Development of Grease
focusing on Improved Energy Efficiency Masamichi Yamamoto Junichi Imai Kyodo Yushi Co., LTD
1. Introduction
In modern times, where energy and resource saving strategies are becoming more important for environmental conservation, the control of air pollutants such as CO2 has been one of the most urgent global issues. Among other CO2 emission sources, plant facilities and automobiles are considered to be the two major contributors. In plant facilities, production machinery and equipment as well as motor-driven fans and pumps consume large amounts of electricity and power plants consequently emit significant amounts of CO2 to cover the electricity demand in the facilities. As for automobiles, there has been a downward trend in the amount of CO2 emissions since the 1990s thanks to the growth of the eco-car. Still, looking at CO2 emission sources by sectors, transportation sector is highly influential against the industrial, residential and energy sectors. Accordingly, electric and automobile sectors are working on energy- and resource-saving strategies including a reduction of power consumption by improving the torque characteristics of an electric motor. To be more specific, component and material designs and lubricants are being reviewed for potential torquereducing effects, and grease in its unique “semi-fluid” state has attracted considerable attention being extensively studied to find out application-specific formulations.
This study describes a design concept and study case to develop a rolling element bearing grease with torque reducing effects.
2. Torque generating factor during rolling element bearing operation The torque during rolling element bearing operation occurs accompanied by friction resistance which is classified roughly into three types: grease’s churning resistance, rolling-viscous resistance and seal resistance1).2). Greases are involved especially in the two torque-generating factors: resistance acting on the rolling contact surfaces of rolling elements in elastohydrodynamic lubrication (EHL) and lubricant’s churning resistance observed at any other site (Fig.1). Considering that these two factors are, basically, dependent on grease base oil viscosity, a low viscosity base oil was conventionally used to decrease bearing torque. However, a low viscosity oil had limited effectiveness as it could only form a thin oil film and shortens the rolling fatigue life of the bearing. Thus, a new low-torque technique without using a low viscosity base oil, or with using a low viscosity base oil not adversely affecting the rolling fatigue life has been of great interest to bearing manufacturers.
- 18 VOLUME 78, NUMBER 4
