Bolted joint

Careful consideration is given to many factors such as temperature, corrosion, vibration, fatigue, and initial preload.

[2] These joints are not tightened to a precise preload, and the tension is mainly used to keep parts together without generating a high clamping force.

This ensures that any applied tensile loads are distributed between the bolt and the clamped parts (see theory section), which has some advantages: The joint should be designed so that the preload always exceeds the external tensile load to prevent separation.

Relative motion between the clamped parts is prevented and thus any fretting wear that could result in the development of fatigue cracks.

These joints are not tightened to a precise preload, and the tension is mainly used to keep parts together without generating a high clamping force.

As bearing type joints rely on this direct contact, they are more susceptible to wear and deformation of the bolt holes under high or repeated loads, which can lead to bolt fatigue or elongation of the holes over time.

Any tensile loads applied to these joints, usually have a secondary effect, but they can reduce available shear strength of the bolt.

The load carried by the bolt and the clamped parts is in proportion to their stiffness because they both experience the same induced strain.

The portion of an external load carried by the bolt is the joint stiffness ratio

The accompanying graph and table illustrate how the relative stiffness of the clamped parts and the bolt affects the portion of applied load carried by it.

Engineered joints require the torque to be chosen to provide the correct tension preload.

[7] The required torque value for a particular fastener application may be quoted in the published standard document, defined by the manufacturer or calculated.

= 0.15, the dimensions used correspond to any size coarse or fine bolt, and the nut factor is K ≈ 0.20, the torque/preload relationship becomes: A study of the effect of torquing two samples, one lubricated and the other unlubricated, 1/2 in.- 20 UNF bolts to 800 lb-in, produced the same mean preload of 7700 lbf.

To achieve the benefits of the preloading, the clamping force must be higher than the joint separation load.

Friction in the threads and under the nut or bolt head uses up some fraction of the applied torque.

Finally, dynamic friction prevails and the torque is distributed in a 50/40/10 % manner as the bolt is tensioned.

If one has a larger bearing area or coefficient of friction it will require more torque to provide the same target preload.

More accurate methods for determining the preload rely on defining or measuring the screw extension from the nut.

Alternatively, measurement of the angular rotation of the nut can serve as the basis for defining screw extension based on the fastener's thread pitch.

[11] Measuring the screw extension directly allows the clamping force to be very accurately calculated.

This can be achieved using a dial test indicator, reading deflection at the fastener tail, using a strain gauge, or ultrasonic length measurement.

Under some circumstances, a skilled operator can feel the drop off of the work required to turn the torque wrench as the material of the bolt begins to yield.

All methods, from the least to most accurate, involve first relaxing the fastener, then applying force to it and quantifying the resultant amount of elongation achieved.

Recent technological developments have enabled tensions to be established (± 1%) by using ultrasonic testing.

Large-volume users (such as auto makers) frequently use computer-controlled nut drivers.

For instance, the FAA has determined that in general cases, at least one thread must be protruding from any bolted connection.

[1] When doing a failure mode analysis for bolts that have broken, come loose or corroded, careful consideration must be given to the below failure modes: Bolted joints may be used intentionally as sacrificial parts, which are intended to fail before other parts, as in a shear pin.

Bolt banging occurs in buildings when bolted joints slip into "bearing under load", thus causing a loud and potentially frightening noise resembling a rifle shot that is not, however, of structural significance and does not pose any threat to occupants.

When the lateral force becomes sufficient to overcome this friction, the clamped elements move until the sides of the holes bear against the shank of the bolt.

This movement – "slip into bearing" – usually starts and stops very suddenly, often releasing elastic energy in the associated elements, resulting in a loud but harmless bang.