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.
When one is tightening bolts on a car, there are torque specs in most cases. Thus one can use a torque wrench to ensure that you're meeting the spec and not putting undue stress on the bolt-plus-nut assembly (and also not cracking the metal parts that bolt and nut are clamping together). Of course we all know that in many cases, in repairs and particularly with home mechanics, bolts are just tightened and the "spec" is just done by eyeballing it (i.e., no torque wrench used). So my question is, is there any analogy for screws? In other words, how to you ensure a screw is tightened properly but not overtightened?
Dave, thanks for letting us get inside the head of a failure analysis specialist. Much like detective work, it's fun to get in on the thought process as you follow the trail to the end resolution. And with this example, it shows you really do have to sweat the small stuff.
The legacy endpoint devices that control our critical infrastructure (utility systems, water treatment plants, military networks, industrial control systems, etc.) are some of the most vulnerable devices on the Internet.
In a switched-capacitor filter, capacitors and switches take the place of resistors and accurately reproduce the characteristics of continuous-time Bessel, Butterworth, and elliptical filters.
From Dell / Intel® New Paradigms in Design Work Scott Hamilton, vertical market strategist for Dell Precision workstations, 5/2/2013 5
Early in my career, I worked as a draftsman and remember the days of drawing on vellum with numbered pencils and Mylar with plastic lead. This was a fun experience in the sense that I ...
I've been using workstations for more than 10 years and love finding ways to get more performance from my system. With demanding professional applications that require more power each ...
A lasting memory from my first job as an engineer in an auto assembly plant is standing on hard concrete at six in the morning, vending-machine coffee clutched in hand, listening to ...
For industrial control applications, or even a simple assembly line, that machine can go almost 24/7 without a break. But what happens when the task is a little more complex? That’s where the “smart” machine would come in. The smart machine is one that has some simple (or complex in some cases) processing capability to be able to adapt to changing conditions. Such machines are suited for a host of applications, including automotive, aerospace, defense, medical, computers and electronics, telecommunications, consumer goods, and so on. This radio show will show what’s possible with smart machines, and what tradeoffs need to be made to implement such a solution.
To save this item to your list of favorite Design News content so you can find it later in your Profile page, click the "Save It" button next to the item.
If you found this interesting or useful, please use the links to the services below to share it with other readers. You will need a free account with each service to share an item via that service.
Tweet This
6 
