Introduction
Fasteners are among the most frequently used components in mechanical design — and among the most frequently misspecified. Understanding thread standards, grades, and tightening requirements is essential for producing reliable, safe designs.
Thread Standards
Metric Threads (ISO 68-1)
The international standard. Specified as M[diameter]×[pitch]. Common examples:
- M6×1.0 — 6 mm diameter, 1.0 mm pitch
- M10×1.5 — 10 mm diameter, 1.5 mm pitch (coarse, standard)
- M10×1.25 — 10 mm diameter, 1.25 mm pitch (fine)
Coarse threads are standard for most applications; fine threads are used where vibration resistance or fine adjustment is needed.
Unified Inch Threads (UNC/UNF)
Used in North America and on some legacy equipment worldwide. Specified as [diameter]-[TPI (threads per inch)] [UNC/UNF]. Example: 3/8″-16 UNC.
When designing new products, use metric threads. Inch threads appear mainly on existing equipment or US-market products.
Bolt Strength Grades
| ISO Grade | Tensile Strength | Proof Load | Typical Use |
|---|---|---|---|
| 4.6 | 400 MPa | 225 MPa | Non-critical, low-load |
| 8.8 | 800 MPa | 600 MPa | General engineering (most common) |
| 10.9 | 1000 MPa | 900 MPa | High-strength joints |
| 12.9 | 1200 MPa | 1080 MPa | Maximum strength (expensive) |
Grade 8.8 is the standard choice for most mechanical assemblies. Use higher grades only when load analysis justifies it — higher grades are more susceptible to hydrogen embrittlement and fatigue failure.
Tightening Torque
Proper torque creates clamp force that holds the joint together. Approximate tightening torque (for grade 8.8, lubricated):
| Thread Size | Tightening Torque (approx.) |
|---|---|
| M6 | 10 Nu00b7m |
| M8 | 25 Nu00b7m |
| M10 | 50 Nu00b7m |
| M12 | 85 Nu00b7m |
Always use manufacturer or design standard torque tables for critical joints. The values above are approximate and depend on friction coefficient.
Preventing Loosening
- Spring washer: simple and widely used, but limited effectiveness under vibration
- Nyloc nut (prevailing torque nut): nylon insert grips threads; reliable for moderate vibration
- Thread-locking adhesive (e.g., Loctite 243): fills thread gaps; highly effective, but requires solvent or heat to disassemble
- Double nut (jam nut): two nuts tightened against each other; effective but bulky
- Serrated flange bolt/nut: serrations bite into mating surface; good for sheet metal
Thread Engagement Length
Minimum thread engagement (length of threaded hole) to match bolt strength:
- Steel into steel: 1.0 × bolt diameter
- Steel into aluminum: 1.5–2.0 × bolt diameter
- Steel into cast iron: 1.25–1.5 × bolt diameter
FAQ
Q. What is the difference between a bolt and a screw?
A. In engineering usage, a bolt is tightened by a nut (passes through a clearance hole); a screw is tightened by rotating its head into a tapped hole. In common usage the terms are often used interchangeably.
Q. When should I specify a fine thread vs. a coarse thread?
A. Use coarse threads (standard) for most applications — they are faster to assemble and more resistant to cross-threading. Use fine threads where axial adjustment precision is needed, where thread engagement depth is limited, or on thin-walled parts where coarse threads would be too few in engagement.
Q. What does the head marking on a bolt mean?
A. ISO grade bolts are marked on the head: “8.8”, “10.9”, “12.9” etc. The first digit relates to tensile strength in units of 100 MPa; the second digit (after the decimal) relates to the proof load ratio. A blank head or no marking often indicates a low-grade (4.6 or below) fastener.
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