Among some of the most inconspicuous and hidden pieces of the construction puzzle, nuts and bolts, and associated hardware such as various types of washers, make up the "glue" that holds everything together.Of course, for many projects, screws or even nails do just fine, but in this article we'll be sticking to the science behind the thicker shanks coupled with nuts that we call bolts.
Bolts are especially advantageous when dealing with larger lumber and steel, due to its strength, wider flexibility of length, and ability to more or less permanently fasten the joint. Aside from the obvious strength issue, the problem with screws and nails is the feared eventuality of them becoming loose over time, and jeopardizing the integrity of joints. This is especially true of joints subject to vibration and movement.
What is the Preload of a Bolt?
For the most part bolts don't face the above problem due to the ability to produce what is called a clamping load, or a preload, which if large enough, will ensure joint integrity. The preload is the force that is created when torque is applied to a bolt to fasten two or more objects together. The bolt is pulled into tension as torque is applied, while the fastened parts experience compression.
This tension, as long as it's within the elastic limit of the bolt, exerts an equal and opposite force (Hooke's Law) called the tensile stress which works to keep the bolt from loosening. Above all secondary safety measures such as lock nuts and washers, the principle of tightening the bolt down sufficiently to begin with, is the most crucial factor in preventing premature loosening.
How Will a Bolt Fail?
Bolts are manufactured in such a way that the bolt shank will fail before the threads strip. In other words, the collective strength of the thread can withstand forces greater than what would be necessary to rupture the actual bolt shank! Of course, in order for this to be true, there must be a minimum amount of bolt thread "engaging" the nut, known as minimum thread engagement.
A bolt can fail in any number of ways. When speaking of a bolt failing, one might assume it's the yielding of a bolt. But a bolt can also "fail" by becoming loose over time, thus compromising the joint it was supposed to hold together. This is perhaps the most common type of bolt failure - where the mechanical load exceeds the preload of the bolt causing it to loosen.
What is Brinelling?
This is what happens when a bolt is tightened down on a material that isn't sufficiently stiff, like wood, causing an impression. (See further below.) This can happen to a low quality washer as well, and causes a loss of preload leading to premature loosening of the bolt. When there is even one applied load that is greater than the yield limit of the material being fastened, brinelling will occur.
What about Bolting Wood?
Unlike harder materials like steel, when fastening wood with bolts, it is important to bear a few things in mind. Like screws and nails, bolts are not exempt from the consequences of movement either. Wood will compress substantially even under small preloads, and will tend to compress further with time and movement.
This is the primary consideration that must be taken into account when attempting to bolt wooden objects together. The degree of compressibility will vary, even significantly, depending on wood type, but wood is wood and it will always be enough to cause problems. There are a few common ways to counter this problem - not all of which are necessarily recommended in all cases.
Lock washers, or split washers. These are washers that are split and bent into a helical shape, putting a spring force on the two surfaces it contacts. Contrary to popular belief or use, split washers don't offer a sufficiently effective "lock" to make them safe for critical applications. The torque required to flatten a split washer is in most cases significantly less than the recommended torque for a given joint. As per Hooke's law, that states that the extension of a spring is in direct proportion to the force applied to it, we know that the bolt tightened down with the lock washer will not be able to resist a rotational force greater than the force applied to tighten it - which as you've probably noticed, isn't all that much.
Elastic stop nuts. There are various kinds of elastic stop nuts, but the concept is the same. The nut thread or some portion of the nut is made to deform elastically, creating friction, which keeps the bolted joint from coming loose. If used properly, with the right amount of applied torque, this can provide a fairly effective locking mechanism.
Thread adhesive, or thread-locking fluid. Applied to bolt threads, thread adhesive can be applied both before or after bolting. There are two kinds: removable and permanent. The removable type can be removed by simply applying heat. Note that bolt thread must be cleaned thoroughly and even primed in some cases before adhesive application.
Rather than lock washers, elastic stop nuts and tapered thread nuts are recommended for applications expecting large loads and movement, due to the above mentioned reasons. In any application where the integrity of bolted joints are critical to structural safety, locking washers of any kind as the primary safeguard against rotation are discouraged.
Other measures you can take when dealing with wooden bolted joints is minimizing movement at the joints by sufficient bracing. Vibration, movement, and worst of all, rotation at the joints, can be reduced to a minimum by transferring loads and forces away from joints and toward the foundation. This is done by properly bracing a structure.
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