Why Nut Selection Matters
The nut in a bolted joint is not an afterthought. The right nut must match the bolt grade (to prevent thread stripping before bolt failure), the application environment (corrosion resistance), the load type (static vs. vibration), and the assembly method (torque vs. tension control).
Standard Hex Nuts (DIN 934 / IS 1367)
The standard hexagon nut is the most common fastening element in engineering. Flat on six faces for wrench grip. Height = ~0.8 × thread diameter (nominal).
Grades: Match to bolt grade. Grade 8 nut with Grade 8.8 bolt. Grade 10 nut with Grade 10.9 bolt. Mismatching (lower-grade nut with higher-grade bolt) causes thread stripping in the nut.
Thread tolerance: 6H for standard applications. 6AZ (oversize) for HDG applications.
Available material: Carbon steel (zinc plated, HDG, plain), SS304 (A2), SS316 (A4), brass, nylon.
Use for: General engineering, structural connections, any static or low-vibration application.
Heavy Hex Nuts (ASTM A194 / IS 6623)
Heavier than standard hex nut — greater width across flats and greater height. Provides more thread engagement and better resistance to thread stripping under high bolt preload.
Required with: ASTM A193 B7 stud bolts (always specify A194 2H heavy hex nuts). IS 3757 HSFG bolts (specify IS 6623 heavy hex nut). Any application using full bolt preload in structural connections.
Available grades: ASTM A194 Grade 2H (plain or HDG), Grade 4, Grade 8M (SS316).
Lock Nuts — Preventing Loosening in Vibration
When a bolted joint is subject to vibration, dynamic load, or thermal cycling, the nut may progressively loosen. Lock nuts prevent this by adding a prevailing torque — the nut resists rotation in the loosening direction.
Nyloc Lock Nut (DIN 985)
The most common lock nut — a standard hex nut with a nylon insert at the top. The nylon ring deforms against the bolt thread when engaged, creating friction that prevents rotation.
Temperature limit: 100°C for standard nylon insert. 150°C with metal insert (DIN 982).
Key characteristic: Creates prevailing torque without distorting the nut body. Can be removed and re-used (2–3 times with reduced effectiveness).
Use for: General vibration-resistant applications in automotive, solar, machinery, and equipment assembly.
All-Metal Lock Nuts (DIN 982 / DIN 6925)
All-metal lock nuts with a distorted thread zone at the top of the nut. No nylon insert — suitable for high-temperature applications.
Types:
- DIN 982: Collar all-metal lock nut (flange at top with distorted thread)
- DIN 6925: Hexagonal all-metal lock nut (no flange)
Temperature: Suitable up to 300°C+. Required where nylon insert temperature limit is exceeded.
Castellated / Castle Nuts (DIN 935)
A hexagon nut with six slots (castles) cut into the top. A cotter pin (split pin) passes through the nut slots and a hole drilled through the bolt shank, physically locking the nut against rotation.
Use for: Applications where loss of nut is dangerous and positive locking is required. Steering components, wheel spindles, pin connections in construction equipment.
Key characteristic: Positive mechanical locking — cannot loosen even at zero friction. Requires drilling through bolt for cotter pin.
Flange Nuts
A hex nut with an integral washer flange at the base. The flange distributes the bearing load over a larger area, providing better grip on thin materials and preventing pull-through.
Serrated flange nuts: Add radial serrations on the flange face — these bite into the mating surface, providing vibration resistance without a separate lock washer.
Use for: Sheet metal fastening, automotive body panels, roofing and cladding screws (matching flange bolt heads).
Coupling Nuts (DIN 6334)
A long hexagonal nut (typically 3× thread diameter in height) used to join two threaded rods or studs end-to-end. Also used to extend a bolt.
Use for: Extension of anchor rods in post-tensioned concrete, joining threaded rod lengths for pipe support systems, adjustable-length assembly connections.
Wing Nuts (DIN 315)
A nut with two wings for finger-tightening. No wrench required.
Use for: Temporary connections requiring frequent assembly/disassembly. Formwork connections, temporary structures, test rigs. Not for structural loading.
Quick Selection Guide
| Application | Nut Type | |-------------|---------| | General engineering | Hex nut DIN 934 | | High-vibration | Nyloc DIN 985 or serrated flange nut | | High-temperature vibration | All-metal lock nut DIN 982 | | Pressure flanges | Heavy hex nut A194 2H | | Safety-critical positive lock | Castle nut + cotter pin | | Sheet metal, thin material | Flange nut | | Join threaded rods | Coupling nut DIN 6334 | | Quick release | Wing nut DIN 315 |
Contact PGS Fasteners for nut supply in all types, grades, and coatings — from standard hex to speciality pressure bolting nuts.