Keeping It Together With Bolted Joints

It's not uncommon for thousands of dollars worth of equipment to be held together by $.10 screws. Because of their low cost and high degree of standardization, screws, studs, bolts, and nuts tend to be thought of as commodity products. As a result, bolted joints too often fail to receive the level of attention they deserve in engineering design. This can lead to unfortunate consequences: if one of those $.10 screws happens to break, thousands of dollars of equipment may be rendered inoperable, and lives even may be put at risk.

First, a few notes on terminology. A bolt or screw is an externally threaded fastener with a head. The head may either be externally driven (for example, a hex head or flange head) or internally driven (for example, a socket head or button head). What's the difference between a bolt and a screw? In the old days, the word "screw" was used for fasteners that were fully threaded along their length, while fasteners that were partially threaded were called "bolts." In current usage, as defined by the Industrial Fastener Institute, a bolt threads into a nut, while a screw threads into a tapped hole. This means that the same fastener can be considered either a bolt or a screw, depending on how it's used. A stud is an externally threaded fastener without a head; in its simplest form, a threaded rod. One end of the stud usually threads into a tapped hole, while the other end threads into a nut. A joint held together by threaded fasteners is called a "bolted joint," regardless of whether the actual fasteners used are bolts, screws, or studs. In this article, I'll use the word "bolt" as a generic term to refer to externally threaded fasteners in general.

How do bolted joints work? Tightening the bolt by applying a torque causes it to stretch elastically, like a spring or rubber band. The tension force in the bolt (usually called "pre-load" or "clamp load") is balanced by an equal and opposite compressive force in the joint. These forces keep the surfaces being joined from being separated by an external load. Unless the applied load exceeds the bolt tension, the surfaces will be held tightly together.

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The amount of clamp load generated for a given amount of stretch depends on the bolt's stiffness, which, for bolts made out of the same material, is proportional to the square of the bolt's diameter. For example, a half-inch bolt will have about four times the amount of clamp load as a quarter-inch bolt with the same amount of stretch.

Since clamp load is only produced by elastic stretching of the bolt, stretching the bolt beyond the point at which it begins to permanently deform won't produce any additional clamp load. Therefore, the maximum amount of clamp load a given bolt can provide is limited by its yield strength. In the fastener world, this is sometimes referred to as "proof load." In previous times, it was tested by measuring the length of a bolt before and after pulling on it with a certain amount of force. If the length was the same before and after, this was proof that the strength of the bolt was adequate (hence the term "proof load"). These days, it's more commonly tested by performing a tensile test on the bolt. The yield strength is then determined from the stress-strain curve.

There are multiple strength grades available for most fasteners. For example, Grade 5 bolts have minimum yield strength of 85,000 lb per square inch; Grade 8 bolts have minimum yield strength of 120,000 lb per square inch. (Don't mistake metric Grade 8.8 bolts with Grade 8 bolts; metric Grade 8.8 is approximately equivalent to Grade 5, while metric Grade 10.9 is approximately equivalent to Grade 8.) In order to determine the proof load for a given diameter of fastener, multiply the minimum yield strength by the tensile stress area, which can either be calculated or simply looked up on a chart.

How much torque does it take to achieve a given clamp load? This depends on two things: the diameter of the fastener and the amount of friction. As much as 80% of the tightening torque is used up overcoming the friction underneath the head and in the threads of the fastener. The friction coefficient depends on lubrication and bolt finish (for example, plain, black oxide, or zinc-plated), as well as the material and condition of the mating threads. If you replace a zinc-plated bolt with a stainless-steel bolt, don't expect to achieve the same clamp load with the same torque. If you put anti-seize on a bolt and try to install it with the same torque you used on a dry bolt, don't be surprise if the bolt breaks!

The best way to determine the correct torque to use in a given bolted joint is by directly measuring the clamp load. This can be done using a strain-gaged washer or a strain-gaged bolt. A strain gage is a variable resistor, the resistance of which decreases with tension and increases with compression. However, installing a strain gage inside a bolt requires you to drill a hole down the centerline of the bolt, which may significantly affect its stiffness, especially for relatively small bolts. Clamp load can also be measured using an ultrasonic bolt tension monitor. These devices measure the elongation of a bolt by sending an ultrasonic pulse along its length.

There's much more that could be said on the topic of bolted joints; for example, in an earlier Design News post, I wrote about the important topic of thread engagement. While bolted joints tend to attract little attention in universities (with the exception of Oakland University's Fastening and Joining Research Institute), they are essential to the safety and reliability of nearly all engineered products. Design engineers ignore the basics of bolted joints at their own risk.