A crankshaft is used to convert reciprocating motion of the piston into rotary motion. The crankshaft in an engine is probably the most complex of all the shafts used in any machinery, and, as the name implies, it is far from being anywhere near a straight shaft. With the help of examples, crankshafts are classified depending on the type of supports as overhung and centre crankshaft or based on the number of throws as single throw or multi-throw shafts. The procedure for design the crankshafts is explained in detail using calculations of the crankshaft strength and stress. The factors affecting the fatigue strength are deliberated. Plots of oil film thickness explain the wear pattern. Inherently, single-, two-, three- and four- cylinder in line engines are not fully balanced for inertia forces or couples; if the cost permits, counter rotating balance shafts are designed to neutralize these forces. Otherwise, these forces are left unbalanced; they cause rigid body vibration of the engine and also are transmitted to the vehicle and to other parts. Vee engine shafts are treated slightly differently from the inline engine shafts. The designed shaft when made does not have mass distribution as per the design because of manufacturing tolerances to forging and machining. This imbalance is removed at a balancing machine within the limits specified in standards. The inertias of individual throws, piston, and connecting rod as well as the flywheel with the stiffness of the shaft result in multiple natural frequencies in the rotational direction. In case of long shafts as in the case of a six-cylinder engine, the torsional vibration can have a resonance frequency in the operating range of speeds and can induce fatigue usually starting from the oil hole in crank pin. Therefore, calculations of moments of inertia, equivalent stiffness and the natural frequencies as well as the amplitude of vibration are important. If the amplitude is sufficiently high, the torsional stress can exceed the fatigue strength limit leading to failure. To avoid the natural frequency in the speed range of operation an oscillator in the form of a torsional vibration damper is added. The new system not only shifts the frequencies but al-so reduces the dynamic magnifier to reduce the torsional stresses. Various parameters like characteristic frequency at fixed points, damping ratio, unit tuning ratio, optimum tuning ratio are introduced and with the aid of a characteristic help-graph, the tuning ratio of a rubber or spring damper can be selected. While rubber is relatively less expensive, heat dissipation in the damper must be carefully predicted to estimate the temperature of the rubber in the damper as the properties of rubber are highly dependent on temperature. The type of rubber is properly selected and manufactured with great care to avoid aging of rubber at the operating temperature. When the heat may not be easily dissipated fluid dampers are useful. Such dampers without a spring only dampen the vibration to save the shaft but do not play any major role in shifting the natural frequency of the shaft system. The engine load can be quickly simulated at specific rigs to estimate the bending fatigue or torsional fatigue. Finally, the importance of the design of bolts and the applied tightening torque are important to hold the flywheel, the crank pulley, connecting rods, bearing caps together, cannot be understated.
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Authors acknowledge with thanks “Advanced Simulation Technologies, AVL List GmbH, Graz, Austria” for their kind permission to use the Figs. 15.5 and 15.6 from reference (Loibnegger and Thomas 2001). Authors wish to thank SAE, Society of Automotive Engineers for granting the permission to use figure no.23-31alongwith the text from reference (Lakshminarayanan et al. 1999) through Copyright Clearance Centre, www.copyright.com. They are thankful to Cambridge University Press, Cambridge, UK, for permitting the use of Fig. 15.39, text and equations in Annexure I from reference (Payne and Scott 1958).
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