A machine has been defined as a combination of bodies used to transmit force and motion. The forces acting on the machine elements arise in a number of ways; for example, in a reciprocating engine there is the gas pressure on the piston and the load torque on the crankshaft, together with the reactions at the bearing surfaces. These forces can be analysed by the methods of statics if it is assumed that the mass of the engine parts can be neglected. However, even if the output speed is nominally constant there will be cyclical variations in the velocities of the piston and connecting-rod, and in consequence the force analysis should take account of mass—acceleration (‘inertia’) effects in these elements. Furthermore it is likely that the load will vary about its mean value, and this in turn will result in fluctuations in the crankshaft speed to an extent depending on the total kinetic energy in the system. It is these dynamic effects that will be considered in this chapter, generally in isolation from the effects of other external forces (including gravity).
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