Determining Robust Friction Coefficient for Bolted Closure System

Dual purpose transport/storage casks for radioactive materials use either a welded or bolted closure system: the latter being easier for the retrieval of the content. For the bolted design, the preload of closure bolts is a key parameter in ensuring a high level of containment integrity. The most practical method to control the preload is based on setting the tightening torque. The specified torque is usually obtained with a calibrated torque wrench. Torque wrenches are easy to calibrate in the controlled areas of the loading and unloading facilities. The value of the global friction coefficient is needed to define the torque required for the adequate preload in the closure bolts. The dispersion of the friction coefficient value has a direct impact on the dispersion of the preload to be considered in the safety analysis.The friction coefficient value can be affected by several variables: the type and the nature of the lubricant, the bolts, nut and washer materials, the engaged length, the torque level… Even for a given configuration (same lubricant and materials), there is a natural dispersion of the friction coefficient. This paper describes how the dispersion of the friction coefficient can be evaluated by tests. Several sets of bolts (screws)/washers/threaded base material are needed to evaluate the dispersion for each fastening configuration. After each tightening/untightening, the assembly is degreased and regreased. The first 5 tightenings are the most significant. Depending on the torque level, during the 5 first tightenings there is a wearing effect of the surface in contact (bearing area and threads) and an associated decrease in the friction coefficient. Robust closure system design can be achieved by reducing the uncertainty on the preload by qualification tests of the friction coefficient.