When oil in gearboxes become contaminated there is a rapid loss of bearing operating life. It is critical that metal particles, dirt, sand, oxidisation products, etc be removed from the oil before they gouge and tear-up the finely toleranced machine parts. Oil replacement, contamination prevention and oil filtration are common practices religiously adopted by the best in the equipment reliability management business. This article explains the fine requirements for successful contamination control to insure long trouble- free equipment life.
Keywords: Beta rating
Wear in moving equipment such as gearboxes, pump bearings and hydraulic rams is a function not only of how much work the item does but how we treat the lubricant at commissioning and during operation.
Looking firstly at dealing with the commissioning phase, a survey quoted in an SKF publication of drummed fresh gearbox oil found surprising levels of particulate contaminants. The action to cover this problem (especially at remote mine sites) is to filter your oil prior to charging a piece of equipment. This leads to the question of what sort of filter? Some filters are wire mesh or paper based, others are made from synthetic fibres locked in position with epoxy resin. Another aspect of the quality of the filter is the fibre size with respect to the gap size. Clearly a filter with fine fibres for a particular pore size will have more pores and will last longer. So cheapest may not be best. When filters are tested, they measure the particles going in verse those coming out. The ratio is known as ‘β’ and the subscript represents the particle size it was tested at. Filters having a β6 rating of 75 or better are suitable for filtering gearbox oil. This code means that 74/75 ths of the particles of 6 microns or bigger are removed.
The recommended 6 microns comes from the concept that in operation, the oil film thickness or the “dynamic clearance” of different types of machine components varies. In journal bearings it is 0.5 to 100 microns and in roller bearings 0.1 – 3 microns. A study by Dr. McPherson showed that as the filter rating particle size dropped from 40 to 3 microns, the roller bearing fatigue life increased exponentially. In plain English that means the more care you take of oil cleanliness, the longer your equipment will last – and it is not just a little bit of extra benefit. The operator who pays the most attention gets the elite performance.
So what do the elite performers do? Apart from taking care on commissioning, they monitor the oil and pay attention during the commissioning phase. A study by SKF on gearboxes showed that hard particles went from 100 on startup to about 8000 after 5 hours, to 10,000 per ml after 38 hours. So change or filter your oil in the early stages after commissioning. Then look after it during operation, because when metallic particles, sand, dirt, etc bridge component clearances, the ‘little blighters’ are going around grinding and grooving their way through the circuit and the machine parts. The more of them that there are – the more damage is being done. Hey its obvious isn’t it! But do we act accordingly?
A team of people at Weyerhaeuser (an American paper mill) systematically set about dissecting problems, introduced precision lubrication systems, contamination control and monitored their efforts by oil and vibration analysis. The result: An average of 25 bearing failures annually for 1980 – 1989 then after the program less than 5 for 1990 –1999. The downtime cost means real money lost in that section of the plant.
Typical dynamic (in motion) clearance values are:
Gear type oil pump: Tooth to side plate: 0.5-5 micron
Teeth tip to case 0.5-5 micron
Vane type oil pump: On sides 5-13 micron
On vane tips 0.5 – 1 micron
Servo valve: 1-4 micron
Roller bearings: 0.1 – 3 micron
From the above figures, it can easily be seen that very small particles are going to make a difference to the life and free operation of equipment using oil.
And another bad thing is … water ingress! Check the table below to see how bearing life is affected terribly by water:
|Water % in oil||% Bearing life remaining|
It is common in industry to use hoses to wash down. So where does this water go? It goes into your gearboxes and pumps.
We recently noted a pattern of failure in the top bearing of a vertical shaft mixer box that lead to massive damage. We believe the cause to be a breakdown of the oil seal followed by all the ongoing ramifications (issues) of shaft seal failure allowing dirt and water ingress. In conjunction with the customer we have implemented a solution to protect that seal from the combination of wash down water, dust and the Western Australian sun. By keeping out the contaminants there should be a marked rise in trouble-free operating life.
Written by Richard Lang of Abonnel PumpWorks Engineering (http://www.makers.com.au)
(Editor: minor editing done for clarity.)
References: Contamination control and filtration fundamentals, Pall Filter technical notes.
Contamination in lubrication systems for bearings in industrial gearboxes, SKF Ball Bearing Journal #242
We (Accendo Reliability) published this article with the kind permission of Feed Forward Publishing, a subsidiary of BIN95.com
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