Essential to proper operation of a ball and roller bearings is lubrication. A proper lubricant will reduce friction between the internal sliding surfaces of the bearings components and reduce or prevent metal-to-metal contact of the rolling elements with their raceways. Proper lubrication reduces wear and prevents corrosion, insuring long service lives for bearings.
Lubrication, especially circulating oil will also remove heat from the bearing.
There are two basic types of bearing lubricants readily available: Oil and Grease. The former is fairly simple to understand being a free-flowing liquid, while the latter is a little more complex. In order to be a lubricant, all greases have oil that is entrained in a thickened base. It is this base that gives the impression that grease is a more viscous type of oil; however, it is the oil in the grease that does the actual lubricating. Each type of lubricant has its own advantages and disadvantages and is selected by the nature of the application. The major advantages of the two basic types of lubricants are:
Advantage : Easy to distribute, lubes other components, less drag, easier to drain out and change. Better for high temperature.
Disadvantage : May leak (environmental concern), then no more lubrication.
Advantage : Remains in place, doesn’t leak out easily, improves sealing,doesnot require monitoring.
Disadvantage : Requires more labor to clean out and replenish. High temperature grease is very expensive.
Each manufacturer of a lubricant can supply a specification sheet for each of their products, and each sheet will have a list of about 20 properties and their values related to this lubricant. The most important property of any lubricant for rolling element bearings is its oil viscosity. If the specification sheet is for an oil, the viscosity values will be for the oil. If it’s a grease, it should refer to “Base Oil Viscosity” or another similar term, depending on the manufacturer. Usually, four viscosity values are shown as follows:
1: cSt @ 40° C(104°F) SI units
2: cSt @ 100°C (212°F) SI units
3: SUS @ 100°F(38°C) Imperial units
4: SUS @ 210°F(99°C) Imperial units
It is very important to select a lubricant that will provide a minimum acceptable viscosity at the bearing’s operating temperature, which will usually be between the lowest and highest reference temperatures shown above. Typically, oil viscosity numbers decrease very rapidly with increasing temperature. Determining a bearing’s operating temperature is a fairly complicated calculation that is beyond the scope of this.Most machines use a lubricant that is selected to match the most severe demand of one component in the machine such as a bearing, gear, etc.
Additives are a very important feature of modern oils and greases and can often mean the difference for successful, long term operation of bearings and other machine components. The selection of any lubricant over another should always consider additives.
From a performance aspect, oil is the best form of lubrication, and it offers several types of delivery methods to the bearings. The simplest form is that of maintaining a static oil level in the bearing’s housing. In several types of equipment, such as those with gears and/or connecting rods, the oil supplied to these components creates a fog or spray that wets the bearing contact surfaces. This is sometimes called “splash lubrication.”
Next in complexity are the oil mist and air/oil systems that are designed to provide the exact amount of oil needed for lubrication, preventing excess oil that could be churned by the bearing, increasing drag and temperature.
For high-speed applications, jetted circulating oil is often necessary. Nozzles inject oil directly into the bearing providing a duel function of lubrication and removal of heat. These systems are complicated and expensive and are selected when absolutely necessary.
Generally, grease lubrication is chosen if the lubrication demands of the bearing will allow it. Typical grease systems are much simpler than oil systems and do not cost as much. Often, the only features needed are grease supply holes and an external grease nipple for replenishment.
When choosing a grease for an application, several of its properties need to be considered for the expected operating conditions. Priority of these properties is:
- Required oil viscosity at temperature of the bearing.
- Grade for the operating temperature.
- Soap base which is best for the application.
- Availability of EP (Extreme Pressure) additives.
The “Grade” level of the grease is an indicator of how stiff the grease is. Grades “0” and “1” are relatively soft and are typically used at low operating temperatures. Grades “2”, “3”, and “4” are used at increasingly higher temperatures. Grade “3” is also usually used in vertical applications to prevent all the grease from settling at the bottom of the bearing.
The different thickening bases have specific advantages so they may be selected for different applications. Some of their main advantages are:
Calcium: Inherent EP ability, corrosion resistance, safe for food processing, low-temperature use only.
Sodium: Lower cost, general purpose, medium-high temperature.
Lithium: Higher temperature, high speeds.
BentoneClay: Heavy loads at high temperature, water washout resistance.
Synthetic: Very high temperature. (High cost)
In many applications, it is necessary to replenish the grease at regular intervals as the old grease will “dry out” from bleeding oil to the moving parts of the bearing, and the thickening base will oxidize. Re-greasing should be an integral part of the design of the equipment, and some bearing types already provide a re-lube feature. Good designers will provide accessible grease passages in the machine itself for getting grease into the bearing. It does very little good to push new grease up against a bearing if the old grease blocks the way. It is far better to introduce new grease in the center of the bearing and let it push the old grease out each side. If this is not possible in the selected bearing, then the grease needs to be applied to one side of the bearing while the other side of the housing cavity provides someplace for the old grease to go. Some machine designs provided a purging vent or allow the old grease to escape under seal lips. Some types of equipment employed in industries that have abrasive particles in the air use the lubricating grease as a filter media to trap these particles. Regular re-greasing of these bearings and their housings purges the contaminated grease out of the bearing housings. It is important to remember that re-lubrication should be done when grease in the bearing is still good.
Important Point for Re-greasing Bearings (Conclusion)
Re-greasing intervals, that will always provide the proper amount of oil to the bearings, cannot always be accurately predicted. We do know that a proper interval is mainly a function of operating temperature, the number of hours in operation per day, and the size and speed of the bearing. Some equipment needs all the bearings re-greased every day, some once a week, some every two weeks, and some once a month. In applications such as this, it is often beneficial to completely wash out the bearings once a year, re-pack with new grease, and continue on with the established re-greasing program. Users are advised to not only inspect the condition of the old grease but to send samples to a laboratory that specializes in analyzing used lubricants. The knowledge gained for each specific application is the best indicator of a proper re-greasing interval.