From "Lubricants and Lubrication"

If possible, different base oils used in industrial gears should not be mixed since it must be expected that the mix will always assume the properties of the inferior product. However, the fact that mixing two base oils leads to unexpected reactions is even more critical. For example, it might happen that mixing mineral oils with polyglycols results in a significant increase in viscosity – as a result of polymerization – which makes it more difficult to pump the mixture through narrow tubes and might lead to the failure of the gears affected due to starved lubrication.

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QUESTION: How long should grease be stored on-site?

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"Should all greased bearings be fitted with a vent (spring-loaded or other?). Most of the 3,500 RPM pillow blocks that I maintain have no vent and the relubrication schedule was arrived at by 'tribal council'. The bearings heat up from about 104 degrees F to around 165 degrees F for about 2 days after relubrication, and then return to normal (104 degrees F) conditions for the rest of the month. Will a vent help?"

The issue of overheating is related to fluid friction which is a result of fluid churning, which is a secondary effect of overfilling the cavity at the time of relubrication. Installing a relief vent port can help, but this is addressing the symptom rather than the cause.

Two issues need to be addressed.

1. Relubrication practices based on tribal knowledge. While the old-school guidelines can sometimes be correct, the evidence here is that something is not quite right. You need to calculate the volume and the frequency based on bearing type, size, speed and operating environmental factors. Correctly gauged interval and volume per relubrication event can help minimize overfilling the housing. There are a variety of texts available that provide formulas for this effort.

2. Lubricant selection for the application. As bearing speeds increase, the oil viscosity requirement decreases. Obviously the grease's oil viscosity decreases with temperature, but that not withstanding, it is imperative to select a grease based on the bearing manufacturer's recommendations for an application, which is based on mean element speed. (nDm = (Speed in RPM * (Bearing ID + Bearing OD)) / 2).

Bearing manufacturer relubrication guidelines are specific about minimum oil viscosities for mean element speeds. Verify that the selected product meets the fundamental viscosity requirement, and then factor slightly to provide a cushion. If the viscosity of the oil at bearing operating temperature is 2 times or more than the minimum operating viscosity (from the bearing manufacturer) then you should reconsider the lubricant selection, especially for bearings operating near 3,600 RPM.

Churning and overheating both contribute to loss of oil in the grease. This shortens grease life and relubrication interval. Properly selected viscosity, volume and frequency each play a key role in sustaining bearing lifecycles.

Mike Johnson, CMRP, Noria Corporation

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