Case History 6
Girth Gear Failure Averted
An Owner in Central Asia had engaged Hevī's participation on a
Girth Gear replacement project. Hydraulic torque wrenches were
used by the project's Mechanical Contractor to tighten the
fasteners. Hevī's Technicians monitored the clamp loads as the
contractor "torqued" the bolts. Most fasteners reached the
specified bolt stress at a hydraulic pump pressure of between
4,000 and 4,900 psi (variations caused by frictional differences
and bolt interactions). However, three critical bolts didn't
achieve even minimal clamp load although the torque wrench
"maxed-out" at 10,000 psi. As a result of Hevī's intervention, these
galled fasteners were identified and replaced.
To help understand this significance of this, it's important to understand that "Torque" is not a measurement of a bolt's "tightness". Torque is only a measurement of the resistance encountered when trying to turn a bolt or nut. Torque specifications are very misleading and thus quite dangerous. "Calibrated" torque tools, insidiously contribute to this false sense of security.
Had Hevī not been involved to monitor and control the actual applied and residual bolt stresses, these damaged fasteners would have gone unnoticed; the Mechanical Contractor would have simply "torqued" each fastener to the same specific torque value. This would have resulted in many bolts not being tightened as per the designer's intention and three of the bolts being seriously under-tightened. The latter would have likely failed during service with a consequent forced shutdown. In the extreme, serious gear and pinion failure could have also occurred.
To help understand this significance of this, it's important to understand that "Torque" is not a measurement of a bolt's "tightness". Torque is only a measurement of the resistance encountered when trying to turn a bolt or nut. Torque specifications are very misleading and thus quite dangerous. "Calibrated" torque tools, insidiously contribute to this false sense of security.
Had Hevī not been involved to monitor and control the actual applied and residual bolt stresses, these damaged fasteners would have gone unnoticed; the Mechanical Contractor would have simply "torqued" each fastener to the same specific torque value. This would have resulted in many bolts not being tightened as per the designer's intention and three of the bolts being seriously under-tightened. The latter would have likely failed during service with a consequent forced shutdown. In the extreme, serious gear and pinion failure could have also occurred.
