Machinery Lubrication

Machinery Lubrication May June 2015

Machinery Lubrication magazine published by Noria Corporation

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52 May- June 2015 | www.machinerylubrication.com ASK the EXPERTS Think of a bearing failure as happening in four stages. During the fi rst stage (or earliest detectable point using vibration analysis), the signals will appear in frequency bands around 250 to 350 kilohertz (KHz). In the second stage, a signal around 500 to 2,000 hertz (its natural frequency) will begin to ring. At the onset of the third stage, the harmonics of the funda- mental frequency will start to be very apparent. Defects in the races are now obvious and will be visible on vibration analysis of the noise signal. At this point, there will also be a signifi cant temperature increase. During the fourth stage, there will be very high vibration. The fundamental frequency and harmonics begin to decrease as the random ultrasonic noise is boosted. Temperatures will start to skyrocket as the bearing self-destructs. So, the short answer is yes. There are defi nitely warning signs of a bearing about to fail. The real question is, "Do you know what to look for?" The most popular technologies today for bearing monitoring are vibration analysis, oil analysis, ultrasonics and thermography. You can use these tools to compare current states to historical data and accurately assess the remaining life of the bearing. Vibration analysis and oil analysis are considered the best at predicting a failure but are not always the most cost-effective. Bearing manufacturers have long known of the relationship between bearing life and temperature. They even have formulas that work very well at calculating safe operating temperatures. These formulas and calculators show that once a bearing starts operating outside its ideal temperature range, its life will begin to degrade at an accelerated rate. Keep in mind that for every 15 degrees C above 70 degrees C that the base oil operates, its life is more than halved. Considering this, why is thermography not a more popular method for bearing life prediction? The monitoring of tempera- tures is not always considered reliable because of the sheer amount of variables that contribute to the heat generation. Ambient temperature, friction, speed variability, load and runtime all have an effect on the temperature that will be measured. Friction is the variable you should be the most concerned with if trying to predict a failure, but how do you separate it from all the others? If you could account for all the variables accurately, the increases you would get in operating temperature could be a great indicator of an impending failure. Perhaps the cheapest and easiest way to spot a bearing failure is to use a non-contact infrared thermometer. The caveat is that you must always account for the other variables as well. "Are there any warning signs of when a bearing is about to fail?" "How important is the kinematic viscosity at 100 degrees C for a vacuum pump oil?" Viscosity is the most impor tant physical proper ty of a lubricant. Whether you are looking at the viscosity at the standard 40 degrees C or at 100 degrees C, it is the viscosity at the machine's operating temperature that is crit- ical to understand. Most bearing temperatures are hotter than 40 degrees C (104 degrees F) at the core, which is where the temperature should be analyzed to match the correct viscosity for the application. Vacuum pumps stress oil in a variety of different ways. Aside from the operating temperature, contamination is prevalent if these machines are not properly set up to exclude the ingress of contaminants. Also, the gas being processed has a huge impact not only on the viscosity but also on the lifespan of the oil in service. Process gases can reduce the oil's viscosity, raise the acid number, increase the moisture level and lead to short lubricant life and low oxidative stability. Most lubricant manufacturers will provide the viscosity for their products at 40 degrees C and 100 degrees C. The amount of change in the viscosity in relation to the temperature is known as the viscosity index of the lubricant. The viscosity index is most often

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