I assembled an outdoor dining table on Sunday, and belatedly realized that I had used only half the washers. The unclear directions had said to place one washer next to the bolt head and the other washer at the far end, just inside the nut. I had overlooked the latter. I don’t feel like undoing twelve bolts and starting over, so it will stay that way.
It occurred to me to wonder what purpose the washer serves. I had a notion that, since it increases the apparent area of the bolt head or the nut, it must reduce the pressure on whatever is being tightened by the bolt in the immediate vicinity of the bolt’s ends.
A washer is a thin plate (typically disk-shaped) with a hole (typically in the middle) that is normally used to distribute the load of a threaded fastener, such as a screw or nut. Other uses are as a spacer, spring (belleville washer, wave washer), wear pad, preload indicating device, locking device, and to reduce vibration (rubber washer). Washers usually have an outer diameter (OD) about twice larger than their inner diameter (ID).
The Straight Dope says:
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.