08/08/2026
Joint Performance Depends on Maintaining Preload Throughout a Structure's Service Life
A bolted connection carries its design load only for as long as it retains its preload. Once that clamping force begins to fall away, the joint behaves differently to how it was specified, long before any visible sign of movement appears.
Three distinct mechanisms drive that fall in preload, and each calls for a different response.
The first is vibration-induced loosening. Cyclic loading, from traffic, machinery, or wind, allows the nut to rotate incrementally against the thread until clamping force drops below the level the joint was designed around. Avlock International's Lockbolt system addresses this at the point of fastener selection. A pin and collar arrangement, with the collar swaged into annular grooves on the pin rather than threaded, removes the rotational element that vibration acts on. It cannot back off the way a nut can.
The second is corrosion-assisted loss. Degradation at the contact surfaces reduces the effective clamping area over time, and with it the load the joint can hold. Several of Avlock's fastener lines are built with this in mind: fully sealed breakstem rivets, and one-piece aluminium alloy fasteners with a water-resistant neoprene seal, engineered to keep moisture out of the joint interface rather than manage its effects afterwards.
The third is thermal relaxation. Repeated expansion and contraction across temperature cycles alters clamping force independent of any vibration, and it behaves differently to the other two. It cannot be engineered out through fastener choice alone, in the way vibration and corrosion resistance can. The established response is a Belleville, or disc spring, washer fitted under the bolt head or nut, which stores mechanical energy and continues to push back on the joint as it tries to relax under heat. A bolt in a hot application can lose a large share of its clamping force to thermal relaxation alone; a properly specified disc spring stack holds that loss to a fraction of it.
This has a bearing on how a fastening system is specified, before it is ever installed. Two of the three mechanisms behind preload loss can be designed out through the fastener itself. The third needs an added component, regardless of which fastener sits beneath it.
Avlock has built five decades of fastening expertise around knowing which failure mode a given environment produces, and specifying accordingly. That distinction, made early, decides whether a joint needs re-torquing within a few years or holds without intervention for the life of the structure.
For engineers specifying connections across South Africa's transport, mining, energy, and heavy industrial sectors, the practical question is which of the three mechanisms applies in a given environment, and whether the joint has been specified to answer it.
