RotoPrecision Miniature and Instrument Bearings

Lubrication




A key element of successful bearing selection is the choice of an appropriate lubricant. The lubricant in a bearing application reduces friction between the balls and raceways by providing a viscous hydrodynamic film that separates them during normal operation. In addition, properly specified lubrication serves to minimize cage wear, prevent corrosion, act as a barrier to contaminants, and can serve as a heat transfer agent in some cases.

The main considerations in selecting a lubricant are operating speed, load, operating temperatures, viscosity of the lubricant across the range of operating temperatures, ball material and the presence of seals or shields.

The two main categories of lubricant used in miniature and instrument ball bearings are grease and oil. Dry lubricants and coatings can be used in specialized situations such as extreme temperatures, radiation, or vacuum conditions but they are generally limited in their speed and load capabilities. The basic lubricating properties of an oil or grease are highly dependent on the base fluid being used. There are a number of options for base fluids, each with their own particular operating characteristics.


Petroleum
Petroleum based lubricants have excellent load-carrying capability in moderate temperature ranges (-25°F to 250°F) but can be limited at high temperatures. They are classified as either naphthenic or paraffinic and are typically enhanced with additives to inhibit oxidation, corrosion, foaming, polymerization, and to improve the stability of viscosity over a wide temperature range.

Super-Refined Petroleum
These bases retain the excellent load-carrying capability of the petroleum bases and, as a result of being super-refined, avoid many of the unwanted characteristics of standard petroleum. Consequently, they are capable of operating under wider temperature conditions (-65°F to 350°F), and can be further enhanced with sophisticated additives to suit a wide variety of operating conditions.

Diester Synthetics
Diester based synthetic oils in their base state do not have the film strength of petroleum oils. But they are much more stable over a wide range of operating temperatures and offer good low temperature starting torque. The normal operating temperature range for diester synthetic based lubricants extends from –65°F to 350°F. For these reasons, synthetics have gained wide use as good general-purpose lubrication.

Slicone Synthetics
Silicone compounds retain a very constant viscosity index over their entire temperature range (-100°F to 400°F), which is particularly applicable for cold start, low torque applications. Film strength is very poor, though, causing dynamic load ratings to be reduced to 1/3 of a bearing’s normal rating. Silicone is also not recommended for high speeds, having a dN limitation in the 200,000 range.

Other
Many other synthetic base oils exist for the production of specialty oils and greases. The selection of these depends on specific operating conditions – usually outside the range of normal industrial applications. One of our Application Support personnel would be pleased to discuss your specific needs or questions.


All these base oil products listed above are offered in either grease or oil form.


1) GREASE LUBRICATION

Grease is made by combining a base oil with a thickening agent such as lithium, clay, urea soap, or a number of other compounds. The solid or semi-solid state that results is what keeps the lubricating base oil within close proximity of the rolling bearing surfaces. Grease is the preferred choice for many miniature and instrument applications since it limits the flow of lubricant out of the bearing. Also, the bearing can be prelubricated at the time of manufacture under clean-room conditions and, with shields or seals, is usually lubricated for life. Grease does possess some limiting characteristics that should be considered when deciding on a suitable lubricant.


• Bleeding

All greases are susceptible to separation of the oil from the thickener over time. The rate of bleed depends on many factors including the viscosity of the base oil and the characteristics of the thickening agent. Bleeding may be a concern when there is lengthy period of time before the initial use or between periods of operation.

• Operating Temperatures

Grease has less ability to transfer heat away from the bearing elements than oil - especially compared with re-circulating oil lubrication systems. This leaves grease lubrication susceptible to heat build-up and oxidation over time. The maximum temperature for most greases is up to 300°F, although additives can raise this to 500°F or higher in some cases.

• Speedability

The plastic nature of grease tends to cause overheating at high dN values, limiting its use in higher speed applications. The value “dN” is expressed as the inside diameter of the bearing in mm multiplied by the speed in rpm. Greases are limited to dN values up to 650,000, depending on the base oil selected. For some situations, special channeling grease can be specified for higher speed applications.

• Torque

Grease increases the start-up torque of a bearing and if not properly specified, can also affect the running torque.

• Fll Rate
Too much grease can be as damaging to bearing performance and life as too little. Excess grease causes churning and shearing and can result in excess heat buildup. A normal fill rate for grease is 25% to 33% of the free volume of a bearing.


Even with all these issues, grease lubrication remains the best choice in the vast majority of cases due to its ease of maintenance. Most of the above issues can be overcome due to the wide array of base oils and additives now available on the market.


2) OIL LUBRICATION
While grease is a more convenient alternative, certain application conditions warrant the selection of an oil instead as a lubricant in miniature and instrument applications. These tend to be situations where speedability values are high, where start-up torque needs to be low, or where the use of an oil flow would assist in dissipating temperature. For long life in challenging application conditions, oil is often delivered through a lubricating system to ensure the correct amount and constant availability of lubrication to the bearing components. A well-designed lubrication system will provide for retention, circulation, filtration and sometimes even cooling of the oil, especially when speed limitations are being reached. 


Whether selecting grease or oil lubrication, it is important to determine the dominant challenge of your application – speed, load, or temperature – and then balance the many options and trade-offs in selecting the optimal lubricant. Our Application Support personnel are ready to assist you with this process at your convenience.

PACKAGING
Packaging is a critical but often overlooked part of ensuring that a precision bearing performs to its intended standard. For this reason, RotoPrecision bearings are transported, stored, and supplied in packaging that delivers the highest standard of protection and cleanliness available. Each bearing is individually packaged and hermetically sealed under clean room conditions in plastic or foil pouches. Should you have any special packaging needs, please discuss them with our Application Support personnel.

CONCLUSION
With all these issues and considerations, the process of bearing selection can be a daunting challenge. It need not be, though. At RotoPrecision we are committed to delivering you the perfect customer experience and would welcome the opportunity to assist you in getting the most out of your miniature or instrument bearings.


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