Machinery's Handbook, 31st Edition
TORQUE AND TENSION IN FASTENERS Table 1. Coefficients of Friction of Bolts and Nuts
1657
Bolt/Nut Materials
Coefficient of Friction, μ ± 20%
Lubricant
Graphite in petrolatum or oil Molybdenum disulfide grease
0.07 0.11 0.15 0.12 0.17 0.15 0.14
Steel a
Machine oil None added None added None added None added
Steel, a cadmium-plated Steel, a zinc-plated Corrosion-resistant steel or nickel-base alloys/silver- plated materials Steel a /bronze
Titanium/steel a
Graphite in petrolatum Molybdenum disulfide grease
0.08 0.10
Titanium
a “Steel” includes carbon and low-alloy steels but not corrosion-resistant steels. Where two materials are separated by a slash (/), either may be the bolt material; the other is the nut material. Preload Relaxation.— Local yielding, due to excess bearing stress under nuts and bolt heads (caused by high local spots, rough surface finish, and lack of perfect squareness of bolt and nut bearing surfaces), may result in preload relaxation after preloads are first applied to a bolt. Bolt tension also may be unevenly distributed over the threads in a joint, so thread deformation may occur, causing the load to be redistributed more evenly over the threaded length. Preload relaxation occurs over a period of minutes to hours after the application of the preload, so retightening after several minutes to several days may be required. As a general rule, an allowance for loss of preload of about 10 percent may be made when designing a joint. Increasing the resilience of a joint will make it more resistant to local yielding, that is, there will be less loss of preload due to yielding. When practical, a joint-length to bolt- diameter ratio of 4 or more is recommended; for example, a 1 ∕ 4 -inch (6.35 mm) bolt and a 1-inch (25.4 mm) or greater joint length. Through bolts, far-side tapped holes, spacers, and washers can be used in the joint design to improve the joint-length to bolt-diameter ratio. Over an extended period of time, preload may be reduced or completely lost due to vibration; temperature cycling, including changes in ambient temperature; creep; joint load; and other factors. An increase in the initial bolt preload or the use of thread- locking methods that prevent relative motion of the joint may reduce the problem of preload relaxation due to vibration and temperature cycling. Creep is generally a high- temperature effect, although some loss of bolt tension can be expected even at normal temperatures. Harder materials and creep-resistant materials should be considered if creep is a problem or high-temperature service of the joint is expected. Mechanical properties of fastener materials vary significantly with temperature, and allowance must be made for these changes when ambient temperatures range beyond 30 to 200 ° F ( - 1 to 93°C). Mechanical properties that may change include tensile strength, yield strength, and modulus of elasticity. Where bolts and flange materials are generically dis similar, such as carbon steel and corrosion-resistant steel, or steel and brass, differences in thermal expansion that might cause preload to increase or decrease must be taken into consideration. Methods of Applying and Measuring Preload.— Depending on the tightening method, the accuracy of preload application may vary up to 25 percent or more. Care must be taken to maintain the calibration of torque and load indicators. Allowance should be made for uncertainties in bolt load to prevent overstressing the bolts or failing to obtain sufficient preload. The method of tensioning should be based on required accuracy and relative costs.
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