Attachments, Case Studies, Maintenance, Maintenance Products, Plant & Equipment, Tips & Advice

Why nut and bolt fastened joints fail

As all quarries and mines use fasteners, understanding why they fail is often misunderstood. The defence and aerospace (D&A) industry certainly does not accept any fastener failures, as reliability in D&A platforms are both critical and paramount.

So why do some industries accept that fasteners can fail during the operational life of their equipment? D&A utilises double and triple redundancies in its platform designs. Mechanical forces seen in quarries and mines are similar to what we see in precision aviation and heavy military equipment.

This feature covers why and how nuts and bolts fail, so quarries and mines can capitalise on eliminating this problem. To help understand the above we need to go back to the basics and look at the following subjects:

  1. How fasteners work.
  2. Torque-tension relationship.
  3. Why nut and bolt fastened joints fail.
  4. The solution.

From Loctite’s experience in all industries, many fitters, mechanics and even qualified engineers are not completely conversant on the above subjects. For example, most mechanically based courses at colleges and universities do not adequately cover the torque-tension relationship that operates when a nut and bolt are torqued up to achieve the desired clamp load.

Insufficient knowledge of these topics can affect final mechanical design(s), which in turn could lead to unscheduled platform failure, resulting in downtime, loss of production and significant costs. Quarries and mines are annually invited to conduct sessions at a major Swinburne University campus to deliver hands on classes covering these (typical) subjects.

The primary purpose of fasteners is, in a cost-effective way, to achieve a desired clamp load (see Figure 1) to assemble or anchor a mechanical structure with the ability to disassemble it, should it be required. While cheaper than using welding (which requires welding equipment and labour), they are not as strong as heavy welding and for mass produced parts, fasteners can be expensive. Hence, the use of rivets and spot welding in manufacturing are quite common alternatives.

However, rivets and spot welding are best suited to small manufactured items that do not need regular or any disassembly.
Structural bonding is a subject that is not covered in this article. However, modern military, business and commercial aircraft are now bonded together with HP (high peel and shear) adhesives. Many quarries and mine sites use (industrial grade) adhesives for bonding light weight UHMW liners and ceramic tiles used in high wear rate areas in chutes, hoppers and relevant areas.

Note the 70 per cent friction that must be overcome to achieve the desired clamp load. As the stretch and tension in the threaded fastener cannot be easily measured, it is accepted to use a calibrated torque wrench to measure the applied torque instead. The type of fastener used is critical to longevity and cost. Soft ferrous mild steels will stretch easily and can fail earlier than high tensile bolts, but the latter incurs higher cost.

Other critical factors such as the type of material, rotation speed and the K factor (lubricity) co-efficient of friction at the torque loading points vary, thus affecting the final clamp load by as much as 40 per cent, even when the same torque is applied.

When should an anaerobic threadlocking compound or anti-seize be used on fasteners?

A “dry” (unlubricated) fastener will not achieve the clamp load for which it was designed, irrespective of the amount of torque applied. On the contrary, fasteners lubricated with an anti-seize compound (common practice within the quarrying and mining industries) can be stretched well into their elastic limit and could potentially be a “disaster waiting to happen”. Hence, it is critical to know the correct K value to work out the correct torque to be applied to achieve the desired clamp load (ie as dictated by the torque-tension relationship).

A good mechanical design engineer takes into account the K factor. The K factor is the lubricity involved when a fastener is tensioned.

Sadly, this factor is often overlooked or “forgotten” by fitters, engineers and maintenance personnel, resulting in the fastener system being either under or over torqued – both of which are unacceptable in critical platforms.

Mechanical engineers use engineering handbooks to work out the torques to be applied to a fastener set. Modern design trends using smaller, fewer, high strength/low yield bolts torqued into the yield region place more and more emphasis on good design practices and understanding of the forces involved.

Large high tensile bolts can stretch under tension, up to 20mm in length. Replacing these with mild steel fasteners (eg used to clamp the top and bottom main shells of a large cone crusher) to reduce cost can prove counterproductive, as this usually results in fastener failure and unscheduled platform shutdowns.

Stainless steel (SS) fasteners are not ductile (limited elasticity) and often experience premature failure, and this outweighs the reason for which these were selected.

SS fasteners also experience dry pick-up, whereby the threads seize upon assembly.

A tub of anti-seize should solve this, right? No, as this completely changes the lubricity (K factor) and needs to be considered.

WHY FASTENED JOINTS FAIL {{image2-A:R-w:300}}The knowledge base for quarries and mines is derived from research, trials and good product technology to ensure fasteners do not fail in critical applications in all industries, including on gearboxes, vibrating and feeding equipment, crushers and conveyors, etc. Failures occur due to:

  • Limited metal contact in threaded fasteners. Only 20 per cent of metal to metal contact exists and 80 per cent is air. We get extreme loads on the thread tips. Expensive, well machined fasteners achieve only 50 per cent metal to metal contact.
  • Vibration. The graph (see Figure 2) shows which fasteners loosen under an extreme vibration due to micro movements. Remove the friction from the spring washers and the fastener unravels! Ever heard of fasteners vibrating tighter?
  • Thermal cycling. With large variations in temperature we get stretching and relaxing of fasteners that can result in their loosening.
  • Mechanical failure. Heavy impact and loads, poor design and incorrect fastener selection contributes to premature failure.
  • Over or under tightening. Torqueing fasteners must be done with the correct lubricity (K factor) to achieve the correct torque-tension relationship. Unlubricated (“dry”) or anti-seize coated fasteners must employ the correct K value or run the risk of not achieving the correct clamp load. Using the correct K factor value is critical to the torque calculation.
  • Corrosion. Air gaps in threaded fasteners have moisture, which promotes corrosion – leading to seizure, preventing disassembly. Anaerobic threadlockers and sealants not only stop loosening but (via 100 per cent gap fill) stop corrosion, micro-movement, moisture, sand and mud entry into the threads.

Maintenance repair workshops cover the traps and pitfalls while offering time proven results for threadlocking, thread sealing, gasketing, retaining and anti-seize lubrication via hands-on relevant training for quarries and mines, available in all states.

Leave a Reply

Send this to a friend