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Why it works – PDF
Over 90% of all bolted joint failures can be attributed to low bolt tension on installation tightening!
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James Walker RotaBolt Ltd
Peartree Business Park
Peartree Lane
During traditional tightening, the tension being achieved is unmeasured – it is the tightening force being
Dudley
applied (torque) that is measured.
West Midlands DY2 0UW
There is no reliable correlation between the equipment tightening power and the residual bolt tension
achieved.
Achieving and maintaining the correct installed design tension will eliminate failures from fatigue, vibration
loosening, structural slip and pressure containment.
The behavior of a bolted joint, whether it is in an aircraft, car, pressure vessel or a wind turbine assembly,
is governed by the same science. Its reliability is dependent on three major parameters:
joint design
component quality
installing bolts to the design bolt tension objective
By measuring and assuring all three factors, bolted joint reliability is guaranteed. The investment in
assured design, fastener procurement and associated quality is substantial. It has one objective – to
deliver the level of bolt tension/joint compression on installation tightening that will assure joint reliability.
There is a direct relationship between installed bolt tension and the four common bolt failure mechanisms:
Pressure Containment Leaks (See diagram above)
Bolted flanged joints are used to contain internal pressure in pipeline or pressure vessels. The bolt tension
developed on tightening creates an equal and opposite compression in the joint which has to withstand the
internal pressure trying to open the flanges. This compression also develops the required gasket seating
stress enabling the gasket to seal effectively. With insufficient bolt tension and low joint compression, the
internal pressure finds a leak path.
United Kingdom
Tel:
+44 (0) 1384 214442
Fax: +44 (0) 1384 455186
sales@rotabolt.co.uk
Fatigue (See diagram above)
This joint diagram illustrates the effect of a cyclic, fatigue working load, FA, on a correctly loaded joint, bolt
tension FV. The bolt feels only a small proportion of this fatigue load, FSA. If the design is correct, FSA
should be lower than the bolts endurance fatigue limit, ±σalt
With a lower bolt tension, the bolt feels more of the fatigue loading thereby increasing the risk of fatigue
failure. If FSA exceeds ±σalt then fatigue failure will definitely occur.
Vibration Loosening (See diagram above)
Bolts can self loosen when subjected to transverse vibration. Bolt tension creates friction grip in fastener
mating surfaces under the bolt head or nut and in the male and female thread interfaces. In the diagram,
with low bolt tension, low levels of friction grip are generated in the bearing interfaces. Transverse
vibration overcomes the grip and the bolt will self loosen. The loosening process is accelerated by the
bolts internal ‘off-torque’; the bolt is like a wound spring that wants to get back to its unstressed position. At
the correct bolt tension, the developed friction grip resists the transverse shake, it cannot self loosen and
the joint remains reliable.
Structural Slip (See diagram above)
The explanation for this mechanism is the same as in friction grip. Bolt tension develops friction grip in
connecting structural members, preventing slippage under service loadings be it static or dynamic. The
bolting type used for this type of connection is commonly called High Strength Friction Grip (HSFG)
bolting. Insufficient bolt tension/friction grip increases the risk of structural slippage/failure where the
structure slips on to the bolt shank putting it in shear.
Any measuring system needs to utilise Hooke’s Law – Stress is proportional to Strain. The system needs
to measure stress or strain; bolt load or bolt extension.
Historically, direct bolt length or extension has been used by industry to control the bolt tension achieved
on tightening. Measurement techniques have involved dial gauges, depth micrometers, callipers and even
ultrasound. All these traditional methods had practical limitations in engineering environments. They also
required operator skill and were found to be time consuming, increasing the overall bolt tightening time
significantly. They have not been cost effective.
RotaBolt measurement technology is used extensively across all heavy engineering industries and is
generally regarded as ‘state of the art’ in terms of accuracy, reliability, durability and ease of use. Its tactile
feel indication requires a minimum amount of operator skill and understanding and its re-usability provides
in service monitoring for the life of the bolt.
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