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NASA administrators traditionally get unhappy when they have to send an astronaut up to tighten a loose screw on a satellite. For much less than the cost of a single manned rocket launch, they commissioned a study on fastener reliability, and made it available to the public as NASA Reference Publication 1228. Here is an excerpt:

The typical helical spring washer shown in figure 14 is made of slightly trapezoidal wire formed into a helix of one coil so that the free height is approximately twice the thickness of the washer cross section. They are usually made of hardened carbon steel, but they are also available in aluminum, silicon, bronze, phosphor-bronze, stainless steel, and K-Monel.

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

You don’t have to trust just the fancypants academics at NASA, though. An unsmiling German guy named Gerhard Junker invented the Junker test to evaluate just how useless split ring lockwashers are, and that was back in the 1960s. Junker took a fastener, held two parts together with the fastener, and shook the assembly until it came loose. The results of the first Junker tests were that split ring lockwashers loosened about twice as fast as if they were not present. In other words, split ring lockwashers actively sabotage your assembly:

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