Bolts and Set Screws: Are They Interchangeable? (Technical Article)

Key things to remember

The essentials from this document in plain words. Check the full document before you rely on a figure.

• ✓Thread length is what separates the two. A bolt has an unthreaded shank; a screw is threaded almost to the head (the plain shank is no more than 3 thread pitches). A stud has no head.
• ✓Bolt thread length in the bolt standards it lists (such as AS 1110.1): 2D + 6mm for bolts up to 125mm long, 2D + 12mm over 125mm up to 200mm, and 2D + 25mm over 200mm, where D is the nominal diameter.
• ✓Shear joints, friction grip joints and combined tension/shear joints: use a bolt, so the plain shank crosses the shear planes. The shank has more shear area than the thread, and a screw is not suitable for these joints.
• ✓Tensile joints: a screw can always replace a bolt, because tensile capacity is set by the thread's stress area, which is the same for both. Swapping a screw for a bolt needs an engineer's advice.
• ✓Joints under vibration and dynamic loads (for example truck chassis or railway carriages) are fatigue critical: screws are preferred, because the longer thread makes them more ductile. Do not replace a screw with a bolt there.
• ✓Class 8.8 and lower bolts are treated as ductile; class 10.9, 12.9 and higher as brittle. A thread crossing a shear plane cuts the shear capacity of a 10.9 bolt further, because the thread acts as a notch.
• ✓EN 14399-3 HR class 10.9 bolts have bolt-length threads and should be fitted with about two threads protruding from the nut. EN 14399-4 HV bolts have shorter threads, suit more shear joint layouts, and get their ductility from the threads stripping in the nut.

Who it helps: Builders, fabricators and engineers deciding whether a set screw can stand in for a bolt in a steel connection.