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Bolting theory

Function of Bolts and Nuts

Threaded fasteners are used across industry to assemble products ranging from pipelinesto heavy-dutyearth movers and from cranes and bridges and many more. Their principle function is tocreate a clampingforce 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 mustbehave like springs.When load is applied, the bolt stretches and tries to return to its original length. Thiscreates compressiveforce across the joint members.

Behavior of Bolts and NutsElasticity is defined in Hooke's Law of Physics: The

stress in a bolt is directly proportional to its strain.The stress-strain of a bolt has an elastic range anda plastic range. In this elastic range Hooke's Law istrue.

All of the elongation applied within the elastic rangeis relieved when the load is removed. The amount ofelongation increases when more load is applied.When a bolt is stressed beyond its proof load(maximum load under which a bolt will behave in anelastic manner), the elastic elongation changes toplastic deformation and the strain will no longer beproportional to 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. Thefurther application of load takes the bolt to a point where it begins to fail this is termed itsultimate tensile strength (UTS). At this UTS-point, if additional force is applied to thebolt it will continue to elongate until it finally breaks. The point at which the bolt breaks iscalled the tensile point.

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Tightening Methods

Principally there are two modes of tightening: "Uncontrolled" and "Controlled".

Uncontrolled TighteningUses equipment and/or procedures that cannot be measured. Preload is appllied to a boltand nut assembly using a hammer and spanner or other types of impact tools.

Controlled TighteningEmploys calibrated and measurable equipment, follows prescribed procedures and iscarried out by trained personnel. There are two main techniques: Torque tightening andBolt Tensioning.

1. Torque Tightening - Achieves preload in a bolt and nut assembly via the nut in acontrolled manner using a tool.2. Bolt Tensioning - Achieves preload in a bolt and nut assembly by stretching the boltaxially using a tool.

Careful attention must be paid to the grade of bolt being used as bolt grades differ in theelastic range.

Preload

The main purpose of a bolt and nut is to clamp partstogether with the correct force to prevent looseningin operation. The term preload refers to the loadingin a bolt immediately after it has been tightened.

The amount of preload (residual load) is critical asthe joint can fail if the load in the bolt is too high, toolow, or not uniform in every bolt.

Uneven bolts can result in:

Some bolts being loose while others areoverloaded. Crushing of the gasket on one side, leakage on

the other side.

Preload is normally dictated by the joint design, (seeEnerpac Bolted Joint Integrity) for information oncommon joint types or contact your localrepresentative.

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Advantages of Controlled Tightening

Known, controllable and accurate bolt loadsEmploys tooling with controllable outputs and adopts calculation to determine the requiredtool settings.

Uniformity of bolt loadingEspecially important on gasketed joints as an even and consistent compression isrequired for the gasket to be effective.

Safe operation following prescribed proceduresEliminates the dangerous activities of manual uncontrolled tightening and requires that theoperators be skilled and follow procedures.

Reduces operational time resulting in increased productivityReduces tightening time and operator fatigue by replacing manual effort with the use ofcontrolled tooling.

Reliable and repeatable resultsUsing calibrated, tested equipment, following procedures and employing skilled operatorsachieves known results consistently.

The right results first timeMany of the uncertainties surrounding in-service joint failures are removed by ensuring thecorrect assembly and tightening of the joint are carried out the first time.

Learning Center 2011 ENERPAC. All rights reserved.

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