Imagine rubbing your hands together 20,000 times a minute. The two surfaces would generate an unbearable amount of heat caused by friction unless you were to use a suitable lubricant. Here, Chris Johnson, of SMB Bearings, explains that lubricants are just as important in bearings and why, if you don’t use the correct lubricant, expensive failure is very likely to result.
Standard lubrication — the generic stuff that is in the bulk of bearings, will do its bit in reducing friction and dissipating heat. However, this lubricant may not satisfy the maximum running speed, torque level and temperature requirements of your application. All is not lost though; plant managers and maintenance engineers can have bearings supplied using bespoke lubrication that is best suited to the specification.
Oil or grease?
Some applications are more suited to oil than grease, although both have advantages. Higher running speeds are possible with oil, but because it tends not to stay in place effectively, continuous re-application is needed.
In many applications, grease is more convenient. Simply defined as an oil mixed with a thickener so it stays inside the bearing for longer, grease benefits from both the properties of the base oil and the added thickener; both affect the performance of the bearing and ultimately its lifespan.
The extent to which you fill the bearings with lubricant can have huge implications on performance. Lots of free space inside a bearing allows heat to radiate away from the contact area between the balls and raceway and makes for low rolling resistance. On the other hand, a very full bearing may suffer from greater heat build-up and more rolling resistance yet the higher grease fill will increase the bearing’s capabilities in handling higher loads.
Lubricants have recommended upper and lower temperature limits that maintenance engineers and designers should abide by. Using a lubricant above its upper limit will cause lubricant breakdown and rapid bearing failure.
Standard greases will be rated up to 100-120º Celsius, which is fine for most applications. For hotter applications, there are grease options that are suitable for constant use up to 288º Celsius and short interval use at 300º Celsius.
Extreme low temperatures can be faced in marine, outdoor and cryogenic applications; the catch is that cold temperatures make oil and grease much more viscous. Below a certain temperature, a grease might become so stiff that the bearing becomes difficult to rotate. At this point, the grease will not lubricate effectively. Ironically, this can generate excess heat which will soften the grease but, before this occurs, the bearing will experience accelerated wear. Choosing the right low temperature grease will allow effective lubrication down to minus 80º Celsius.
Low torque requirements
A bearing with high frictional torque requires more effort to spin; sometimes referred to as rolling resistance. Many applications require the bearing to spin easily with extremely low frictional torque without generating excess heat. Dry lubricant is one way of meeting low torque requirements. Despite being in the solid phase, dry lubricants are able to reduce friction between two surfaces without the need for oil or grease.
Using very low viscosity grease with a reduced fill can achieve a similar outcome but will also permit much higher running speeds.
High speed applications
Bearings that endure a large number of revolutions per minute, and are lubricated with an overly viscous grease, can suffer from excessive heat build-up. As the grease overheats, it becomes too thin and can start to leak out of the bearing, causing further problems. Therefore, low viscosity base oils are desirable in high speed greases, along with smooth thickeners to generate less heat and retain oil better. In addition, the standard grease fill of 25 to 35% of the internal space of the bearing should be reduced for high speeds.
So, there you have it. You can rub your hands together in anticipation of setting up your own equipment for the ultimate success. The right bearing lubrication is the foundation of long term productivity, overall equipment effectiveness and reduced repair costs.