1 / 81

Lubricated and Journal Bearings

Lubricated and Journal Bearings. Professor John J. Mills Department of Mechanical and Aerospace Engineering The University of Texas at Arlington. Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366. Outline. General comments about lubricated bearings Lubrication

kalei
Télécharger la présentation

Lubricated and Journal Bearings

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Lubricated and Journal Bearings Professor John J. Mills Department of Mechanical and Aerospace Engineering The University of Texas at Arlington Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  2. Outline • General comments about lubricated bearings • Lubrication • Viscosity • Journal bearing design • Reynold’s equation • Long bearing solution (Sommerfeld) • Short bearing solution (Ocvirk and DuBois) • Numerical solution Raimondi and Boyd) • The Norton/Ocvirk approach • Problems with this approach Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  3. Lubricated Bearings • A complex subject • A confusing subject • Multiple approaches • Different sets of assumptions • Different levels of detail Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  4. Before Roller Bearings • The simple lubricated bearing for a wheel and and axle • Around since the invention of the wheel • Problem • Carry a load over rough territory • Solution • The wheel and axle • A revolute pair or joint • Contains a journal bushing pair • Many contexts • Still a powerful solution to many problems Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  5. Comparison to Roller Bearings • Roller bearing advantages over journals • Can support both radial and thrust loads • Are less sensitive to interruptions of lubrication • Have • Low starting and good operating friction • No self excited instabilities • Good low temperature starting • Can seal lubricant within bearing – lifetime lubrication • Typically require less space in axial direction Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  6. Comparison to Roller Bearings • Roller bearing disadvantages over journal bearings • Roller bearings • May eventually fail from fatigue • Require more space in the radial direction • Have • Poor damping ability • Higher noise level • More severe alignment requirements • Higher cost • Higher friction Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  7. Definition of a Journal Bearing • Consists of a rotatable shaft contained within a close fitting cylindrical sleeve • Rotatable shaft is the journal • The cylindrical sleeve is the bearing • The journal and the bearing are separated by a thin film of lubricant • The separation of the two surfaces (the clearance) has four functions • to permit assembly of the journal and bearing • to provide space for the lubricant, • to accommodate unavoidable thermal expansions • to tolerate any misalignments Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  8. Purpose of the Journal/bushing Pair • To allow relative rotary motion • To constrain the nature of that motion Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  9. Key Component of the Wheel and Axle • Lubrication • Otherwise there would be no relative rotary motion • An immense topic in its own right • A true cross-disciplinary research area • Mechanics • Chemistry • Surface physics • Rheology • Fluid mechanics • heat transfer Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  10. Objective of Lubrication • Reduce • Friction • Heating of rubbing/sliding machine parts • Wear Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  11. A Lubricant - a Definition • Is any substance which, when introduced between two surfaces moving relative to each other, accomplishes the objectives of lubrication Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  12. Types of lubrication • Five distinct forms of lubrication may be identified: 1 Hydrodynamic 2 Hydrostatic 3 Elastohydrodynamic 4 Boundary 5 Solid-film Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  13. Hydrodynamic Lubrication • Also called • Full film • Fluid lubrication • The load bearing surfaces are separated by a relatively thick layer of lubricant • Prevents metal to metal contact • Stability determined by the laws of fluid mechanics Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  14. Hydrodynamic Lubrication • Requires an adequate supply at all times • Does not require the lubricant to be supplied under pressure • Requires a relative motion between the two surfaces • The lubricant is pulled into the gap between the surfaces by their relative velocity • If the velocity is sufficiently high • A wedge shape film is created that supports the load • The film separates the two surfaces Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  15. Hydrodynamic Lubrication • On start up of the bearing • The journal pumps lubricant under itself • The load is then born by the lubricant which is under pressure created by the pumping action Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  16. Hydrostatic • Occurs when the lubricant is introduced into the gap between the surfaces at a pressure high enough to separate the surfaces • Does not require a relative motion of the surfaces • Not dealt with in this book • Can be important for • Bearings with low or zero velocity • Absolute mimimal friction Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  17. Elastohydrodynamic • Occurs when a lubricant is introduces between two surfaces which are in rolling contact • rolling bearings • mating gears Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  18. Boundary • Occurs when one of the previous mechanism is not valid or fails • The highest asperities of the surfaces are separated by only a few molecules of the lubricant • Mostly chemical in nature • Depends on the nature of the surfaces • Like • Unlike • The change from another mechanism to Boundary is not abrupt Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  19. Comparison of Boundary and Hydrodynamic Coefficient of Friction Boundary mixed hydrodynamic mN/P Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  20. Solid-film • Occurs when a solid film • such as graphite or Molydenum disulphide is used to separate the surfaces • or a plastic like nylon or Teflon forms one of the surfaces • Mostly used for high temperatures Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  21. Combinations of Lubrication • Some systems operate with more than one lubrication type • Elastohydrodynamic • Mixed • Boundary Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  22. Stable Lubrication • Graph from actual test data • Right of AB, lubrication stable • Variations self correcting • The the left of AB, lubrication unstable Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  23. Condition for Stable Lubrication • Units are reyn, rev/s and psi • For centipoise, rev/min and psi, the number is 30 Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  24. Key Factor in Most Lubrication • A key factor in all except solid film lubrication is the viscosity of the medium placed between the surfaces Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  25. Viscosity • Let a plate A be moving with velocity U on a film of lubricant with thickness h • Imagine the lubricant is made up of layers and the force F moving the plate cases them to slide over each other like a pack of cards Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  26. Viscosity • The Newton's law of viscous effect says that • Where m is the constant of proportionality • The absolute or dynamic viscosity • If the rate of shear is constant, then Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  27. Units for Viscosity • SI units • Pascal-second or Newton-second/square meter • inch pound second (ips) system • Reyn • pound-force-second/square inch • Named after Sir Osborne Reynolds • centimeter gram second (cgs) system • Poise • dyne-second/square centimeter • Not an official ASME unit but still used extensively • Centipoise (0.01 poise) is a convenient number in lubrication Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  28. Units for Viscosity • ASTM standard • Saybolt Universal Viscosity • the length of time it takes for 60 ml of the fluid at a specified temperature to run through a tube 17.6 mm in diameter and 12.25 mm long • kinematic viscosity • unit is the stoke and is in square centimeters/ second • based on fluid flow through tubes • One can convert between any of these with the appropriate equation Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  29. Different Ways of Measuring Viscosity • Rotating • disc • cup • cone • Flow through a tube • Saybolt method Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  30. Viscosity Examples • Viscosities of various lubricants 1 reyn is 1.4 10-7 poise Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  31. A Journal Bearing • A journal bearing is simply a shaft with a special special bushing surrounding it Bearing Lubricant Journal Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  32. Types of Journal Bearings • Three basic types • Non-self contained • Full bearing • The bearing fully encloses the journal • Partial Bearing • The bearing only partially supports the journal • Self contained • No sump for oil • Pressure fed • Oil is fed under pressure to bearing Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  33. How Design for Journal Bearings • Given information • After designing gears or other devices • Forces and speed • After designing shaft • Shaft diameter • Sometimes • the viscosity of the lubricant • The operating temperature Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  34. How Design for Journal Bearings • Need to specify • Bearing length • Length to diameter ratio • Bearing materials • Clearance ratio • Lubricant viscosity • Sometimes specified by customer • Minimum film thickness • Power loss and hence operating temperature • Ocvirk/Sommerfeld Number • Need theory and observed results • Design with multiple unknowns Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  35. How Design for Journal Bearings • Some guidelines available • Trumpler’s criteria • He was a leading designer several decades ago Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  36. How Design for Journal Bearings • Strategy • Start with length • Relatively insensitive • Estimate the clearance • Determine the viscosity of the lubricant needed to support the load • Assume some operating temperature • Calculate the power loss due to friction • Estimate the temperature rise of the lubricant • Calculate the heat loss in the gearbox and hence the average oil temperature • Redetermine the oil viscosity and iterate Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  37. How Design for Journal Bearings • Calculations not simple! • Theory and experiment in reasonable agreement • No simple analytical equations • We use dimensionless numbers and lots of charts • Just the tip of the iceberg!! • Start with some definitions Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  38. Bearing Length • The length of the bearing on the shaft, l • Film thickness, h Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  39. Bearing Length • The length of the bearing, l, is determined from the load • BUT • Other considerations are dominant Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  40. Bearing Length • Long bearing lengths have a different analysis than short bearings • Long bearings are often precluded by packaging concerns • Not enough space on shaft • Limit shaft deflection Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  41. Bearing Length • Long bearing lengths have a different analysis than short bearings • Long bearings • Are often precluded by packaging concerns • Not enough space on shaft • Limit shaft deflection • BUT • Give lower film pressures • Support larger loads • Necessary when alignment is critical Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  42. Clearance • Various definitions • Radial clearance, cr • The difference in the radius of the shaft and the bearing • Diametric clearance, cd • The difference in the two diameters • Film thickness for an unloaded bearing, h = cd/2 Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  43. Clearance • When the shaft is loaded, the clearance is not the same all the way around the circumference Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  44. Clearance • The film thickness is thus a function of location • Note that the minimum is not directly under the point of application of load Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  45. Clearance • The eccentricity e is the distance between the center of the shaft and that of the bearing • Maximum value is cr • Define a dimensionless eccentricity, e • e = e/cr • This varies from 0 at no load to 1 when the shaft is forced to touch the bearing by a load P Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  46. Other Terms • Radial clearance, cr • The bearing radius - the journal radius • Diametral Clearance, cd • The bearing diameter – the journal diameter • The minimum film thickness, ho, is given by • c = ho+efrom geometry • 1= ho/c+e/c = ho/c+e • e= 1- ho/c Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  47. Clearance and Fluid Pressure • Approximately, the film thickness is given by: Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  48. Clearance Design Principle • We are designing the bearing to ensure that the minimum film thickness under load has an adequate safety factor • The journal does not touch the bearing in operation • Clearance ratio • Fluid viscosity • Surface roughness • The journal is supported by the fluid pressure underthe journal Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  49. Clearance • Clearance is important because • If it is too small • Inadequate lubrication flow • Temperature rises • Bearing may overheat • Minimum film thickness may become too small • Journal rubs on bearing • Design to shaded area Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

  50. Clearance • Clearance is important because • If it is too large • Minimum film thickness becomes small again • Shaft “rattles” in bearing • High noise • Design to shaded area Professor John J. Mills: Email: jmills@mae.uta.edu; Tel (817) 272-7366

More Related