Thrust 1 - Efficiency
Title
1G.1: Tribofilm Structure and Chemistry in Hydraulic Motors
Project Leader
Paul Michael, Research Chemist (MSOE)
Statement of Project Goals
The research objective of this project is to improve hydraulic fluid power efficiency by systematically investigating tribofilm structure and chemistry in hydraulic motors.
Project's Role in Support of the Strategic Plan
This project will increase the efficiency and energy density of hydraulic fluid power systems by identifying how tribofilm structure and chemistry affect starting torque and low-speed friction in hydraulic motors. Understanding this relationship will make possible the formulation of hydraulic fluids that improve motor efficiency. This has strategic importance within the CCEFP because low-speed motor efficiency often determines the minimum displacement (size) and operating pressure of mobile hydraulic equipment. The findings will be combined with the piezoviscosity research of Bair and static friction research of Martini to improve efficiency in both the hydraulic hybrid passenger vehicle and excavator test beds.
Description and explanation of research approach
This project targets hydraulic motor efficiency by investigating the fluid properties that influence friction and tribofilm formation. In previous research we determined that boundary friction, thin-film traction, pressure-viscosity and thermochemical fluid properties influence hydraulic motor efficiency under low-speed, high-torque (LSHT) conditions.(1) Boundary friction coefficient, as measured in a High Frequency Reciprocating Rig (HFRR), was found to correlate with motor efficiency through a broad range of motors and temperature conditions.(2,3) In this next phase of research, we will compare the tribochemical films formed in a reciprocating tribometer to those formed in hydraulic motors. It is hypothesized that similarities in tribofilm structure and chemistry account for the apparent correlation between motor efficiency and HFRR boundary friction measurements.
Translating HFRR boundary friction measurements to full scale equipment is a challenge. Boundary friction is affected by many factors including load, entrainment velocity, surface topography, metallurgy, and lubricant chemistry. Even with these parameters defined, it is difficult to replicate real tribological contacts in a benchtop tribometer due to local contact geometries, temperatures and time dependencies.(4) Characterizing tribofilm structure and chemistry presents its own set of challenges. Table 1shows that boundary lubrication film thickness varies from tens to hundreds of nanometers. These features are not only smaller than normal design tolerances; they push the detection limit of procedures such as Energy Dispersive Spectroscopy and Fourier Transfer Infrared Spectroscopy.
Table 1: Dimensions of tribological features.
Parker Hannifin, Poclain Hydraulics and Sauer-Danfoss supplied orbital (geroler), cam-lobe, and bent axis motors for our research project. Two motors of each type were provided, along with extra rotor sets, vanes, and pistons. The efficiency of each motor will be determined in MSOE's hydraulic motor dynamometer. After efficiency testing, one of the motors of each type will be disassembled. The surface topography and tribofilm chemistry of rotors, vanes and pistons from the motors will be evaluated via mechanical profilometry, Scanning Electron Microscopy and Energy-Dispersive Spectroscopy (SEM/EDS). Initially two simple zinc-containing hydraulic fluids will be evaluated. These fluids were selected because zinc dialkyldithiophosphate (ZDDP) produces a relatively thick tribochemical film. (7) It is hoped that a thicker film will facilitate our efforts to locate the areas where tribochemistry is occurring. Film structure and composition will be compared to those produced by ZDDP in the High Frequency Reciprocating Rig. If we are able to locate ZDDP tribofilms in the motors, we will repeat this testing with a friction modified ZDDP formulation. The conventional wisdom is that friction modifiers reduce friction by forming Langmuir-Blodgett monolayer films. Recent investigations indicate that friction modifiers work synergistically with ZDDP to produce low-friction tribofilms that are thinner than those produced by pure ZDDP, but not monolayers. Understanding the nature of the tribofilm in motors is key to reducing friction.
References
1) Devlin, MT; Michael, P; et al "Boundary Lubrication and Fluid Thermal Property Effects in Low-Speed High-Torque Motors," presented at the STLE Annual Conference, Session 6A, Las Vegas, (2010)
2) Michael, P; Wanke, T; Devlin, MT; et.al. "An Investigation of Hydraulic Fluid Properties and Low-Speed Motor Efficiency," Proceedings of the 7th International Fluid Power Conference, Aachen, Germany, Vol. 3, pp. 341-353 (2010)
3) Michael, P; Burgess, K; Martini, A; Garcia, J; and Bair, S; "Lubricant Effects in Hydraulic Motor Starting Efficiency," Proceedings of the STLE/ASME International Joint Tribology Conference, Paper IJTC2010-41262 (2010)
4) Lee, PM; "Consideration of Test Parameters in Reciprocating Tribometers Used to Replicate Ring-On-Liner Contact," Tribology Letters 39:81-89(2010)
5) Tysoe, WT; Kaltchev, M; et al, "An Investigation of the Tribological Properties of Thin KCL Films on Iron in Ultrahigh Vacuum," Wear 252:595-606 (2002)
6) Yamaguchi, ES; Roby, SH; "Film Formation by Non-Phosphorus Wear Inhibitors," TT, 52: 706-717 (2009)
7) Devlin, MT; Turner, TT; Milner, J; Hewette, C.; Sheets, R; Ryan, H; and T-C Jao, "Wear Prevention by Phosphorus Species the form Thin Tribofilms," International Conference on Tribology, Parma, Italy (2006)
8) Michael, P; Wanke, T; Kimball, A; and Blazel, B; "Atomic Force Microscopy of Geroler Motor Wear Debris Ferrograms," Journal of ASTM International, 6(1): Paper ID JAI101628, (2009).
9) Goldstein, J; Newbury, D; et al Scanning Electron Microscopy and X-Ray Microanalysis, 3rd Ed., Springer, NY, p 70, (2003)
10) Isaksson,O; "Particle counter and neural network used to detect sliding wear and pitting in a radial hydraulic motor," International Journal of Fluid Power 11, No.2 pp. 5-13(2010)
11) Garcia, JM; Martini, A; and Lumkes, JH; Paper 1(ii), Proceedings of the 6th FPNI PhD Symposium, West Lafayette, IN, (2010)
12) Bair, S; Michael, P; "Modelling the Pressure and Temperature Dependence of Viscosity and Volume for Hydraulic Fluids," International Fluid Power Journal, No. 11, Vol. 2, pp. 37-42 (2010)