Choosing Fastener Material, Size and Strength
In short: The right fastener depends on the load, the materials being joined and the environment. Size, strength grade and material all matter, and hardness is not the same as strength.
See: Hardware, Heavy EngineeringTalk to our technical team
Choosing a fastener is more than picking one that fits the hole. The wrong size, grade or material is a common cause of failure.
Choosing the size
- Diameter: sized for the load the joint must carry, in shear and tension.
- Length: enough to pass through the parts with full thread engagement in the nut, but not so long that it protrudes unnecessarily.
- Thread engagement: too little engagement risks stripping. See Fastener Threads Explained.
Choosing the material
- Carbon steel: strong and economical, usually zinc-plated or galvanised against corrosion.
- Alloy steel: for high-strength grades such as 8.8, 10.9 and 12.9.
- Stainless steel: for corrosion resistance, but generally lower strength than high-tensile steel.
- Aluminium: light, used in rivets and light assemblies.
- Match the materials being joined where possible, to avoid galvanic corrosion. See Fastener Corrosion and Stainless Steel.
Hardness versus strength
They are related but not the same.
- Strength is how much load a fastener can carry before it yields or breaks.
- Hardness is resistance to surface indentation and wear.
Harder fasteners are generally stronger, but very hard fasteners can be brittle and more prone to sudden failure, including hydrogen embrittlement. The right choice balances strength with toughness for the application.
Avlock’s technical team can help specify the right fastener. See the hardware range and, for high-load vibration-prone joints, lockbolts.