the minimum axial load the balls skid; the catalogue Ca of 47.5 kN never saves a bearing that is running too lightly loaded at 2,600 r/min.
- Never substitute a double-direction bearing drawing part-for-part: the single-direction 51209 stack height of 20 mm and washer set differ from the double-direction execution.
- The Ca 47.5/C0a 105 kN ratings and 4.75 kN Pu here are NTN Ball and Roller Bearings CAT.No.2203E figures for 51209; 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 51209?
51209 is a single-direction thrust ball bearing to ISO 104: 45 mm shaft-washer bore, 73 mm outside diameter, 20 mm stack height (1.772 x 2.874 x 0.787 in). From NTN Ball and Roller Bearings CAT.No.2203E: axial Ca 47.5 kN, C0a 105 kN and a 4.75 kN fatigue load limit Pu; the grease limiting speed is 2,600 r/min and the oil rating 3,700 r/min; mass about 317 g.
How is the 51209 designation structured?
512 is the dimension series (medium-section 51200 thrust family); 09 encodes the 45 mm bore (bore code x 5). The 73 mm OD and 20 mm height follow the ISO 104 boundary plan for that series.
Can 51209 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 51209?
At the 2,600 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 51209?
The 45/73/20 mm envelope is ISO 104 interchangeable, but washer bore codes, raceway osculation and ratings are maker-specific. Re-verify Ca 47.5 kN / C0a 105 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.