Many application-relevant fluids exhibit shear thinning, where viscosity decreases with shear rate above some critical shear rate. For hydraulic fluids formulated with polymeric additives, the critical shear rate is a function of the molecular weight and concentration of the polymers. Here we present a model for predicting the critical shear rate and Newtonian viscosity of fluids, with the goal of identifying a fluid that shear thins in a specific range relevant to hydraulic pumps. The model is applied to predict the properties of fluids comprising polyisobutene polymer and polyalphaolefin base oil. The theoretical predictions are validated by comparison to viscosities obtained from experimental measurements and molecular dynamics simulations across many decades of shear rates. Results demonstrate that the molecular weight of the polymer plays a key role in determining the critical shear rate, whereas the concentration of polymer primarily affects the Newtonian viscosity. The simulations are further used to show the molecular origins of shear thinning and critical shear rate. The atomistic simulations and simple model developed in this work can ultimately be used to formulate polymer-enhanced fluids with ideal shear thinning profiles that maximize the efficiency of hydraulic systems.

This paper describes an investigation of the effects of fluid properties on hydraulic motor efficiency through experimentation and modeling. Synthetic ester, straight-grade mineral oil, and VI improved hydraulic fluids were evaluated. Fluid properties, including viscosity, shear-stability, density, and traction coefficients, were characterized. A model for relating motor mechanical efficiency to fluid properties was developed. This model combines the viscosity parameter of Stribeck with the surface adsorption model of Michaelis–Menten. The results revealed that fluids with a low traction coefficient improved the low-speed efficiency of motors by transitioning out of the boundary lubrication region at a lower Stribeck number.

The US Army uses MIL-PRF-2104 SAE 10 hydraulic fluid in its construction and material handling equipment. This oil meets basic performance requirements but has not been optimized for hydraulic system efficiency. Hydraulic system efficiency is important because fuel transportation has unique costs and dangers in military applications. Two energy efficient hydraulic fluids were compared to MIL-PRF-2104 SAE 10 in dynamometer testing. The higher efficiency fluids reduced internal leakage flow losses and decreased low-speed motor friction. Fleet-wide fuel savings estimates for US Army construction and material handling machines were derived from engine fuel consumption models, vehicle mission profiles, hydraulic circuit analysis and dynamometer test results. The savings due to reduced fuel consumption was estimated to be $8,000,000 per annum.

The ASTM D7721 standard practice for determining the effect of fluid selection on hydraulic system performance was applied to the study of two excavators and a hydraulic dynamometer. Six fully formulated fluids spanning two viscosity grades and three viscosity index (VI) levels were evaluated using the dynamometer. Four of these fluids were also evaluated in the excavators. A common monograde ISO VG 46 hydraulic oil served as the reference fluid in both studies. In dynamometer testing, high-VI, low-traction fluids reduced hydraulic motor torque losses by 9 to 35 N·m. In excavator trials, multigrade hydraulic fluids shortened the average bucket filling time by 5.1% and increased the average bucket payload by 2.8%. Based on the physical properties of the test fluids and fundamental principles of fluid mechanics, the observed performance gains are believed to be the result of reduced viscous shear, boundary friction and traction losses. The results presented explore correlations between fluid properties and hydraulic system performance, enabling formulation strategies that can help reduce parasitic losses and improve machine productivity.