| The intention of this article is to provide a basic | | | | engagement required to make sure the bolt |
| knowledge of the thread engagement between a | | | | breaks rather than strip the threads depends |
| bolt and a cut tapped hole. Thread engagement | | | | upon the strength of the material that the bolt |
| here refers to two things, Percentage, and Length | | | | and the threaded hole are made of. If a new bolt |
| of Thread Engagement. | | | | is used with a new properly graded nut for that |
| Percentage of Thread Engagement refers to the | | | | bolt, the length of thread engagement is usually |
| cross sectional amount of thread available in a | | | | less than one bolt diameter. If the bolt is screwed |
| tapped hole. It is dependant upon the drill size | | | | into a cut tapped hole with equal strength |
| used to drill the hole. The larger the drill size, the | | | | materials, a length of engagement of one bolt |
| smaller the percentage of thread engagement and | | | | diameter is normally strong enough. However, it is |
| the weaker the thread. However, the drill must be | | | | common for the bolt to be the stronger of the |
| sharp and the geometry properly ground for it to | | | | two materials. In that case, it is advised that the |
| drill the specified size. If not, it could drill an | | | | length of engagement be greater than one bolt |
| oversize hole with the end result of less | | | | diameter. Take low carbon steel for example. |
| percentage of thread engagement. Tap drill size | | | | Low carbon steel is a mild steel commonly used in |
| tables specify drill sizes for a specific percent, | | | | building manufacturing equipment. It's used for |
| 75% is common. | | | | things like machine bases and is welded together |
| Length of Thread Engagement refers to the | | | | to form the structure. The most common |
| distance a bolt extends into the threaded hole. | | | | rule-of-thumb for minimum length of thread |
| More specifically, it is the axial distance over which | | | | engagement with low carbon steel is 1 1/2 times |
| an external thread is in contact with an internal | | | | the diameter of the bolt. |
| thread. In other words, the length of full thread | | | | Accurately calculating the required length of |
| engagement. Corner breaks, chamfers, and | | | | thread engagement is a complex problem. The |
| threads not fully formed either on the bolt or the | | | | formulas for that require information related to |
| tapped hole do not count in that length. | | | | the strength of the material of threads that meet |
| One of the main guidelines in designing fastener | | | | proper thread class specifications. In the case of |
| joints is that the length of engagement should be | | | | older equipment that has been maintained multiple |
| long enough so that the bolt will break rather than | | | | times, there isn't always optimum thread quality |
| strip the threads in the tapped hole if the joint is | | | | and the strength of the material that the threads |
| overloaded. It's generally easier and more cost | | | | are cut into may be an unknown. In the case of |
| effective to replace a broken bolt than to have | | | | the unknown, a common rule-of-thumb may be |
| to repair or replace the part with the tapped hole. | | | | worth considering. After all, would it still be |
| As long as the threads meet all of the proper | | | | common in these modern times if it didn't work? |
| thread class specifications, the length of | | | | |