Everything you need to know about washers?

Nuts and bolts work work by placing a threaded shaft under tension. This tension, or preloading, is essential to strengthen the joint and reduce bolt fatigue due to fluctuating (variable) loads, as in the case of a machine part subject to vibration. If any of the parts involved deform under load--for example, if the contact area in the threads has any creep (or "plastic" flow), or any high spots on the contact surfaces between the threads flatten out, or the bolt or shaft lengthens microscopically, or the fastened parts compress--the joint will lose some of its tension. That's why metal car parts separated by a gasket need periodic re-tightening--as the gasket shrinks, the tension on the bolt decreases, and the parts aren't held together as tightly.

Lock washers can help with this situation by applying a spring force against the nut or bolt head. Any stretching or flattening of parts is likely to be much less than the compression of the spring washer, so the fastener won't loosen much.

Wavy washers are another type of washer meant to apply spring force, but they're not widely used as the spring force tends to be relatively small. Belleville washers, often called "Belleville springs," are washers shaped like a shallow cone or dish. The shape of these washers allows them to exert a constant spring force on a joint, even if the bolt or joint has significant slip or stretch after initial tightening. Belleville washers are typically used in high-stress applications such as structural steel.

Because the lock washers I've been describing generally have smooth surfaces, some people don't consider them true "locking" washers. They think the only real lock washers are the ones that, ironically, you turned up your nose at. Star-shaped lock washers help lock parts together because the points of the star both compress and dig into the mating surfaces when tightened. Thus they help combat the other enemy of the joint, loosening of the nut or bolt due to rotation. The downside, of course, is that these washers can damage the parts they're helping hold together. Sometimes people put smooth washers between sharp star washers and the parts to try to protect the surface, but that defeats the purpose, eliminating the washer's ability to grip the mating surfaces and allowing it to exert only a small spring force on the bolt. As a compromise, sometimes people use a washer with a rough-patterned surface to provide extra friction without damaging the working surfaces.

Why are washers called that? Using the Oxford English Dictionary (OED), we found that "washer," in the sense of "a perforated annular disc or flattened ring of metal, leather, or other material placed between two surfaces subject to rotative friction, to relieve friction and prevent lateral motion and unsteadiness," is "of doubtful origin." The earliest cite in the OED is from 1346 (spelled "whashers"), but the first quote that describes their use is from 1611:

‘an iron hoope, amongst gunners called a washer, which serues to keepe the iron pin at the end of the axeltree from wearing the naue.“

Why do nuts and bolts seem to magically loosen themselves? The most common reasons include corrosion in the threads (for metal fasteners), vibration or cyclic movement of the parts causing the nut or bolt to "walk" loose, and compression of the parts. Temperature changes can cause fasteners to loosen if the coefficients of expansion for fasteners and fastened are sufficiently different. Plastic fasteners are notorious for creep and "plastic" deformation (i.e., stretching) under load, loosening the nuts. If the parts being held together are also plastic, they too can deform, further reducing what little pre-loading exists. Some plastic fasteners are sensitive to temperature change, which could lead to loosening.

Notes
1. Some engineering manuals warn not to tighten split-ring lockwashers till they're flat. For example, the NASA Fastener Design Manual says:

The lockwasher serves as a spring while the bolt is being tightened. However, the washer is normally flat by the time the bolt is fully torqued. At this time, it is equivalent to a solid flat washer, and its locking ability is nonexistent. In summary, a lockwasher of this type is useless for locking.

I've thought about this and discussed the matter with engineering colleagues, and confess I don't understand why a split-ring lockwasher is useless when flat. As far as I know, a spring's resistance is greatest when fully compressed. It's possible the manuals mean that, once a split-ring lockwasher is flat, its cut ends no longer dig into the surfaces being fastened together and thus don't prevent the nut or bolt from turning, as the star washes discussed later in this report do. But the manuals don't explicitly say that.

References
Parmley, Robert O., Standard Handbook of Fastening and Joining, 1989
Spotts, M.F., Design of Machine Elements, 1985
"washer" (definition), Oxford English Dictionary Online, accessed April 28, 2005