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This paper presents an experimental study of external gear pump efficiency based upon an analysis of the Stribeck values. The volumetric, mechanical, and overall efficiencies of a variety of external gear pumps were measured under steady state conditions. Straight grade antiwear hydraulic fluids were evaluated at 50°C and 80°C. Stribeck values for mechanical, volumetric, and overall efficiency were compared to classic pump efficiency curves. The experimental curves for pump volumetric and overall efficiency were consistent with the classic pump efficiency model. Mechanical efficiency diverged from model behavior at low Stribeck numbers; declining at low speeds and high pressures as contact conditions transitioned from the hydrodynamic to the mixed-film lubrication regime. Lubrication of external gear pumps can be enhanced by using hydraulic fluids that optimize the Stribeck value. A simple expression for relating the Stribeck number to volumetric and mechanical efficiency is presented.

Multiple field trials and nearly a decade of laboratory studies have demonstrated that shear stable multigrade hydraulic fluids improve fuel economy. These studies have determined that fuel efficiency is dependent upon temperature, fluid viscosity and shear stability. This paper presents a viscosity classification system proposed by the National Fluid Power Association (NFPA) Fluids Technical Committee. This system is analogous to the SAE J300 viscosity classification system for engine oils. The letter “L” is used in place of “W” as the designation for the low temperature grade. Under this new classification system, NFPA 32L-68 fluids will provide the low temperature viscosity properties of an ISO VG 32 hydraulic fluid and the high temperature viscosity properties of an ISO VG 68. In addition, fluids that meet the requirements of the proposed NFPA Energy Efficient classification system increase fuel economy and productivity while reducing CO2 emissions.

This paper examines the compatibility and filterability characteristics of mineral oil and bio-based hydraulic fluids. Due to advantages in renewability and environmental acceptability, bio-sourced, biodegradable hydraulic fluids are increasingly used in fluid power applications. Conversion from mineral oil based fluids to biodegradable fluids can be expensive because hydraulic systems often must be drained and flushed several times during the conversion process. The goal of this research is to investigate the use of the ISO 13357:1 wet filterability procedure as a method for determining the compatibility of hydraulic fluids. Mineral oil, vegetable oil and synthetic ester based hydraulic fluids mixtures have been evaluated. An additive combination that is known to have marginal compatibility has been evaluated under conditions of varying temperature, duration, and concentration.

This paper presents an investigation of the fluid viscosity properties that affect the Low-Speed High-Torque (LSHT) efficiency of hydraulic motors. Low speed torque efficiency is important because it often determines the minimum displacement (size) and operating pressure of mobile hydraulic equipment. The viscosity, viscosity index, high-shear viscosity, piezoviscosity and shear stability of prototype fluids have been characterized. These hydraulic fluids have been evaluated in a full-scale hydraulic motor dynamometer. Geroler, axial piston and radial piston motors have been evaluated. Low-speed (1 RPM) testing has been performed under constant pressure conditions in accordance with the ISO 4392-1 standard test method. Startability testing has been performed under constant load conditions in accordance with ISO 4392-2. In startability and 1 RPM testing, the axial piston motor exhibited a 6 percent efficiency gain with a high viscosity index hydraulic fluid.

Off-highway mining and construction equipment typically converts all the power output of the engine to hydraulic power, with this power then used to perform the earth-moving operations, and also to propel the vehicle. This equipment presents significant opportunities for a new type of powerplant designed to deliver hydraulic power directly. An alternative to the conventional engine driven pump is a free-piston engine-pump (FPEP). The FPEP incorporates the functions of both an internal combustion engine and a hydraulic pump into a single, less-complex unit. The design presented in this paper utilizes two double-ended, reciprocating, opposed pistons, with combustion at one end of each piston and pumping at the opposite end. The opposed piston layout provides balance and also facilitates uniflow scavenging through intake and exhaust ports in the combustion section of the engine. An important feature of this FPEP design is the rebound accumulator circuit.