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Stopping spin damage: Replace the bearing, not the assembly

Many quarries utilise high quality bearings with lubrication/grease, backed up by temperature and vibration monitoring and automated lubrication systems. Furthermore, mean time between failure (MTBF) estimates are utilised to plan shutdowns to facilitate planned bearing changeovers (preventative maintenance).

While this has made a big improvement to bearing life, unfortunately, for many reasons, bearings still fail before MTBF is achieved, at the most inconvenient time. Sound familiar?

Sadly, most catastrophic (unscheduled) bearing failures result in damaged mechanical assemblies, which require replacement, along with high labour costs to facilitate removal/replacement.

In Loctite’s experience, it is well known that fretting corrosion can turn into cold fusion welding, resulting from micro-movements between mating surfaces. By avoiding micro-movement caused by vibration, thermal expansion and shock impact, aggravated by further galvanic corrosion, oxidisation and spinning on the bearing journals, we can eliminate damage to shafts and housing assemblies.

Precision metal assemblies only achieve 30 per cent metal to metal surface contact. The solution to this is to fill the remaining gaps (voids) with an anaerobic retaining compound.

The aviation industry has been doing this since the 1950s, capitalising on the high strength, low stress and lightweight design benefits this solution offers. Filling the voids increases the surface contact to 100 per cent, which helps spread the load and stress. See Figures 1 and 2 for an example of limited 30 per cent metal contact.

Many fitters and engineers miss the true benefit retaining compounds (RCs) offer, tending to believe that if the manufacturers of gearboxes, drive assemblies and other machinery do not use it, why should they? The good news is that many manufacturers are now using RCs to help improve reliability and service.


A damaged bearing assembly in a vibrating screen exciter gearbox drive 

The temperature sensors indicated a sudden rise in temperature on a large finished product handling screen bearing, operating at a major quarry. It was a Tuesday and the decision was made to monitor the temperature until the scheduled Sunday maintenance shutdown.

Unfortunately, the bearing failed on the Thursday, with the automated control system shutting down the entire screen house. The maintenance team discovered that the fixed side bearing’s (non-float) inner race had been spinning on the shaft before it completely seized and fusion welded to the shaft.

The disassembly process involved six hours with two fitters. A new bearing and housing assembly (valued at $2500) was found in the store. A new shaft was considered (valued at $4500) to speed up the reassembly. Unfortunately, a new shaft was not available for five working days, so it was decided to repair the existing shaft.

The damaged shaft journal needed to be machined down below the damaged surface to a precision tolerance and fitted with a sleeve via an interference shrink fit. The sleeve could then be machined back to a sliding transition fit for the bearing/shaft assembly. The total process was estimated to take six hours.

The Loctite solution was to apply Loctite 680 – a fast curing, high strength anaerobic RC – between a machined clearance gap of 0.15mm, using a precision machined sleeve. This allowed a relaxed shaft journal finish, low tolerance and fast assembly. Once assembled, the sleeve was allowed a two-hour fixture time, at an ambient temperature of 28°C, so the reassembly could commence (note that a full cure requires 24 hours at 22°C).

To also ensure no micro-movement was possible with the new bearing assembly, Loctite 680 was applied between the fixed bearing’s inner race and the new sleeve that was fitted to the shaft.

The screen was assembled late that afternoon and restarted at 6am the next day, which allowed a 75 per cent cure strength of the RC. This screen has been running for more than two years now. It cost less than $100 for the combined Loctite product and labour time to seek Loctite’s advice and apply the product (see Figures 3 and 4).

Loctite runs maintenance repair workshops that cover the traps and pitfalls of mechanical assemblies while offering time proven results for thread locking, thread sealing, gasketing, retaining and anti-seize lubrication, via hands on, relevant training for quarries and mines, available in all states.

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