Washers are often used with threaded fastener. The washer distributes the stress around the bolted joint. Additionally, the washer reduces friction as the bolt turns. Generally, it is best to have the washer under the turning part of the fastener, either the nut or the head. An example of washer use is under the head of the crank bolt. The washer distributes the pressure on the aluminum arm, and allows the bolt to tighten fully.
So called “lockwashers” typically do not have a flat surface, but use a deformed or star shaped surface. The concept is that the washer will “bite” into the joint material and help in preventing any loosening. A “split lockwasher” applies a mild push to nut and bolt as it is compressed, which in theory helps hold the fastener. If the thread is very poorly tightened, a lockwasher may help in further loosening. However, lockwashers add nothing in terms of “holding power” to a joint that is fully tightened. The pre-load of a fully tightened bolt far exceeds the stress added by a lockwasher.
Safety-wire is thin wire that is used to hold fasteners in place. This is a useful technique to prevent damage from a bolt backing out and striking other machinery. The aircraft and automotive racing service industries have used safety-wire extensively. A bolt knocking about an engine compartment would prove damaging. The bolt head or shank is drilled with a hole. Wire is passed through the hole and routed so tension is maintained. This thin wire does not, however, keep the bolt from loosening. The purpose is simply to keep the bolt in place if it were to loosen.