Oil lubrication is recommended for many applications, as different supply methods can be adapted to suit different operating conditions and the machine’s design. When selecting the most appropriate oil lubrication method for a bearing arrangement, the following application requirements should be considered:
- required quantity and viscosity of the oil
- speed and hydrodynamic frictional losses
- permissible bearing temperature
The typical relationship between oil quantity / oil flow rate, frictional losses and bearing temperature is shown in diagram 1. The diagram illustrates the conditions in different regions:
The oil quantity is insufficient to create a hydrodynamic film between the rolling elements and raceways. Metal-to-metal contact leads to increased friction, high bearing temperatures, wear and surface fatigue.
A larger quantity of oil is available and a cohesive, load-carrying oil film of sufficient thickness to separate the rolling elements and raceways can be formed. Here, the condition is reached where friction and temperature are at a minimum.
A further increase in oil quantity increases frictional heat due to churning and bearing temperature rises.
The oil flow quantity increases such that equilibrium between frictional heat generation at the bearing and heat removal by the oil flow is achieved. Bearing temperature peaks.
With increasing oil flow, the rate at which heat is removed exceeds the frictional heat generated by the bearing. Bearing temperature decreases.
Maintaining low operating temperatures at extremely high speeds generally requires either an oil-air lubrication system or a circulating oil lubrication system with cooling capabilities. With these systems, the operating conditions shown in regions B (oil-air) or E (circulating oil) can be maintained.