1 / 16

WORCESTER POLYTECHNIC INSTITUTE

WORCESTER POLYTECHNIC INSTITUTE. MECHANICAL ENGINEERING DEPARTMENT. DESIGN OF MACHINE ELEMENTS ME-3320, C’2003. Lecture 20-22-23 February 17, 2003 C. Furlong. Journal bearings (short/long) and bushings. Thick-wall journal bearing. (Typically used in low-speed applications). Hard material.

woods
Télécharger la présentation

WORCESTER POLYTECHNIC INSTITUTE

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. WORCESTER POLYTECHNIC INSTITUTE MECHANICAL ENGINEERING DEPARTMENT DESIGN OF MACHINE ELEMENTS ME-3320, C’2003 Lecture 20-22-23 February 17, 2003 C. Furlong

  2. Journal bearings (short/long) and bushings Thick-wall journal bearing (Typically used in low-speed applications) Hard material Soft material Bearings and lubrication Journal bearings(short/long) -- sliding type Short journal bearings

  3. Friction Relative velocity Bearings and lubrication: friction Journal bearings(short/long) -- sliding type Friction as a function of velocity

  4. Note hardness difference between shaft and bearing material Bearings and lubrication: materials Journal bearings(short/long) -- sliding type

  5. Shear stresses: and for constant thickness, h, where, - is absolute viscosity - is linear (tangential) velocity - is film (lubricant) thickness Linear (tangential) velocity: (Analogy) Film (lubricant) thickness Bearings and lubrication: shear stresses in lubricant Journal bearings(short/long) -- sliding type

  6. Absolute viscosity: • Units of absolute viscosity in English system: lb-sec/in2 (reyn) • Units of absolute viscosity in SI system: Pa-sec • Typical values are expressed in µreyn and mPa-sec (1 centipoise (cP) = 1mPa-sec) • Kinematic viscosity ( ) and absolute viscosity are related as: • Units of kinematic viscosity are: in2/sec (English) and cm2/sec (stoke - SI) The term “viscosity” without modifiers refers to absolute viscosity Bearings and lubrication: viscosity Absolute/Relative

  7. Diametral clearance: Radial clearance: Eccentricity: (Maximum value of eccentricity is: ) Eccentricity ratio: Minimum and maximum film thickness: (Analogy: used to derive mathematical models) Bearings and lubrication: shear stresses in lubricant Reynold’s equation for eccentric journal bearings Definitions Film thickness as a function of angular position ():

  8. Reynold’s equation: Neglecting pressure gradient in the axial direction (assumption): Bearings and lubrication: shear stresses in lubricant Reynold’s equation for eccentric journal bearings Close-form solutions for Reynold’s equation do not exist, however, it can be solved numerically (e.g., with finite element methods). Particular close-form solutions for simplified Reynold’s equation do exist Involves solution of simplified Reynold’s equation:

  9. Pressure distribution, p, is: Total load, P,is: Average pressure, Pavg,is: Bearings and lubrication: Reynold’s equation Long bearing assumption: Sommerfeld solution No-leakage of lubricant at both ends of journal Important dimensionless quantity: Sommerfeld number, S ( n’ is rotational speed in revolutions per second)

  10. Bearings and lubrication: Reynold’s equation Short bearing assumption: Ocvirk solution Incorporates leakage of lubricant at both ends of journal Pressure distribution, p, is: Total load, P,is: Coefficient, K (dimensionless), is: Important dimensionless quantity: Ocvrick number, ON

  11. Bearings and lubrication: Reynold’s equation Short bearing assumption: Ocvirk solution Incorporates leakage of lubricant at both ends of journal Correcting eccentricity ratio (correction based on experimental observations):

  12. Bearings and lubrication Pressure distribution in journal bearings with Power loss:

  13. %P- distributions (approximations) in short and long bearings: (Short-bearing solution) %Pressure (Long-bearing solution) Bearings and lubrication Pressure distribution in short journal bearings

  14. Bearings and lubrication: design considerations Journal bearings(short/long) -- sliding type If shaft has been designed (for stress and/or deflection) If shaft has not been designed • Diameter and length of bearing can be found from iteration of bearing equations with and assumed Ocvirk number • A trial lubricant must be chosen and its viscosity defined Chose ratio l/d of bearing (based on packaging considerations) • Larger l/d ratios give lower film pressures • Clearance ratio (Cd/d) are typically in the range of 0.001 to 0.002 • Larger clearance ratios increase load number ON • Higher ON numbers give larger eccentricity, pressure, and torque • Larger clearance ratios used for better lubricant flow (increasing cooling conditions) • An Ocvirk number is chosen and lubricant is then selected • After bearing has been designed, heat and momentum (flow) analysis can be done… not in this course... Note: ON 30 ( =0.82) moderate loading (recommended) ON 60 ( =0.90) heavy loading ON 90 ( =0.93) severe loading

  15. Examples • Review and Master: Example 10-1

  16. Reading • Chapters 10 of textbook: Sections 10.0 to 10.13 • Review notes and text: ES2501, ES2502, ES2503 Homework assignment • Author’s: 10-2 • Solve: 10-9, 10-13, 10-19

More Related