Note 1: The values given are 50 and 75 percent of theoretical yield strength of a bolt material with a yield of 120 ksi. This value is usually 75 percent of theoretical yield, to prevent inadvertent yielding of the fastener through torque measurement inaccuracies. Since the exact yield torque cannot be determined without extensive instrumentation, it is not advisable to torque close to the bolt yield point.įastener proof load is sometimes listed in the literature. The higher load will increase the vibration resistance of the bolt, but the bolt will yield and unload if its yield point is inadvertently exceeded. Higher torques (up to theoretical yield) are sometimes used for bolts that cannot be locked to resist vibration. (A similar table was published in Product Engineering by Arthur Korn around 1944.) The torque values in table V have been calculated as noted in the footnotes, by using formulas from reference 3. Running torque should be included in the values listed in the tables because any torque puts shear load on the bolt. 2 gives a variety of friction coefficients.) Since calculated torque values are a function of the friction coefficients between mating threads and between the bolthead or nut and its mating surface, it is vitally important that the torque table values used are adjusted to reflect any differences in friction coefficients between those used to calculate the table and the user's values.
The coefficient of friction can vary from 0.04 to 1.10, depending on the materials and the lubricants being used between mating materials. Whether "running torque" for a locking device should be added to the normal torque Interaction formulas to be used for combining simultaneous shear and tension loads on a bolt (Should friction loads due to bolt clamping action be included in the interaction calculations?) Relative spring rates of the structure and the bolts Once agreement is reached on item 4, how to accurately determine this value The percentage of bolt tensile strength to be used for preload The effect of bolt coatings and lubricants on the friction coefficients The coefficient of friction between the bolthead (or nut) and its mating surface The coefficient of friction between mating threads Some of the many variables causing problems are Determining the proper torque for a fastener is the biggest problem in fastener installation.
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