This article describes the grease lubrication mechanisms in rolling bearings which are described in a new book written by SKF engineers , which was also addressed in a grease course co-hosted by ABMA and STLE at the 2013 STLE Annual Meeting.
Phases in grease lubrication
Grease lubrication is a dynamic process that can be divided into three phases as shown in fig. 1. During the churning phase, most of the grease will flow onto the seals or onto the bearing ring shoulders (into the unswept volume of the bearing) or will end up attached to the cage. From here, the grease will slowly provide the raceways with lubricant by either bleeding or shear. The lubricating film will be determined by balance between feed and loss of lubricant . At some point, the reservoirs may be exhausted or deteriorate to the point that replenishment can no longer happen. If no relubrication has taken place, severe film breakdown will take place, called the end of grease life, which subsequently leads to bearing damage and failure.
Fig. 1: The phases in grease lubrication of rolling bearings.
Grease Reservoir Formation
The rate at which the reservoir formation will take place is given by the flow properties of the grease, also called “rheological properties”. Fig. 2, shows the various exiting models At very high shear rates the grease viscosity may approach the base oil viscosity. Such high shear rates occur in the lubricating films between rolling elements and raceways.
Fig. 2 (right): Schematic representation of the viscosity-shear rate curves for lubricating greases on a double logarithmic scale.
The lubricating film thickness in grease-lubricated bearings is determined by layers formed by thickener hR and by the hydrodynamic action of the viscous grease/base oil hEHL [2,4].
In the case of starved lubrication the film thickness is primarily a function of the thickness of the lubricant layers on the contacting surfaces (fig. 3), given by oil bleeding [3, 13], sideflow [11,12], replenishment [6,7] and shear and drag due to ball spin .
Fig. 3: Schematic representation of film thickness and pressure in a fully flooded and starved EHL contact .
Grease Life and Relubrication: Safe Operation
Lubricating greases are developed to operate in a distinct temperature window. The maximum temperature, called high-temperature limit (HTL), is the temperature where the grease loses its structure irreversibly. This temperature may not be exceeded at any time!
The low temperature limit (LTL) is determined by the temperature at which the grease will enable the bearing to start-up without difficulty. The “safe” temperature window is given by the Low Temperature Performance Limit (LTPL) and the High Temperature Performance Limit (HTPL). In this window the life of the grease follows “Arrhenius behaviour” and is predictable .
Grease Life and Relubrication: Grease Life Models
All models are empirical, based on grease life tests. In general grease life is defined as the L10 life: the time at which 10 % of a large population of bearings have failed. If relubrication is possible then this should be carried out much earlier. Fig. 4 shows the grease life for lightly loaded capped deep groove ball bearings as a function of rotational speed, mean bearing diameter, operating temperature and grease type (grease performance factor). Correction factors can be applied for the impact of load .
Fig. 4 (right): Grease life in lubricated-for-life standard radial deep groove ball bearings operating at light loads (C/P ≥ 15) [1,8]. Courtesy of SKF.
The lubrication system should be designed to handle a grease that is best suited for the bearing. The design of the lubrication system needs to take into account the resistance to flow, compressibility, pressure venting, rheology, flow pressure, oil separation, seal-base oil compatibly and hardening and functioning of the pump.
Piet M Lugt, is a Senior Scientist in the SKF Group Technology Development, at the SKF Engineering Research Centre, in Nieuwegein, the Netherlands. Paul Conley is a Chief Engineer at SKF Industrial Market, Strategic Industries, in St. Louis, USA. Dave M. Pallister, is a Chemist at SKF Global Laboratories, in Plymouth, USA. You can find their contact information in our membership database.
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