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Cast Iron Pulleys, The pulleys are generally made of *cast iron, because of their low cost. The rim is held in, place by web from the central boss or by arms or spokes. The arms may be straight or curved, as shown in Fig. 19.1 (a) and (b) and the cross-section is usually elliptical., , Design of Cast Iron Pulleys, The following procedure may be adopted for the design of cast iron pulleys., 1. Dimensions of pulley, (i) The diameter of the pulley (D) may be obtained either from velocity ratio consideration or, centrifugal stress consideration. We know that the centrifugal stress induced in the rim of the, pulley,, σt = ρ.ν2, where, ρ = Density of the rim material, = 7200 kg/m3 for cast iron, ν = Velocity of the rim, = πDN / 60, D being the diameter of pulley and, N is speed of the pulley., The following are the diameter of pulleys in mm for flat and V-belts., 20, 22, 25, 28, 32, 36, 40, 45, 50, 56, 63, 71, 80, 90, 100, 112, 125, 140, 160, 180, 200, 224,, 250, 280, 315, 355, 400, 450, 500, 560, 630, 710, 800, 900, 1000, 1120, 1250, 1400, 1600,, 1800, 2000, 2240, 2500, 2800, 3150, 3550, 4000, 5000, 5400., The first six sizes (20 to 36 mm) are used for V-belts only., (ii) If the width of the belt is known, then width of the pulley or face of the pulley (B) is taken, 25% greater than the width of belt., ∴, B = 1.25 b ; where b = Width of belt., According to Indian Standards, IS : 2122 (Part I) – 1973 (Reaffirmed 1990), the width of, pulley is fixed as given in the following table :
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The following are the width of flat cast iron and mild steel pulleys in mm :, 16, 20, 25, 32, 40, 50, 63, 71, 80, 90, 100, 112, 125, 140, 160, 180, 200, 224, 250, 315, 355,, 400, 450, 560, 630., (iii) The thickness of the pulley rim (t) varies from, for single belt, and, , for double belt. The diameter of the pulley (D) is in mm., , 2. Dimensions of arms, (i) The number of arms may be taken as 4 for pulley diameter from 200 mm to 600 mm and 6, for diameter from 600 mm to 1500 mm., Note : The pulleys less than 200 mm diameter are made with solid disc instead of arms. The, thickness of the solid web is taken equal to the thickness of rim measured at the centre of the, pulley face., (ii) The cross-section of the arms is usually elliptical with major axis (a1) equal to twice the, minor axis (b1). The cross-section of the arm is obtained by considering the arm as cantilever, i.e. fixed at the hub end and carrying a concentrated load at the rim end. The length of the, cantilever is taken equal to the radius of the pulley. It is further assumed that at any given, time, the power is transmitted from the hub to the rim or vice versa, through only half the, total number of arms., Let, T = Torque transmitted,, R = Radius of pulley, and, n = Number of arms,, ∴ Tangential load per arm,, , Maximum bending moment on the arm at the hub end,, , and section modulus,, , Now using the relation,, σb or σt = M / Z, the cross-section of the arms is obtained., (iii) The arms are tapered from hub to rim. The taper is usually 1/48 to 1/32., (iv) When the width of the pulley exceeds the diameter of the pulley, then two rows of arms, are provided, as shown in Fig. 19.4. This is done to avoid heavy arms in one row., 3. Dimensions of hub, (i) The diameter of the hub ( d1 ) in terms of shaft diameter ( d ) may be fixed by the, following relation :, d1 = 1.5 d + 25 mm, The diameter of the hub should not be greater than 2 d., (ii) The length of the hub,, , The minimum length of the hub is 2/3 B but it should not be more than width of the pulley, (B).
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Example. A cast iron pulley transmits 20 kW at 300 r.p.m. The diameter of pulley is 550, mm and has four straight arms of elliptical cross-section in which the major axis is twice the, minor axis. Find the dimensions of the arm if the allowable bending stress is 15 MPa., Solution. Given : P = 20 kW = 20 × 103 W ; N = 300 r.p.m. ; *d = 550 mm ; n = 4 ;, σb = 15 MPa = 15 N/mm2, Let, b1 = Minor axis, and, a1 = Major axis = 2b1, ...(Given), We know that the torque transmitted by the pulley,, ∴ Maximum bending moment per arm at the hub end,, , and section modulus,, , We know that the bending stress (σb),
