What is a Self-Locking Nut? Why is it often used instead of a traditional nut?
- 翰君 陳
- Sep 30
- 3 min read
When people hear Self-Locking Nut (SPN / Press-Fit Nut, Self-Locking Retaining Ring), they often assume it has internal threads like a conventional nut—but that’s not the case.
It is actually a threadless elastic retaining ring that locks onto the shaft by its own spring force.
Compared with traditional nuts, it is thinner, space-saving, and quick to install, especially suitable for applications where the shaft has no threads but still requires anti-loosening and fixation.
Common uses include: securing bearings on motor shafts, rotating joints in fitness equipment, or pulleys in vending machines.
If you see a smooth shaft with no threads but parts are still held firmly—it’s very likely using a self-locking nut.
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How to Select the Right Size?
The key factor is the shaft diameter.
Self-locking nuts are designed according to JIS B 1216 standard, where each size corresponds to a specific shaft diameter.Choosing the wrong size means it could be too loose (cannot lock) or too tight (cannot be installed).
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What to Check Before Buying?
Measure the shaft
Always confirm the shaft diameter. (e.g., Ø4 mm shaft → use SPNS-4)
Mind precision
If the shaft tolerance is off, the nut may not hold tightly or may loosen quickly.
Choose the right material
Indoors → Carbon steel is sufficient.
Outdoors or humid environments → Stainless steel (304 / 316) is recommended for durability.
Consider frequency of assembly
Occasional use → hand press is enough.
Frequent use → prepare tools (pliers, press, sleeve) to avoid wear and extend lifespan.
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Installation Steps at a Glance
Confirm size → The shaft diameter must match the nut spec (e.g., Ø4 mm shaft → SPNS-4).
Align → Position the nut hole with the shaft, straight and centered.
Press in → Use your fingers to lightly seat it, then press fully with a tool (pliers, press, socket).The “legs” of the nut will expand slightly and grip the shaft firmly.
Check → After pressing down, rotate or pull slightly to confirm it holds securely.
Installing a self-locking nut is similar to pressing on a bottle cap or earring clasp—no threading required, it locks by elasticity.
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Application Illustration
Item | Description |
Inner Diameter | e.g., 3.9 mm → Fits Ø4 mm shaft |
Applicable Shaft | e.g., 4 mm → Only for shafts Ø4 mm |
Side View Structure | Shows thickness and elastic tabs, which expand and grip during press-fit |
Installation Note | Like pressing a bottle cap or clasp—just push it on, no threading required |
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Common Applications of Self-Locking Nuts
Fan motor bearings → Prevents bearings from shifting during rotation
Gears and pulleys → Secures parts in drive systems, prevents loosening
Agricultural and machine tool shafts → Stops parts from sliding under load
Everyday equipment → Fitness machines, vending machines, smooth shafts without threads
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Material Options and Features
Material | Feature | Suitable Applications |
Carbon Steel (Black Oxide) | Economical and durable, but prone to rust in outdoor/humid conditions | Indoor machinery, general equipment |
Stainless Steel SUS304 | Good corrosion resistance | Outdoor or humid environments (e.g., vending machines, gym equipment) |
Stainless Steel SUS316 | Excellent corrosion resistance | Coastal, chemical, or marine environments |
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Why Choose Sheng Fong Precision?
Wide material range → Carbon steel, SUS304, SUS316, with surface treatments (oxidizing, plating, passivation)
Full specifications → Complies with JIS B 1216, standard sizes always in stock
Reliable quality → Each batch inspected for size and elasticity to ensure secure fit
Fast lead time → Supports both mass orders and small-lot urgent requests
Selection guidance → Professional advice on the best material/size for your application
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