Reducing Maintenance Downtime: Better Ways to Secure Non-Threaded Components

The majority of unexpected downtime in rotating machinery does not commence with a grand failure. It commences with some motion that is not supposed to be there: a bearing that has moved slightly in its housing, a gear that has generated a minor knock, a bushing that has started to rotate. By the time anyone becomes aware of these issues, it is already too late, and part of the unrecognized potential of machine failure has occurred.

The normal reaction is to remove, change, and reassemble the component. What is rarely done is asking the question: why did the component move? This is the question that is worth pursuing.

The Contact Problem Inside Every Press-Fit

Even with a well-machined press fit you’re only going to get contact across roughly 20-30% of the mating surfaces. The rest is air pockets sitting between the microscopic peaks and valleys of each metal face. Under load, those voids allow micro-movement. That micro-movement generates heat and oxide debris, a process called fretting corrosion, which progressively destroys the interface and loosens the joint.

Keyways and splines are particularly bad here. They concentrate stress at specific points rather than spreading it evenly around the shaft, which creates stress risers, localized weak points where fatigue cracks will start to form over time. In high-torque applications, this is often the actual root cause of shaft failures that get misdiagnosed as material defects or overloading.

The mechanical fit itself isn’t the problem. The assumption that metal-to-metal contact alone is sufficient is.

How Chemical Bonding Changes the Equation

Retaining Compounds are anaerobic adhesives, they cure in the absence of air when confined between metal surfaces. What that means practically is that the resin fills every void in the interface, achieving close to 100% surface-to-surface contact rather than the partial contact a mechanical fit produces.

That gap filling does two things simultaneously. First, it distributes load evenly across the entire mating surface, eliminating the stress concentrations that cause fatigue. Second, it seals the interface against moisture, which cuts off the fretting corrosion cycle at the source. Seized bearings caused by corrosion-welded interfaces are one of the most frustrating maintenance problems in plant environments, and one of the most preventable.

Industry testing by major adhesive manufacturers shows a solvent-free retaining compound can increase the load-carrying capacity of a light-duty press-fit by two to three times its original mechanical strength. That’s not a marginal improvement.

Slip-Fit Assemblies and the Cost of Over-Engineering

There’s also a practical case for using these compounds to move away from high-tolerance interference fits. Getting a bearing into a housing with a true interference fit requires either a press and significant force, or thermal methods, heating the housing, freezing the shaft, or both. That equipment isn’t always available. The process isn’t always controlled. And if you damage the component during assembly, you’ve created the exact failure mode you were trying to avoid.

A slip-fit assembly with an applied retaining compound gives you a unitized assembly that behaves as a single rigid unit under load, without the assembly complexity. Disassembly is still possible when needed, localized heat breaks the bond and allows components to be separated with standard tools. That matters for maintenance teams who need to overhaul equipment without writing off the housing every time.

Selecting the Right Compound For the Application

It’s important to stress that not all retaining compounds are the same and you cannot interchange one for the other. Get the choice wrong and you’ll either under-bond the joint (it still moves) or over-bond it (impossible to disassemble without causing damage). The key things to look at are:

Viscosity: this needs to match the diametrical clearance. You’ll use a thin, low-viscosity compound for close-tolerance fits where the gap is smallest; for bigger gaps you’ll be looking for a higher-viscosity product that has enough body to stay in place without running out before it cures.

Operating temperature: this dictates the cure chemistry and the long-term performance. If the assembly is expected to get hot, the compound needs to be rated well above the peak operating temperature (most standard products are good up to around 150°C but you can get specialist formulations that will go much higher).

Substrate metals: this affects the rate of cure. Anaerobic adhesives will cure more quickly on ‘active’ metals (such as iron and steel) but more slowly on ‘passive’ metals due to the absence of oxygen during the cure process. This includes stainless steel, aluminium, and certain coatings. In these cases, you might need to apply an activator first. When joining two dissimilar metals you should also check their Coefficient of Thermal Expansion, as a big disparity can put additional stress on the bond while the assembly heats and cools in use.

Shear strength rating: this must match the application load. Using a high-strength compound on a component that will need to be regularly serviced or maintained will exacerbate the problem more than it will solve it.

Building Reliability Into the Assembly

Deciding to swap reactive maintenance for reliability-centred maintenance pretty much boils down to what you choose to do when the component’s in your hands, not when it’s busted. Opting to use a retaining compound on cylindrical assemblies (bearings, rotors, bushings, gears) isn’t a ‘quick fix’ because wear has made everything sloppier. It’s a proactive decision to eliminate known failure modes before they become a problem.

If fretting corrosion, interface slippage, and fatigue cracking are the villains in your maintenance schedule, the solution isn’t to replace the part sooner. It’s changing how the joint is built in the first place.