Prevention of fastener loosening part 1
Threaded fasteners are an unavoidable part of building any engine. When we want to harness the energy contained in fuels, liberating this energy via a rapid series of small explosions, we need to be sure our engine assembly stays in one piece. The forces involved, and the rapid accumulation of damaging stress cycles, means we place great demands on bolted joints, and we need to know they are not going to come loose, as the consequences can be financially costly.
The best way to ensure that a fastener remains tightly fastened is to understand its behaviour, and to tighten it accurately. The most critical fastener in any reciprocating engine with split con rods are the con rod bolts, and these are rarely equipped with any positive method to prevent loosening. However, con rods are a rare case where we can accurately measure pre-load (indirectly via measurements of bolt extension and a knowledge of the stiffness of the fastener).
For most other applications we do not have this luxury. There are many ways of preventing a fastener from loosening; some are chemical and some are mechanical. The chemical solutions to fastener tightening are based on either wet or dry thread-locking compounds. There are various dry thread-locking compounds, but they aren’t very widely used in comparison with the wet thread-locking chemicals that many of us are probably familiar with. They come in various strengths and levels of temperature resistance. The weakest is specified for fasteners that require routine disassembly, while the strongest types are really only used for applications where the intention is for the fastener never to be removed. The very strong grades also often require heat to weaken them before studs can be removed.
Such thread-locking compounds are basically anaerobic adhesives, which rely on an absence of air in order for them to cure. Their cure time is affected by temperature, and although there are chemical accelerants available to reduce cure times, the final strength of the thread-locking compound is often compromised by their use, although new activators have been developed by some companies that bring the cured strength back to 100% of the ‘unactivated’ strength.
The strength of the bond is also affected by the material to which the compound is applied. When applied to a bare steel substrate, such compounds are often much stronger than when applied to fasteners that have been chromated, for example. The bond gap also has a large effect on the cure time and the final cured strength of the thread-locking compound, with larger gaps showing increased cure times and significantly diminished performance, as measured by the torque required to loosen the fastener.
Experience of using thread-locking compounds in trials where clamp load was measured for a critical application shows it is important to develop a process for the use of the compound, detailing how much is used, where it is applied, how much delay there is before fitting the bolt and how it is tightened. During some trials I organised, the clamp load – particularly with high-strength thread-locking compounds – showed much greater variation for a given torque than for oil-lubricated fasteners. It was clear from the trials that the thread-locker was beginning to cure and to exert some resistance before the joint was fully tight.
Written by Wayne Ward