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2.72 Elements of Mechanical Design Lecture 07: Rolling contact bearings
2 © Martin Culpepper, All rights reserved Schedule and reading assignment Quiz Constraints Topics Bearing types & failure modes Experimental results to modeling Bearing load-life-reliability Start spindle exercise Reading assignment None, work on getting bearings done Quiz Tuesday Bearings (conceptual) www.renault4.co.uk/ Besides the children’s toy (sorry bout that), what do you notice about the bearing that should be of import?
3 © Martin Culpepper, All rights reserved Loose ends Thermal stability Function of separation distance and contact geometry - cancellation Will you always have bearing death if it is not thermally stable? Mounting of races: Inside rotating vs. outside rotating Friction torques Look at the tapered roller example from lab
4 © Martin Culpepper, All rights reserved Rough design steps – Inherently iterative Step 1: Functional requirements DOF Stiffness Lifetime/reliability Etc… Step 2: Bearing type/layout Step 3: System design & mfg issues Housing geometryShaft geometry Shaft deflectionPreload ThermalTolerances Step 4: Assembly specifications Iteration
Types of rolling contact bearings
6 © Martin Culpepper, All rights reserved Examples of rolling element bearings static.howstuffworks.com www.machinedesign.com www.gm-exports.com/ www.bearingsworld.com
7 © Martin Culpepper, All rights reserved Inner RingRace Outer RingRace Diameters OuterBore Ball/roller Cage/separator Face Width Elements: Rotary rolling contact bearing
Bearing failure: Causes and failure mode
9 © Martin Culpepper, All rights reserved Brinelling plus cracks www.amstedrail.com Subsurface face fatigue crack that has progressed to surface. Magnification 100X Cracks in bearings elements Brinelling of surface
10 © Martin Culpepper, All rights reserved Failure mode: Spalling http://www.tsb.gc.ca
11 © Martin Culpepper, All rights reserved Causes of spalling Spalling Surface fatigue that occurs as a result of contact Seeds of failure Crack growth Inclusions Impact Cyclic high stress Degradation of the lubricant Once it starts, what happens? Minor spalling + correct problem may slow/stop Typically increases in size with continued service http://www.tsb.gc.ca Steel quality key to making long-lasting bearings
13 © Martin Culpepper, All rights reserved Model ball-groove contacts as six balls on flats Important relationships for ball-flat contact Ball-flat elastomechanics: Hertz contact F Hertzian contacts act as non-linear springs Hertz 1857-1894
14 © Martin Culpepper, All rights reserved Location, magnitude of max shear stress Max Max shear stress occurs below surface, in the member with largest R if ball and flat of same material Equivalent modulus Equivalent radius
15 © Martin Culpepper, All rights reserved Preload increases stiffness A little preload goes a long way Classes of preload, as % of static load capacity Heavy5% Medium3% Light2% Sensitivity of contact stiffness to F Contact stiffness 0 05 10 20 30 02505007501000 Fn [N] k [N/micron] Large k for small Preload
16 © Martin Culpepper, All rights reserved How do you preload a bearing? Direct Nuts pressing directly on races Uses compliance of contacts Internal Oversized balls Uses compliance of ball-race contacts Nuts-springs Spring in series with bearing Primarily uses compliance of spring Think in terms of relative stiffness because… Sensitivity of force to the displacement… Many bearings come preloaded “out of the box” Check to make sure so that you do not add preload that will act to overload
Bearing life and reliability
18 © Martin Culpepper, All rights reserved Bearing life depends on: Load and revolutions From experimentation: For a given R, linear log behavior Reliability = % chance that the part will function as expected Where a = 3 for ball bearings a = 3.333 for roller bearings Bearing life at rated reliability log(F) log(L)
19 © Martin Culpepper, All rights reserved Regression fit to experimental data Manufacturers provide a match of: Cycle rating: RevolutionsFor example 10 6 or 90 6 revs Load rating: ForceAnything… that defines bearing failure for a given reliability Given these two numbers, and: This may be used to extrapolate behavior at different loads and revs Units of force Constant Units of revolutions Why C vs. F? This is for common reliability
20 © Martin Culpepper, All rights reserved Design life in terms of hours or revolutions? We can think in terms of life as time if: Dynamic load rating Revs at failure Be careful with units
21 © Martin Culpepper, All rights reserved Reliability vs. life Reliability often well-predicted via Weibull distribution x o = minimum guaranteed value of x θ = corresponds to 63.2 percentile of the variate (stochastic variable) b = a shape parameter (controls skew, large = right) For bearings, we use this as: r(x) x xoxo Commonly used to fit experimental data; b & come from fit This is for common load on bearings
22 © Martin Culpepper, All rights reserved BUT the catalogue never tells me what happens for…. My exact desired load My exact desired life Situations when I want a reliability that is different than R = 0.90 C 10 x Design R = 0.90 Relationship between load, life and reliability log(x) log(F) A D B 2 1 1 2
23 © Martin Culpepper, All rights reserved Relationship between load, life and reliability Load as a function of reliability & vice versa for given: C 10 L 10 R = 0.90; x = 1 when L = L 10 C 10 x Design R = 0.90 log(x) log(F) A D
24 © Martin Culpepper, All rights reserved Reliability of multi-bearing sets What is the reliability of: One bearing? A spindle with two bearings? With N bearings? For first order design, how should individual bearing reliability scale as a function of N?
25 © Martin Culpepper, All rights reserved How to handle combined loading So far we have only considered radial loading… What about combined radial, Fr, and axial loading, Fa? Use an equivalent load, Fe, that does the same amount of damage. Where V = 1.2 for outer ring rotation and 1 for inner ring This has to do with the fact that outer ring fails more often X i and Y i are a function of the Axial load, Fa Static load rating, Co
26 © Martin Culpepper, All rights reserved Other issues, Shigley/Mischke covers well Life recommendations (hrs) Aircraft engines0 500 – 2 000 24-hour critical service100 000 – 200 000 Application factors General commercial1.1 – 1.3 Moderate impact1.5 – 3.0
27 © Martin Culpepper, All rights reserved Group exercise Work on your spindle housing-shaft-bearing design Constraint layout Loads Preload Thermal stability Cost
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