June 6, 2016
MACHINERY VIBRATION PROBLEMS – PART 4 BEARINGS
By Stan Riddle
In industrial machinery, the centerpiece of vibration analysis is the ability to detect bearing problems BEFORE the bearing fails. Even more beneficial is detecting the root cause of a bearing problem before damage is done.
The bearing really is the heart of the rotating machine. There’s a plethora of bearing types and designs. There are numerous manufacturers, training programs, and books on bearings. Diagnosing bearing failures early is one of the hallmarks of vibration analysis.
But the cause(s) of bearing failures are what’s most important. If you can minimize the causes of bearing failures, you can minimize replacing bearings, reduce “wrench time”, and improve reliability.
Bearings need five basic things:
- Proper fits, mounting, and loading
- Proper application
- Correct operating temperature range and speed
These five requirements sound simple enough. Most any experienced maintenance mechanic would agree to this list. Any bearing manufacturer, training facility, or instruction manual would as well.
So, why do bearings fail? Usually because one or more of these things isn’t happening. There are many studies available that quantify common causes of bearing failures, but let’s just use this list of five.
Cleanliness. Bearings must be manufactured, packaged, transported, stored, handled and installed like the extremely precise machinery components they are. Buy good bearings-not just the “cheapest” bearing. When bearings are stored for future use, moisture and contaminants must be avoided. New bearings should be stored in a clean, dry environment. Do not open a bearing box until you plan to use it.
Keep your hands and bearing assembly tools clean. Clean the areas where the bearing is to be mounted-don’t just “wipe it with a shop towel”. You may be thinking “everyone knows this”. They know it-but do they practice it? Every maintenance mechanic knows “the way it’s taught, and the way it’s done” can be different! But there are no shortcuts to precision bearing cleanliness and handling. As in many things, you have to “work slow to work fast”.
Lubrication. As mentioned previously, lubrication is critical. And the proper lubricant is a must. The lubricant should of the proper weight, viscosity, additive package, and operating temperature range and quantity for the application. Lubricants should not be mixed. They should be clean, and its cleanliness confirmed by testing or filtering. The tools used to add lubrication should be clean-laboratory kind of clean. How many people have picked up a dirty grease gun, hanging on the side of a dirty machine, shoved it onto a dirty grease fitting, and pumped grease into a bearing, without even wiping anything off? Lubrication – CLEAN lubrication, is a must.
Proper Fits, Mounting, and Loading. Bearing fits are often very precise, many times to tolerances of less than 0.001”. Bearings normally should not “drop in”, nor should they be “beat in”. The mounting surfaces should be concentric and clean, but not overly sanded, filed, or polished. They should be MEASURED with a micrometer or gauge. Proper bearing mounting tools should be used, not just brass bars and hammers. Some bearings have clearances that must be set as part of the bearing installation.
Proper Application. There are many different types of bearings – sleeve, plan (babbitt), tilting pad, ceramic, sintered bronze, balls, rollers, single row, double row, thrust, tapered bore, cone and cup, needle – and on and on. Why? Because bearings do different things. A bearing designed for a hammer mill is not the same as one for a high speed gas turbine. Proper bearing application should be done by bearing manufacturers and engineers, not just by whether the bearing fits or not.
Correct Operating Temperature Range and Speed. Bearings can get hot, because of friction and loading. Bearings grow as they get hotter. As they do, clearances within the bearing can change. Engineers design bearings with this in mind, but bearings have their limits-both in speed, load, and temperature. When replacing or specifying a bearing, pay close attention to the operating speeds, static and dynamic loads, and temperatures. Be sure the bearing/bearing housing can disperse heat- 6 inches of dust on a bearing housing acts as a blanket. If the bearing utilizes cooling, make sure the coolant system is clean, and working. If the bearings are lubricated from a reservoir, make sure it is clean, and the heat is dissipating.
OTHER PROBLEMS SOMETIMES INCORRECTLY DIAGNOSED AS BEARINGS
- Bearing seals or shields – usually rubbing
- Rubs within a machine
- Lack of lubrication
- Couplings – especially lubricated couplings