Book Bits

From "Practical Lubrication for Industrial Facilities":

Automatic single point grease lubricators should be used judiciously. These lubricators have their place but cannot be applied indiscriminately. They are quite useful in keeping bearing housing grease cavities full, but this is an advantage only if the bearing is constructed and installed so as to avoid detrimental overgreasing. Although automatic single-point grease lubricators are attractive in inaccessible locations, there may be no acceptable solution to the grease separation problems which are frequently observed in plants. One ASPGL manufacturer suggested use of low-temperature or extreme-pressure greases instead of the premium high-temperature greases recommended by most electric motor manufacturers. Also, it would seem prudent to look at cost justifications before using ASPGLs for every lubrication point in the plant. Moreover, there is some concern that field-refillable ASPGLs may be refilled with the wrong type of grease unless special precautions are taken to ward off this possibility.

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Today's Tip

• As the temperature of a gear oil increases, its ability to support a load decreases. This is due to the thinning effect that temperature has on the viscosity. The rate of change differs for each oil. The rate of change is expressed in an oil's Viscosity Index (VI) number. The higher the VI number, the lower the rate of change.

  If you have a hot gear box that seems to have a low reliability rating, you might check to see if the oil has a suitable VI, and is capable of holding up under the high temperatures. If the product's VI is less than 120, as shown on the product data sheet, then you might consider an alternative oil for the hi-temperature, heavily loaded application.

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Q & A

"How hot is too hot for a grease-lubricated bearing?"

In the final analysis, if the bearing is hot enough to evaporate the oil off of the grease between relubrication cycles, then it is too hot for grease lubrication.

The answer to this question, though, depends on several factors. Let's address a few of them.

When dealing with elevated temperatures we should first ask "What is the source of the heat?". Is it ambient? Is it from the process (transmitted down the shaft)? Is it wear related? Is it lubrication related?. These questions have a lot of impact on our selection of the grease type and in determining the appropriate answer to the question.

If the temperature is from the process or atmosphere, and we know that we have to deal with that condition perpetually, then we should adjust the grease composition to meet the requirements. Perhaps we should select a synthetic oil with a non-soap thickening system that gives us a low evaporation rate. If the temperature is not process or atmosphere related, then we should make sure that we are not over-filling the bearing with grease. Too much grease in a cavity causes churning and excessive temperatures. Too little grease, or too infrequent relubrication, may cause friction-generated heat.

If we are using precisely the right volume and we are sure that the heat is internally generated rather than atmospheric or process related, then perhaps we should examine whether the grease has sufficient strength to support the dynamic load. In this case we might look at selecting a grease with a heavier viscosity base oil.

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