Electro-Rheological (ER) fluids are fluids which change their properties in the presence of an electric field. They are used in the design of advanced control systems (drives, clutches, dampers etc.). A common ER fluid consists of a suspension of polymeric micro-spheres in a light silicone base oil. When an electric current is applied to this suspension, it behaves as a Bingham plastic; it requires a finite applied stress to initiate shearing.
Problems are experienced in the processing and transport of these fluids. They generally show poor lubricating properties; and cause the failure of associated hydraulic machinery. In this work, the behaviour of unexcited ER fluids has been investigated in elastohydrodyanamic lubricated contacts (the kinds of contacts that occur in bearings, gears, cams etc.)
The figure shows an elastohydrodynamic contact between a ball rolling on a glass plate (the photograph is taken through the plate and shows the film formed). The isolated spots show where micro-particles have entered the contact.
The base oils are low viscosity fluids such that they do not form a thick film between the rolling elements. It is the entry of the suspended particles which causes separation of the surfaces. Thus lubricating performance is dependent on this particle entrainment process.
Particle entrainment is dependent on both the size of the particles and the contact rolling speed. Faster motion tends to cause fewer particles to enter the contact. The drag force on the suspended particles is higher and they tend to be swept around the contact sides.
The photograph shows the track left on the glass surface after the passage of a ball. The upper track has been left by a slow rolling ball and shows a region of particle concentration. The lower track has been left by a faster rolling ball and is depleted of particles.
The results show that slow speed contacts are at risk of failure from inadequate lubrication. In addition smaller particles are less likely to become entrained into a contact than larger ones. In such instances, base oil viscosity should be increased or active (EP) additives considered.
- Dwyer-Joyce, R. S., Bullough, W.A., and Lingard, S., (1996), "Elastohydrodynamic Performance of Unexcited Electro-Rheological Fluids", International Journal of Modern Physics B, Vol. 10, Nos. 23 & 24, pp. 3181-3189, World Scientific Publishing Company.