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Thread Engagement
The intention of this article is to provide a very basic knowledge of thread engagement. It's focused on the thread engagement between a bolt and a cut tapped hole with the standard steel bolts most commonly used on manufacturing machinery. Thread engagement here referrs to two things, percentage of thread engagement, and length of thread engagement.
Percentage of Thread Engagement
Percentage of thread engagement in a tapped hole is dependant upon the drill size used. The larger the drill size, the smaller the percentage of thread engagement and the weaker the thread. The Inch & Metric Tap Drill Size Table presented on this website specifies drill sizes for approximately 75% of thread
engagement. The drill must be sharp and the geometry must be properly ground or it will probably drill a larger hole than specified. This will result in less percentage of thread engagement. This may or may not be a problem, according to the Machinery's Handbook, there is minimal decrease in thread strength when utilizing 65% thread engagement versus 75% thread engagement.
Length of Thread Engagement
Length of thread engagement referrs to the distance a bolt extends into the threaded hole. One of the main guidelines in designing fastener joints is that the length of thread engagement should be long enough so that the bolt will break rather than strip the threads in the tapped hole if the joint is overloaded. It's generally
easier and more cost effective to replace a broken bolt than to have to repair or replace the part with the tapped hole. When we referr to length of engagement here, we are referring to length of full engagement. Corner breaks, chamfers, and threads not fully formed do not count in that length.
As long as the percentage of thread engagement is acceptable, the length of engagement required to make sure the screw breaks rather than strip the threads depends upon the strength of the material that the bolt and the threaded hole are made of. If a new bolt is used with a new properly graded nut for that bolt, the length of thread engagement is usually less than one bolt diameter. If the bolt is screwed into a cut tapped hole with equal strength materials, a length of engagement to a depth of one bolt diameter is normally more than strong enough. However, it is common for the bolt to be the stronger of the two materials. In that case, it is advised that the length of engagement be greater than one bolt diameter. One of the arguments for that is we aren't always dealing with optimum thread quality as in the case of older equipment being maintained multiple times.
Accurately calculating the required thread engagement length is a complex problem. There are a number of different formulas for that that can be found by a web search. The following length of thread engagement values are some rules-of-thumb that are being used in the manufacturing industry but, it is the user's responsibility to verify that the provided information is appropriate for their application before using it.
Gray Cast Iron
Gray Cast Iron is a common material used on machine tools. It is not ductile, it is brittle. Theoretically all the threads in engagement should share the load. In actuality, due to inaccuracies in thread spacing associated with how ductile the materials are, the first pair of threads take the greatest share of the load. Since gray cast iron is brittle, if the first thread breaks then that load is passed on the the next thread and so on. Commonly, a minimum of 1 1/2 times the bolt diameter is recommended for length of full thread engagement. If one is going to torque down a Socket Head Cap Screw according to the torque tables presented on this site, consider using 2 times the bolt diameter for the minimum. In other words, a 12mm bolt would have 24mm of full thread engagement. If that much engagement cannot be achieved, reducing the applied torque might also be considered depending upon the quailty of the threads. We can easily use new bolts but the thread quality of the tapped hole may not be up to specs anymore on older equipment.
Low Carbon Steel
Low Carbon Steel is another common material used on manufacturing equipment. It is used for things like machine bases and is welded together to form the structure. The most common rule-of-thumb for minimum length of thread engagement with low carbon steel is 1 1/2 times the diameter of the bolt. One of the mild steels in use is 1020. 1020 can have a yield strength as low as 30,000 psi or 207 Mpa depending upon the manufacturer. If the strength of the steel is unknown, consider using 2 times the diameter under the same conditions as mentioned above with Gray Cast Iron.
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