Preload indicating washers
Preload indicating washers that provide an accurate method of measuring the preload (clamp force) in a bolted joint have been developed by SPS Technologies.
"Using PLI washers to achieve preload allows designers to maximise joint strength", says SPS product engineer Steve Foster. "Frequently, fewer fasteners or fasteners of a smaller diameter can be used thus reducing joint reliability. The washers also are useful with joints that are inaccessible to torque wrenches. No special tooling is required in their application."
Three types of the washers have been made available. The standard product is designed for use under lock-nuts to induce an average preload of 80% of the yield strength in bolts ranging from 80Ksi to 260Ksi minimum ultimate tensile strength. An under-the-head version is designed for use in applications where space limitations make it impossible to install an under-the-nut washer. An oversize version is for use with reworked joints.
