2015 09 18

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Master of Science in Mechanical Engineering Politecnico di Milano Course of Machine Design 2 (Prof. S. Beretta, Prof.ssa L. Vergani) Written test – 18.09.2015 Name: Surname: Polimi-ID: Do not write here 1 2 3 TOTAL Exercise 1. Figure 1 shows the simplified structure of a washing machine with two supports (A and B) . A belt transmission gives power to the shaft (on the left side) : the corresponding forces are S0 and S N, together with the motor torque M t which is constant. At the right side, there is the basket. At its centre of mass (point C), it is applied: 1) the force P1, its weight due to mass M1; 2) the force P2, due to the linen. This corresponds to the eccentric mass M2; 3) the resistant torque Mr, corresponding to the resistance of the fluid to the motion. Figure 1: Scheme of the washing machine. Questions: 1) Plot the diagrams of bending and torque moments on the shaft; 2) Perform the assessments of the shaft; 3) Evaluate the maximum displacement of the basket. Data: Mt = 5 Nm Motor torque n = 800 rpm Rotational speed a = 100 mm; b = 80 mm; c = 120 mm Geometrical data d = 20 mm Diameter of the shaft D = 500 mm Diameter of the basket S0 = 600 N; SN = 100 N Belt forces M1 = 10 kg Mass of the basket M2 = 1.5 kg Mass of the linen UTS = 800 MPa; YS = 600 MPa Mechanical properties of the steel of the shaft S0 SN Mt A B a b c D P1 P2 C Mr Exercise 2. A surface crack with a depth of 2 cm is found in a cast iron rod of an old steam engine (Figure 2). Every weekend the machine is used for 8 hours for demonstration purposes, d uring which it is run at 15 rpm. T he load in the rod varies from +6.4×105 N to -6.4×105 N. Consider that the Paris relation is sufficiently accurate until failure. Questions: 1) Is it safe to use this machine for demonstrations and for how long?…