Why Joint Fatigue Is the Silent Killer of Complex Industrial Equipment
Joint fatigue is a silent destroyer. It doesn’t give you grinding noise, a visible crack, or a warning light. One day your equipment is working perfectly. The next day, a fastener is loosening, a shaft is misaligning, and instead of a simple fix, you’re facing an emergency shutdown that translates to thousands of dollars lost in production. Fatigue causes up to 90% of all metal/component and mechanical failures in machinery and plants. And most of that fatigue failure can be traced back to the joint.
Why Mechanical Fasteners Are Part Of The Problem
Bolts and rivets are so ubiquitous in heavy equipment assembly that we almost take them for granted. But we shouldn’t.
Every hole you drill through a metal part in order to use a fastener is a point of stress concentration: the material directly surrounding that hole must take more than its fair share of any force on the joint. Again, this isn’t a problem under static conditions. As soon as you subject the joint to repeated, fluctuating forces – in other words, the conditions for most machines in use around the world – you create a series of starting points for cracks in the metal to begin. And since the entire advantages of riveted and bolted joints are predicated on the fact that they can carry extremely high loads, these cracks can grow fast.
These fatigue cracks themselves don’t have to be immediately visible to be deadly. Microscopic cracks in the metal part will literally rip through the base material with each loading cycle until they grow large enough to pose a hazard. As all of those hidden cracks multiply, the remaining metal can’t bear the load. Suddenly, the part completely fails, even if a maintenance worker did a full inspection one day prior.
In addition to making the joint prone to cracking, each fastener depends on the clamping force to maintain the joint’s integrity. If that force goes away – whether from the machine vibrating, from the temperature going up and down, or just from material stretching under a load – then the joint isn’t acting as a joint anymore. It’s moving.
Distributing Stress Instead Of Concentrating It
A mechanical joint transmits longitudinal load from one connected component directly to its mating counterpoint. Whether you’re discussing welded steel plate or a simple through-bolt, the bearing surfaces receive load solely at the contact points where the fastener passes.
By design, the fastener hole in the bolted assembly contains clearance relative to the shank or thread of the fastener that occupies it. That clearance means that when subjected to dynamic loads – say from an engine or an external vibration source – the fastener is free to move slightly within the joint. The result can be a phenomenon known as fretting corrosion.
When you bond two mating components using Industrial Adhesives rather than relying solely on bolt clamping force, the solid bank of cured polymer represents a far smaller volume than a physical hole, and one that is impregnated with the solid adhesive itself. The technique, when properly employed, can provide a near-perfect seal against the elements and inhibit water from entering and initiating the electrochemical reaction that causes traditional fretting.
How Fretting Corrosion Quietly Destroys Joint Integrity
Tiny motion between bolted connections is difficult to prevent, especially in high-vibration applications. This motion is enough to remove the protective oxide layers from the bolt material and flange at the contact point, resulting in the raw metal being exposed to moisture and oxygen in a process called fretting corrosion. This corrosion increases the rate of surface degradation right at the location where the joint remains in tension.
The effects of fretting corrosion tend to be underestimated, but its effects are insidious because they tend to be self-reinforcing. The oxide debris generated from initially damaged surfaces essentially becomes a fine abrasive pressed between the surfaces in subsequent motions, rapidly escalating any wear the joint might undergo. Initially, the joint appears undamaged from the outside, while failing internally.
The fatigue limit of the affected components also is reduced by fretting. Thus, even if the components theoretically are capable of handling the design load stress for many years, once the surface condition is deteriorated by fretting the component will fail prematurely because of fatigue.
What Maintenance Teams Can Do Now
One solution for reducing fatigue-related joint failures could be rethinking the way we design them into equipment to begin with – but it’ll be years before we see the first results from any such effort. In the meantime, maintenance and reliability engineers can put proven rebuild and assembly best practices to work right now.
Thread locking compounds are a simple way to remove a common vibration loosening root cause without spending engineering hours redesigning to eliminate the use of threaded fasteners in the first place. We recommend making threadlockers standard for any fastener that’s tied to a piece of rotational equipment, or that’s part of a conveyor system, pump assembly, or gearbox.
If a vibrating fastener has ever failed and put you in an unplanned downtime situation, use a threadlocker; it’s a no-brainer risk mitigation step. The same goes for failing to pull product on a machine because bearings were seized up on an unlocked threaded fastener.
For press fit assemblies, bore geometry photomicrographs suggest that vibrational loosening that slowly abrades press fit connections is also at least a common root cause of premature production downtime. If experiencing that type of failure on a part that mates by press fit, consider having a retaining compound added there at rebuild.
