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Effect of multiple loads/Energy (or Rayleigh’s) method Multiple loads on SSB • To find the natural frequency of a beam on which multiple loads are acting, we find the deflection caused by each load separately and find the total effect.
Effect of self weight/Dunkerley’s method • The weight of a beam is taken as an UDL acting on top of existing loads.
A shaft 50 mm diameter and 3 metres long is simply supported at the ends and carries three loads of 1000 N, 1500 N and 750 N at 1 m, 2 m and 2.5 m from the left support.The Young's modulus for shaft material is 200 GN/m2. Find the frequency of transverse vibration. Given d = 50 mm = 0.05 m ; l = 3 m, W1 = 1000 N ; W2 = 1500 N ; W3 = 750 N; E = 200 GN/m2 = 200 × 109 N/m2
Moment of inertia of shaft • Static deflections due to point loads,. • 1000 N load • 1500 N load
Static deflections due to point loads • 750 N load • Frequency of transverse vibrations
2. Calculate the natural frequency of a shaft 20 mm diameter and 0.6 m long carrying a mass of 1 kg a its mid-point. The density of the shaft material is 40 Mg/m3 and Young’s modulus is 200 GN/m2. Assume the shaft to be freely supported. Given d = 20 mm = 0.02 m ; l = 0.6 m ; m1 = 1 kg ; ρ = 40 Mg/m3 = 40 × 106 g/m3 = 40 × 103 kg/m3 E = 200 GN/m2 = 200 × 109 N/m2
Moment of inertia of shaft • Mass per unit length • Static deflection due to point load
Static deflection due to self weight-UDL • Natural frequency of shaft
Assignment Calculate the natural frequency of a shaft 30 mm diameter and 0.6 m long carrying a mass of 1 kg a itsmid-point and 2kg at 0.4m from the left end. The density of the shaft material is 30 Mg/m3 and Young’s modulus is 200 GN/m2. Assume the shaft to be freely supported.