Lecture 6 Numerical (unit 3)
- 1. Department of Mechanical Engineering Subject Name : Machine Design-II (RME603) Topic covered: Numerical Sliding Contact Bearing (Unit-3) Dr. Ashish Kumar Srivastava Professor My Sincere Gratitude to: Dr. APJ Abdul Kalam Technical University Lucknow (UP) India 1
- 2. Table for Design values for journal bearings 2 Refer your design data book
- 3. Example 1 Design a journal bearing for a centrifugal pump from the following data : Load on the journal = 20000 N; Speed of the journal = 900 r.p.m.; Type of oil is SAE 10, for which the absolute viscosity at 55°C = 0.017 kg/m-s; Ambient temperature of oil = 15.5°C ; Maximum bearing pressure for the pump = 1.5 N/mm2. Calculate also mass of the lubricating oil required for artificial cooling, if rise of temperature of oil be limited to 10°C. Heat dissipation coefficient = 1232 W/m2/°C. Given : W = 20 000 N ; N = 900 r.p.m. ; t0 = 55°C ; Z = 0.017 kg/m-s ; ta = 15.5°C ; p = 1.5 N/mm2 ; t = 10°C ; C = 1232 W/m2/°C First of all, let us find the length of the journal ( l ). Assume the diameter of the journal ( d ) as 100 mm. From design data table, we find that the ratio of l/d for centrifugal pumps varies from 1 to 2. Let us take l/d = 1.6. l = 1.6 d = 1.6 × 100 = 160 mm Ans. Since the given bearing pressure for the pump is 1.5 N/mm2, therefore the above value of p is safe and hence the dimensions of l and d are safe. From Table , we find that the operating value of Reference : Text Book of Machine Design by R.S. Khurmi published by Lakshmi Publication
- 4. The minimum value of the bearing modulus at which the oil film will break is given by Since the calculated value of bearing characteristic number (12.24) is more than 9.33, therefore the bearing will operate under hydrodynamic conditions From Table, we find that for centrifugal pumps, the clearance ratio (c/d) = 0.0013 Coefficient of friction (k = 0.002)
- 5. 5 We see that the heat generated is greater than the heat dissipated which indicates that the bearing is warming up. Therefore, either the bearing should be redesigned by taking t0 = 63°C or the bearing should be cooled artificially. Mass of lubricating oil required for artificial cooling Reference : Text Book of Machine Design by R.S. Khurmi published by Lakshmi Publication
- 6. 6 Example 2 A full journal bearing of 50 mm diameter and 100 mm long has a bearing pressure of 1.4 N/mm2. The speed of the journal is 900 r.p.m. and the ratio of journal diameter to the diametral clearance is 1000. The bearing is lubricated with oil whose absolute viscosity at the operating temperature of 75°C may be taken as 0.011 kg/m-s. The room temperature is 35°C. Find : 1. The amount of artificial cooling required, and 2. The mass of the lubricating oil required, if the difference between the outlet and inlet temperature of the oil is 10°C. Take specific heat of the oil as 1850 J / kg / °C. Given : d = 50 mm = 0.05 m ; l = 100 mm = 0.1 m; p = 1.4 N/mm2 ; N = 900 r.p.m. ; d/c = 1000 ; Z = 0.011 kg/m-s ; t0 = 75°C ; ta = 35°C ; t = 10°C ; S = 1850 J/kg /°C The coefficient of friction Reference : Text Book of Machine Design by R.S. Khurmi published by Lakshmi Publication
- 7. 7 Since the value of heat dissipation coefficient (C ) for unventilated bearing varies from 140 to 420 W/m2/°C, therefore let us take C = 280 W/m2/°C Heat dissipated, Reference : Text Book of Machine Design by R.S. Khurmi published by Lakshmi Publication
- 8. End of Lecture Thank you 8