Investigation of micro surface shaping effects for improving the lubrication performance of water based high pressure external gear machines Divya Thiagarajan (presenter) Prof. Andrea Vacca Maha Fluid Power Research Center School of Mechanical Engineering Purdue University, USA 14th October 2015
Outline #2 Introduction to water hydraulics Designing water based EGMs and concerns Simulation models used Micro-surface shaping effects Linear wedge gears Step + wedge gears Results Conclusion
Introduction #3 Why is water an attractive alternative hydraulic fluid to oil? Fluctuating oil prices Current issues with oil Leakage issues Cost of disposal Environmental hazards Safety Availability Potentials of water Non-flammable, environment friendly, non-toxic Negligible contamination problems Easy availability and relatively low cost Lower friction losses due to lower viscosity Superior cooling capacity High bulk modulus
#4 Potential Applications for water hydraulics Mining Offshore Food processing Metal Production Desalination Waterways Agriculture Potential medical applications...many more possible Source : Krutz, G. W., & Chua, P. S. (2004, February). Water hydraulics theory and applications 2004. In Workshop on Water Hydraulics, Agricultural Equipment Technology Conference (AETC 04) (pp. 8-10).
#5 Water hydraulics - Major Challenges Low viscosity creates poor lubrication characteristics and poor sealing abilities Can cause problems of corrosion Risk of cavitation due to high vapor saturation pressures can cause erosion problems Low viscosity of the fluid makes surface finishes and surface conformity critical Risk of freezing Microbial growth Problems can be solved by High pressure water hydraulics key for promoting the technology Most critical design component - Positive displacement machines Lubrication and sealing characteristics of the working fluid are the biggest challenges in high pressure positive displacement machines
State of the art Water hydraulic pumps/motors #6 Danfoss PAH/MAH series pumps/motors Nessie Series The Water Hydraulics Co. Ltd Wittkop, W., & Samland, U. (1992). High-pressure water pump having a polyetheretherketone cylinder bushing for pure water. U.S. Patent No. 5,131,818. Washington, DC: U.S. Patent and Trademark Office. (Claim : Pressures upto 450 bar) Many low pressure low cost water hydraulic external gear pumps Existing water hydraulic pumps/motors are limited to a maximum operating pressure of 160 bar No water hydraulic external gear machine operating at pressures > 50 bar currently exists in the market
External Gear Machines outlet #7 High tolerance to contamination Well suited for open center solutions Lateral gaps Relatively compact and low cost design Lower cavitation problems inlet Lesser noise pulsations Radial gaps Lateral lubricating interface in external gear machines is the first critical component to address while designing high pressure external gear machines using water Main functions of the lateral gap + Sealing +Bearing loads Main source of power losses - Mechanical Losses (Viscous friction) - Volumetric Losses (Leakage flows)
Designing water based EGMs Lubricating interface design Low leakages low gap heights No wear, Low shear losses gap high enough to prevent boundary lubrication and wear #8 lateral bushes (pressure plates) gears casing Goal : To carefully design the lubricating interface of water hydraulic external gear machines that Supports high pressure loads Minimizes leakages axially balanced bushes F out F in lateral lubricating gap
Axial Balance Forces Representation Direction #9 High Pressure Balance Area Seal F balance Towards gears F TSV + Gap Away from gears Low Pressure Balance Area Determining balancing areas can design the axial balance of EGMs! Is hydrostatic balancing sufficient? Hydrodynamic effects are necessary
Methodology #10 HYGESim tool for simulating external gear machines TEHD model for the lateral lubricating interfaces of external gear machines Vacca, Guidetti (2011) Dhar, Vacca (2014) Advanced numerical tools can be carried out in the investigation of micro surface shaping effects
#11 Micro-surface shaping effects Shot peened gears Shot peened lateral bushing Waved gears Shot peening does not appear to improve the lubrication performance Waved gears can be difficult to manufacture Wedged gears is the potential option can be implemented with water Lubricant film thickness scaled upto ~10000 times for visibility Wedged gears - Dhar (2014)
#12 Wedged gears Oil as the working fluid Idea : Linear sloping wedge on either surface of the gear teeth To have positive load carrying capacity, lubricant film thickness must decrease in the sliding direction. FSI-EHD model results with wedged gears with maximum wedge depth = 1 µm Maximum improvement in power loss ~ 66 % Axial balance needs to be optimal when changes with surface shaping as well as working fluid is changed An automatic numerical procedure for optimizing the axial balance of external gear machines has been developed. - Thiagarajan, Vacca (2014) Dhar (2014)
Optimal Axial Balance Flowchart #13 Operating Condition 1 Operating Condition 2... Operating Condition N Initialize grid of design parameters FSI-EHD model for lateral lubricating gaps Run simulations using design grid Evaluate objective functions I. Power Losses II. Gap Non Uniformity Index Iterative procedure Delivery pressure Shaft speed Fluid temperature Find best feasible solution in the current grid Convergence criterion reached? No Define modified new grid Yes C++, OpenFOAM, GSL, Shell scripting Optimized balance area design obtained
#14 Objective Function 1 Total power loss from the lateral gaps Velocity field in the gap/leakage This is then integrated over the boundaries to find leakage losses P loss = Q leak P Axially balanced bushing design Low leakages low gap heights low losses due to leakages No wear, Low shear losses gap high enough to prevent boundary lubrication and wear Viscous Friction/ Shear losses τ zx = h p 2 x μu g h τ zy = h p 2 y μv g h T = i 2A i (r i τ i ) P loss = T. ω Optimal balance between the two opposing sources of power losses is desired
#15 Results : Wedged gears with water as the working fluid (preliminary) Gap film thickness results 200 bar 2000 rpm 80 bar 1000 rpm 13.31 % decrease in total losses (volumetric + mechanical) with respect to original balance with wedged gears
Conclusion #16 Initial steps towards designing a water hydraulic external gear machine was presented with a focus on the lubrication performance Wedged gears were shown to improve the lubrication performance of water hydraulic external gear pumps Future work involves prototyping and testing the EGM for water as the working fluid
#17 THANK YOU Divya Thiagarajan, Andrea Vacca Maha Fluid Power Research Center, Purdue University, West Lafayette, IN, USA dthiagar@purdue.edu, (765)-409-9672