A commonly used rule of thumb states that the thread engagement length for a screw in a tapped hole should be 1.5D, where D is the major diameter of the screw. A variation on this recommends an engagement length of 1.0D for steel, 1.5D for cast iron, and 2.0D for aluminum.
A more accurate approach is suggested in a technical bulletin from the Industrial Fastener Institute titled "Calculating Thread Strength." This bulletin explains how to determine the needed thread engagement length based on the shear strength of the tapped material.
Shear strength is a material property which is not often listed on datasheets. If you don't have data, you can assume that the shear strength is 50 percent of the ultimate tensile strength. The stress on the internal threads must not exceed this value.
To calculate the stress on the internal threads, divide the tensile load by the internal thread stripping area. Internal thread stripping areas for standard fastener sizes can be found in IFI Inch Fastener Standards, 7th Edition; or in IFI Metric Fastener Standards, 3rd Edition. For the tensile load, you can use the proof load of the screw, or, for a more conservative approach, multiply the tensile strength of the screw by its tensile stress area (also found in the IFI Fastener Standards books). The second approach ensures that the screw will always break before the threads do, no matter what.
This approach assumes that the load is divided evenly among all of the engaged threads, which isn't completely true. Also, internal thread areas in cast materials may be less than the published values due to porosity. However, this calculation is a good starting point.
Designing joints with the proper thread engagement length can help ensure that threads don't strip or yield. This can prevent parts from failing on the assembly line or in service.
Palmer is correct about Helicoils and similar products. But they only are beneficial when used on material thick enough to allow them to be installed correctly, which usually does not include sheetmetal. although it is common to put self threading screws into pierced holes in thin sheetmetal, on manyoccasions the result is an inadequate joint, only strong enough to hold correctly until the product is first used. When the hole for the screw is punched instead of being pierced the joint integrity is usually even worse.
Thread engagement in tapped holes in material much thicker than sheetmetal is a totally different case. Many folks assert that most of the load is carried by the first two threads, but they don't realize is that as soon as those threads deform just a bit that the load is then shared by quite a few more threads. Also, that conceptis most true for "avaerage quality" threads. It is not that much extra effort to produce threads that have greater engagement and can carry a larger load than standard threads. So it is a better approach to select threaded connections to be at least 2 screw diameters deep.
When I worked on cars, for 30 years on my Hot Rod cars (all daily drivers), I always thoroughly cleaned all fasteners and all threads before re-assembly.All fastener threads were properly lubricated, "Loctited", anti-seize used (exhaust fasteners), or silicon sealant used (head bolts (screws) threading into the engine block).Every fastener (screw) that went into a tapped hole was sequentially tightened using a torque wrench.The nuts were always tightened on the bolts using a torque wrench for all critical applications such as suspension and brakes.Such thoroughness meant I rarely had a problem on the road, and had much better success than other home mechanics I knew.
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