The Design of Rolling Bearing Mountings

63–64 Ship shaft thrust blocksThe thrust block is located directly behind a ship's engine.It transmits the propeller thrust to the ship.Apart from a small radial load from the shaft weightthe bearing is loaded by a purely concentric thrustload. Depending on the direction of rotation of thepropeller, it acts either forward or backward. Duringsternway the thrust load is lower and usually occursonly seldom. Three bearing arrangements are commonlyused for these requirements:Fig. 63a illustrates a thrust block arrangement withtwo spherical roller thrust bearings for small shaft diametersin a SGA plummer block housing.Fig. 63b illustrates a thrust block arrangement withtwo spherical roller thrust bearings and one radialspherical roller bearing in an FKA flanged housing.Both bearing arrangements are used when the axialload carrying capacity of a radial spherical roller bearingis insufficient when sternway is very frequent. Thespherical roller thrust bearings accommodate the propellerthrust during forward motion and the propellerpull during sternway. In 63a the thrust bearings accommodatethe weight also while in 63b the weight ofshaft and propeller is supported by a radial sphericalroller bearing.Fig. 64 shows ship shaft thrust blocks each with aspherical roller thrust bearing and a radial sphericalroller bearing:a: – in SGA housing, b: – in SUB housingThe curvature centres of the outer ring raceways of theradial and axial bearings coincide. The bearings aretherefore self-aligning and thus misalignment andbending of the shaft and hull are compensated for. Inthese thrust blocks only the propeller thrust is accommodatedby the spherical roller thrust bearing duringforward motion. The radial spherical roller bearingtransmits the weight of the shaft and the propeller pullduring sternway. The spherical roller thrust bearingnot under stress is preloaded by springs so that it doesnot lift during sternway. A constant axial minimumload is thus ensured.Machining tolerancesFig. 63a:Spherical roller thrust bearing Shaft m6; housing H7Fig. 63b:Spherical roller thrust bearing Shaft n6; housing reliefturnedRadial spherical roller bearing Shaft n6; housing F7Fig. 64a, 64b:Spherical roller thrust bearing Shaft m6; housing reliefturnedRadial spherical roller bearing Shaft m6; housing H7Dimensioning of bearingsThe diameter of the thrust block shaft is determinedaccording to the guidelines of the Classification Societies.Taking the power output into account the nominallife L h [h] and the resulting index of dynamic stressingf L are calculated. An f L value of 3 – 4 is recommendedfor the rolling bearings in ship shaft thrustblocks. Particularly with utmost cleanliness in the lubricatinggap, ship shaft thrust blocks reach endurancestrength according to the adjusted life calculation.DesignShip shaft thrust blocks are supplied as complete unitsFAG BEHT.DRL. The unit includes bearings, housingwith sealing and thrust block shaft with loose flange.The FAG thrust block housings are supplied either insplit design SGA (figs. 63a and 64a) or in unsplit designFKA (fig. 63b) or SUB (fig. 64b).Order example for unitFAG BEHT:GRL:110.156680, consisting of:1 Plummer block housing FAG SGA9322.1566781 Thrust block shaft with loose flangeFAG DRW110 x 610.1566782 Spherical roller thrust bearings FAG 29322E1 Locknut FAG KM261 Lock washer FAG MB26Oil lubricationFAG 86