The product tables typically provide two speed ratings:
- the reference speed, which is based on thermal conditions
- the limiting speed, which is based on mechanical limits
Both speed ratings are cautionary limits, rather than strict prohibiting limits, but approaching either of them signals that deeper analysis of the operating conditions is required.
For bearings with contact seals, no reference speeds are listed in the product tables. Typically, the limiting speed determines the maximum speed for these bearings.
Approximate thermal speed limit based on ISO standard conditions
The reference speed listed in the product tables is based on the SKF friction model and derived from thermal equilibrium under the ISO 15312 standardized operating and cooling conditions. Its main purpose is to provide a quick assessment of the speed capabilities of a bearing. You can also use it to estimate a thermal speed limit.
The ISO reference speed is valid for open bearings only, operating under the following conditions:
- predefined reference heat dissipation
- radial load P = 0,05 C0 for radial bearings
- axial load P = 0,02 C0 for thrust bearings
- nominal temperature increase of 50 °C (90 °F) above an ambient reference temperature of 20 °C (70 °F)
oil lubrication with mineral oil without EP additives
- ISO VG32 for radial bearings
- ISO VG68 for thrust bearings
- clean conditions
- sufficient operating clearance (→ Selecting initial internal clearance)
- horizontal shaft, rotating inner ring and stationary outer ring
The ISO standard does not provide reference conditions for sealed bearings.
The ISO standard, established for oil lubrication, is also valid for grease lubrication, provided a lithium based grease with mineral base oil having a viscosity between 100 and 200 mm²/s is used. Grease lubricated bearings may however undergo a temperature peak during initial start-up, requiring a running-in period before they reach their steady-state operating temperature.
Adjusted reference speedThe ISO reference speed is valid for a standardized set of operating conditions including standardized heat dissipation. Therefore, SKF recommends calculating the adjusted reference speed considering the actual load and lubricant viscosity in your application. Do this using the SKF Bearing Calculator. However, this reference speed adjustment does not include the data regarding the actual heat dissipation for your application, so a conservative approach to the result is recommended. To include effects from heat dissipation, a detailed thermal analysis is required.
Mechanical speed limit
The limiting speed indicated in the product tables is a maximum speed valid for the standard bearing execution that should not be exceeded unless the bearing design and the application is adapted to a higher speed.
The limiting speed is determined by:
- the form stability or strength of the cage
- lubrication of the cage guiding surfaces
- centrifugal and gyratory forces acting on the rolling elements
- other speed-limiting factors, such as seals and the lubricant for sealed bearings
NOTESome open ball bearings have very low friction, and the reference speeds listed for them might be higher than their limiting speeds. Do not use only the mechanical speed limit. Also calculate the adjusted reference speed. The lower of the two sets the speed limit.
Speeds above the reference or limiting speed
It is possible to operate a bearing at speeds above its reference speed, its adjusted reference speed, or even the limiting speed. Before doing so, first make a detailed thermal analysis, and take whatever further measures may be required, such as use of special cage executions, or consider using high precision bearings. Regarding management of the effects of increased speed, consider the following options:
- Control the resulting increase in bearing temperature by additional cooling.
- Compensate for any reduction in bearing clearance resulting from increased bearing temperature.
- Revise the housing fitting tolerance choice to ensure that the influence of increased bearing temperature does not impair the axial displaceability of non-locating bearing outer rings.
- Revise the bearing tolerance class, together with the geometrical precision of the shaft and housing seats, to ensure these are sufficient to avoid excessive vibration.
- Consider using an alternative cage execution that is suitable for higher speed operation, in particular when approaching or exceeding the limiting speed.
- Ensure that the lubricant and lubrication method used are compatible with the higher operating temperature and the cage execution.
- Check that the relubrication interval is still acceptable, particularly for grease lubricated bearings. Oil lubrication may be required.