64 wwwenerpaccom Bolting Theory Function of Bolts and Nuts Threaded fasteners are used across industry to assemble products ranging from pipelines to heavyduty earth movers and from cranes to bridges and many more Their principle function is to create a clamping force across the joint which is able to sustain the operating conditions without loosening Correctly tightened bolts make use of their elastic properties to work well they must behave like springs When load is applied the bolt stretches and tries to return to its original length This creates compressive force across the joint members Behavior of Bolts and Nuts Elasticity is defined in Hookes Law of physics The stress in a bolt is directly proportional to its strain The stress strain of a bolt has an elastic range and a plastic range In the elastic range Hookes Law is true All of the elongation applied within the elastic range is relieved when the load is removed The amount of elongation increases when more load is applied When a bolt is stressed beyond its proof load maximum load under which a bolt will behave in an elastic manner the elastic elongation changes to plastic deformation and the strain will no longer be proportional to the stress In the plastic deformation a part of the elongation will remain after the load is removed The point where this permanent elongation occurs is called the yield strength The further application of load takes the bolt to a point where it begins to fail this is termed its ultimate tensile strength UTS At this UTSpoint if additional force is applied to the bolt it will continue to elongate until it finally breaks The point at which the bolt breaks is called the tensile point Careful attention must be paid to the grade of bolt being used as bolt grades differ in the elastic range Hookes Law of Physics Preload The main purpose of a bolt and nut is to clamp parts together with the correct force to prevent loosening in operation The term preload refers to the loading in a bolt immediately after it has been tightened The amount of preload residual load is critical as the joint can fail if the load in the bolt is too high too low or not uniform in every bolt Uneven bolt loads can result in Some bolts being loose while others are overloaded Crushing of the gasket on one side leakage on the other side Preload is normally dictated by the joint design see Enerpac Bolt ed Joint Int egrity for information on common joint types or contact your local representative Uniform preload residual load Elastic Range T ensile P oint Str ain Plastic Range Bolt F ailure Ultima teT ensile P oint Proof Load Str ess 1 Bolt loosens due to cycle loads of vibration 2 Sealing face surface damage 3 No compression 4 Cracking 5 Flange rotation 6 Yielding of bolts 7 Over compression of gasket Over tight Under tight