The next time you walk through the plant, take a good look at your lubrication practices. Do you see bearing housings that have collected enough dust and dirt to start a vegetable garden? Are handfuls of old grease lying outside the housings? Both of these are typical of too many lubrication programs.
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Proper lubrication is not difficult. It requires selecting the proper lubricant for each application, applying it correctly and adhering to a lubrication schedule that meets the needs of critical plant equipment. So why isn't lubrication done properly?
In the good old days, most plants had a group dedicated to lubrication. It selected, housed and applied oils and greases for the entire plant. More importantly, the group had the knowledge and experience needed to perform this critical function. Today, everyone,and, by default, no one,is responsible for lubrication. As a result, too many plants have seriously inadequate lubrication programs.
Grease lubrication of rolling element bearings
Many pieces of equipment continue to use grease-lubricated bearings. Generally, this bearing type is limited to relatively slow speed and low temperature applications. Two methods are used for lubricating this bearing type: hand-packing and field lubrication.
Hand packing bearings
As the name implies, the technician applies the grease to the bearing by hand. However, it's not as simple as one might expect. Follow these guidelines to ensure proper bearing operation and achieve useful life.
Fill each bearing completely with fresh grease. If the bearing has been used, clean it thoroughly before applying new grease.
When the bearing is installed, grease should not occupy more than one-half to three-quarters of the total available free space within the housing.
When one or more bearings are mounted horizontally, fill each bearing and the space between bearings completely, but leave one-quarter to one-half of housing free.
With vertically mounted bearings, fully pack bearings, but leave one-half of the top cover and one-quarter of the bottom cover free of grease.
Relubrication of rolling element bearings
Best practices for lubricating installed grease-type bearings are predicated on hand application, not pressure grease guns. Gun users have a tendency to overfill the bearing. Proper lubrication requires precise quantities of grease.
Two methods may be used to lubricate installed rolling element bearings. The first removes the bearing and repacks it as outlined above. The second puts additional grease into the housing. In both cases, remove old lubricant and accumulated contamination from the bearing and its housing.
With the first method, removing the grease and contamination removal is relatively easy. Thoroughly clean the bearing and its housing, and then repack with fresh lubricant.
Some housing designs have two access ports to facilitate the process. One port is used to pump fresh grease into the housing, while the second is used to purge the old grease. With this method, however, it's difficult to adhere to rules of proper hand packing. It doesn't provide a positive means of keeping space within the housing free of grease,something that's necessary for proper bearing operation and heat dissipation.
In too many cases, lubrication methods are inadequate and result in either too little or too much grease. It's virtually impossible to flush the old grease and contaminants from the housing without overpacking the bearing. In this instance, too little lubrication is more desirable than too much. When the bearing or housing is overfilled, the resultant friction and heat limits bearing life severely.
As a rule, the replenishment charge for a grease-type bearing should not exceed five percent of its original charge, and open the vent or purge port for a few minutes after start-up to permit excess grease to escape.
Oil lubrication of rolling element bearings
One would think that best practices for oil lubrication would be easier. After all, you don't have to worry about leaving enough free space in the bearing housing. You just need to follow a periodic lubrication schedule and make sure there's always an adequate quantity of fresh oil in the housing at all times.
Unfortunately, it's not quite that simple. Like with grease-type bearings, contamination is a problem. As a result, lubricating oils must be reconditioned or replaced on a regular basis to ensure proper operation and acceptable useful bearing life. Therefore, the lubrication program must provide a means of maintaining the oil's lubricating properties.
On large, common reservoir systems, a filtration system removes water and other contaminants from the oil as it circulates through the machine-train. In general plant applications, each bearing is isolated to a single, small reservoir, i.e., bearing housing. It's not cost-effective to remove, clean and return these oils periodically. Therefore, these applications are limited to "topping off" the oil cups or oilers mounted on the housings. For most applications, these are acceptable practices. However, both are dependent on the assumption that each bearing is receiving adequate quantities of fresh lubricating oil.
In many cases, that's a bad assumption. Oil lines, including those in individual oil cups or oilers, have a tendency to plug, which prevents oil from flowing into the bearing. An effective program must include a means to ensure proper bearing lubrication. One way to accomplish that is to coordinate the lubrication and vibration monitoring programs. If a bearing is not receiving adequate oil, the vibration signature shows a resultant change in bearing frequencies. Another method is to track oil usage by location. This works for applications that have separate reservoirs for each bearing, or a small population of bearings. By recording the amount of oil used for each application, a decrease in volume indicates that one or more bearings are not receiving adequate quantities of oil.
Proper lubrication is not difficult, but it requires following specific guidelines.
Contributing Editor Keith Mobley can be reached via email firstname.lastname@example.org.