FRANCO AND BROZEK: OIL RING SELF-LUBRICATING SLEEVE BEARING Predicting Oil Ring Self-Lubricated Sleeve Bearing Temperature Rise and its Effect on Shaft Criticals ALBERTO FRANCO, MEMBER, IEEE, AND ROBERT J. BROZEK, MEMBER, IEEE Abstract-Several steps are involved in predicting the temperature rise within the babbitt/oil film of sleeve bearings. Initially, the viscous friction loss developed in the oil film is determined. This loss depends on the speed, length, and diameter of the journal, oil viscosity, and diametral clearance. A heat balance study then compares heat loss generation and dissipation rates. The final consideration is the ability of the oil rings to deliver the necessary oil flow rate. An approach for predicting sleeve bearing temperature rise and the adequacy of oil ring lubrication is described. It is shown that some of the design variables affecting the bearing temperature rise also influence the bearing oil film stiffness. The value of this stiffness plays a vital role in determining the shaft critical speed. INTRODUCTION FOR MOST horizontal petrochemical applications, motors of medium to large horsepower are commonly specified with sleeve bearings. Two areas of concern exist in applying these. The first is the determination of the expected operating temperature of the bearing babbitt and the necessary lubri- cation. In analyzing this, values must be established for the following: 1) the Btu/h loss developed in the oil film; 2) the heat dissipation available; 3) the required oil flow; 4) the actual oil ring flow available. The above, along with the operating eccentricity ratio, decide if the bearings can be self oil ring lubricated or whether some external forced lubrication system is necessary. The second consideration is the effect of the oil film on the shaft critical speed. The oil film is a spring with some finite value, and its presence tends to reduce a shaft critical value from a rigid bearing approach. Some of the design variables that affect the temperature/lubrication performance of the bearing also influence the shaft critical. Furthermore, the impact may not be a positive one in both areas of concern. The purpose of this paper is to outline an approach for the determination of bearing babbitt temperature and the oil film influence on the shaft critical speed. The Nomenclature lists the definitions of symbols used in this paper. BEARING DIAMETER AND LENGTH The bearing bore diameter and babbitt length selection are the starting points for the analysis. In most cases, manu- Paper PID 81-58, approved by the Petroleum and Chemical Indus- try Committee of the IEEE Industry Applications Society for presen- tation at the 1980 Petroleum and Chemical Industry Technical Con- ference, Houston, TX, September 8-10. Manuscript released for publi- cation April 28, 1981. The authors are with Electro Dynamic Division, General Dynamics Corporation, 130 Avenel St., Avenel, NJ 07001. facturers will have fixed parameters developed over the years, or some novelty may arise forcing a newly designed bearing to be used. In either case, the bearing inside diameter is usually chosen based on the torque to be transmitted by the shaft journal. Equation (1) gives the approximate journal diameter required at a given horsepower for an allowable shear stress of 5000 lbf/in2 at rated torque. / (p 1/3 D = 4 N (1) The use of the 5000-lbf/in2 design limit at full load torque depends on the shaft material used and the factor of safety necessary to take the higher torque transients that are often encountered during bus transfer or fast reclosing operations. Thus, a 2000-hp 3600-r/min-motor would give a journal inside diameter of 3.288 in and a bearing bore diameter of this plus the design diametral clearance. Once the bearing bore diameter has been decided, the length of the bearing babbitt is obtained by evaluating several interdependent factors. Two of these are the bearing loading and characteristic number. One can choose a length to give a predetermined bearing loading, as in (2). The bearing charac- teristic number CN, calculated independent of length, is then compared along with the bearing LID value for determining eccentricity ratios. If acceptable relationships for eccentricity ratios are not obtained, a variable can be changed and the process repeated: w L= (D)(P) (cl)2 (W)(I 0)6 (D)4 (ZXN) (2) (3) Small values of CN indicate lightly loaded bearings, and are common on two pole machines. Slower speed motors tend to have higher bearing characteristic numbers and loading. The eccentricity ratio plays an important role in deciding the bearing length. Low eccentricity ratios signify that the journal will run almost centered within the bearing. Too low a ratio is associated with very lightly loaded bearings and can lead to oil film instability. High eccentricity ratios denote that the journal center is displaced considerably from the bearing center. Film thickness decreases and a limit is reached when the surface roughness peaks exceed the minimum film thick- ness, and journal to babbitt contact begins. Fig. 1 illustrates eccentricity ratio. 0093-9994/81/0700-0433$00.75 © 1981 IEEE 433