Roulements - 2011-02-04
Research by NSK shows that the costs associated with bearing replacement as a result of premature failure due to contamination can be considerable. In addition to the replacement cost of the bearing itself, and the time and effort involved in sourcing and physically replacing it, there is the cost of unplanned plant and equipment down time, and the negative effect on production schedules. In fact, if the true, ‘all inclusive costs' associated with premature bearing replacement are multiplied by the frequency of replacement over an extended time period, then the costs to a company can be quite staggering.
A rolling bearing subject to contamination may require replacement at a point in its life-cycle that is a mere fraction of the life expectancy that would otherwise be achieved if operating in a clean environment. This point is underlined by the incidence of bearing failures in problem environments such cement manufacture, steel production, quarrying and agriculture, where machinery is exposed to moisture, dust, dirt and assorted processed material.
NSK counsels its customers to always bear in mind that bearings are precision engineered components with very small internal operating clearances: a fact that makes them particularly sensitive to contamination. In many cases, it is the service life of the rolling bearings that dictate the required maintenance effort and, hence, the availability for service of a machine.
Cement is manufactured in both wet and dry processes, so both moisture and dust are a problem for bearing users in the industry. Of the two, dust is probably the primary problem, because that produced by cement is highly abrasive. Cement dust is hygroscopic, which means that it absorbs the lubricating oil from the bearing grease, resulting in an abrasive paste of contaminated grease and cement dust.
In addition, the ‘blotting paper’ effect of dust ingested into a bearing has the effect of drawing oil away, leaving the bearing with minimal lubrication at the same time as it is struggling to cope with increased friction resulting from the circulation of dust debris. The overall effect of this is that severe internal wear of cage pockets and raceways may occur, and shafts may no longer be rigidly located as a result. This has a knock-on negative effect on the life expectancy of other components in the power train, such as pulleys, chains, sprockets etc, all of which may require early replacement, adding to unnecessary costs.
Moisture is a problem, also, because it can be drawn into rolling bearings by the vacuum created as bearing internal air pockets cool after machines are shut down. With bearings at standstill, moisture saturates the grease lubricant, congregating around the points of contact between balls and raceways and exposing these areas to developing corrosion. This is evidenced by internal inspection of moisture-contaminated bearings, which will often reveal patches of corrosion on the raceways at the ball spacings. The result of this would be noise and rough running, eventually leading to premature bearing failure.
NSK also warns of a further consequence of increased internal friction in bearings as a result of contamination: creep of the bearing rings, causing them to rotate in their housings or on the shaft. Severe wear to the housing bore or shaft is a likely consequence and this itself can be a cause for mechanical inaccuracy and vibration, not to mention possibly resulting in a more extreme mode of bearing failure, such as seizure. In addition, shaft journal damage from bearing inner ring fretting or bearing seizure may necessitate shaft replacement; the expense of this operation possibly dwarfing all other replacement costs.
The key to determining the impact of contaminated operating conditions on bearing service life is an analysis of rolling bearing failure modes, through individual bearing examinations combined with consideration of bearing replacement history and applied loads.
With this data an assessment can be made of the relative costs of the problem to a business. Moreover, once the problem is recognised it is usually easy to overcome, by using bearings that are optimised for use in their respective operating environment.
Bearing optimisation can be achieved as part of a structured, company-wide initiative, using NSK’s Asset Improvement Programme (AIP). AIP is designed to deliver real benefits in operating costs, efficiency and profitability to the cement manufacturing and extraction industries. These benefits are achieved by delivering tangible savings to assets, such as equipment and machinery, and also by improving the working knowledge of maintenance and engineering personnel.
AIP is simple to implement; it is broken down into stages in order for people to understand the programme. The process usually starts with a site survey, which is very detailed. NSK engineers visit a customer’s premises, talk to them; get to understand what their difficulty is and work out what NSK can do to support them.
As part of the AIP programme, all aspects of a company’s operations, including systems, stock, specific problem applications, lubrication, and even training issues are audited. An analysis of the data acquired is then performed and this leads to recommendations for improvements. In practice, these recommendations could relate to training: about how to examine or look after bearings; how to spot failure modes; and how to handle bearings correctly - as that is one of the biggest causes of premature failure.
In addition, NSK might need to look at the lubrication of bearings; or recommend part substitution for a bearing that would work more effectively. The overall aim is to provide added value through the provision of engineering solutions that, when documented, can represent substantial savings on operating and maintenance budgets. This provides management with firm figures that give them reassurance that they have saved money and improved profitability by taking the AIP route.
The effectiveness of AIP was demonstrated recently in a screw pumping conveying system at a cement plant. The pumping system was experiencing regular failures due to large angular contact bearings experiencing dust contamination.
Hygroscopic dust was wearing the raceways and increasing clearance to the point that, to avoid catastrophic failures, the bearings had to be changed prematurely, and on a regular basis.
NSK assessed the application under its AIP programme and recommended a material change in the bearing specification to Hi-TF Steel™, a material that offers increased resistance to contamination. Since installation, bearing consumption at the cement plant has reduced by more than 50%. As a result of this success, the plant has converted other locations to Hi-TF bearings.
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