(November 1st, 2010)

Rolling bearings are critical components in heavy-duty agricultural machinery.

Selecting superior-quality bearings is therefore essential in ensuring that any costly, unforeseen breakdowns and repairs are avoided.

High-value, heavy-duty agricultural machinery such as tillage and sowing machinery, harvesters, graders and tractors, must all be designed to withstand the harsh environments in which they operate.

Invariably, machine components will have to cope with a variety of conditions, including high humidity levels, sand, dirt, dust, heavy clay soil and shock loads.

Increasing working speeds and higher-performance capacity of tractors and harvesters means it is now critical that engine, drive train, wheel, axle, seed discs and other machine components are robust, reliable and not prone to breakdowns or premature failure.

Purchasing lower-cost, inferior-quality components may look desirable in the short term, but will inevitably result in increased maintenance and servicing costs over the long term.

Rolling bearings - including supports and seals - are particularly important in helping to increase the service life and maintenance intervals of agricultural machinery.

This in turn will help to minimise any unforeseen breakdowns, but even if a breakdown does occur, bearings can be designed to ensure that the removal and mounting process is quick and easy.

To a farmer, an unforeseen breakdown to a key piece of machinery during the harvesting period could result in thousands of pounds worth of 'lost' crop or late deliveries.

Sourcing and selecting suitable rolling bearings is therefore critical.

Making a purchase decision based on price alone will almost certainly have damaging repercussions on the end user.

Specialist agricultural contractors should therefore demand superior, high-quality bearings that will require minimal maintenance and servicing and prolong the life of the customer's machinery.

Consideration should be given to bearing lifecycle costs rather than purchase price alone.

Selecting or replacing an existing bearing with a lower-cost, inferior-quality alternative can have very damaging consequences for the agricultural machine contractor or farmer.

Likewise, the use of counterfeit bearings on high-value capital machinery will almost certainly endanger the operating reliability of a machine, since these bearings will have been manufactured to inferior quality standards.

While the offer price for inferior or counterfeit bearings may initially look attractive, buyers must ask themselves what the potential hidden costs are in terms of product liability and credibility with their customers if their product turns out to be counterfeit and things inevitably start to go wrong.

Buyers therefore need to take into account the cost of any machine downtime if the bearings fail early.

Inferior or counterfeit bearings fail because they are normally manufactured from inferior-grade materials rather than high-grade steels.

These bearings often fail early due to inadequate or incorrect lubricants being used, including a failure to machine lubrication grooves into the outer raceways.

Other inferior-quality bearings may suffer from poor sealing, non-hardened or poorly ground raceways, poor cage designs, and poor corrosion protection.

When it comes to reliability and service life, inferior bearings are also much more likely to fail early, often shortly after the running-in stage.

Other problems include inappropriate packaging of the bearing or incorrect storage prior to use.

Purchasers of counterfeit bearings also run the risk of not knowing how the bearings have been stored, in what conditions and for how long.

So, even if the bearing is a genuine product, the manufacturer of the bearing may not support product liability and the warranty will become void.

The message here is always buy from an authorised distributor or direct from a reputable manufacturer, otherwise the risks are too high.

There are many ways in which bearing failures can occur.

Flaking (or spalling) occurs as a result of fatigue at the contact points of the rolling elements and raceways, usually when the bearing has reached the end of its normal life.

Classic fatigue begins with subsurface fissures, which progress to the surface and cause material fragments to break loose.

Once started, the problem increases as the loose fragments are scattered around the raceway and the bearing becomes unserviceable, with increasing vibration and noise, as well as possible eventual fracture.

Premature fatigue can be caused by excessive loads, inadequate lubrication, improper handling or assembly, poor installation design (incorrect shaft and housing tolerance limits), misalignment, contamination or a reduction in hardness at high temperatures.

Selection of appropriate mounting based on the operating conditions, adequate sealing, accurate installation, the use of proper fitting techniques, lubrication and maintenance should help the bearing to achieve its design fatigue life.

Wear is the loss of material from sliding or rolling contacting surfaces due to abrasion.

Normally this is caused by inadequate lubrication, which produces a shiny worn surface, or the ingress of small abrasive particles (dull, worn areas).

Inadequate lubrication can also cause dislocation due to overheating.

When wear does occur, engineers should ensure that the lubricant is of the correct type, that it is clean and that a sufficient quantity is reaching the bearings.

Sealing should also be improved where possible.

Smearing (or skidding) is caused by inadequate lubrication and can occur at the contact surfaces between rollers and their guide ribs.



This usually occurs if the rolling elements are subjected to severe acceleration on their entry into the load zone, as well as in situations where the bearing load is too light with respect to operating speed.

Improved lubrication and careful selection of the bearing to suit the operating conditions should prevent the occurrence of smearing.

Bearing rings should also be fitted with sufficient interference.

Seizure is caused by extreme smearing, leading to friction-welding of bearing components.

As this is due to metal-to-metal contact caused by failure of the oil film, adequate lubrication under all operating conditions will normally prevent this occurring.

Distortion of bearing cages can be caused by improper mounting techniques.

Inadequate lubrication or contamination by abrasive particles can also result in wear of the pockets and to the inner and outer diameters.

Cage fractures are primarily caused by vibration, excessive running speeds or acceleration, seizure, high temperatures (for plastic cages) and damage during mounting.

These failures can be reduced by employing the appropriate fitting techniques, adequate lubrication and sealing, as well as matching of the cage type to the operating conditions (speed, temperature).

Rust and corrosion can be caused by poor storage conditions or incorrect packaging; insufficient rust inhibitor; penetration by water, acids and other aggressive media; or by the incorrect handling of the bearings.

Measures should be taken to prevent rusting while the bearings are in storage and to improve the sealing performance.

Lubricating oil should be inspected periodically and tested for signs of contaminants.

Reputable bearings manufacturers now offer a variety of maintenance-free bearing solutions and seals for agricultural machinery, designed specifically to increase service and maintenance intervals, minimising machine downtime and MRO costs.

These bearings are available in various configurations, including needle, cylindrical, spherical and tapered roller bearings.

Housed bearing units are also common, as are angular contact and radial insert ball bearings.

Special, custom-designed bearing solutions for specific agricultural machinery can also be provided.

Bearings for harvesters, for example, have to cope with plant remnants and sap, the acidic contents of which can cause ingress of dirt, leading to possible seal damage.

On crop harvesters, high rotary speeds result in high mechanical stress on the bearings; contamination and aggressive, acidic corn sap can affect the seals.

To solve these problems, superior-quality, ready-to-fit bearing units with triple-lip seals can be specified.

Angular contact needle roller-bearing units for seed discs are also now available, which not only enable high load carrying capacity and tilting rigidity, but also use multi-lip cassette seals, which protect the bearing against contamination and prevent loss of grease.

These clearance-free units can be supplied with the maximum quantity of grease, ensuring maintenance-free operation without any downtime.

Special coatings can also be applied to bearings to further protect them against harsh operating environments, particularly extreme dirt and heavy shock loads.

Schaeffler's INA and FAG brands offer a wide choice of bearings for applications in gears, power transmission systems and axles.

Bearings can be manufactured in premium quality 'X-life' standard, with improved surface and geometrical accuracies, resulting in increased performance, improved reliability and longer bearing life.

To enable faster, easier installation and simplified removal of machine components on seed discs, ready-to-mount bearing units can be provided.

Here, the attachment screws for the seed discs are usually integrated in the flanged bearing.

The seed disc is initially attached to the bearing.

After this, the entire unit is screwed to the machine quickly and easily via a central screw.

Seed disc change simply involves loosening the screws.

This is all designed to make life easier for the operator of the machinery.

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