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6. (Now withdrawn) Nuts with specified proof load values – Fine pitch thread [7] 7. Torsional test and minimum torques for bolts and screws with nominal diameters 1 mm to 10 mm [8] With exception to part 7, which defines test standards, the parts of this standard define properties for fasteners made of carbon steel and alloy steel.
The preferred bolt preload for structural applications should be at least 75% of the fastener's proof load [7] for the higher strength fasteners and as high as 90% of the proof load for permanent fasteners. To achieve the benefits of the preloading, the clamping force must be higher than the joint separation load.
The most common usage is to describe the load applied to a fastener as a result of its being installed, i.e., before any external loads are applied (e.g., tightening the nut on a bolt). Preload in such cases is important for several reasons.
A proof test is a form of stress test to demonstrate the fitness of a load-bearing or impact-experiencing structure. An individual proof test may apply only to the unit tested, or to its design in general for mass-produced items. Such a structure is often subjected to loads above those expected in actual use, demonstrating safety and design margin.
Multiple techniques exist for preload control to ensure that the tension in the bolt is close to the one specified in the design (some bolt-to-bolt statistical variations are inevitable): [1] torque-controlled tightening is a simple and most popular approach: the fastener is tightened until the torque limit is reached.
ASTM A325 is an ASTM International standard for heavy hex structural bolts, titled Standard Specification for Structural Bolts, Steel, Heat Treated, 120/105 ksi Minimum Tensile Strength. It defines mechanical properties for bolts that range from 1 ⁄ 2 to 1 + 1 ⁄ 2 inches (13 to 38 mm) in diameter.