Detailed methodology for determining torque limits to maximize preload for high-strength threaded fasteners.

摘要

High-strength threaded fasteners are an amazing product of engineering, and one of the most common methods of making structural attachments. They are frequently used in structural joints because of two important features: such fasteners allow disassembly and reassembly of the joint, and they can generate high clamping loads called preload.;To achieve the full benefit of threaded fasteners, the initial preload must be maximized. The benefits of high preload are well documented and include improved fatigue resistance, increased joint stiffness, and better resistance to vibration loosening. For joints in shear, preload resists relative motion, or slip, of the clamped members. Many factors, like elastic interactions and embedment, tend to lower the initial preload placed on the fastener. These factors provide additional motivation to maximize the initial preload. Also, in aerospace applications, maximized preloads help to achieve the full structural capacity from the fasteners, thereby minimizing weight. Of the available methods for controlling fastener preload, "torque control" is the most common. Unfortunately, determining the torque to maximize preload is problematic and greatly complicated by the large preload scatter generally seen with torque control.;This dissertation presents a detailed methodology for generating torque limits to maximize preload for high-strength threaded fasteners. The methodology accounts for the large scatter in preload found with torque control, and therefore, addresses the statistical challenges of dealing with preload uncertainty. In developing this methodology, the probabilities of yielding the bolt during installation were calculated. To calculate these probabilities, the expected scatter in effective stress had to be determined. The scatter (or uncertainty) in the bolt's effective stress was shown to be less than the scatter in bolt preload. This result was observed in the torque-tension test data and also predicted by propagation of uncertainty theory.;Prevailing torque is the torque required to overcome a locking feature in a fastener. The locking feature, typically added to the nut, helps to maintain preload and provide resistance to vibration loosening. Using this methodology, the prevailing torque of each locking nut is measured at installation to ensure that it is within specification. However, the measured prevailing torque is not added to the specified torque limit because its effect is already included in the torque limit value. Prevailing torque was shown to slightly reduce the achievable preload.;Finally, the methodology was developed for specific fastener combinations; defined as a specific bolt type, nut, washer, and lubricant. Since friction is known to be the main influence on preload and preload scatter, testing specific fastener combinations limits the variables that affect friction. Once a torque limit is determined for a specific fastener combination, it can be applied to any joint using that combination. Taking this approach eliminates the need and the cost of lot-specific or joint-specific testing. A wide range of torque-tension fastener testing was conducted in support of this work. The results from this research should enable design engineers and structural analysts to produce more optimized bolted joints, leading to better structural designs.