Thermal equilibrium
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.
Generated heat
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. → Bearing friction, power loss and starting torqueHeat 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
- compressors
- 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.
Dissipated heat
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
