Jim Bryan 
EASA Technical Support Specialist (retired)

One key method to improve the re­liability of bearings is through proper lubrication. We should always strive to use the best available lubricant in the motors and other rotating equipment we repair. 

Researching quality and compat­ibility – up front – will pay off in the long run. Refer to resources such as Table 1 to determine if the grease you use is compatible with ones that the customer uses (if that information is available). 

What If A Problem Arises? 
Hopefully this will never hap­pen, but what if you find a situation where you’ve made a repair and the bearings are failing after only a few months of service? What has gone wrong? Rooting out this problem is typi­cally time-consuming and difficult. After serious data collection, you might find that the problems did not begin until the customer relubricated the machine. Compatibility might be an issue, so check Table 1 again and confirm that your grease is clearly compatible with what the customer is using in his plant. For example, you may use Polyurea Shear Stable and the customer uses Polyurea Conventional. Since the table indicates that they’re compatible, you must dig deeper for the answers. 

We know the performances of the lubricants are comparable; both have similar dropping points, viscosities and oxidation resistance. Other criteria on the specification sheets line up as well. It is obvious now that something is happening when the greases are mixed. Alas, the table we have con­sulted only tells us that the thickeners used in the lubricants are generally compatible.

Compatibility is impor­tant not only in thickeners, but also in additives. Greases with the same thickener can be incompatible due to the interaction of the additive pack­ages of the two [1].

Resolving The Problem 
Now that you understand the problem, what should you do about it? First, you could minimize the risk by using greases with similar thickeners, use grease that is properly designed and applied for the machine and environment, and re-lubricate often [2]. This option acknowledges that the lubricants may not be completely com­patible and attempts to compensate by replenishing the lube often. Actually, though, this could lead to over-greas­ing problems as well, merely shifting issues from one mode of failure to another. 

You could also try to replace all the old grease by flushing new grease through the system. This method cannot replace all the grease hiding in the nooks and crannies of the bear­ing cavity, so there will always be a mixture of some ratio. Further, this is the very definition of over-greasing as you’re completely filling the grease cavity under pressure. Any good me­chanic will tell you that over-greasing is bad. 

Confirm Compatibility Through Lab Testing 
One way to confirm compatibility is through lab testing. This is costly, time consuming and often not entirely conclusive [3]. To determine compat­ibility, find how the lubricants mix physically and chemically [4]. The procedure for doing this is outlined in ASTM Standard D6185, Standard Practice for Evaluating Compatibility of Binary Mixtures. Various ratios of the greases are mixed and subjected to three tests. The tests defined within are: Dropping point (Test Method D 566), Shear Stability (Test method D 217), and Storage Stability (Test Method D 217). Any one of these tests could find a problem that the other two would miss. These tests should be done by a qualified lab with the equipment and experience neces­sary to properly perform them [1]. Of course, even with this testing, the ultimate proof is the performance and reliability of the machine [5]. 

Ask The Manufacturers 
A less-expensive alternative is to ask the direct question of the manu­facturers of both greases in question: Is your grease “x” compatible with grease “y” from manufacturer “fill in the blank?” If BOTH grease manu­facturers tell you, in writing, that those two greases are compatible, you have a reasonable certainty that they are. If EITHER one replies in the negative or will not commit in writ­ing, avoid mixing them. A reputable supplier will only be able to do this if the tests have already been performed. Results from these tests should be available. 

The example we’ve used applies to a grease-lubricated machine, but the same questions apply to oil. If two turbine oils are mixed, one with an acidic rust inhibitor and the other with a basic rust inhibitor, the resulting salt from the acid/base interaction could clog filters and form deposits that interfere with lubrication. Another ex­ample is that less than 0.2% engine oil can form emulsions in turbine oil [6]. 

Service Improvement Opportunities 
The goal is to provide a valu­able service to our customers. Part of this service should be the reasonable and prudent care to insure that there will be no compatibility issues with the lubricants used in the repair of their machines. One definitive way to accomplish this is to use the same grease they are using. There can be no compatibility issue if there is no mixture of greases. If their grease is comparable in performance to yours and that of the original manufacturer, why not use it? If it is not, you have a great opportunity to prove your value and help the customer improve overall plant reliability by educating them and recommending a better lubrication solution. 

Are there issues and/or implica­tions to this? Of course there are. How do you stock the multitude of lubri­cants that our customers might use? How do you know what lubricants customers are going to use on any giv­en day? These are difficult questions but we in EASA are in the service business. The service and value we provide defines our success. The extra effort we expend to provide better reli­ability and performance by educating and accommodating our customers’ lubrication needs will be appreciated. One service center’s solution was to post a board, provided with hooks to hang grease guns, and labels for each grease gun (and its correspond­ing position on the board). Beneath the specific grease name was a list of their customers who required that particular grease. The reward we will receive is the on-going, and hopefully expanded, services that customers will call upon us to provide. 

Categories: Bearings, Lubrication

Tags: Bearings

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