Lubricating a precision bearing optimizes the success and extends the lifespan of all applications. When choosing a lubricant for your application, one of the first questions you need to answer is whether you need grease or oil.
We will cover the technical details of grease and oil in this article so you can choose the right one for your application.
Find Answers to These Lubrication Questions
Answering these questions can help you identify if you need a grease or oil lubricant:
- Is oil already being used in the system?
- What temperatures are the bearings exposed to?
- What bearing life is required or is there a maintenance cycle?
- What’s the lubrication budget?
In looking at an internal combustion engine for vehicles or stand by generators, the bearings within the starter pulley are most likely grease lubricated. These bearings are relatively open to the atmosphere and creating the oil circulating system would be very difficult. Conversely, most turbochargers on these engines already have oil circulation for both the bearings and cooling effects.
Another interesting application is CNC spindles. Metalworking spindles traditionally use oil lubrication with set feed rates, return lines, and a reservoir for cooling the oil. These high duty cycle metal working spindles sometimes require the added cooling effect on the bearings that this oil circulation provides.
Conversely, most woodworking spindles now use grease with a sealed bearings. The two main reasons for this:
- The seals keep the grease incorporated in the bearings to prevent contamination from wood/plastic/etc.
- The cost of the relubrication system is removed. This creates a much lower price point for woodworking spindles than many metalworking spindles.
Temperature & Bearing Lubrication
Application temperatures also play a big roll in the precision bearing lubrication choice.
Oil circulation can provide temperature removal within an application. In applications with extremely low-temperature requirements, a highly engineered grease to prevent large viscosity swings might be more optimal.
Many bearings that are used in space have a grease lubricant because of the low temperature and no-maintenance requirements. Greases designed for space applications have a very low viscosity even at low temperatures to keep efficiencies as high as possible.
Both oils and greases have capabilities for extremely low and high temperatures. The differences will be in viscosity, overall temperature usage range, chemical compatibilities, chemical resistances, and maintenance cycle and bearing life requirements.
A properly greased ball bearing can have a successful application life of years. A key metric in a long precision bearing life with grease lubrication is the initial set up. First, bearings are either delivered pre-greased from the factory or with a preservative oil lubrication film. If you are greasing the bearing yourself instead of a bearing industry professional, you need to be positive that the grease being utilized is compatible with the oil preservative from the factory. If it is not compatible, the grease will not have proper adhesion to the bearing surfaces and will slide out of the bearing causing premature failure. This is common with PTFE and other specialty greases that are not compatible with many preservative oils. To grease a bearing with PTFE, the surface needs to be cleaned with a non-residual solvent, a compatible preservative put on the bearing surfaces and the specialty grease properly applied. This will ensure good lubrication adhesion to the precision bearing surfaces, which will in turn provide a long application life. Another aspect that is absolutely pivotal for grease lubricated precision ball bearings is the break in procedure. A proper greased bearing break in procedure will ensure the right amount of grease remains in the raceways, a good grease collar is formed, and temperatures are stabilized. There is a good precision ball bearing break in procedure here.
A similarly long life can be achieved with a properly executed oil lubrication system. Oil/Air or Air/Oil systems will delivery small amounts of oil to a high-speed precision bearing. Most oil systems for high speed bearings delivery as little as 1 drip of oil a minute up to a small flow of oil. Sealing of the oil system, a good oil filter, and correct oil choice will also contribute to the application.
As hinted at above, the costs of a complex oil system can be significant. These systems can include but are not limited to: the oil pump, tubing, reservoir, chiller, customized nozzles, etc.
A wild card in the ball bearing lubrication conversation is dry lubrication. This category can include hard coatings, malleable coatings, or even precious metals.
Some common materials are Molybdum DiSulfide, Tungston DiSulfide, Dicronite, Gold, and Silver. The cost for these lubrications can be quite expensive and not all bearing companies have the equipment and/or experience to apply them properly.
A downside to these lubrications is minimum application life. Generally, one should expect less than 1000 hours of lubrication life, much less if the application is high RPM. Dry lubricants are used quite often for touch down bearings, some space applications, and commonly in environments with high levels of exposure to chemicals – such as refrigerants.
Brand Lubrication Selection Options
Now that you have potentially chosen the type of lubrication for your application, the next task is figuring out which is best. Commonly companies will have a few ‘go-to’ lubrication options that are used internally, this is a great starting point. Other aspects to take in to account of application/bearing temperatures, n*dm speed factor (RPM * diameter of balls, provided in catalog here), time before usage, cost, availability, etc.
Because of the application, your bearing lubrication choice may be simple, or maybe even made for you. But if not, be sure to fully flush out the myriad of options to properly lubricate a precision ball bearing. GMN Bearing USA Engineers are here to support your efforts with our decades of experience in both methods and ball bearing lubrication choice.
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