Bearings are a basic machine component found in a large number of items from cars to computers and everything in between. Their basic functions are to reduce friction between moving parts, support moving loads, and maintain alignment. Bearings can come in a variety of shapes and sizes. They can be “Plain” or Journal bearings with no moving elements or they can contain roller elements.
JOURNAL BEARINGS
Journal bearings can carry high radial loads because of the greater surface area; but they cannot handle thrust. These bearings can be lubricated with oil or grease. The proper oil viscosity for any bearing should be specified by the OEM and can range from ISO Viscosity Grade (VG) of 5 for very light, low pressure, high speed loads up to 1000VG for slow speed and heavy loads. Lubricant is applied to these bearings through holes and distributed by grooves.
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Chamfered oil grooves keep oil in place and allow for greater flow to cool the loaded area of the bearing. In applications where an oil film is needed at start up the oil can be fed under pressure or hydrostatically which will support the load at startup. Many operations provide continuous lubricaton by discs or rings.
Both of these work by riding on the shaft, picking up oil in the bath and dropping it into the oil hole on the bearing. These are typically slower speed operations since high speeds will throw the oil away from the hole. In these systems the ring should be 1.5-2 times the size of the shaft and the oil level should be 1/8 to 3/8 inch above the inside of the bottom of the ring.
Grease lubrication of jounal bearings can help to keep the lubricant in place and seal out contaminants, however they have several disadvantages. Grease cannot be filtered, does not transfer heat well or remove contaminants. There are situations where a grease can be used to extend the life of the bearing if a change cannot take place when needed; a low NLGI grease like 000 or 00 can be used to help support the shaft until maintenance can be performed.
Representative Journal Bearing Failure Modes:
- Babbitt Fatigue – eventually all bearings will fail. The babbitt is the inner liner of the bearing and is typically made of softer material than the shaft. This prevents the shaft from getting damaged.
- Lack of lubrication and improper grooving – If the lubricant is not getting to the shaft/bearing interface the bearing will eventually fail due to heat from friction.
- Abrasive particle damage – if oil applied is not being filtered properly there can be hard particles that contaminate the load zone and cause premature wear.
- High temperature oil Burn – This is usually the result of the wrong viscosity lubricant, too much load, too high speed, too small reservoir, or expired lubricant life.
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ROLLER ELEMENT BEARINGS
Roller Element bearings like ball bearings, tapered roller bearings, and needle bearings provide some advantages over journal bearings. The major ones being the significant lower startup friction, higher speed capabilities and the ability to handle thrust (tapered roller bearings). These bearings consist of several elements including inner and outer rings, the roller elements and seals.
The selection of an oil’s viscosity for a particular bearing can be calculated by knowing the speed, size, type, and operating temperature. There are also many other factors that go in to determining the oil used to lubricate bearings. The environment and ability to maintain the oil by filtering and cooling along with the loading of the bearings all play a part determining the additives the oil will need. In an oil bath system the oil level should be maintained halfway up on the bottom element.
Multiple bearings with constant uses may nessitate the need for an automatice lubrication system to help eliminate lubrication problems.
Greases are used many times in roller element bearings due to the fact that they are frequently sealed or operate in isolated conditions where oil is not practical. One major problem with grease is the effects of over greasing. Not only is grease wasted but if there is too much grease in a bearing the heat cannot disspate and the oil cannot properly separate from the thickener to provide lubrication. Single point grease lubricators and centralized grease systems are also available to supply grease to multiple bearings in high use facilities. Your lubricant supplier can assist in determining if these are needed or beneficial.
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Roller Elements Have Multiple Components and Failure Issues:
- Poor Maintenance
- Poor Design
- Ineffective sealing
- Wrong bearing for application
- Overload or excessive speed
- Insufficient Lubrication
- Incorrect Lubrication
- Oil Deterioration
- Temperature variation
- Contamination
- Incorrect assembly/installation
- Misalignment
- Incorrect clearances
- Improper seating
- Vibration
There Are Many Proper Care Practices For Bearings My Three Favorite Are:
- Dip to clean in solvent bath
- If you drop it, throw it away the stressing of the materials from the drop will cause certain failure when you don’t want it.
- If you touch it, oil it. Acid from skin can cause initial corrosion and spread to more problems.
This white paper was prepared by Joe Goecke, STLE® Certified Lubrication Specialist and Manager of Industrial Services for Apollo Oil. Apollo Oil is a Premier Multibrand Distributor providing Lubricants and Metalworking fluids for all industrial users. Apollo services all of the Southeastern United States. Contact Apollo at 859-744-5444 jgoecke@apollooil.com. Website www.apollooil.com
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