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Function of a crankshaft

crankshafts are made from forged steel or cast iron. crankshafts for high-volume, low-load production vehicles are 
generally constructed from nodular cast iron, which has high strength (see Appendix D). Fuel-efficient engines require 
a high power-to-displacement ratio, which has increased the use of forged crankshafts. The proportions of the materials 
used for crankshafts in car engines in 2003 were estimated to be, cast iron 25%, toughened (quenched and high-temperature 
tempered) or normalized steel 20%, and micro-alloyed steel 55%. Table 8.2 shows the chemical compositions of steel crankshafts.
2.2.2 Torsional damping of driven plate
Crankshaft torsional vibration(Fig. 2.6) Engine crankshafts are subjected to torsional wind-up and vibration at certain speeds due 
to the power impulses. Superimposed onto some steady mean rotational speed of the crankshaft will be additional fluctuating torques 
which will accelerate and decelerate the crankshaft, particularly at the front pulley end and to a lesser extent the rear 
flywheel end (Fig. 2.6). If the flywheel end of the crankshaft were allowed to twist in one direction and then the other 
while rotating at certain critical speeds, the oscillating angular movements would take up the backlash between meshing 
gear teeth in the transmission system. Consequently, the teeth of the driving gears would be moving between the drive 
(pressure side) and non-drive tooth profiles of the driven gears. This would result in repeated shockloads imposed on 
the gear teeth, wear, and noise in the form of gear clatter. To overcome the effects of crankshaft torsional vibrations 
a torsion damping device is normally incorporated within the driven plate hub assembly which will now be described and 
explained.
Multistage driven plate torsional spring dampers may be incorporated by using a range of different springs having various 
stiffnesses and spring location slots of different lengths to produce a variety of parabolic torsional load-deflection 
characteristics (Fig. 2.7) to suit specific vehicle applications.
The amount of torsional deflection necessary varies for each particular application. For example, with a front mounted 
engine and rear wheel drive vehicle, a moderate driven plate angular movement is necessary, say six degrees, since the 
normal transmission elastic wind-up is almost adequate, but with an integral engine, gearbox and final drive arrangement, 
the short transmission drive length necessitates considerably more relative angular deflection, say twelve degrees, 
within the driven plate hub assembly to produce the same quality of take-up.
Construction and operation of torsional damper washers (Figs 2.2, 2.3 and 2.8) The torsional energy created by 
the oscillating crankshaft is partially absorbed and damped by the friction washer clutch situated on either side 
of the hub flange (Figs 2.2 and 2.3). Axial damping load is achieved by a Belleville dished washer spring mounted 
between one of the side plates and a four lug thrust washer.
The outer diameter of this dished spring presses against the side plate and the inner diameter pushes onto the lugged 
thrust washer. In its free state the Belleville spring is conical in shape but when assembled it is compressed almost 
flat. As the friction washers wear, the dished spring cone angle increases. This exerts a greater axial thrust, but 
since the distance between the side plate and lugged thrust washer has increased, the resultant clamping thrust remains 
almost constant (Fig. 2.8).
Vibration analysis of a shafting system for a marine diesel generator set including dynamic characteristics between shell and housing of generator bearing
W.H. Kim, ... W.H. Joo, in 10th International Conference on Vibrations in Rotating Machinery, 2012

3.1 Crankshaft
The 9H21/32 crankshaft was modelled using idealized Timoshenko beam elements, in which the crank pin was idealized by taking an 
equivalent mass and inertia moment. To take account of the mean kinematic energy that would be induced by the reciprocating 
masses and moments of inertia under running condition, the total reciprocating mass and moments were attached at the two ends 
of crank pin. Table 2 presents the calculated masses and inertia moments of crank shaft.

Table 2. Mass and inertia moment of crankshaft
Mass [kgf] Moment of inertia [Kg-m2]
Crank pin 49.41 1.26
Reciprocating element 34.38 1.54
For the solid structure of crankshaft in which the crank-throws are not arranged in the same plane, bending stiffnesses 
are different as rotating angle changes. Therefore, the beam model for the crank pin is necessary to check the percentage 
error with crank structural natural frequencies obtained by impact tests. Figure 3 shows mode shapes of idealized crankshaft 
system with error percentages to impact results. The impact tests were performed at 45 points with an accelerometer and 
impact hammer. Error percentages within second bending mode shape do not exceed by 3 %. Considering the operating speed of 
diesel engine, 15 Hz (900 rpm), higher natural frequencies of crankshaft would not affect vibration characteristics.

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