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Fastener inspections ensure safety on mines
Product News Wednesday, November 28, 2012: Swagefast (Pty) Ltd.
Fastener inspections are a critical part of maintenance on mines and should be regularly conducted by mine inspectors as joint failure could be catastrophic for mining operations, states fastening system solutions provider Swagefast financial director Jay Rossouw.
“No matter what the cause of fastener failure, the cost of redesign, repair, downtime, labour, injuries, legal is astronomical,” he says.
Bolt and nut systems com- monly fail because of the nut loosening owing to vibration. This condition will often lead to structural weakening and possible failure of mine structures, Rossouw explains.
He warns that fastener breakage in the shear, tensile or bending modes should not be confused with fatigue failure. Breakage takes place when the tensile strength of the fastener is exceeded owing to the improper assessment of the load, incorrect fastener selection or a manufacturing defect.
Fatigue failure is a common problem in connections that are subject to dynamic shock loading and vibration. If the correct fastener is selected at the design stage, the cause of failure is generally an incorrect installation and underachievement of clamp force or fastener tension.
“When fastener failure occurs, a failure assessment is critical to identify the cause,” says Rossouw.
Failure assessment entails ensuring the correct fastener grade and size selections in the mining structures or equipment were used, potentially redesigning or revising the connection and removing, repairing and refitting the components affected by the failure. Recommissioning of the equipment or mining structure may also be necessary.
Rossouw adds that technical installation specifications differ according to fastener type.
“Many comparisons have been made between Swagefast fasten- ing systems and nut and bolt sys- tems offered by other companies, but there are significant differ- ences in the force applied to these systems and the action of their application to fastener components during installation,” he says.
Swagefast fastening systems, such as the Swagebolt installation, are more efficient for fastening applications as sheet gap (the space in between the application material) is removed and tension is maintained, while nut and bolt systems rely on tension to create proper clamp force which leads to varying degrees of torsional and tensile stresses within the bolt.
Rossouw says the mining industry places high value on fastener inspections as it realises the cost implications of fastener failure.
The ‘turn of the nut’ procedure specified by the SANS 1282 standard is the generally accepted tightening method that com- panies employ in the absence of other direct tension indication devices.
However, Rossouw states that the procedure is open to significant operator abuse and constantly requires supervision to ensure the correct matchmaking of the nut and bolt is achieved after the correct tightening level is achieved.
“The consequence of incorrectly tensioned bolts range from vibratory loosening to bolt fatigue,” says Rossouw.
Source: Mining Weekly