In the midst of the 2017 US harvest season a farmer lost his large self-propelled combine harvester after it caught fire and had to be written-off. The subsequent investigation undertaken on behalf of the insurance company concluded that the fire emanated from the failure of a bearing. It appeared that the failure occurred when the cage collapsed, allowing the balls to rub together, and friction increased the temperature, which can reach over 1,600? C. The harvester was valued at US $400,000 (€341.000), and although the farmer would have received compensation for loss of the machinery, it is unlikely that the farm could have easily obtained a replacement machine in the middle of the season, and part of the crop could have been lost as a consequence.
The global bearings market is generally viewed as the sales of rolling bearings, comprising ball and roller assemblies of various designs. Bearing manufacturers suggest that one percent of bearings fail before reaching their L10 estimated operating life, with 36% of premature failures being due to poor lubrication and another 14% due to oil/grease contamination.
Just as bearings have a life expectancy, so too does grease. Although blenders add oxidation inhibitors, the protection is slowly consumed, and unless the bearings come pre-lubricated and sealed, they will need periodic renewal of grease. It is an important part of an active routine maintenance programme.
So how can one improve the chances that bearings won’t harbour one of the 1% that doesn’t complete its expected operational lifetime? Lubricant selection is critical and will affect life, torque, speed, noise, grease migration out gassing, temperature and bearing rust prevention. The use of free flowing oil where bearings are subjected to high speeds or high temperature mitigate heat dispersal; but the oil requires frequent renewal, so in the vast majority of cases grease is the natural choice.
Price, either high or low, should not be the arbiter in the type of grease that is used for a particular purpose; but thickening agent or soap type, certainly is a key factor to take into account. The consideration should always be selecting the type that best meets the test ‘fit for purpose’; type of additives, consistency of blend, and results from field testing.
Greases are made from mineral or synthetic oil base stock with the addition of a thickening agent and protective additives. Thickening agents can include lithium, lithium-complex, PTFE, clay, polyurea, sodium or calcium; but more complex greases for high temperature use are turning to bentonite and silica aerogel thickening agents. In simple terms, the thickening agent is analogous to a natural sponge, absorbing oil and slowly releasing it to the moving parts. The inclusion of solid lubricants such as graphite, aluminium, and molybdenum disulphide enable the creation of a secondary layer of lubrication between the sliding surfaces, thereby preventing metal-to-metal contact in the event that the lubricant becomes too thin to function properly.
Rolling bearings depend on the presence of a very thin film of lubricant between balls and races, and between the cage, bearing rings, and balls. Care must be taken to ensure that neither too much nor too little grease is loaded into the bearing. Too much grease can cause bearing temperatures to rise, especially at high speeds, while too little grease results in harmful metal-to-metal contact. Failures are typically caused by restricted lubricant flow or excessive temperatures that degrade the lubricant’s properties.
The National Lubricating Grease Institute (NLGI) has a table that rates the relative hardness of greases used for lubrication. The full classification scale 000 to 6 is analogous to a food consistency continuum, the lower the number the softer the lubricant, refer to Table A. Grade numbers 000 to 1 are low viscosity fluids with good pumping ability at low temperature, and would typically be used in low-speed gear boxes. The grease packed into rolling bearings normally complies with NLGI numbers 1, 2, and 3. Where maximum protection against penetration of water or solid contaminants is required, NLGI number 3 is commonly used, while the higher numbered greases are firmer and remain in place, consequently they have a role where leakage is a concern.
|NLGI classification of grease consistency number |
|Appearance at room temperature||Consistancy of food analogy|
|000||very fluid||cooking oil|
|1||very soft||tomato paste|
|3||medium hard||vegetable shortening|
|5||very hard||smooth pȃté|
|6||extremely hard||cheddar cheese|
The price of a failed bearing is more than the cost of a replacement. It must also include loss of production – cost of engineer’s time – and the priceless loss of client confidence. Yet Italian company Des-Case cite a survey that found “Less than 0.5% of a plant’s maintenance budget is spent purchasing lubricants, but the downstream effects of poor lubrication can impact as much as 30% of a plant’s total maintenance costs each year.”2
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