In January 2000, a tragic event occurred off the coast of California. Alaska Airlines Flight 261 was flying to San Francisco from Puerto Vallarta, Mexico. When the pilots realized the unexpected response from their flight controls, they first attempted to troubleshoot out at sea to minimize the risk to people on the ground. In the terrifying last moments, the pilots tried heroically to fly the plane upside down after the uncontrollable horizontal stabilizer had caused the plane to invert. All aboard were lost.
The investigation began with the recovery of the wreckage, including retrieval of the horizontal stabilizer from the ocean floor. Incredibly, the investigation team was able to recover grease from the stabilizer jackscrew for analysis. The grease analysis, along with inspection of the jackscrew threads, revealed that the stabilizer control had been completely lost as the threads stripped away. The root cause was determined to be inadequate lubrication of the threads and deferred maintenance inspections, which included measuring wear on the threads.
Among the issues discussed in the investigation was a change in the grease used in the jackscrew. Over the history of operating these planes, the manufacturer presented an alternate product as being approved for use, but there was no documentation of any compatibility testing between the previous grease and the new one. While not a contributing factor in the failure of Flight 261, the investigation suggested that product changeovers could create the condition of mixed lubricants if the previous product was not completely removed, and that this should be a concern for future maintenance activities.
Most lubrication actions are not life-or-death decisions, but the same sort of damage that led to this tragedy is seen on a daily basis in grease-lubricated components around the world. The result of their failure can be unexpected downtime, higher maintenance costs or even personnel safety risks. In the worst cases, human lives may be at stake. It is time to stop treating grease as some simple substance that just needs to be pumped into machines at some random frequency and then hoping for the best. Machine greasing must be a systematic and carefully planned process to ensure safe operation of assets and to achieve maximum equipment life.
Whether your asset mission is critical, or you are just looking to optimize operating costs, the following steps are important for trouble-free grease lubrication:
“Grease is just grease.” The death of many machines begins with this statement of ignorance. This perception is not helped by oversimplified instructions from original equipment manufacturers. “Use a good grade of No. 2 grease” is the extent of guidance given for some equipment. However, if long, trouble-free asset life is the goal, then the selection of grease must include the proper base oil viscosity, base oil type, thickener type, NLGI grade and additive package.
Some machine locations have a prominent Zerk fitting, and the choice of where and how to apply grease seems obvious. But is there just one fitting? My dad is a farmer, and when he purchases a new implement, his first action is to review the manual or survey all parts of the machine to determine the number of greasing points. He then creates his “lubrication procedure,” which consists of writing the total number of fittings and hints on where the tricky ones are hidden with a permanent marker on the machine.
In other cases, the application point may not be obvious or may require special tools for proper application. For threaded applications, like the jackscrew mentioned previously, achieving sufficient coverage of the threads can be challenging. Tools exist to help ensure complete coverage of valve stem threads, for example, which can make a big difference.
Unfortunately, many maintenance programs decide on the grease lubrication frequency out of convenience. Rather than consider the conditions of each machine and how quickly a specific grease will degrade or be contaminated, some generic frequency is selected and applied equally to all. Perhaps a route is created to grease all machines once per quarter or once per month, and a few shots of grease are applied at each point. However, “one size fits all” rarely fits any optimally. Tables and calculations exist for identifying the correct frequency based on speed and temperature, and adjustments can be made according to estimates of contaminant levels and other factors. Taking the time to establish and then follow a proper lubrication interval will improve machine life.
Once the right grease has been selected and an optimized relubrication schedule developed, it is still necessary to evaluate and adjust as needed due to differences in field conditions. One way to test lubrication effectiveness is with the use of ultrasonic monitoring. By listening for sounds generated by asperity contact in ineffective bearing lubrication and determining the amount of grease required to restore the bearing to the correct lubricated condition, you can make adjustments to the calculated values and achieve precision lubrication.
In addition to the use of ultrasonic monitoring, feedback on greasing effectiveness can be obtained through grease analysis, but first a representative sample must be taken. New tools and techniques for grease sampling have recently been developed. Although grease analysis doesn’t happen as often as oil analysis, it can prove beneficial in monitoring the equipment condition, lubricant condition and lubricant life.
Maximum equipment life can be achieved by ensuring grease lubrication is effective. This also results in minimal wear. Detection of wear quantities and modes can help you make adjustments and discover problems earlier. It is important to monitor in-service grease consistency, as grease that softens too much can run out of the machine or fail to stay in place. Grease that hardens can provide inadequate lubrication and increase the load and electrical consumption. Grease mixing with the wrong product is one of the most common causes of failure. Early detection of this condition can allow purging and restoration before significant damage takes place. Tests to measure the quantity of moisture and particle counts in grease have been developed. Utilizing them to identify contaminant ingression, or just plain dirty greases, can present the opportunity for life extension through the use of clean greases and more effective sealing mechanisms.
While even a single bearing failure is regrettable, it is worse still when the opportunity to learn from it is squandered. I’m often told there is “no time” to save bearings and document as-found conditions following a failure. The focus is on restoring production. Broken parts are thrown away or put in the parts washer where the evidence of the failure is washed away. If a failed part and the grease can be recovered from the ocean floor, you should be able to save these components following a plant failure.
Understanding the reasons a failure occurred doesn’t just impact the restoration of the machine but can have a multiplied effect on the reliability and life of other components across the enterprise. Ensure that root cause failure analysis includes inspection of the bearing surfaces, but first start with preservation and then removal of the grease for analysis. Combining results from the lubricant analysis with the bearing analysis will create a more comprehensive picture of the failure and help you determine which corrective actions can be used to prevent it from happening in the future.
|35%||of lubrication professionals never inspect the grease discharge from bearings and other machine components at their plant, based on a recent survey at MachineryLubrication.com|