operation of journal bearing

Operation of journal bearingA journal bearing is used to support the rotor in horizontalmachines and to restrict radial movement in verticalmachines. Most oil lubricated journal bearing consist of astationary cylindrical body (sleeve) separated from therotating shaft by a layer of lubricant. In a journal bearing suchas plain cylindrical, lobe or tilting pad bearings pressure orhydrodynamic lift is generated within the thin lubricant oil filmwhich separates the shaft and the bearing, thus preventingmetal-to-metal contact. The nomenclature of oil lubricatedsleeve bearing is shown in Fig. 1. In oil lubricated sleevebearings oil may be supplied to the bearings by gravity feed,external lubrication system or oil rings. Oil adheres to thejournal due to its viscous properties and pumping action asjournal rotates. Fluid pressure is generated due to thehydrodynamic action of the fluid film. This pressurecounteracts the weight of the rotor and lifts the journal from978-1-4244-3800-6/09/$25.00 ©2009 IEEE2c. Unstable Orbitb. Steady State orbit with unbalancethe bearing surface preventing metal to metal contact. Thereare several key operating and geometric parameters whichinfluences the generation of reliable oil film such as operatingspeed, load, clearance between journal and bearing, oilviscosity, surface finish as well as the radius and length ofthe bearing.Some of the important parameters involved in the designof oil lubricated sleeve bearings systems are summarizedbelow and have been divided into various sub-categories:A. Operating Parametera) Bearing Load (W)b) Operating speed (N)c) Oil viscosity ()B. Geometric Parametera) Length of bearing (L)b) Diameter of bearing (D)c) Radial clearance between journal and bearing(C)C. Performance Parametersa) Eccentricity (e)b) Eccentricity ratio () = e/Cc) Minimum film thickness ( hmin )Usually operating parameters are determined by the rotorload and motor speed, which are known to the mechanicaldesigner. In order to obtain optimized performanceparameter for a given set of operating parameters, thedesigner can control items such as geometric and operatingparameters such as oil viscosity to obtain the optimal andmost reliable bearing system possible.B. Motion of the Rotor Inside the bearing clearanceRotor center can exhibit following type of trajectory inside thebearing clearancea) Steady state orbit without unbalanceb) Steady state orbit