Straight cylindrical ball and roller bearings are machine elements well-suited to heavy loads and high speeds. Under at least “normal” operating conditions, there’s little need to worry about temperature rise in bearings. However, there are a number of different conditions that can turn the temperature of the rolling elements into a problem. For example, exposure to extremely high loads and speeds, high ambient temperatures, and hot process fluids can be problematic. Here are five factors engineers can adjust for cooling roller bearings:
Cooling Roller Bearings
The Bearing Type Factor in Cooling Roller Bearings
There are several reasons a rolling-element bearing heats up: micro-slip of the rolling elements on the races; hysteresis of the rolling elements and race materials due to contact stresses; slip between the cage and the rolling elements or the guiding surfaces; slip between the rollers and the guide flanges; and shearing and turbulence in the lubricant.
The amount of heat from each of these sources varies greatly depending on the bearing’s geometry, load, speed, and the type, viscosity, and quantity of the lubricant. Under light loads and high speeds, ball bearings generally run cooler than roller bearings; at low speeds and higher loads, roller bearings may run cooler. While the choice of bearing type usually depends on cost and life factors, differences in temperature rise can also influence the decision in some cases.
The Oil Type Factor in Cooling Roller Bearings
Grease lubrication offers simple design, low cost, and reliable operation, but it doesn’t carry heat away. Circulating oil, on the other hand, provides an effective means of temperature control. It’s especially useful in applications such as gas turbines and pumps for hot fluids, particularly where process heat is introduced into the bearing. Laboratory tests show that bearing temperatures are nearly the same for oil-mist lubrication and grease lubrication.
The Oil Flow Factor in Cooling Roller Bearings
Rolling-element bearings only need a thin film of oil for satisfactory lubrication. High flow rates cool the bearing but don’t otherwise improve lubrication. However, the relationship between bearing temperature and oil flow rate is complex. While higher flow rates increase heat transfer, they also increase the amount of heat generated by turbulence and oil churning.
In general, bearing temperatures tend to drop with increasing flow rates, then level off or even rise again. In high-speed applications such as gas turbines, most of the oil can be circulated around the bearing to provide cooling while reducing oil turbulence.
For existing machinery, the optimum oil flow can be determined experimentally. For new designs, especially those involving active heat sources, computer simulation is preferred.
Oil Level:
In systems using an oil bath or splash lubrication, bearing temperatures are quite sensitive to the oil level in the sump. Setting the nominal oil level at the center of the lowest ball (so that the oil depth or height, h, divided by the bearing diameter, d, is 0.5) is common practice and will work in most cases.
However, even this level may not be adequate at extremely high speeds. And if the h/d value is below 0.5, poor maintenance or abnormal operating conditions risk causing oil starvation and catastrophic bearing failure. In such cases, an oil mist generator should be used. It creates an oil mist without precisely controlling the oil level.
Airflow:
In systems that don’t use circulating oil, most bearing cooling occurs by convection from the bearing housing to the surrounding air. Since moving air has a much higher convection coefficient than still air, bearing temperatures can be significantly reduced by moving air around the housing. However, airflow should not be directed into the housing itself, since it brings in dirt that shortens bearing life. Sealed, grease-lubricated bearings help keep air from entering the housing.
Air cooling is used as a quick fix, since it can be retrofitted without changing the design. In new designs, adding an axial-flow fan wheel mounted on the shaft extension is a better alternative. It not only moves cooling air over the housing, but also acts as a heat sink on the shaft, and prevents the outer ring from creeping in the bearing housing.
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