Page 137 - Rollingbearings

Speed limitations
Further Approximate thermal Adjusted reference speed
temperature-related speed limit based on The ISO reference speed is valid for a stand-

checks ISO standard ardized set of operating conditions including
standardized heat dissipation Therefore,
After you have estimated the operating tem- conditions SKF recommends calculating the adjusted
perature, check: reference speed considering the actual load
The reference speed listed in the product and lubricant viscosity in your application
• that the temperature assumption for cal- tables is based on the SKF friction model and Do this using the SKF Bearing Calculator
culating bearing life (operating viscosity) derived from thermal equilibrium under the (skf com/bearingcalculator) However, this
was correct ISO 15312 standardized operating and cool- reference speed adjustment does not include
• the lubricant selection and temperature ing conditions Its main purpose is to provide the data regarding the actual heat dissipa-
limits a quick assessment of the speed capabilities tion for your application, so a conservative
• the grease or oil change interval of a bearing You can also use it to estimate a approach to the result is recommended To
• the cage and seal material limits thermal speed limit include effects from heat dissipation, a
The ISO reference speed is valid for open detailed thermal analysis is required
bearings only, operating under the following
conditions:
Speed limitations • predeined reference heat dissipation Mechanical speed

• light loads limit
The speed capability of a bearing is normally – radial load P = 0,05 C for radial
0
determined by the bearing operating tem- bearings
perature However, for certain bearing types – axial load P = 0,02 C for thrust bearings The limiting speed indicated in the product
0
and arrangements, the mechanical limits of • nominal temperature increase of 50 °C tables is a maximum speed valid for the
the bearing components may have a signii- (90 °F) above an ambient reference tem- standard bearing execution that should not
cant inluence perature of 20 °C (70 °F) be exceeded unless the bearing design and
The product tables typically provide two • oil lubrication with mineral oil without EP the application is adapted to a higher speed
speed ratings: additives The limiting speed is determined by:
– ISO VG32 for radial bearings Operating temperature and speed
• the reference speed, which is based on – ISO VG68 for thrust bearings • the form stability or strength of the cage
thermal conditions • clean conditions • lubrication of the cage guiding surfaces
• the limiting speed, which is based on • suficient operating clearance (Selecting • centrifugal and gyratory forces acting on
mechanical limits initial internal clearance, page 183) the rolling elements
• horizontal shaft, rotating inner ring and • other speed-limiting factors, such as seals
Both speed ratings are cautionary limits, stationary outer ring and the lubricant for sealed bearings
rather than strict prohibiting limits, but
approaching either of them signals that The ISO standard does not provide reference B.5
deeper analysis of the operating conditions conditions for sealed bearings
is required The ISO standard, established for oil lubri-
For bearings with contact seals, no refer- cation, is also valid for grease lubrication, NOTE
ence speeds are listed in the product tables provided a lithium based grease with min- Some open ball bearings have very low
Typically, the limiting speed determines the eral base oil having a viscosity between 100 friction, and the reference speeds listed
maximum speed for these bearings and 200 mm 2 /s is used Grease lubricated for them might be higher than their
bearings may, however, undergo a tempera- limi ting speeds Do not use only the
ture peak during initial start-up, requiring a mechanical speed limit Also calculate
running- in period before they reach their the adjusted reference speed The lower
steady-state operating temperature of the two sets the speed limit

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