1 / 28

Tutorial 7 - Design of a Journal Bearing

Tutorial 7 - Design of a Journal Bearing. Goals : • Design a journal bearing. • Calculate important operating parameters of hydrodynamic bearings. Problem Statement.

Olivia
Télécharger la présentation

Tutorial 7 - Design of a Journal Bearing

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. Tutorial 7 - Design of a Journal Bearing Goals: • Design a journal bearing. • Calculate important operating parameters of hydrodynamic bearings.

  2. Problem Statement Given: A heated roll is used to evaporate water from pulp in the production of paper. This roll is mounted onto a two inch diameter shaft for which a journal bearing needs to be designed. The roller sees a total load of 1500 pounds, which is distributed evenly between two identical bearings. The roll speed is 2000 rev/min and SAE 10 oil is readily available (it is used throughout the paper mill and is in abundant supply). The oil inlet temperature is thought to be around 110°F. Because of clearance issues, the bearing width must be one inch or less. Find: 1.) The radial clearance of the bearing for optimum load carrying capacity. 2.) The surface finish required on the bearing. 3.) The temperature rise, friction coefficient, flow rate and side flow rate of oil through the bearing. (These are needed to prescribe heat exchangers for the oil reservoir.) 4.)Comment on the importance of the inlet temperature, that is, what effect on the bearing performance would occur if the inlet temperature were 130° F?

  3. Solution Outline • Preliminary Calculations • Determination of Bearing Characteristic Number • Determination of required clearance • Determination of bearing parameters and surface roughness • Effect of inlet oil temperatures • Concluding Remarks Note: The approach presented is only one of many approaches which can be pursued. Although the indicated steps are in a logical order, they are not to be considered the essential order.

  4. Preliminary Calculations Since the bearing width is restricted to one inch or less, it will be taken as one inch. The reason for this is that there is no advantage from an operating standpoint to have a smaller width, and if the bearing will be manufactured through a grinding operation, the cost of finishing a one inch wide surface on a large roller is insignificantly larger than a 0.75 inch bearing, for example. Therefore, the diameter to width ratio for the bearing is 2 in    2 1 in j

  5. Determine the bearing characteristic number and the dimensionless film thickness

  6. Determination of Bearing Characteristic Number The bearing characteristic number is Bj=0.35. The dimensionless film thickness parameter is hmin/c=0.42. See the Next Slide for details of the analysis!

  7. Analysis Details These values can be directly read from Figure 2.28, since it is known that the diameter to width ratio is 2 and the bearing is to be designed for maximum load carrying capability: From the chart, the bearing number Bj is approximately 0.35 and the dimensionless film thickness variable is hmin/c is 0.42. The bearing number will be used to obtain the clearance once the average viscosity is calculated.

  8. Determine the dimensionless coefficient of friction variable for the bearing.

  9. Dimensionless Coefficient of Friction Variable The dimensionless coefficient of friction variable is rbm/c=9.5. See the Next Slide for details of the analysis!

  10. Analysis Details The coefficient of friction can be read from Figure 12.30, since the bearing number is known to be 0.35: The dimensionless coefficient of friction variable can be seen to be around 9.5.

  11. Determine the dimensionless volume flow rate for the bearing.

  12. Dimensionless Volume Flow Rate The dimensionless volume flow rate is Q=5.1. See the Next Slide for details of the analysis!

  13. Analysis Details The dimensionless volume flow rate is obtained from Figure 12.31: The dimensionless volumetric flow is 5.1.

  14. Determine the side-leakage flow ratio.

  15. Side Leakage Flow Variable The side leakage flow variable is qs/q=0.73. See the Next Slide for details of the analysis!

  16. Analysis Details The side leakage flow variable can be obtained from Figure 12.32: The value is qs/q=0.73.

  17. Calculate the temperature rise in the lubricant in the bearing and the average lubricant viscosity.

  18. Temperature Rise and Lubricant Viscosity The temperature rise is 113°F. The average lubricant viscosity in the bearing is µ=1.16 x 10-6 lbf-s/in2. See the Next Slide for details of the analysis!

  19. Analysis Details The dimensionless load on the bearing is Therefore the temperature rise can be calculated from Equation (12.91b): The average lubricant temperature is then 110°F + 113°F/2=166°F = 74°C. The oil viscosity is from Figure 8.13, µ0=0.008Ns/m2, or using the conversion from Table 8.2, µ0=1.16 x 10-6 lbf-s/in2.

  20. Calculate the required radial clearance, the minimum film thickness and the required shaft surface roughness.

  21. Journal Information The required radial clearance is 540 µin. The minimum film thickness is 230 µin. The maximum surface roughness is 23 µin. See the Next Slide for details of the analysis!

  22. Analysis Details The required radial clearance, now that the viscosity is known, is obtained from the Bearing number (Equation 12.85): The minimum film thickness is obtained from the dimensionless film thickness parameter previously determined: To maintain full film lubrication, the surface finish should be at most one-tenth the film thickness, or 23µin. Fortunately, this is obtainable through standard grinding operations (see Table 8.1).

  23. Calculate the friction coefficient of the journal bearing.

  24. Friction Coefficient The friction coefficient is 0.005. See the Next Slide for details of the analysis!

  25. Analysis Details The dimensionless friction coefficient has been previously obtained as rbµ/c=9.5. Since the bearing radius is 1 in and the clearance is 540 µin, the coefficient of friction is simply 0.005.

  26. Comment on the importance of the inlet oil temperature. Specifically, what effect would an inlet temperature of 130°F have on the bearing performance?

  27. Inlet Temperature Importance As can be seen from Figure 8.13, the effect of increasing the oil temperature would be minimal for such a small temperature rise. However, if the viscosity were to fall significantly, then the film thickness would be small enough to allow the film to break down and have surface-to-surface contact. This would lead to quick failure of the bearing. Also, care must be taken so that the lubricant does not become excessively heated and degrade chemically.

  28. Concluding Remarks The design of the bearing was relatively easy, using the figures from the textbook. Fortunately, this problem did not require consideration of a number of different bearing widths, which is normally the case in design. Also, this problem resulted in clearances, surface finishes and bearing dimensions which were reasonable and easily obtained, so the bearing design did not require successive iterations.

More Related