TRC Project 400124-00078 TRC-B&C-02-2017 May 2017 Year III A TEST RIG FOR EVALUATION OF FOIL THRUST BEARINGS: DYNAMIC STIFFNESS OF A METAL MESH THRUST FOIL BEARING Travis Cable Graduate Research Assistant Luis San Andrés Mast-Childs Chair Professor
Justification In oil-free rotating machinery, foil bearings are low maintenance elements that dispense of expensive lubrication systems, saving on footprint, weight, and cost. Experience with metal mesh dampers and small metal mesh foil bearings show their large damping capability. Underspring structures that are easier to manufacture and design motivate engineers to use foil bearings. MMFTB, 1
Objective and Tasks Bring metal mesh foil bearings (radial & thrust) to a commercialization level 1. Revamp test rig for the dynamic load characterization of metal mesh pads. 2. With radial MMFB (5 pads): measure lift-off speed, break away & drag torque for tests with shaft speed to 40 krpm and static load (~180 kpa, W 300 lbf). 3. Design and manufacture a novel metal mesh foil thrust bearing. 4. Design, construct and troubleshoot a test rig for evaluation of foil thrust bearings and perform static (no rotation) tests with a metal mesh foil bearing. MMFTB, 2
Components of a Metal Mesh Thrust Foil Bearing Components: Solid metal support Stainless steel Rayleigh-step top foil Metal mesh underspring (circular sheets) MMFTB, 3
Metal Mesh Sheets & Mass Ratio Circular metal mesh layers laser cut from larger sheets of mesh Mass ratio (compactness) for a metal mesh pad: MR m A MM w copper MM d MR = 44, 51 and 59% CR > 30% typical MMFTB, 4
Industrial Metal Mesh (OPI: openings per inch) Three types of metal mesh procured (20, 30 and 40 OPI). Larger OPI : finer mesh and higher mass ratio Manufacturer delivered 11 sheets of each type of mesh, each within ±0.10 g Circular Mesh Sheets from TWP Inc. Parameters for metal mesh sheets. Density of Copper, ρ c = 8,960 kg/m 3. 20 OPI 30 OPI 40 OPI Wire Diameter, w d [mm] 0.406 0.305 0.254 Square Opening Size [mm] 0.864 0.533 0.381 Measured Mass per Circular Sheet, m MM [g] 15.00 (±0.10) 13.00 (±0.10) 12.55 (±0.10) Area, A MM [cm 2 ] 93.4 93.4 93.4 Mass Ratio, MR [-] 0.44 0.51 0.59 MMFTB, 5
A Rayleigh Step Top Foil Bump-type Foil Thrust Bearing Metal Mesh Foil Thrust Bearing Corrugated bump foil strips support sector shaped top foils Hydrodynamic wedge formed by taper in top foil Structural damping from deformation of bumps and dry friction between the bump foil, top foil and bearing support Uniform circular mesh layer support a Rayleigh-step top foil with multiple pads Hydrodynamic wedge formed by sharp step, etched into the metal top foil Structural damping from material hysteresis and dry friction between wire connections and the bearing support and top foil MMFTB, 6
Manufacturing a Rayleigh Step Top Foil Vacco-Etch manufactured step top foils via a photo-chemical etching process (https://www.youtube.com/watch?v=se2s_lsi6jk ). 1 2 Starts with a 12 x 12 thin metal sheet 3 Etch forms the pads and fixture locations (holes) A second etch cuts a step into each of the six pads MMFTB, 7
Manufacturing a Rayleigh Step Top Foil Vacco manufactured 9 top foils for prototype MMTFB: 3 top foils with step 15 from leading edge, 3 with step 22.5, and 3 with a negative slant orientation. (a) Step location Θ L / Θ P = 1/3 (b) Step location Θ L / Θ P = 1/2 (c) Negative Slant, 0 Θ L 45 MMFTB, 8
Assembled Metal Mesh Thrust Foil Bearing Dimensions of a prototype Rayleigh step thrust foil bearing with a metal mesh substructure. Material Bearing Support 316 Stainless Steel Mesh Substructure Copper Inner Diameter 45.72 [mm] 50.8 [mm] Outer Diameter 120.65 [mm] 120.65 [mm] Thickness 9.53 [mm] ~0.40 [mm] Material Coating Coating Thickness Rayleigh Step Top Foil 316 Stainless Steel Parylene N 3 [μm] Number of Pads, N PAD 6 [-] Outer Pad Diameter, D PO Inner Pad Diameter, D PI 101.6 [mm] 50.8 [mm] Pad Arc Extent, θ P 45 [ ] Step Depth, h S Top Foil Thickness, t tf 19.1 [μm] 0.127 [mm] 6-Pad Metal Mesh Foil Thrust Bearing 6-Pad Bump Type Foil Thrust Bearing [1] MMFTB, 9
A Test Rig for Thrust Foil Bearings MMFTB, 10
A Test Rig for Thrust Foil Bearings [b] Cross-section view MMFTB, 11
A Test Rig for Foil Thrust Bearings Test Rig specs Shaft Speed, R PO Ω Motor Torque Bearing OD Limit Static Load Dynamic Load Cooling Flow Thrust Bearing Performance Measurements: Bearing static stiffness (load vs deflection) Dynamic axial force coefficients Static break-away torque Operational drag torque Pad Temperatures Load capacity 0-40,000 rpm, 212 m/s 1.1 N.m 101.6 mm 0-580 N 0 100 N 0 500 SLPM Measurements can be performed for various amounts of cooling flow (0-500 SLPM). MMFTB, 12
Components of the Foil Bearing Test Rig Electromagnetic Shaker Aerostatic Loading Plenum MMFTB, 27 Router Motor, Motor Base and Thrust Collar Load Shaft and Test Bearing
Functionality of Aerostatic Load Plenum Functionality: 7.9 bara shop air supplies to the guide bushings at two locations. The guide bushings float the shaft (no friction) while also sealing the central chamber. A precise (CV= 0.03) control valve regulates static pressure in the central chamber by varying outlet flow. Supply flow path Cooling flow path Resultant load Static pressure in the central chamber exerts a load on the shaft (and test bearing) F ~ ΔP A SL MMFTB, 13
MMFTBs Load vs. Static Deflection [b] Bearings with 2 MM sheets Increasing pad compactness (OPI) increases bearing stiffness. Structural stiffness ranges between 1-85 MN/m. MMFTB, 14
MMFTBs Load vs. Static Deflection [a] Bearings with 1 sheet [c] Bearings with 3 sheets Bearing with a single sheet (not practical) is very stiff MMFTB, 15
Comparison to a bump-foil TB [Stahl] Foil bearings should have similar structural stiffness to compare their performance with rotor speed. Three test MMFTBs have similar structural stiffness to that of BFTB of Stahl. Stahl, B.J., 2012, Thermal Stability and Performance of Foil Thrust Bearings, MS Thesis, Case Western Reserve University, Cleveland, OH. MMFTB, 16
Dynamic Stiffness of MMTFB Test setup for dynamics 1 DOF Model m z F F t 1 1 MMFTB i t F F e W z Z e z t i t 0 0 t 1 MMFTB F m A F K i K Z K F m A Z 1 i K Measured dynamic force, load shaft acceleration, and relative displacement between thrust collar and bearing. Modeled as a one degree of freedom system. MMFTB, 17
MMTFB dynamic stiffness. 3 Sheets of 20 OPI Mesh Static load W/A = 7.7, 19.7 and 33 kpa Z = 5 μm 3 Sheets of 40 OPI Mesh 3 Sheets of 30 OPI Mesh Stiffness (K) increases with applied static load and mesh compactness (OPI). Not a function of frequency MMFTB, 18
MMTFB Quadrat stiffness. 3 Sheets of 20 OPI Mesh Static load W/A = 7.7, 19.7 and 33 kpa Z = 5 μm 3 Sheets of 40 OPI Mesh 3 Sheets of 30 OPI Mesh Bearing quadrature stiffness increases with applied load and mesh compactness (OPI). Changes with frequency. MMFTB, 19
Material Loss Factor. 3 Sheets of 20 OPI Mesh K K Static load W/A = 7.7, 19.7 & 33 kpa Z = 5 μm 3 Sheets of 40 OPI Mesh 3 Sheets of 30 OPI Mesh Average loss factor γ ~ 0.2 for 20 OPI mesh, and slightly higher for 30 and 40 OPI meshes. is a measure of energy dissipation (damping) in a foil bearing Not a function of load. MMFTB, 20
Break-Away Torque & slide friction coefficient f T WR mid Bearing with a Parylene coating has f=0.22, lesser that f for uncoated top foil bearing. Torque measuring mechanism Tests w/o rotor speed MMFTB, 21
Post test condition of top foils (a) MMTFB prior to testing (b) Coated MMTFB post testing (c) Uncoated MMTFB post testing Both coated and uncoated foils show signs of wear (galling) after the break-away static torque measurements. MMFTB, 22
Questions (?) TRC-B&C-02-17 A TEST RIG FOR EVALUATION OF FOIL THRUST BEARINGS: DYNAMIC STIFFNESS OF A METAL MESH THRUST FOIL BEARING Travis Cable and Luis San Andrés Thanks to TRC for their support MMFTB, 24