Using Bolts Correctly - Amateur
- Adam Wrigley
Last updated - July 2006

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Build Responsibly.

Bolts, also known as threaded fasteners to the engineering world, are often misused. There are two basic types of force a bolt can carry; shear, and tensile. Shear is a force across the bolt, and tensile is a force through the bolt:

The above diagram shows shear force(bad), while the diagram below shows tensile force(good).

What is commonly misunderstood, is that threaded fasteners should not be used to take shear force. Bolts used to hold on weapon teeth for example, are generally used incorrectly. Bolts are simply not designed to take that type of force. This is why you see so many bolts holding on weapons.

The way to get a bolt to hold a shear load, is to transfer the force to something else, using friction. See the example below:

At first glance, it would seem as though the bolts were taking all of the shear force. However, friction should keep the bolts from experiencing any shear at all. Bolts can take a tremendous amount of tensile force, and this should be used to the designer's advantage. By tightening the bolts in this example, a large amount of static friction will develop between the plates. This friction force will keep the plates from sliding apart, not the shear strength of the bolts. The plates in this example are NOT threaded, there should be nuts places on the opposite end of the bolts. Threads should be avoided as often as possible, as they create undue stress concentration points in the material behind held by the bolts(also note that it would not be possible to have all three of these plates threaded, as it would not allow the bolt to expand while being tightened, and the threads in the plates could be misaligned).

If you are securing weapon teeth, other methods besides bolts should be used to hold the teeth in place. Using pockets in your weapon that allow the tooth to "sit" into it are a good idea. This way, the force is directed straight from the tooth to the disc, and not through any bolts. Common weapon tooth construction is similar to the above diagram, but doesn't allow the plates to have free movement in the vertical direction. This causes no friction to be generated, and all of the shear to transfer into the bolts. If using a design like this, it should be noted that the plates must have some vertical movement allowed, so that friction can be developed (or, use a pocketed tooth design, where the tooth and weapon fit together like a puzzle).

By designing your bot to correctly use threaded fasteners you can save time and weight by not using as many(and save money by not breaking them when you do use them).

 

© 2006 by T.i. Combat Robotics unless otherwise noted.