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. For bearings more highly loaded, please contact American’s sales department for adjusted speed limits.
The design of the machine and its environment effects the way heat can be removed from the bearing, thereby lowering its temperature. Generally, most heat is removed through the bearing’s outer race through the machine’s housing. Thin, exposed housings result in lower bearing temperatures than thick, confined housings. Free movement of air around the housing plus a low ambient temperature allows the bearing to run cooler also.
The problem with “hot” bearings is the breakdown of the lubrication in two ways. The first is loss of oil viscosity with increased temperature allowing metal-to-metal contact of the bearing components, while the second is a physical breakdown of the base oil itself and its additives. Generally, most users prefer to keep their housings below 60 C (140° F). Temperatures higher then this require specialized oil, constant level checks, and much shorter re-greasing intervals when grease is the chosen lubricant. To determine the limiting speed, Nlim, of any bearing, find the Ndm value for the specific bearing type in Table II below. Any operating speed that is within 80% of the limit should be scrutinized further.
Calculate the bearing’s Pitch Diameter (dm) from this formula:
The limiting speed, Nlim, in RPM is simply calculated:
An NU1996MC3 bearing (480mm x 650mm x 78mm) needs to operate at 800 RPM with static oil lubrication. Is this acceptable?
The bearing’s Pitch Diameter is:
Its Width to Section Height ratio:
This is a narrow Cylindrical Roller bearing with a machined brass Cage.
Its Speed Limit Factor is 550,000mm/Min., from Table II.
Its limiting speed – Nlim =
Since the intended operating speed of 800 RPM is 82% of the speed limit, further investigation is required.
In some applications, the established rotational shaft speed and the size of the bearing selected results in an Ndm value that exceeds the limiting speed, Nlim, of the bearing, sometimes extensively. The solution to this situation often employed is the installation of a jetted, circulating oil system, which lubricates and removes heat from the bearing. Such systems usually involve a heat exchanger to cool the oil after it is removed by a suction line from the machine. When the oil is recycled back to the bearing, it is significantly cooler, possessing a higher viscosity that better lubricates the bearing.
When the bearing’s operating speed is slightly higher than its limiting speed with circulating oil, a standard two-piece machined brass cage is usually acceptable. However, when the operating speed is much greater than its limiting speed, a cage of stronger construction is necessary. This is due to increased stresses within the cage due to centrifugal effects.
Whenever a bearing is selected that operates within 80% of its limiting speed, please contact American’s sales department so our engineering department can analyze the application and make proper recommendations regarding the method of lubrication and the bearing’s cage.
*Wide, single row bearings have a Width/Section Height ratio of 1.10 to 2.50. Section Height = (O.D. – Bore)/2. For 2 row bearings, use the third column.
High Rotational Acceleration/Deceleration
It has been observed in equipment that rapidly changes RPM, both up and down, that the bearing’s cage is subjected to high inertial loads. This can result in a fatigue failure of the cage and a functional end to the bearing’s life, even though no fatigue spall has developed. When observed, the problem has been fixed by supplying a more “robust” cage design. If such a condition exists with your equipment, or you suspect it might occur, contact American’s sales department for bearing selection advice.