Bolt tightening and tensioning - how it works

8/22/2019 Bolt tightening and tensioning - how it works

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Introduction - How a RotaBolt works.

The sole function of bolts is to clamp or, less frequently, pin joint members together. The

behaviour and life of that joint usually depends as much, if not more, on the correctness of

the clamping force (or tension) holding the parts together than on any other design factor,

often outweighing the importance of such factors as the selection of joint materials, joint

dimensions, tolerances, finish, etc ..

Introduction to Design and Behaviour of Bolted Joints by John H. Bickford.

Achieving and maintaining the correct clamping force, or bolt tension, is essential in

assuring bolted joint integrity. 85%-90% of all bolted joint failures are caused by

insufficient bolt tension at installation. When a standard bolt is traditionally tightened,the tension being achieved is completely unknown because it is the tightening force

(torque) that is being measured, not tension. RotaBolt fasteners are measuring devices

highly accurate indicators of clamping force or tension.

Safety

Throughout all bolt assembly and tightening processes it is essential to follow standard

safety procedures and instructions on any equipment. Protective safety clothing should be

worn at all times. See morehere.
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Pre-Assembly

Pictured are 8 RotaBolt all thread studs. 1.1/8 diameter by 8 long, 16 nuts and washers.

The studs were calibrated at manufacture with 12 tonnes pre-load.

NB: RotaBolt studs have been used rather than RotaBolts to give a detailed illustration ofcorrect stud assembly and tightening. When using either RotaBolts 1, 2 or the RotaBolt

Vision, the assembly process would be simplified. (seeRotaBolt products)

The Flange is an ASME B16.5 series.
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Assembly of RotaBolt Studs

A washer is put over the 'top' of the first stud and a nut is threaded on leaving three threads

visible above the nut. This is the standard height setting for a RotaBolt stud.

NB: If the washer being used has a raised face on one side, it should be positioned with the

raised face towards the nut.

The assembly of stud, nut and washer is inserted into the first flange hole. This is repeated for each RotaBolt stud assembly. A washer and nut is now threaded onto the bottom of the first stud and hand

tightened.

This is repeated for each RotaBolt stud.


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Checking Studs

Before the preliminary and main tightening cycles, each stud is checked again to see that 3

threads are showing above the top of each nut

The height can be adjusted by loosening both nuts and turning the stud until the correct

height is achieved, before re-tightening by hand.

An alternative option is to check by measuring. From the base of the nut to the top of the

stud should be 1.3/8. This represents 1 for the nut and 3/8 for three threads (1 thread =

1/8).

Numbering Studs

To ensure that the flange is tightened properly, the studs have to be tightened in a set

diagonal sequence.

Each stud is numbered to reflect the order of the sequence to be followed.

Each stud number is then chalked onto the outward face of the flange where it can be seen

easily.


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Calculating Tightening

It is the air gap technology built-in to each fastener that will provide the accurate

measurement during the tightening process.

The fastener will measure the tension (or clamping force) being achieved across the joint.

Each RotaBolt stud has been load test calibrated at manufacture with a pre-load of 12

tonnes to achieve the desired tension across this particular flange.

It will be when the RotaCap on top of each RotaBolt stud locks that the correct tension is

achieved. However, it is still necessary to estimate the tightening force (torque) that needs

to be used, so that the tightening tool selected has sufficient torque power capacity and for

any pre-set requirement.

Preliminary Tightening

To uniformly bed-in the RotaBolt studs, a manual torque wrench (in this case set to 40

Newton Metres) is used to tighten the first stud in the sequence, with a spanner applied tothe bottom nut to prevent turning.


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The same tightening is then done to each stud following the correct numbered sequence.

Main Tightening Cycles

For this flange assembly, two tightening cycles were completed using a hydraulic torquewrench.

In the first cycle the torque wrench was set to 75% of the desired torque and the studs

tightened in the correct sequence.

In the second tightening cycle, the hydraulic wrench was set to 100% of the estimated

torque. Each stud was tightened in number sequence until the RotaCap on top of each stud

locked, indicating that the correct tension had been achieved.

Contacting RotaboltJames Walker RotaBolt Ltd

Peartree Business Park

Peartree Lane

Dudley

West Midlands DY2 0UW

United Kingdom

Tel: +44 (0) 1384 214442

Fax: +44 (0) 1384 455186

[email protected]

http://www.rotabolt.co.uk
mailto:[email protected]:[email protected]://www.rotabolt.co.uk/http://www.rotabolt.co.uk/http://www.rotabolt.co.uk/mailto:[email protected]

Bolt tightening and tensioning - how it works

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