Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/325287
56 | May - June 2014 | www.machinerylubrication.com percent of bearing failures are due in part to contamination. Similar studies show the benefits of controlling particle contamina- tion. For instance, Nippon Steel was rewarded for its contamination control efforts with a nearly 76-percent reduction in pump replacement frequency, a 75-percent reduction in oil consumption, an 80-percent reduction in hydraulic repairs and a 50-percent reduction in bearing purchases. Likewise, after BHP Billiton improved filtration at one of its mills, production increased nearly 3.5 times. These are just a few examples of the benefits to be gained from controlling particle contamination. "Bright" refers to the presence of water. Whoever made the state- ment that oil and water do not mix was just plain wrong. Nearly all oils have a certain quantity of dissolved water, and this dissolved water will not be evident by conducting a visual test. Oil will contain a quantity of water up to the saturation point and still appear clear. According to SKF, "The presence of water in lubricating oils can shorten bearing life down to 1 percent or less, depending on the quantity present." Oil can actually carry up to 2,000 parts per million of dissolved water before reaching its saturation point and beginning to appear cloudy. In his Machinery Lubrication article on how water causes bearing failure, Jim Fitch explained several modes of failure caused by water in oil. Among these are hydrogen-induced fractures, corro- sion, oxidation, additive depletion, oil flow restrictions, aeration and foam, impaired film strength, and microbial contamination. A few of these modes are less obvious as to how they operate than others. In regard to impaired film strength, lubricating oils have a unique property known as the pressure-viscosity coefficient. Simply put, the higher the pressure, the higher the viscosity. The pressure in the load zones for rolling-element bearings is often in excess of 500,000 psi. This causes oil to almost become a solid, and it will maintain the separation between the rolling element and the raceway. The viscosity of water is one centistoke, and regardless of pressure, it stays essentially at one centistoke. Therefore, water will not be sufficient for maintaining the separation between the rolling element and the raceway. It should now be readily apparent that it is not a sound practice to simply utilize the clear and bright criteria for determining whether oil is acceptable to use or for deciding whether equipment can be placed into service. As far as I know, the Navy currently has no means of tracking the number of bearing failures it experiences over the course of a year. It would be interesting to see these statistics and the associated costs. If your organization is using the same methodology as the U.S. Navy, you likely are losing a large amount of money every year. 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 I certification and a Machine Lubricant Analyst (ML A) Level III certification through the International Council for Machinery Lubrication (ICML). Contact Loren at lgreen@noria.com. BACk PAge BAsICs COMPONENT CLEARANCE Rolling-element Bearings 0.1 to 3 microns Journal Bearings 0.5 to 100 Gears 0.1 to 1 Engines • Ring/Cylinder 0.3 to 7 • Rod Bearing 0.5 to 20 • Main Bearings 0.8 to 50 • Piston Pin Bushing 0.5 to 15 • Valve Train 0.0 to 1.0 • Gearing 0.0 to 1.5 Pump, Gear • Tooth to Side Plate 0.5 to 5 • Tooth Tip to Case 0.5 to 5 The USS San Antonio was commissioned on Jan. 14, 2006. Shortly after being placed into service, the ship began to develop serious mechanical issues with its main propulsion diesel engines. The failures were the result of several factors, which were not all contamination-related. The Navy conducted an investigation into the causes of the engine failures. Several issues were discovered, with a good number of them resulting from poor contamination control. In the investigation's report, it was noted that the lube oil service system design used muslin bags and that particles smaller than 25 microns would pass through the filters. During inspections of the engine oil sumps, welding slag, paint chips, inorganic fibers and free water were found. It was also reported that the system configuration allowed contaminants to recirculate through the pump. These contaminants would be macerated until they were able to pass through the filter. Case Study: USS San Antonio