Grease Quantity Having Influences on Bearing

May 21, 2016

Sealed bearing has already been fulfilled with appropriate grease during manufacturing. And open bearing is also equipped with sealing device in case of leakage of grease. Then, there is problem: how much grease is proper.

Too little grease quantity can easily cause lacking of grease of dry friction, declining service life. Too much grease quantity can result in temperature of bearing rising. Large leakage of grease pollutes environment and causes waste. For this, under the condition maintaining bearing fully lubricated, less grease quantity is better. 

Bearing should not be added with too much grease
Grease has good adhesiveness, abrasive resistance, temperature resistance, rust protection and lubricity, which can highly improve high temperature oxidation resistance, extending ageing, dissolving carbon deposition, preventing metal abrasive dusts and oil contamination, enhancing abrasion resistance, pressure resistance and corrosion resistance of machinery. 

Understand motor process of grease in bearing. A conclusion comes out: grease in bearing should not be too much. Too much grease is not only wasteful, but also harmful. The higher revolving speed of bearing is, the larger hazard is.

The more grease quantity is, the large friction torque is. Having the same quantity, the friction torque of sealed bearing is larger than that of open torque. When grease quantity is equivalent to 60% of internal space volume of bearing, friction torque does not obviously grow any more. Because most of grease in opening bearing has been extruded out and grease in sealed bearing has already leaked.

With grease quantity increasing, temperature of bearing rises perpendicularly. Having the same grease quantity, temperature rise on sealed bearing is faster than that on open bearing.

In general, grease quantity on sealed ball bearing cannot exceed than about 50% of internal space at most. Test of Shawki and Mokhtar shows that 20%-30% of grease quantity is the most appropriate on ball bearing.

How to extend service life of bearing
Firstly, lubrication is the key. Rational lubrication system and high quality grease should be chosen. Lubricant film separates oil film surfaces which contact each other due to load bearing, providing necessary protection for corrosion and abrasion. For all of rotation and reciprocating motion motors, especially bearing and gear, lubrication film is indispensable. For splash lubrication or circulating lubrication, oil film can also transmit heat. Unnecessary closing down occurs or equipment deteriorates because of inappropriate lubrication. Insufficient oil supply can result in abrasion increasing and temperature rising, which can cause excessive wear, initial failure or damage of bearing. Excessive lubrication, especially for high speed running equipment, will cause overmuch heat caused by oil stirring, making grease cause chemical degradation or bearing damage. 

Rational lubrication and periodical maintenance can avoid bearing damage caused by lubrication. For reasonable lubrication, the following measures should be taken:
1. Follow operating guide on every equipment set by manufacturer;
2. When adding grease, the grease should be filled between rolling parts and shell of bearing, ensuring that sufficient grease can be filled. Surface of key rollaway also should be lubricated fully; 
3. When to re-lubricate should be noted; 
4. Observe indicating instrument on equipment to discover problems as early as possible such as temperature fluctuation and abnormal high temperature;
5. Pay attention to equipment noise or abnormal vibration;
6. Note leakage of grease;
7. Sample grease regularly to check pollution.

Secondly, to improve performance of bearing, rational adjustment is very critical. During installation and disassembling, if internal clearance of bearing is too large or too small, or preloading is overlarge under some situations, early damage may be caused and service life of bearing is declined. Besides closing down and repairing having high cost, improper assemble and installation may cause more negative effects, such as influencing operation of other parts and reducing service life of bearings.

Assembling, installation and periodic maintenance based on service manual supplied by manufacturer are the best ways for precaution. If bearing is adjusted too tightly, it may cause super heating damage. And bearing must be replaced. The processes of assembling bearing must be accurate, repeatable and it also should be able to verify. Parameters of other parts should also be taken into consideration to optimize performance of system.
Grease Quantity Having Influences on Bearing

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