Reducing Bolt Preload Variation with Angle-of-Twist Bolt Loading

Critical high-pressure sealing joints on the space shuttle reusable solid rocket motor require precise control of bolt preload to ensure proper joint function. As the reusable solid rocket motor experiences rapid internal pressurization, correct bolt preloads maintain the sealing capability and structural integrity of the hardware. The angle-of-twist process provides the right combination of preload accuracy, reliability, process control, and assembly-friendly design. It improves significantly over previous methods. The sophisticated angle-oftwist process controls have yielded answers to all discrepancies encountered while the simplicity of the root process has assured joint preload reliability. Pressurized joint component deflection controls O-ring or gasket function, and thus joint sealing capability. A tight joint will seal acceptably to lower temperatures than a looser one will. Deflection control requires precise bolt-clamping force (preload). Several methods for preloading critical fasteners have been used over the years. Design requirements, flight environments, and manufacturing concerns have driven the bolt preload to a narrower range of variation than that afforded by traditional torque methods. Angle-of-twist provides accurate preload without the need for overly complex, difficult-to-diagnose/verify, or difficult-tocontrol process alternatives. Previous bolt loading methods resulted in a preload variation of ± 35%. This variation level was too high to ensure minimum preload and avoid overloading the joint at the same time. An ultrasonic loading method was implemented in an effort to resolve the issue. The ultrasonic process on the assembly line revealed that variations of ± 35% or higher could occur on an infrequent basis. Thiokol invented the ultrastrain bolt to simultaneously measure preload with 1% strain gages. The AOT process was implemented with significantly reduced joint preload variation (± 12 % to ± 19% depending on joint type vs. ± 35% for torque). Changes in Space Shuttle launch commit criteria, the ability to tolerate higher component wear/damage, and the elimination of elusive ultrasonic discrepancy reports have resulted from the AOT process. AOT torque limits are currently being fine tuned to better reflect flight torque populations. This will reduce torque related discrepancies that do not impact preload while continuing the excellent preload performance of AOT.