- Buyer's Guide
"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 two 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. This is a secondary effect of overfilling the cavity at the time of relubrication. Installing a relief vent port can help in this situation, but this would be only addressing the symptom rather than the cause.
Two issues need to be addressed. First is the matter of 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 the bearing type, size, speed and operating environmental factors. A correctly gauged interval and volume per relubrication event can help minimize overfilling the housing. There are a variety of texts available that can provide formulas for this effort.
Second, you must consider the 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 notwithstanding, it is imperative to select a grease based on the bearing manufacturer's recommendations for an application, which is based on mean element speed. This is calculated with the following formula:
NDm = Speed in rpm * ((bearing bore + bearing outside diameter) / 2)
Bearing manufacturer relubrication guidelines are specific about minimum oil viscosities for mean element speeds. You should verify that the selected product meets the fundamental viscosity requirement and then factor in a slight cushion. If the viscosity of the oil at the bearing operating temperature is two 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 the loss of oil in the grease. This shortens the grease’s life and the relubrication interval. Properly selected viscosity, volume and frequency each play a key role in sustaining bearing lifecycles.