51305 bearing: complete specifications & equivalents
The 51305 is a ISO 104 radial bearing sized 25 x 52 x 18 mm (bore x OD x width). Verify dimensions against the official catalog before ordering.
Specification table
| Parameter | Value |
|---|---|
| Brand | NTN |
| Bore diameter d | 25 mm |
| Outer diameter D | 52 mm |
| Width B | 18 mm |
| Material | GCr15 |
| Seal / shield | open |
| Internal clearance | CN |
| Tolerance class | P0 |
| Basic dynamic load rating C | 35.5 kN |
| Basic static load rating C0 | 61.5 kN |
| Fatigue load limit Pu | 2.77 kN |
| Limiting speed (mechanical) | 3200 rpm |
| Operating temperature | -40 to +120 °C |
| Mass | 176 g |
| Manufacturer designation | NTN 51305 |
| 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 & 51300-series range
| Bore d | 0.984 in |
|---|---|
| Outer diameter D | 2.047 in |
| Width B | 0.709 in |
| ISO bore code | 05 |
| Radial section height (D−d)/2 | 13.5 mm |
| Smallest bore in 51300-series | 25 mm |
| Largest outer diameter in 51300-series | 340 mm |
| Width range across 51300-series | 18–110 mm |
| Bore position in 51300-series (smallest first) | 1 of 25 |
| Bore above smallest 51300-series size | 0 mm |
Cross references & equivalents
| Equivalent model | Brand | Match level | Note |
|---|---|---|---|
| 51305-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 51305 model number is decoded
51305 is the 25 mm member of NTN's heavy-section 51300 thrust family: 52 mm across the washers, 18 mm tall. Decoding 51305: 513 is the dimension series (heavy-section 51300 thrust family), with 05 encodes the 25 mm bore (bore code x 5). Catalogue axial dynamic rating Ca is 35.5 kN against C0a 61.5 kN — a static/dynamic ratio of 1.73 that is the signature of a thrust raceway. Within the thrust-ball plan, 511 is the light section and 514 the heaviest for a given bore; moving up a series raises the C0a from 61.5 kN class figures at the cost of OD and height. Mount the stack in order and confirm the shaft washer sits against its shoulder before loading: pressing through the balls brinells the raceway on the spot.
Typical equipment & failure modes
Common applications
- valve actuator and gearbox thrust seats
- food-machine vertical auger shafts
- laboratory centrifuge and mixer thrust ends
- small vertical pump and agitator thrust positions
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
- washer face corrosion under long standstill in humid service, later printed into the raceway as evenly spaced pitting
Selection pitfalls — read before you substitute
- Small thrust washers of 25 mm bore bend if the shaft shoulder is rough — check burrs before seating, and never press on the housing washer to drive the shaft washer home.
- The 513 heavy section buys capacity with a larger 52 mm OD and taller stack — check the housing space before upgrading a 512 position to this series.
- Grease in a horizontal-washer seat stays only if the housing is designed to hold it; open vertical arrangements need an oil bath or a grease-retaining cover.
- Never substitute a double-direction bearing drawing part-for-part: the single-direction 51305 stack height of 18 mm and washer set differ from the double-direction execution.
- Swapping the shaft and housing washers is an easy assembly error on the 25 mm size — the bores differ only slightly, but the wrong pairing walks off-centre under load.
- The Ca 35.5/C0a 61.5 kN ratings and 2.77 kN Pu here are NTN Ball and Roller Bearings CAT.No.2203E figures for 51305; 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 51305?
51305 is a single-direction thrust ball bearing to ISO 104: 25 mm shaft-washer bore, 52 mm outside diameter, 18 mm stack height (0.984 x 2.047 x 0.709 in). From NTN Ball and Roller Bearings CAT.No.2203E: axial Ca 35.5 kN, C0a 61.5 kN and a 2.77 kN fatigue load limit Pu; the grease limiting speed is 3,200 r/min and the oil rating 4,600 r/min; mass about 176 g.
How is the 51305 designation structured?
513 is the dimension series (heavy-section 51300 thrust family); 05 encodes the 25 mm bore (bore code x 5). The 52 mm OD and 18 mm height follow the ISO 104 boundary plan for that series.
Can 51305 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 51305?
At the 3,200 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 51305?
The 25/52/18 mm envelope is ISO 104 interchangeable, but washer bore codes, raceway osculation and ratings are maker-specific. Re-verify Ca 35.5 kN / C0a 61.5 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.