Sealed bearings save in the long run

Avoiding the costs of bearing contamination can safeguard production - and a company's bottom line, says Roger Turner of NSK.

The costs associated with premature bearing replacement as a result of 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 effective 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, the costs to a company can be quite staggering.

A rolling bearing subject to contamination may require replacement at a point in its lifecycle that is a mere fraction of its 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 as food processing, sewage treatment, and agricultural, where machinery is hosed down or exposed to dust, dirt and assorted processed material.

What must always be borne in mind when using bearings is that they 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 which dictate the required maintenance effort and, hence, the availability for service of a machine.

In addition to being forcibly injected by means such as high-pressure hosedowns, contamination in the form of moisture can be drawn in to rolling bearings by the vacuum created as bearing internal air pockets cool after machines 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.

Where the problem is dust rather than moisture, the problem may be different but the overall result, bearing failure, is the same.

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.

A further consequence of the incidence of increased internal friction in bearings as a result of contamination is 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 being likely to dwarf 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.

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Moreover, once the problem is recognised it is usually easy to overcome using bearings protected with sealing which is appropriate to the respective operating environment.

The past decade has witnessed a huge leap forward in the design of bearing seals.

Today OEMs and users can protect the operation of their machines in all types of conditions using innovative designs such as NSK's Triple-Lip and Flinger seals, and for high speed applications, the patented, noncontact labyrinth V seal.

In contrast to conventional seals, V types are noncontact seals that eliminate seal lip friction.

Consequently, bearings equipped with these seals can be used at the same high speeds as bearings with shields.

However, compared with shields, V type seals provide better grease retention and greater resistance to contamination.

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