Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/258778
www.machinerylubrication.com | January - February 2014 | 19 Once the normal operating speed is reached, full-film or hydro- dynamic lubrication is achieved. In this regime, the two metal surfaces are separated by a lubricant film to such a degree that the asperities no longer come in contact. It makes perfect sense that if you maintain full separation of the metal surfaces with a lubricant in between, no mechanical wear will occur. In fact, it has been stated that as long as this condition exists, these bearings can operate indefinitely without wear. This process can be compared to water skiing. While the boat is idle, the skier is in the water, which is equivalent to a boundary condition with the lubricant providing no support to the shaft. As the speed increases, the skier rises out of the water. This is similar to the mixed-film regime, as the water is providing some support to the skier. Once the boat is up to speed, the skier is fully out of the water and riding across the surface (full-film or hydrodynamic lubrication). Fluid pressure is generated in the lubricant film, which is able to support load due to its viscosity. Lubricating oils have a significant pressure-viscosity coefficient. This means that the greater the pres- sure on the lubricant, the higher the viscosity at the pressure point. In the case of rolling-element bearings, this pressure is high enough to raise the lubricant's viscosity to the point where it will deform the bearing's rolling elements. This pressure-viscosity coefficient is what provides the load-carrying capacity of a journal bearing. With a better understanding of lubricant films, let's now look at the two equations for determining the lubricant oil feed to ensure a sufficient quantity of lubricant to support the load in a journal bearing. The equation you use to calculate the proper circulating flow will depend on whether you are working in gallons per minute or in drops per minute. The equations can be seen in the box above. While most of the variables in these formulas are straightfor- ward, the clearance factor (m) may be confusing for some. It can be determined by calculating the diametral clearance (2C), which is equal to the bearing bore diameter minus the journal diameter. Obviously, the clearance will be much smaller than the journal diameter (D), so this value is multiplied by 1,000 to make calcula- tions easier. Therefore, the clearance factor is: m=1,000(2C/D). Returning to the original question about establishing the proper oil flow to a journal bearing, in this instance, the clearance was known. The diameter and length should be easy to obtain either by taking a measurement or by checking the documentation. The speed was also known, so the only value left to find is the load (W). This is simply a matter of determining the weight of the rotating element divided by the number of bearings. Once all the values have been identified and the appropriate equation selected for gallons or drops per minute, you just need to enter the numbers into a calculator. references Campbell, W.E. (1969). "Boundary Lubrication: An Appraisal of World Literature." ASME. Rippel, Harry C. (1960). Cast Bronze Bearing Design Manual. Cast Bronze Bearing Institute Inc. About the Author Loren Green is a technical consultant with Noria Corporation, focusing on machinery lubrication and maintenance in support of Noria's Lubrication Program Development (LPD). He is a mechanical engineer who holds a Machine Lubrication Technician (MLT) Level II certification and a Machine Lubricant Analyst (ML A) Level II certification through the International Council for Machinery Lubrication (ICML). Contact Loren at lgreen@noria.com. Typical journal bearing