Mixing Lubricants: A Recipe For Trouble

Ashley Mayer
Tags: lubricant storage and handling

Making a lubricant is a bit like baking a cake. Unlike other forms of cooking, in baking, measuring ingredients with "a sprinkle of this and a pinch of that" doesn't cut it - they need to be precisely measured and combined. Mixing lubricants is similar to taking two cake recipes and attempting to combine them; sometimes it works, other times it flops. This is where the analogy ends, because a wrong lubricant mixture can result in a lot more trouble than a flopped cake.

In my experience working in different plants, I'm amazed at how endemic lubricant mixing can be. In a study conducted by BHP Billiton, it was found that 23 percent of gearbox failures were attributed to "wrong lubricants and lack of oil."

While I am not aware of the percentage of wrong lubricants as opposed to low oil levels, there is no doubt that using the wrong lubricant, including mixtures of lubricants, is a major cause of machine and lubricant failure.

In most cases, lubricant mixing occurs due to a lack of knowledge about the possible negative consequences, as well as just a lack of attention to detail.

Figure 1. BHP Billiton Reasons for Gearbox Failure

The Problems Caused by Mixing Lubricants
Mixing lubricants can cause issues for the lubricant itself and the machine. These issues include:

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The Causes of Lubricant Mixing
Why is so much lubricant mixing taking place? Some of the more common reasons include:

Issues to Consider when Faced with Mixing Lubricants
Situations arise when mixing lubricants is unavoidable, which typically occurs when changing from one supplier to another; when needing to change, for operational reasons, from one lubricant to another; or when an inadvertent lubrication mix has already taken place. Faced with one of these situations, the following must be considered:

A good place to start when considering a mixture of lubricants is with one or both of the lubricant OEMs. Their engineering departments should have access to compatibility studies, both between their own products and competitor's products, and may be able to provide a solution. Always use this as the first resource.

If the lubricant OEMs are not able to provide the answers, it is up to the user to conduct the testing. It is important to use the proper test method to gain the most accurate results, because most commonly performed oil analysis tests typically do not provide the user with relative information.

Strategies for testing mixed lubricants include:

Avoiding Lubricant Mixing
In my experience, the biggest single cause of lubricant mixing is probably the lack of lubricant identification on both the lube-handling side (tanks, drums, containers) and on the machine side. The answer is simple and cheap to implement - institute a color-coding and tagging system. Once this is in place, the only chance of unplanned lubricant mixing comes from improper handling.

Again, it is easy to implement a comprehensive policy to manage transfer and storage practices. As far as lubricant storage issues are concerned, ensure that transfer equipment (pumps, containers, hoses, filter carts) are dedicated to lubricant types as much as possible, and when cross-use of equipment is unavoidable, proper flushing procedures are followed. Once good lubricant storage and handling practices are in place, virtually the only instances of mixing will come from planned lubricant changes, and with the benefit of foresight, these can be easily and safely planned. 

Reference
Noria Corporation. Machinery Lubrication training course.