Strength of Threads
Two fundamentals must be considered when designing a threaded connection.
- Ensure that these threaded fasteners were manufactured to some current ASTM, ANSI, U.S. Government or other trusted standard.
- Design bolts to break in tension prior to the female and/or male threads stripping. A broken bolt is an obvious failure. It’s loose. However, when the threads strip prior to the bolt breaking, we may not notice the failure until after the fastener is put into service.
As was shown on page 1, the strength of bolts loaded in tension can be easily determined by the ultimate tensile strength. To determine the amount of force required to break a bolt, we multiply its ultimate tensile strength by its tensile stress area, As. Determining the strength of the threads is more complicated. Since the male threads pull past the female threads, or vice-versa, the threads fail in shear and not in tension. Therefore, the stripping strength depends on the shear strength of the nut and bolt materials.
To determine the force required to strip the threads we must multiply the shear strength by the cross sectional area, which must be sheared. Determining the cross sectional area in which the shear occurs is a problem. Here are three possible alternatives for the failure.
- The nut material is stronger than the bolt material. In this example, the nut threads will wipe out the bolt threads. The failure will occur at the root of the bolt threads.
- The bolt material is stronger than the nut material. With this example, the bolt threads will tear out the nut threads. The failure will occur at the root of the nut threads.
- The nut and bolt are the same material. With this example, both threads will strip simultaneously. This failure will occur at the pitch line.
The tensile strength of most fasteners is a common specification whereas shear strength is not. In order to avoid shearing in the threads, we must insure that the length of engagement between the bolt and nut, or tapped hole, is long enough to provide adequate cross-sectional thread area. The typical failure for both alternative #1 and #2 would be a tensile failure of the bolt provided proper engagement.
With conventional steel nut and bolt materials, a length of engagement of about one nominal diameter of the bolt is typical. A longer thread length engagement will be needed when dealing with tapped holes in soft material.