Fastener Selection: Practical Engineering Guide (SPM / Machine Design)

Selection of Fastener – Practical Engineering Guide (SPM / Machine Design)

Fastener selection is a design decision, not a purchase decision. In machines & SPMs, the right fastener ensures stiffness, accuracy, safety, and life.

1. Define the Purpose of the Joint

Ask what the fastener must do:
Clamp parts together
Carry load (tension / shear)
Allow adjustment or frequent removal
Resist vibration

 Rule: Bolts clamp; dowels locate.

2. Identify Load Type

Load Design Focus：
Tension Bolt tensile strength
Shear Prefer friction via preload
Dynamic / fatigue Higher preload + higher grade
Shock Increase safety factor

3. Select Fastener Type

Hex bolt – general purpose
Socket head cap screw – compact, high strength
Stud & nut – frequent assembly
Shoulder bolt – pivot / guide
Self-tapping – sheet metal

4. Choose Material

Condition Material:
Normal industrial Carbon steel
High strength Alloy steel
Corrosion Stainless steel
Lightweight Aluminum (low load)

Be noticed: Stainless steel ≠ high strength by default.

5. Select Strength Grade (Metric)

Grade Typical Use:
4.6  Light duty
8.8  General machinery
10.9 SPM / automation
12.9 Compact, high load

SPM default: 8.8 or 10.9

6. Decide Size (Diameter & Length)

Diameter from load calculation
Length to ensure proper thread engagement
Avoid bolt bottoming in blind holes

7. Thread Engagement Rule

Base Material Min Engagement
Steel 1 × dia
Aluminum 1.5 × dia
Cast iron 1.5–2 × dia

8. Locking Against Vibration

Prefer preload + friction
Use thread locker / nyloc nut
Avoid spring washers for vibration-prone joints

9. Assembly & Maintenance

Standardize bolt sizes
Ensure tool accessibility
Mention torque value in drawings

Golden Engineering Rules
- Preload matters more than bolt size
- Threads usually fail before bolt shank
- Fewer fastener sizes = better maintenance
- Poor fastener choice = machine failure