Principles of naval engineering - Historic Naval Ships Association

Chapter 5- FUNDAMENTALS OF SHIP PROPULSION AND STEERINGelements. Both roller and ball bearings aremade in different types, some being arrangedto carry both radial and thrust loads. In thesebearings, the balls or rollers generally areassembled between two rings or races, thecontacting faces of which are shaped to fit theballs or rollers.The basic difference between ball and rollerbearings is that a ball at any given instantcarries the load on two tiny spots diametricallyopposite while a roller carries the load on twonarrow lines (fig. 5-9). Theoretically, the areaof the spot or line of contact is infinitesimal.Practically, the area of contact depends on howmuch the bearing material will distort underthe applied load. Obviously, rolling contactbearings must be made of hard materials becauseif the distortion under load is appreciablethe resulting friction will defeat the purpose ofthe bearings. Bearings with small, highly loadedcontact areas must be lubricated carefully ifthey are to have the antifriction properties theyare designed to provide. If improperly lubricated,the highly polished surfaces of the ballsand rollers soon will crack, check, or pit, andfailure of the complete bearing follows.Both sliding surface and rolling contactbearings may be further classified by theirfunction as follows: radial, thrust, and angularcontact(actually a combination of radial andthrust) bearings. Radial bearings, designedprimarily to carry a load in a direction perpendicularto the axis of rotation, are used tolimit motion in a radial direction. Thrust bearingscan carry only axial loads; that is, a forceparallel to the axis of rotation, tending to causeendwise motion of the shaft. Angular-contactbear ings can support both radial and thrust loads.The simplest forms of radial bearings arethe integral and the insert types. The integralSPOT CONTACT LINE CONTACT LINE CONTACT77,66Figure 5-9.— Load-carrying areas of ball androller bearings.type is formed by surfacing a part of the machineframe with the bearing material, while the insertbearing is a plain bushing inserted into and heldin place in the machine frame. The insert bearingmay be either a solid or a split bushing, andmay consist of the bearing material alone or beenclosed in a case or shell. In the integral bearingthere is no means of compensating for wear,and when the maximum allowable clearance isreached the bearing must be resurfaced. Theinsert solid bushing bearing, like the integraltype,has no means for adjustment due to wear,and must be replaced when maximum clearanceisreached.The pivoted shoe is a more complicated typeof radial bearing. This bearing consists of ashell containing a series of pivoted pads orshoes, faced with bearing material.The plain pivot or single disk type thrustbearing consists of the end of a journal extendinginto a cup-shaped housing, the bottom ofwhich holds the single disk of bearing material.The multi-disk type thrust bearing is similarto the plain pivot bearing except that severaldisks are placed between the end of the journaland the housing. Alternate disks of bronze andsteel are generally used. The lower disk isfastened in the bearing housing and the upperone to the journal, while the intermediate disksare free.The multi-collar thrust bearing consists ofa journal with thrust collars integral with orfastened to the shaft; these collars fit into recessesin the bearing housing which are facedwith bearing metal. This type bearing is generallyused on horizontal shafts carrying lightthrust loads.The pivoted shoe thrust bearing is similar tothe pivoted shoe radial bearing except that it hasa thrust collar fixed to the shaft which runsagainst the pivoted shoes. This type bearing isgenerally suitable for both directions of rotation.Angular loading is generally taken by usinga radial bearing to restrain the radial load andsome form of thrust bearing to handle the load.This may be accomplished by using two separatebearings or a combination of a radial and thrust(radial thrust). A typical example is the multicollarbearing which has its recesses entirelysurfaced with bearing material; the faces of thecollars carry the thrust load and the cylindricaledge surfaces handle the radial load.95