The Design of Rolling Bearing Mountings

76 Rope sheave of a pulley blockIn pulley blocks it is customary to arrange severalsheaves on a common shaft. To achieve minimumoverall pulley block width, the sheaves and their bearingsshould, therefore, be as compact as possible.Bearing selectionFor the rope sheaves of pulley blocks the wrap angle is180°. Thus the radial load on the bearing is twice therope pull. Thrust loads, resulting from a possible inclinedrope pull, and the moments caused by them arelow and can be neglected for bearing life calculation.Adequate bearing spread for load accommodation isachieved by mounting either two bearings or onedouble-row bearing. Deep groove ball bearings aresatisfactory for accommodating the loads in this application.The bearings are mounted on a sleeve, forming aready-to-mount unit with the sheave which can beeasily replaced.Lubrication, sealingThe sheave bearings are lubricated with lithium soapbase grease of penetration class 3 (Arcanol L71V). Highloads (load ratio P/C > 0.15) require a lithium soapbase grease of penetration class 2 and EP-additives(Arcanol L186V). One grease filling normally lasts forseveral years.The rope sheave in this example is sealed by springsteel seals (Nilos rings).Operating data and bearing dimensioningRope pull S40 kNBearing loadF = 2 · S80 kNSpeed n 30 min –1Speed factor f n 1.04Bearings mounted 2 deep groove ball bearingsFAG 6220Dynamic load rating C = 2 x 122 kNEquivalent dynamic load P = F/2 = 40 kNIndex of dynamic stressing f L = C/P · f n= 122/40 · 1.04 = 3.17Nominal rating life L h = 16,000 hUsually, an index of dynamic stressing f L = 2.5...3.5 isused for rope sheaves. This corresponds to a nominalrating life of 8,000 to 20,000 hours.Thus the bearings are adequately dimensioned comparedwith established field applications.Machining tolerancesThe mounting is a so-called hub mounting, i.e. thepulley, with the outer rings, rotates about a stationaryshaft. The outer rings carry circumferential load and arepress-fitted: hub to M7.The inner rings carry point load allowing a loose fit orsliding fit: shaft sleeve to g6 or h6.76: Rope pulleys with deep groove ball bearingsFAG 108