Bolt Shank Diameter and Thread Diameter

Introduction:
Bolts are commonly used fasteners for joining two or more components together. The shank diameter of a bolt is a critical dimension that determines the strength of the fastener. There is a relationship between the shank diameter and the thread diameter of a bolt.

Thread Diameter:
The thread diameter of a bolt is the major diameter of the external threads. It is the largest diameter of the thread that is measured across the crests of the thread. The thread diameter is used to determine the size of the nut that will be used to tighten the bolt.

Shank Diameter:
The shank diameter of a bolt is the diameter of the unthreaded portion of the bolt between the head and the threaded portion. It is the portion of the bolt that is subjected to shear and tension forces.

Uniform Strength:
In bolts of uniform strength, the shank diameter is made equal to the minor diameter of the threads. The minor diameter of the threads is the smallest diameter of the thread that is measured across the roots of the thread. This ensures that the bolt will fail in tension before it fails in shear.

Why Minor Diameter?
The minor diameter of the threads is chosen because it represents the smallest cross-sectional area of the bolt. This means that the bolt will fail in tension at this point before it fails in shear. This is important because a failure in tension is less catastrophic than a failure in shear. In tension, the bolt will stretch and deform before it breaks, whereas in shear, the bolt will fail suddenly and without warning.

Conclusion:
In summary, the shank diameter of a bolt is made equal to the minor diameter of the threads in bolts of uniform strength. This ensures that the bolt will fail in tension before it fails in shear, which is important for safety reasons.