FIGURE 14.20 A helical spring lock washer would appear to be a fairly inefficient way to resist selfloosening;
but recent research—described in the text—shows that this washer twists and rolls when it’s
fully loaded. Since that requires a clamping force that can equal 65% of the proof load of the bolt, this
washer can, indeed, provide significant resistance to self-loosening.
14.6.7.3 Helical Spring Washer
At first glance the helical spring washer shown in Figure 14.20 would appear to be of as little
value as the wave washer; unless the cut ends manage to bite into joint and nut and resist
relative motion the way the toothed washer does. Research at the Lawrence Technological
University in Southfield,Michigan, however, suggests that this device is more effective than it
appears to be [22]. Dr. Clarence Chambers has shown that, while this washer is flattened by
bolt tension equal to only 5% of its proof load, increasing bolt preload to 70% of proof will
cause the trapezoidal cross section of the washer to roll and twist down on the outside
diameter, which also grows. This complex action results in a washer spring rate which can
approach 65% of the spring rate of the fastener. That spring rate will dominate the behavior
of the fastener under load, and will reduce the amount of preload lost under a given applied
load. Retaining preload, of course, is an effective way to resist self-loosening.
