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Sealing solutions eliminate contamination to boost MTBF

Sealing solutions eliminate contamination to boost MTBF

More than half of all bearing failures are caused by contamination of the bearing oil due to inefficient sealing. With the availability of cost-efficient sealing solutions which can eliminate contamination and increase MTBFs from months to years, bearings failure should be the exception, not the norm, says Chris Carmody of AESSEAL.

Bearings failure accounts for roughly 40% of all failures in pumps and other rotating equipment, leading to costly downtime and equipment repair and replacement. Almost 50% of those bearings failures are caused by contamination of the lubrication oil, most commonly by water, process fluid or particles of dust and dirt. The ingress of moisture or particles into the bearings chamber might be minute, but the impact on efficient operations can be anything but.

Research indicates that water contamination as low as 0.002% – that’s just a single drop of water in a typical bearings chamber – can reduce bearing life in some oils by as much as 48%. Another example: an oil-lubricated 45 mm radial bearing running at constant load and speed under ultra-clean conditions (nc = 1) has been calculated to complete 15,250 operating hours. Introduce contaminated conditions where nc = 0.02 and its mean time between failure (MTBF) plummets to just 287 hours.

With reliability and cost efficiency high on the agenda for most companies, the logical response should be to look at the root cause of bearings contamination - and in most cases the culprit is the conventional lip seal.

Lip seals have been used to contain lubricants and exclude contaminants from bearings for over 70 years and, while many variations are now available, the design principle has changed little: a flexible elastomer forms a seal between a stationary member and a rotating member through pressure-contact with both. A fluid film of lubricant at the contacting interface helps to reduce friction and delay wear or damage to the contacting surfaces.

However, this pressure-contact means from the moment the lip seal is put into service it starts to deteriorate. Even in ideal operating conditions it will endure just two to five months of continuous operation before oil starts leaking out, contaminants start entering the bearings and replacement is necessary – pricey in terms of equipment, human resources and lost production. More seriously, it begins fretting the shaft or shaft sleeve, which can require costly replacement.

In addition, lip seals are inherently one-way devices and even when a double lip is used to seal in both directions, the outer lip is running dry, which can accelerate the lip wear and shaft fretting, and cause excessive heat build-up.

Most maintenance teams are well aware that lip seals are inefficient and that advanced sealing solutions are available which significantly increase MTBF at little extra cost. The fact is that force of habit means they simply accept the routine of halting production, plucking another inefficient lip seal off the shelf – and allowing the whole cycle of leakage, contamination and failure to begin again.

This presents a clear anomaly in what are often otherwise perfectly efficient maintenance plans. And the solution is simple: break the lip seal habit and install labyrinth-design bearing protector seals. While lip seals are the weakest link in the operation and maintenance of rotating equipment, labyrinth bearing protector seals are the bedrock, maximising the reliability of equipment, improving operational efficiency and achieving long term cost savings.

If bearing seals are to work to optimum efficiency, they must facilitate the ‘breathing cycle’ required by rotating equipment - allowing the oil/air mixture to move through the bearing seal out into the atmosphere when it heats and expands with the rotating equipment, and then sucking air from the external atmosphere back into the bearings housing as it cools.

Unlike lip seals, the most advanced labyrinth bearing protection seals incorporate dynamic lift technology to facilitate this breathing cycle whilst preventing the ingress of contaminating dust and moisture. This involves using the centrifugal force of rotating equipment to open a temporary micro gap, allowing expansion of the oil/air mixture in the bearing housing and allowing the equipment to ‘breathe’. When the equipment stops rotating the micro gap immediately closes, forming a perfect seal against potential contaminants.

An integral, self-adjusting axially energised shut-off

O-ring is made from a highly resilient elastomer material, which exhibits near-zero wear, further enhancing the lifespan of the seal.

While many lip seals are machined integrally into the bearing plate by the OEM, particularly in older equipment, the most efficient bearing protector seals can be retrofitted onto shafts. Once fitted they require minimal maintenance and will last until the bearings reach the end of their natural lifespan – an important feature when considering ongoing maintenance costs.

With the risk of damaging leakage eliminated, reliability engineers can be confident of making other cost-effective decisions, for example by upgrading to the more expensive, but more efficient, synthetic bearing lubrication oil.

Bearing protector seals may require a higher initial outlay, but this is swiftly offset by significantly improved bearing, and therefore pump, reliability and more cost-efficient maintenance procedures. Take all the benefits of bearing protector seals into account and it’s clear that the case for upgrading should be not so much a matter of choice, as simple common sense.

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