Preload indicating washers

Anyone ever heard of a preload indicating washer?

Looks very simple. It’s a washer that crushes when the fastener reaches 80% of yield. According to Sandia Report SAND2008-0771 they are accurate to +/-10% of clamping force compared to +/-35% for a torque wrench and it directly measures clamping force unlike a torque wrench that is mostly measuring the friction being overcome by the nut heat and the threads.

It appears that there are 3 versions. One has slots where you can stick something and attempt to turn it. When it won’t turn, the preload has been reached. With the second type, you measure the gap (see if you can stick feeler gauge in it). When the gap is gone, preload has been reached. The third type loads the gaps with RTV. When the RTV squirts out of the joint, preload has been reached.

Sounds like a good, cheap solution to ensure proper clamping force at least with bolted joints. I love the fact that since it measures clamping force directly it avoids issues with damaged/galled/contaminated threads that can fool you into thinking the joint is tightened properly even with a torque wrench. They also look relatively inexpensive. But frankly I’ve never seen these in use anywhere except for critical connections on steel structures. Has anyone used these before? Care to share any experiences? CLICK to answer.