Water-based hydraulic fluids : rheology and elastohydrodynamic lubrication

Sammanfattning: Experiments have been carried out to measure traction, antiwear and rheological properties of water-based hydraulic fluids. Four types of fluids have been examined, emulsions, micro-emulsions, synthetic solutions and water-glycols. A numerical solution of the elastohydrodynamic lubrication problem for pure rolling with non-Newtonian lubricants is also outlined. The non-Newtonian rheological model used is a power-law, ... Non-Newtonian behaviour has been established for fluids with polymeric viscosity improvers. These non-Newtonian effects are less pronounced at high temperatures. The surface charge for emulsion droplets increases the viscosity considerably but on the other hand ionic contaminations decrease the viscosity. The viscosity increase with pressure is much lower for water-based fluids than for mineral oils. For fluids with low n-values (n < 0.5) the pressure profile in elastohydrodynamic contacts approaches the Hertzian pressure distribution and the minimum film thickness will be that given by the base fluid viscosity. The wear-rate is lowest for those fluids having the highest pressureviscosity coefficient along with a Newtonian behaviour. The water-glycols give low traction coefficients but will not prevent wear at high loads. There is a decrease in both wear rate and traction for emulsions having a less pronounced droplet stability. The same improvement is reached using micro-emulsions, with smaller droplets.