TRC Project 32513/1519N1 May 2015 IDENTIFICATION OF STRUCTURAL STIFFNESS AND MATERIAL LOSS FACTOR IN A LARGE DIAMETER METAL MESH FOIL BEARING Luis San Andrés and Travis Cable
Justification Foil bearings are low maintenance mechanical elements that dispense of expensive lubrication systems, saving on footprint, weight, and cost. Experiments with metal mesh dampers and small metal mesh foil bearings show promising damping capabilities. OEMs/TM Users wish to extend metal mesh foil bearings to large turbomachinery applications.
Objective and Tasks Current Progress since Jan 2015 Bring metal mesh foil bearings (radial & thrust) to a commercialization level 1. Construct jigs to manufacture metal mesh pads of various lengths: top foil and bearing cartridge, along with a means to verify the pads and (assembled in) bearing static structural stiffness and material damping (loss factor). 2. Document manufacturing procedure and detail steps for verification. 3. Build two low cost test rigs using commercial router motors (25 krpm) to evaluate the static load performance and drag torque of radial and thrust MMFBs. 4. Measure rotor lift-off speed and break away torque, touchdown speed and stall torque, load versus minimum film thickness, and drag power losses, over a range of shaft speeds to 25 krpm.
Components of an Assembled Metal Mesh Bearing Top foil t MM Metal mesh pad 105.4 mm Journal 90.17 mm Bearing clearance t MM Bearing cartridge Components: Bearing cartridge Stainless steel top foil Metal mesh pads = underspring structure
Components and Dimensions of the Test Metal Mesh Bearing L C D o,r ~9 cm 2 Parameter Components: Steel bearing cartridge Stainless steel top foil Metal mesh underspring structure (pads) Magnitude Unit Rotor mass, M R 3 kg Diameter, D o,r 90.17 mm Length, L R 101.6 mm Bearing cartridge mass, M C 6.75 kg Outer diameter, D o,c 166.6 mm Inner diameter, D i,c 105.4 mm Axial Length, L C 81.3 mm Top foil thickness, t tf 0.254 mm Length, L tf 327 mm Elastic modulus, E tf 214 GPa
Metal Mesh Pad Dimensions &Compactness Ratio Two sets of five pads constructed [a] 6.5 mm thick pad [b] 7 mm thick pad CR = 33% CR = 30% Compactness ratio for a metal mesh pad: CR ~ 30% is desirable CR m copper MM V MM
Industrial Metal Mesh Overlapping mesh makes for inconsistent thickness when compressed [a] Inconsistent wire mesh Copper mesh from TWP Inc. Parameter Mesh Size Wire Diameter Opening Magnitude 16 per in 0.011 in 0.051 in [b] Wire mesh from TWP Inc. [5] Weight/square foot 0.14 lb/ft 2 Density of copper 557 lb/ft 3
Pad Forming 1 1. A length of copper mesh is cut and weighed until achieving the desired mass (86 or 90 g) Digital Scale (+/ 0.05 g) 2 2. The strip of mesh is then folded end over end to form the precompressed pad 3 3. Folded, pre-compressed pad
Pad Forming Jigs Problem: Pads are curved geometries, making conventional machining of pad compression jigs prohibitively expensive (wire EDM). Solution: 3D print pad forming jigs (3) Full Assembly (1) Bottom Piece (dimensions in mm) (2) Top Piece (dimensions in mm)
Pad Compression 1 A hydraulic press applies a load of 2,000 psi for 5 minutes to compress the metal mesh pad. 2 After 5 minutes, pad removed from the compression jig and its thickness measured. If pad is thicker than desired, pad recompressed under a load of 500 psi until the desired thickness is obtained.
Pad Compression Compressed 2 sets of five metal mesh pads, one set with a 6.5 mm thickness and the other with a 7 mm thickness. [a] Face on view [b] Side view Dimensions of compressed metal mesh pads (ρ = 0.00892 g/mm 3 ). 7 mm Pads, 30% CR PAD 1 PAD 2 PAD 3 PAD 4 PAD 5 Nearly identical thickness and compactness ratios Weight [g] 87.6 87.3 87.3 87.6 87.5 Average Thickness [mm] 6.99 7.11 7.05 7.01 6.95 Compactness Ratio [%] 30 29 30 30 30 6.5 mm Pads, 33% CR PAD 1 PAD 2 PAD 3 PAD 4 PAD 5 Weight [g] 90.1 90 90.1 90.1 90.1 Average Thickness [mm] 6.54 6.52 6.56 6.51 6.49 Compactness Ratio [%] 33 33 33 33 33
Load vs. Deflection to Verify Pad Uniformity The underspring structure (metal mesh) defines the performance of a gas foil bearing. The mesh and air film act as springs in series. Top foil Air film Rotating Journal ω K 1 Metal mesh pad K 2 Bearing cartridge Static load versus deflection measurements for the individual pads characterize the structural stiffness of the underspring structure and provide another metric for pad uniformity.
Load vs. Deflection to Verify Pad Uniformity Load: An instrumented section of pipe applies static load on one mesh pad. Measure: Static load via a strain-gauge load cell. [a] Schematic of static loader [b] Photographs of static loader Relative displacement between tool and static journal via two eddy current sensors (one at each axial end).
Load vs. Deflection to Verify Pad Uniformity [a] 6.5 mm thick pad [b] 7 mm thick pad CR = 33% CR = 30% 1. Large initial displacements up to 100 N [W/(LD) ~ 20 kpa)] for both sets of pads. 2. Decreasing pad thickness by ~7% gives a decrease in average mesh deflection of ~17% (0.66 to 0.55). 3. Pads demonstrate similar, but not identical structural stiffness.
Load vs. Deflection for Assembled Bearing Load: PUSH-PULL on BEARING [a] Assembled bearing on lathe Measure: Static load via a strain-gauge load cell Relative displacement between bearing cartridge and static journal via two eddy current sensors (one at each axial end) [b] Photograph of bearing on lathe
Load vs. Deflection for Assembled Bearings [a] 6.5 mm thick pads [b] 7 mm thick pads CR = 33% CR = 30% 1. Bearing with 6.5 mm pads show a clearance (dead band), while the bearing with 7 mm pads has none (interference fit). 2. Neither bearing shows calculated diametral clearance (c D = 1.7 mm for 6.5 mm thick pads or c D = 0.7 mm for 7 mm pads). 3. Bearing with clearance shows rotation about pinned top section (large weight).
Results for Assembled Bearings (Clearance Removed) 1. Mechanical preload (interference fit) affects MMFB deflection (7 mm thick pads). 2. Bearing with 6.5 mm thick pads has clearance and swings when pushed. Displacements are due to a rotation and not deformation.
Static Stiffness for Assembled Bearings A 3 rd order polynomial curve fit models the load versus deflection data for the assembled bearings (deadband removed) F x K K x K x K x 2 3 0 1 2 3 The bearing structural stiffness is: df Ks K1 2K2x 3K3x dx 2
Static Stiffness for Assembled Bearings Suspect results due to rotation about top. [a] 6.5 mm thick pads [b] 7 mm thick pads CR = 33% CR = 30% 1. The maximum stiffness for the bearing with 7 mm thick pads is nearly 10 times that of the bearing with 6.5 mm thick pads (K max ~ 23 MN/m vs. K max ~ 2.8 MN/m) 2. Bearing with 6.5 mm thick pads shows similar stiffness during load and unload processes.
Assembled Bearing Material Loss Factor A bearing material loss factor (γ) gives a measure of material damping: Edis 1 FdS K K where: e 2 2 e E dis Mechanical energy lost in a load and unload cycle F Applied load K e Effective pad linear stiffness δ Maximum bearing displacement
Assembled Bearing Material Loss Factor Effective Stiffness, K e [MN/m] Maximum Displacement, δ [mm] Loss Factor, γ [-] Bearing with 6.5 mm pads (CR=33%) 1.0 0.34 0.14 Bearing with 7 mm pads (CR=30%) 9.3 0.05 0.22 MMFB with mechanical preload (radial interference) shows TYPICAL material loss factor Prediction for other bearing (one with clearance) is highly suspect.
Brief comments on a Thrust MMFB Test Rig An existing radial gas bearing rig provides an ideal platform to construct a thrust gas bearing rig. [a] Radial BFB test rig [b] Motor controller A 50 krpm motor and controller, as well as a T-slotted base and instrumentation are available for use and modification.
Brief Comments on a Thrust MMFB Test Rig A test rig in under design phase:
Continuation Proposed Work 2015-2016 1. Construct metal jigs to manufacture metal mesh pads. 2. Determine a more accurate means of classifying metal mesh pad dimensions and verification of assembled bearing clearances. 3. Design and manufacture a thrust metal mesh foil bearing. 4. Construct a test rig for thrust metal mesh foil bearings. 5. Measure rotor lift-off speed and break away torque, touchdown speed and stall torque, load versus minimum film thickness, and drag power losses, over a range of shaft speeds to 25 krpm.
TRC Budget 2015-2016 (9 month) Support for GS (20 h/week) x $ 2,300 x9months $ 20,700 Fringe benefits (2.7%) and medical insurance ($377/month) $ 3,952 Tuition three semesters ($ 363 credit hour x 24 ch/year) $ 8,712 Test rig components and instrumentation $ 4,500 Manufacture metal jigs and purchase metal mesh $ 1,000 Travel and registration to a technical conference $ 2,000 Total Cost: $ 40,864
Questions (?) TRC-B&C-03-15 IDENTIFICATION OF STRUCTURAL STIFFNESS AND MATERIAL LOSS FACTOR IN A LARGE DIAMETER METAL MESH FOIL BEARING Travis Cable and Luis San Andrés Thanks to TRC for their support since January 2015
References [1] Zarzour, M. and Vance, J., 2000, Experimental Evaluation of a Metal Mesh Bearing Damper, ASME J. Eng. Gas Turbines Power, 122, pp. 1-4. [2] Chirathadam, T. A., and San Andrés, L., 2012, A Metal Mesh Foil Bearing and a Bump Type Foil Bearing: Comparison of Performance for Two Similar Size Gas Bearings, ASME J. Eng. Gas Turbines Power, 134, pp. 10250. [3] De Santiago, O. and Solórzano, V., 2013, Experiments with Scaled Foil Bearings in a Test Compressor Rotor, Proc. ASME Turbo Expo, June, San Antonio, Texas, GT2013-94087, pp. 1-8. [4] Lee, Y.B., Kim, C.H., Kim, T.H. and Kim, T.Y., 2012, Effects of Mesh Density on Static Load Performance of Metal Mesh Foil Bearings, J. Eng. Gas Turbines Power, vol. 134, pp. 1-8. [5] TWP Inc., 2014, Copper Wire Mesh, from http://www.twpinc.com/wiremesh-material/copper.