When to replace lubricants
Q. How can I tell if a lubricant is worn out and no longer doing its job?
A: For machines that require periodic lubrication, you need to know the proper lubrication interval. Not lubricating often enough risks lower efficiency, excessive wear, or even component failure.
Some machine manufacturers publish suggested lubrication intervals. However, machines don’t always operate as intended. Therefore, you may need to establish your own lubrication intervals through periodic monitoring of both the lubricant and equipment.
Equipment monitoring. An easy way to check a lubricant’s condition is to monitor operating temperatures of the lubricated parts. As the lubricant ages, friction increases, leading to higher temperature. This method can also detect over-lubrication. After lubrication, for example, bearing temperature increases as the new grease works through the bearing. Within a short time, the bearing settles back into steadystate operation and the temperature stabilizes. If the bearing is over-lubricated, the temperature never stabilizes and eventually causes damage to the bearing or its seal.
Another easy way to check the lubricant is to monitor the energy being used by the lubricated equipment. If the loads on the equipment haven’t changed, any increase in energy use is probably due to increased friction. For example, as the grease deteriorates in electric motor bearings, the motor must provide additional energy to overcome higher friction losses, thus increasing the motor’s current draw.
Though the above methods are very useful, changes in operation and maintenance can give false indications of the lubricant’s condition. More accurate methods include vibration and shock pulse testing of the rotating equipment, Figure 1.
Vibration testing detects damage, out-ofround conditions, and poor lubrication of bearings, gears, and shafts. Shock pulse testing is used specifically for rolling element bearings. As a lubricant breaks down, its ability to keep bearing contact surfaces apart diminishes and the accelerations or shock pulse intensities increase. In Figure 1, for example, the large accelerations in the 45,000 to 48,000 cycles-per-minute (cpm) range indicate poor lubrication.
Although these two methods are more sophisticated, they are easy to use — requiring only that a transducer be attached to the rotating component. These methods can be used for one-time troubleshooting or long-term monitoring of problem areas.
Oil monitoring. The most common way to monitor a lubricant’s condition is called oil analysis. This method is used for gearboxes and other equipment with circulating oil or oil sump lubrication.
The user takes an oil sample and sends it to an experienced outside lab. The lab report gives the viscosity, acid or base content, and the amount of additives, as well as the amounts of water, solid contaminants, and wear metals.
Through periodic testing, you can detect trends that indicate oil degradation. Warning signs include a large increase in viscosity (typically 20% or more depending on the application), increased amounts of wear metals (suggested limits range from 3 to 150 ppm for iron and 0 to 4 ppm for chromium, depending on the application), and contamination by hydraulic fluids or water (typically 5% by weight for oil, somewhat higher for grease). A significant loss of additives also indicates oil degradation.
Wear particle analysis, another useful technique, determines the number, size, and composition of wear particles in an oil sample. Because this method doesn’t measure changes in viscosity, acid or base number, or additives, it is a less-accurate indicator of the lubricant’s condition. But, it is particularly useful for monitoring equipment that is subject to high wear due to inadequate lubricant film between contact surfaces, or due to high loads or vibration. But, it is particularly useful for monitoring equipment that is subject to high wear because the lubricant can’t keep the running surfaces separated, or because of high loads or vibration.
Grease monitoring. Greases are harder to monitor than oils and require different analyses. Because greases are commonly used in sealed-for-life applications or inaccessible areas, grease samples are difficult to obtain and sampling is infrequent.
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Tests for grease deterioration include apparent dynamic viscosity (5 times increase is generally acceptable), wear particles ( as little as 0.05% can affect performance), and water contamination (up to 20% allowed, depending on the type of grease). Other parameters and their suggested limits include worked stability (decrease of one NLGI grade), hardening (increase of one NLGI grade), and total acid number (increase of 2 points).
Systems that use periodic or continuous relubrication generally replenish the grease often enough to ensure a fresh supply to the lubrication point.
Q. Is there a “seat-of-the-pants” way to evaluate lubricant condition?
A: Don’t discount your senses and experience. The way a lubricant looks, feels, and smells, or the way a bearing sounds or feels, can indicate lubricant deterioration. Symptoms include grease that changes from white to black, turns to “tar,” or stiffens up; bearing grease that dries up or leaks from the seals; and bearings that become noisy.
Next: Effect of water on lubricant performance.
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