Silicon Nitride Balls for Improved Bearing Performance Presented
Silicon Nitride Balls for Improved Bearing Performance Presented by Steve Dunlap Product Manager Society of Manufacturing Engineers - High Speed Machining, IAMS May 2 -5, 2000 1 www. CERBEC. com
Topics • What is a Hybrid Ceramic Bearing? • What is a Silicon Nitride (Si N ) Ball? • Why do Si N Balls Improve Bearing Performance? • Applications that Benefit from Si N Balls. • Case Studies. • Hybrid Bearing Failure and Design Considerations. • Machine Tool Spindle Market Trends. 3 3 4 4 3 2 4 www. CERBEC. com
What is a Hybrid Ceramic Bearing? Steel Outer Raceway Lubricant Steel Inner Ball Cage Raceway (Retainer) Silicon Nitride Balls 3 www. CERBEC. com
Dm N, Relative Bearing Speed: Review Do Pitch Diameter, Dm = (Do + Di )/2 Actual Speed, N = RPM Dm Di Relative Speed, Dm N = N x Dm 4 www. CERBEC. com
What is a Silicon Nitride (Si 3 N 4 ) Ball? 5 www. CERBEC. com
Material Properties are Derived from Atomic Bonding Metals Polymers Ceramics Metalic Bonding Covalent Bonding Ionic Bonding Covalent Bonding 6 www. CERBEC. com
Ceramic Materials and Bearing Compatibility Oxides* Al 203 Zr 02 Rolling Contact Fatigue Nitrides* Si 3 N 4 Carbides* Si. C B 4 C Poor Good Poor Density g/cc 3. 9 6. 1 3. 2 3. 1 2. 5 Flexural Strength, MPa, RT 900 1400 980 550 300 4 -5 8 -10 5 -7 ~4. 5 2 -3 9 10 2. 9 4. 4 5. 8 17 13 15 26 28 Fair Good Poor Fracture Toughness MPa-m 1/2 Thermal Cof. Exp. 10 -6/ °C Hardness, HV (10 kg load) Thermal Shock Resistance, °C Silicon Nitride is an Optimal Rolling Element Material *Typical Values 7 www. CERBEC. com
Si 3 N 4 vs Steel as a Bearing Material 8 www. CERBEC. com
ksi 760 el @ ks i Ste 5 0 95 @ Si 3 N 4 10 M 50 20 @ 760 ks 50 3 N 4 80 Sialon Si Probability of Failure (%) 95 i Si 3 N 4 vs Steel: Rolling Contact Fatigue 5 10 20 50 100 Stress Cycles (Millions) The Weibull Plot Shows The Superior Fatigue Life Of Silicon Nitride Balls. The M 50 RCF Tests are 760 ksi. Failure is Instantaneous at 950 ksi. 9 www. CERBEC. com
Why do Si 3 N 4 Balls Improve Bearing Performance? 10 www. CERBEC. com
Ball Motion in a Bearing • Ball is in Unconstrained Motion (X, Y, Z). • Point Contact Causes Rolling, Sliding & Skidding Ri R 0 11 www. CERBEC. com
Effects of Balls and Sliding • • • Steel on Steel Sliding Causes “Cold Adhesion” or Micro-Welding Ball and Raceway Surfaces are Damaged Ball Accelerates Deterioration of Raceways Contamination from Wear Affects Lubrication Performance Excess Heat is Generated by Friction and Contact at Damaged Zone Further Promoting Degradation of Lubrication 12 Si 3 N 4 on Steel Minimizes Wear, Eliminates Adhesion Ball Remains Intact Reducing Running Damage Cooler, Smoother Contact Surfaces Lengthen Lubrication Capability High Ball Hardness Helps “Push” Contaminates from Pathway www. CERBEC. com
Surface Finish, Geometry and Wear + Surface 70% Lower Coefficient of Sliding Friction Finish Less Friction, Heat Generation, Ball/Raceway Wear Composite Surface Roughness (CSR) Ball and Raceway Fbrg = (Frace 2+Fball 2)1/2 • Typical Raceway Finish: ~1 -2µin RA • Typical Steel Ball Finish: ~0. 8 -2µin RA • Typical Si N Ball Finish: < 0. 15µin RA 3 4 Minimum Lubrication Thickness ~1. 5 X CSR + Higher Precision Better Bearing Geometry, Lower Vibration Lower Noise, Longer Bearing Service Life 13 www. CERBEC. com
Si 3 N 4 Balls: Effects on Lubrication 1. 5 X CSR Lubrication Raceway 4 4 Micro-Welding Higher Friction Surface Asperities Higher Temps. Between Ball & Raceway 4 4 Dissimilar Material Fine Surface Finish Low Surface Asperities Low Friction Coefficient OCatalyzation of Hydrocarbon Based Lubrication is Significantly Reduced O Adhesive Wear is Eliminated 14 www. CERBEC. com
Ball Stiffness and Bearing Performance + 50% Stiffness Smaller Ellipse (Contact Patch) Ball on Raceway 2 2 Ball/Raceway Footprint Reduced Friction, Ball/Raceway Contact 15 www. CERBEC. com
Ball Weight and Bearing Performance - 60% Mass Lower Gyroscopic Moment Lower Centrifugal Forces Lower Raceway Stress, Less Ball Skidding 16 www. CERBEC. com
Silicon Nitride Balls: Effects from Density Silicon Nitride Balls Allow Bearings to Operate at Higher Speeds Courtesy of SKF Corporation 17 www. CERBEC. com
L 10 Fatigue Life, 106 Revolutions Silicon Nitride Balls: Effects on Damaged Raceway Hybrid Bearing Brinelled Steel Bearing Undamaged Damaged Hybrid Bearings Outlast Undamaged Steel Bearings >3 X Courtesy of SKF Corporation 18 www. CERBEC. com
Material Advantages and Bearing Wear Benefits Summary • Stiffness Maintains Contact Angle, Contact Ellipse as Temp • Dissimilarity Reduces Degradation of Lubricant • Eliminates Vibration Induced “False Brinelling” • Eliminates Adhesive Wear from Poor Lubrication • High Corrosion Resistance • Low Thermal Expansion Minimizes Ball Deformation • Lighter Weight Reduces Centrifugal Force and Gyroscopic Moment at High Speed 19 www. CERBEC. com
Finish Advantages and Bearing Wear Benefits Summary • Internal Friction is Reduced up to -30% –Reduced Cage Wear –Reduced Raceway Wear –Lower Temperature • Ball Roundness Lowers Vibration, Noise • Ball-to-Ball Variation Improves Bearing Precision –Lower Non-Repeatable Run-out 20 www. CERBEC. com
Wear (measured in ring weight loss) Silicon Nitride Balls: Effects on Wear Silicon Nitride Balls Substantially Reduce Raceway Wear Steel Bearing Courtesy of SKF Corporation Hybrid Bearing 21 www. CERBEC. com
L 10 Fatigue Life, 106 Revolutions Silicon Nitride Balls: Effects in Contaminated Oil Steel Bearing Hybrid Bearings Outlast Steel Bearings >3 X in Contaminated Oil Courtesy of SKF Corporation 22 www. CERBEC. com
Vibration Level Silicon Nitride Balls: Effects in Contamination Low Medium High Frequency Range Vibration Levels are Lower in Hybrid Bearings After Contamination Courtesy of SKF Corporation 23 www. CERBEC. com
Silicon Nitride Balls: Effects of Increasing Speed • • Bearing Contact Angles Shift as Temperature Increases Steel Balls/Raceways Expand • • 4 Increased Contact Ellipse 4 Increased Friction 4 Compound Temperature Rise 4 Lower Speed Capability 4 Lower Life Si 3 N 4 Ball Remains Stable Contact Patch Remains Small 4 Lower Bearing Temperatures 4 Higher Speed Capability 4 Longer Bearing Life Ball/Raceway Footprint 0 Contact Angle at Outer Ring I Contact Angle at Inner Ring 24 www. CERBEC. com
Silicon Nitride Balls Allow Bearings to Operate at Higher Speeds Temperature Change ΔT o. C Silicon Nitride Balls: Effects on Speed and Temperature 30 25 Steel Bearing 20 Hybrid Bearing 15 Or at Similar Speeds and Lower Operating Temperatures ΔT { 10 5 0 5, 000 10, 000 15, 000 (. 7 x 106 Dm N) Courtesy of SKF Corporation 20, 000 (1. 5 x 106 Dm N) Bearing RPM 25 www. CERBEC. com
Silicon Nitride Balls: Effects on Radial Stiffness (N/µm) 140 Steel Balls Si 3 N 4 Balls 130 120 110 100 90 80 70 0. 5 1. 0 Courtesy SNFA 1. 5 2. 0 10 6 Dm. N 26 2. 5 3. 0 www. CERBEC. com
Applications that Benefit from Si 3 N 4 Balls 27 www. CERBEC. com
Application Validation of Si 3 N 4 Balls • Liquid Oxygen Pumps - Space Shuttle - Reliability • Dental Handpiece - 400, 000 RPM - Durability • Long Range Satellites - Critical Mission Success • Machine Tool Spindles - Speed Capability and Safety • Machine Tool Ball Screws - Higher Speeds, Life • Ball Screws - Speed, Durability, Corrosion Resistance – Boeing 777, 737 & Airbus Wing Flaps - Longer Life • In-Line Skates - Low Start-up Torque When Bearings Must Perform, Engineers Turn to Si 3 N 4 Balls for Reliability and Durability 28 www. CERBEC. com
2000 Hybrid Bearing Applications 29 www. CERBEC. com
Case Study Example: Aircraft Generator Bearings • Temperature Decrease of 40 F Using Si N • Decreased Surface Wear on Raceways • Increase Life of Lubricant/Bearing System • Reduced Maintenance Schedule 3 Courtesy The Torrington Co. 30 www. CERBEC. com 4
Case Study Example: Can Seaming Roll • Lower Drag on the Seaming Roll System • Higher Quality Seam on Can • Lower Maintenance: 10 hours vs 10 Minutes • 2 1/2 Hour Increase in Production per Day: • 11, 700, 000 Additional Cans: • $2. 9 Million Production Increase Courtesy Angelus Co. 31 www. CERBEC. com
Case Study Example: Centrifugal Pump • Eliminated Fretting Corrosion • Reduced Vibrations by 50% • Significant Bearing Life Increase • Reduced Pump Maintenance • Higher Productivity at Lower Cost Courtesy Ingersol - Dresser Pump Co. 32 www. CERBEC. com
Hybrid Ball Screws • • 45% Lower Friction from Ball-to-Ball Contact Reduces Steel Ball-to-Ball Wear Eliminates Si 3 N 4 -to-Ball Contact Ball “Idler Ball” Rolls-No Sliding • • • Reduced Lubrication Requirements Higher Speed Capability - to 150 m/min Longer Bearing Service Life Cost Effective Upgrade Consistent and Lower Temperature Across Screw • Lower Machine Down Time Steel Design Courtesy Umbra Cussinetti. Si 3 N 4 Design 33 www. CERBEC. com
Design Considerations: Hybrid Bearing Load Capability • • • Direct Swapping Si 3 N 4 for Steel Reduces Bearing Load Si 3 N 4 Balls Increase Fatigue Stresses in Steel Raceways Reduced Load Capability is due to Steel Limitations Increased # Balls Reduces Stress, Stiffens Bearing Machine Tool Bearing Operate Substantially Below Max. Load Conditions Hybrid Machine Tool Bearings Rarely Fail from Ball Related Problems 34 www. CERBEC. com
Design Considerations: Hybrid Bearings: L 10 Life The Adjusted Bearing Life Calculation Lna = a 1 a 2 a 3 (C/P)p Where: a 1 is a Reliability Adjustment Factor a 2 is a Material Adjustment Factor a 3 Operating Condition Adjustment Factor C is Basic Dynamic Load Rating P is the Equivalent Dynamic Load P = 3 for STEEL Ball bearings The Traditional L 10 Calculation May NOT Apply to Hybrid Bearings 35 www. CERBEC. com
Bearing Failure: Spalling · Premature Fatigue Failure · Heavy Ball Wear Path · Overheating · Lubrication Breakdown 36 www. CERBEC. com
Design Considerations: Hybrid Bearing Fatigue Failure • Silicon Nitride Balls Fail via Spalling • Hybrid Bearings are Limited By Steel Fatigue Capability Less than 3% of All Bearings Fail from Fatigue 37 www. CERBEC. com
Design Considerations: Si 3 N 4 Ball Qualification Testing Bearing Endurance Testing • Contact Stress During Endurance Test = 3 GPa • Contact Stress Required to Fail Steel Ball ~5 -7 GPa • Crush Strength of Si N Balls is 4 -5 x Greater than Steel • Contact Stress Required to Fail Si N Ball ~25 -30 GPa • “Defective” Si N Failed at ~15 GPa 3 4 3 3 4 4 • Rolling Contact Fatigue Life of Si N 3 38 4 is Superior to Steel www. CERBEC. com
Machine Tool Bearing Market Trends 39 www. CERBEC. com
Machine Tool Spindle Advantages • Hybrid Bearings Aid Spindle Designers in: –Higher Spindle Speeds –Reduced Spindle Vibration Levels –Cooler Spindle Operation –Improved Spindle Run-out –Simpler Spindle System Designs –Longer Spindle Life –Lower Cost Crashed Spindle Repairs There is NO Downside Risk of Using a Hybrid Bearing in a Spindle Application 40 www. CERBEC. com
Hybrid Bearings: Machine Tool Current Usage • 10% of Spindles use Hybrid Ceramic Bearings Higher Speeds Dry Machining Aluminum Machining Longer Bearing Life High Acel/Decel Higher Part Accuracy • Bearing Speeds Above 1, 000 d from Hybrid Bearings. • Increasing Usage at 500 K-650 K d Life. m. N Substantially Benefit m. N Due to Longer Service • Hybrid Bearings Allow Elimination of Oil Mist Systems in Some Applications. • Hybrid Roller Bearings are a Special Product Line. 41 www. CERBEC. com
Machine Tool Hybrid Bearing Trends D m. N <500 K D m. N >1000 K The Standard Better Bearing Special Conditions Bearing = Bearing Manufacturers 42 www. CERBEC. com
Machine Tool Market - Hybrid Bearings <48 Hour Delivery = Bearing Manufacturer 1 -2 X Steel Price 3 -4 X Steel Price 6 -9+ Week Delivery 43 www. CERBEC. com
Machine Tool Hybrid Bearings: Future Directions • Market Demands for Improved Durability and Precision. • >50% of Spindles will use Hybrid Bearings. • Hybrid Bearings Easily Obtainable Through Distribution. • Availability Comparable to Steel. • Magnetic Bearing Systems will use Hybrid Bearing Back-up. • Hybrid Roller Bearings will Become Standard Product. • Machine Tool Ball Screws will Primarily use Silicon Nitride Balls for Higher Speeds. “It Must not be thought that where conventional bearings fail hybrid bearings will automatically work. ” -Bruno Pizzi, Bearing Designer 44 www. CERBEC. com
Silicon Nitride Balls Make Bearings Better Higher Speed Lighter Weight Longer Life Harder & Stiffer Smoother Surface Lower Thermal Expansion Corrosion & Electrical Resistance Cooler Temperature Cooler Lubrication 45 www. CERBEC. com
References Many thanks to those who have contributed to the development of this presentation, it is a culmination of information provided, in part, from the following sources: • • • • • SKF Bearing Corporation The Torrington Company Umbra Cuscinetti New Hampshire Ball Bearing Timken Super Precision Bearing FAG Bearing and The Barden Company SNFA Bearings Paul Mueller, GMN Bearings 46 NSK Corporation Koyo Bearings Kaydon Bearings IBAG Spindle Corporation Pratt & Whitney Aircraft Fischer Spindle Company Precise Spindle Company Bryant Grinder Corporation “Rolling Bearing Analysis” Tedric Harris www. CERBEC. com
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