The bolts had been locked by spring washers which simply could not rise to the challenge of securing bolts on a piece of equipment subjected to continuous vibration and high shear forces. This resulted in the loosening – and ultimately the breaking off – of anchor bolts and the collapse of concrete footings.
Spring washers – also known as lock or helical spring washers - are one of the most popular mechanical devices for securing threaded fasteners against self-loosening. The washer is squashed flat when the nut is tightened against the mounting surface so that its sharp edges dig in to prevent the threaded fastener unwinding.
In practice, a spring washer may delay the length of time it takes for the bolt to loosen, but it will not permanently prevent it. The main reason is that the split washer does not solve the gapping issue, the free space remaining between the threads of the nut and the bolt.
On a typical threaded assembly there is just 15% efficient metal to metal contact between the threads. Everything else is empty space that enables plenty of side to side movement for the bolt. Additionally, being metal itself, a spring washer can cause damage to contact faces and corrode in place.
For the Australian mining operation, this problem was exacerbated by the size of the bolts. The bigger the bolts, the bigger the gap between threads which leads to more vibrational impact on the assembly. This realisation resulted in the company changing to adhesive thread locking with Henkel’s Loctite 270. This is a high-strength formulation that completely fills all gaps between the thread and prevents any movement of the bolt within the nut. Secured this way, the assembly is completely vibration-proof, but can still be dismantled with hand tools for repair and maintenance. Additionally, the liquid thread locker seals the threads against humidity and dust, preventing corrosion and surface erosion.
This simple adjustment to working practice has not only underpinned productivity but contributed to a safer working environment. One 50ml bottle of liquid thread locker is sufficient to secure around 850 M10 bolts but, unlike spring washers, can be used on any bolt size.