Abstract External force system acting on the pulley outer surface is non-linearly varying frictional force as produced by the belt on the pulley. This paper deals with dynamics of arm of belt drive pulley using unbalanced couple approach, by computer simulation, and with analysis of armload variation. Equilibrium of rim is considered assuming it as thin. The arm is subjected to varying load. The expression for the intensity of the frictional force per unit length of the arc of contact is found. This arc of contact is divided into angle of iteration taken at a time for analysis that is further sub-divided into number of divisions. Resultant frictional force is found by summing up the elemental frictional force coming on elemental portion of rim. Here an iterative algorithm is suggested to find armload variation. From equilibrium of this elemental portion of rim armload is calculated. There are two possible solutions of this equilibrium equation: (1) Resultant frictional force is passing through the tip of the arm at the arm–rim interface. (2) Resultant frictional force is not passing through the tip of the arm at the arm–rim interface, but somewhere away from it. In this case the arm has to supply some couple to maintain the equilibrium. When the arm is unable to provide or some other mechanism is unable to provide the reaction couple at arm–rim interface, there is momentary loss of contact between the belt and the rim, causing belt flutter. The stress under vibration is then calculated and compared with previous results. The results of this approach are more refined as compared with previous results.