How Lubricants Thermally Degrade

Today's Tip: Sight Gauge Simplifies Inspection

Consider adding a vertical sight gauge to critical gearboxes that are not fitted with a dipstick or a sight glass, where the only way to check the oil level is to take out the level plug. Fitting a sight gauge allows the oil to be checked on the run, and can indicate the oil level even if the gearbox is mounted in an unusual orientation. However, there are some important points to remember:

1. Fit the gauge to a low point (preferably the drain) and not the level plug, as you could overfill the box. 2. To avoid mistakes, mark on the gauge the static level as well as the running level on the gauge. 3. Make sure the gearbox has a breather, otherwise a partial vacuum could be set up also giving a false reading. (Submitted by Andy Monk, Manging Director, Lubrisolve Engineering Solutions Ltd. Thanks Andy.)

Dave Yunkers, Analysts, Inc. wins last week's Readers Challenge - Bewildering Oil Analysis Results. See his answer, as well as other answers.

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Book Bits: How Lubricants Thermally Degrade

From "Oil Analysis Basics."

Lubricants can thermally degrade for a variety of reasons and causes. Unlike oxidation, thermal failure can occur in new lubricants with healthy additive packages. However, many of the symptoms of oxidation are also symptoms of thermal degradation.

One of the most common causes of thermal failure in hydraulic fluids and some lubricating oils relates to aeration, i.e., entrained air bubbles. These bubbles can become rapidly compressed in hydraulic pumps and in the squeeze zones of bearings. This results in extremely high localized temperatures (adiabatic compression) leading to sudden formation of carbon fines. Hot surface carbonization is another form of thermal failure. When an oil thermally degrades, problems associated with sludge, varnish, deposits, viscosity change and additive decomposition will often occur.

More information about the book "Oil Analysis Basics"

Lube-Trivia: The Many Uses of Filter Carts

Test your knowledge and prepare for ICML lubrication and oil analysis certification.

QUESTION: Name four applications for a filter cart.

Get the answer.

Q & A: Why Filters Affect Oil Analysis

"How does the quality of a filter impact oil analysis results (contamination and ferrous wear)? Is it correct to say that using filters with different beta ratios in the same equipment will result in different analysis results? Is there any correlation in setting alarm levels and filter beta ratio?"

The quality of the filter is directly proportional to the cleanliness and wear rate of the system. A higher beta for the same micron pore size will result in a faster cleanup rate; thus, it is better able to control ingression of solids in the system. The main system ingression sources typically are seals, breathers and oil top-ups, and wear debris. When catalytic wear metals are taken out of the oil, the lubricant will experience longer oxidative life.

Therefore, expect to see better cleanliness results on the hard particle measurements, as well as lower levels of wear debris in the system, assuming that the oil is in good condition and there are no other problems like water ingress, misalignment or imbalance.

In terms of setting alarm levels, the first step is to set the alarm levels to achieve a reasonable and realistic gain in lubricant and machine life. Using any of several methods, this can be done to optimize your requirements based on financial, safety, environmental and reliability goals.

On that basis, the appropriate filter can be selected. Most filter manufacturers have methods to determine the required element size and pore rating to not only meet the cleanliness target, but also to match the lubricant viscosities at operating temperature that would affect the flow rate. Most companies have established a beta ratio of 200 for their quoted pore size, although recent changes mean that they must also quote a beta value for a number of size ranges to establish performance. (additional information on this topic)

Over the life of the machine, a better-quality filter (generally more expensive) will prove to be more cost-effective than a cheaper filter. It will provide better filter performance and better system wear rates. To add value, many high-performance filters offer an improved dirt-holding capacity.

Martin Williamson, Senior Technical Consultant, Noria UK Ltd.