Approximately 50% of the torque applied when tightening a bolt is absorbed by under-head friction where the mating surfaces of a nut and flange are being forced together. This makes the interface between the nut and the flange a crucial bearing surface and means that washers play a vital role in accurate tightening of bolted[…]
‘Bolt failure’ – a phrase often accompanied by images of twisted metal, close-ups of corrosion or stress cracks and text throwing suspicion on sub-standard materials, design shortcuts and construction incompetence – but things are rarely as simple as they may seem… Yes, these elements have played their part in numerous failures but even a perfect[…]
Selecting identical bolts and then applying the same tightening load to each one sounds like a sensible approach to creating a safe bolted joint – but things are not as simple as they may seem… Material and manufacturing differences between apparently identical bolts can cause large differences in the bolt tension achieved on installation because[…]
The issue of low elastic strength in stainless steels and non-ferrous alloys has raised its head several times, none more so than with B8 austenitic stainless steel. Data from the British Stainless Steel Advisory Service indicated that for structural purposes designers need to estimate a bolt capacity based on 59% of the specified 0.2% proof[…]
When a bolt yields, it will plastically/permanently deform. The bolt’s effectiveness as a spring clamp is impaired. Apart from being closer to tensile failure there is a resultant loss in tension/clamp force in the joint. The yield point is the bolt’s operational capacity and engineers must ensure that the design tension on installation and in[…]
A previous RotaBlog looked at the effect of torsional stresses during bolt tightening. One school of thought suggested that these combined stresses actually limit or even prevent utilisation of the full elastic strength of the fastener at installation. The yield strength in combined stresses is lower than uni-axial tensile yield. The research referenced clearly shows[…]
The pre-requisite for high temperature measurement of bolt loads in service is measurement and control on installation tightening under ambient conditions. Bolted joint reliability depends on assuring the joint design is correct, that all joint component quality is assured and then the design bolt tension/joint compression/gasket seating stress is assured on installation tightening. If all[…]
If you ask for clarification when you hear people say “ torsional stresses generated during tightening are dangerous to the well being of the fastener” what they usually say is that the combined tensile and shear forces lower the effective yield strength compared to the pure tensile yield, and this drop in strength is what[…]