For bearings with low-viscosity petroleum oil lubricant, the maximum operating speed is dictated by cage type and material, and centrifugal ball loads. In this ideal case the maximum operating inner ring rotation speed is given from the speed factor dN, where dN is the product of the bearing bore (d mm) times the maximum speed (N RPM). Depending on bearing design, dN can have values up to 600,000.
The maximum allowable rotational speed for any individual bearing can be determined from two simple equations and the Table II. These speed limits are considered thermal limits that have been determined from empirical data from many typical applications. The ability of bearings to operate at high speed is further established by the type of lubrication employed. All the limits shown are based on static oil lubrication, with the oil level set at the center of the bottom most ball or roller when standing still. If grease lubrication is desired, then the speed limit is 66% of the oil limit.
|Bearing Type||Speed Limit Factor|
|Ball, Deep Groove||500,000||-||400,000|
|Ball, Angular Contact||450,000||-||400,000|
|Cylindrical, 2 Piece Brass Cage||550,000||500,000||475,000|
|2 Piece Steel Cage||450,000||435,000||380,000|
|Stamped Steel Cage||330,000||300,000||-|
|1 Piece Brass Cage||600,000||420,000||-|
|End Ring Cage||80,000||60,000||60,000|
|Tapered Roller, Pin Type Cage||400,000||350,000||300,000|
|Brass, Land Riding Cage||450,000||420,000||400,000|
|Spherical, Brass Finger Cage||220,000||200,000||-|
|Ball, Angular Contact||200,000||-||-|
|Cylindrical, 2 Piece Cage||220,000||200,000||-|
|Cylindrical, Milled Pocket Cage||240,000||220,000||-|
|Tapered Roller, 2 Piece Cage||180,000||160,000||-|
|Tap. Roller, Milled Pocket Cage||200,000||180,000||-|
|Tap. Roller, Pin-Type Cage||220,000||200,000||-|
|Tap. Roller, Full Comp.||60,000||50,000||-|
Besides speed, the load on the bearing has a significant effect on the generation of heat and temperature. In other words, higher loads generate more heat and higher temperatures. The Speed Limit Factors shown in Table II are applicable when the calculated rating life equals or exceeds 100,000 hours.
During my frequent plant visits, I often am asked about the temperature at which bearings should operate. Inevitably, the bearings that seem to be running the hottest are the ones that rotate the fastest. For example, on a recent trip, I inspected an overhanging fan. This fan was belt-driven at a 1-to-1 ratio from a large electric motor.
The speed of the motor was set at 1,750 revolutions per minute (rpm). Since there was no reduction or increase in pulley size, it is safe to assume the speed of the bearings was quite similar. These bearings were greased with a product that was much too thick for them, leading to the generation of excess heat and shortening the bearing life. By matching the grease properties more closely to the bearing needs, you can help prolong the life of the bearing.
While this example paints a picture of a type of machine in most plants (fans), it is common to find high-speed applications in other components as well. For instance, some pumps that are directly coupled to a motor and have grease-lubricated bearings may spin in excess of 2,000 rpm.
The same holds true for certain mixers, agitators and blowers. These components may suffer if a multi-purpose grease is simply applied without much regard to the needs of the bearing. To understand what the bearing requires in terms of lubrication, you must first learn how to determine the speed factor of a bearing.
6 Factors for Selecting a High-speed Grease
Base Oil Viscosity – Ensure the viscosity adequately provides the lubricating film but is not too thick to cause excessive heat and drag.
Channeling Characteristics – The grease should be able to channel so excess heat isn’t generated from grease churning.
Dropping Point – The dropping point of the grease should exceed the operating temperature by a wide margin to avoid excessive bleed and possible bearing failure.
Thickener Type – Choose a thickener that can provide the proper dropping point, channeling and bleed characteristics. Also, if you use multiple greases, check the thickener types for compatibility in case of accidental mixing.
NLGI Grade – The consistency of the grease will have an impact on the bleed characteristics and channeling properties of the finished lubricating grease.
Additive Load – Most applications require additives to help the oil lubricate. For greases, a wide variety of chemical and solid additives can be blended to aid in film strength and reduce friction and wear.