When you select SKF bearings, you need to pay attention to operating temperature and speed. The relationships between the temperature and power loss of components within an application is complex and these factors, in turn, have interdependencies with many others such as bearing sizes, loads and lubrication conditions.
They influence many performance characteristics of an application and its parts, and do so in various ways depending on the operational state, such as at startup or in normal operation, when steady-state conditions have been reached.
Estimating the operating temperature and verifying speed limitations is a critical aspect of the analysis of an application.
This section provides details of these primary relationships, and guidance on what to consider.
SKF bearing operating temperature and heat flow
Temperature has a major influence on many performance characteristics of an application. The heat flow to, from and within an application determines the temperature of its parts.
The operating temperature of a bearing is the steady state temperature it attains when running and in thermal equilibrium with its surrounding elements. The operating temperature results from:
- the heat generated by the bearing, as a result of the combined bearing and seal frictional power loss
- the heat from the application transferred to the bearing via the shaft, housing, foundation and other elements in its surroundings
- the heat dissipated from the bearing via the shaft, housing, foundation, lubricant cooling system (if used) and other cooling devices
The bearing operating temperature depends as much on the application design as on the bearing generated friction. Therefore, the bearing, its adjacent parts and the application should all be thermally analysed.
SKF bearing size, operating temperature and lubrication conditions
For a given bearing type, the bearing size, operating temperature and lubrication conditions are interdependent as follows:
- Bearing size is selected based on bearing load, speed and lubrication conditions.
- Operating temperature is a function of the bearing load, size, speed and lubrication conditions.
- Lubrication conditions depend on the operating temperature, the viscosity of the lubricant and the speed.
The operating temperature of a bearing reaches a steady state when there is thermal equilibrium – i.e. there is a balance between generated heat and dissipated heat.
Provided that the load ratio C/P > 10 and the speed is below 50% of the limiting speed nlim, and there is no pronounced external heat input, then cooling via the surrounding air and foundation is usually sufficient to result in an operating temperature well below 100 °C (210 °F). Where these conditions are not met, perform a more detailed analysis, as additional heat dissipation may be required.
The heat generated is the sum of:
- heat generated by the bearing, as a result of the combined bearing and seal frictional power loss
- heat flow from adjacent parts or processes
Bearing frictional heat (power loss)
Bearing friction consists mainly of rolling friction, sliding friction, seal friction and oil drag losses.
Heat flow from adjacent parts or processes
In many applications, the bearings are in locations where they receive:
- heat from working parts of the machine (e.g. caused by friction in gears or shaft seals)
- external heat (e.g. from hot steam going through a hollow shaft)
The operating temperature of the bearings is influenced by this, in addition to their self-generated heat. Examples of such applications include:
- drying cylinders in paper machines
- calender rolls in plastic foil machines
- hot gas fans
The heat input from adjacent parts within the application or from the process can be very pronounced and is typically very difficult to estimate. The rule is to insulate the bearing, as far as possible, from the additional heat flow.
The heat dissipated is the sum of:
- heat dissipated by the shaft, housing and ambient airflow (e.g. cooling effects in arctic conditions)
- heat dissipated via the lubricant or lubrication system
SKF bearing friction, power loss and starting torque
SKF bearing friction is not constant and depends on certain tribological phenomena that occur in the lubricant film between the rolling elements, raceways and cages.
Bearing frictional moment as a function of speed
The diagram shows how friction changes, as a function of speed, in a bearing with a given lubricant. Four zones are distinguishable:
- Zone 1 – Boundary layer lubrication condition, in which only the asperities carry the load, and so friction between the moving surfaces is high.
- Zone 2 – Mixed lubrication condition, in which a separating oil film carries part of the load, with fewer asperities in contact, and so friction decreases.
- Zone 3 – Full film lubrication condition, in which the lubricant film carries the load, but with increased viscous losses, and so friction increases.
- Zone 4 – Full film lubrication with thermal and starvation effects, in which the inlet shear heating and kinematic replenishment reduction factors compensate partially for the viscous losses, and so friction evens off.
SKF model of bearing friction
In the SKF model for calculating bearing friction, the total frictional moment, M, is derived from four sources:
M = Mrr + Msl + Mseal + Mdrag
|Mrr||the rolling frictional moment, and includes effects of lubricant starvation and inlet shear heating [Nmm]|
|Msl||the sliding frictional moment, and includes the effects of the quality of lubrication conditions [Nmm]|
the frictional moment from integral seals [Nmm]
Where bearings are fitted with contact seals, the frictional losses from the seals may exceed those generated in the bearing.
|Mdrag||the frictional moment from drag losses, churning, splashing, etc., in an oil bath [Nmm]|
Estimating bearing operating temperature
If you are able to estimate a value for the heat dissipation from a bearing, Ws, then you can use the formula given in SKF model of bearing friction for calculating the bearing frictional power loss, Ploss, to estimate the operating temperature, Tbear, for a bearing in thermal equilibrium, under steady-state conditions, using
Tbear = (Ploss / Ws) + Tamb
|Tbear||estimated average bearing operating temperature [°C]|
|Ploss||bearing frictional power loss [W]|
|Ws||total heat dissipation per degree above ambient temperature [W/°C]|
|Tamb||ambient temperature [°C]|
Should the value of the estimated bearing operating temperature be too high for the application requirements – for example, resulting in a κ value that is too low, or a relubrication interval that is too short – a possible solution may be to reduce the operating temperature by means of a circulating oil lubrication system.
The speed capability of a bearing is normally determined by the bearing operating temperature. However, for certain bearing types and arrangements, the mechanical limits of the bearing components may have a significant influence.
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 SKF bearings with contact seals, no reference speeds are listed in the product tables. Typically, the limiting speed determines the maximum speed for these bearings.