Smooth operation of motor cars becomes increasingly important as average driving-speeds become higher and as the public demands greater luxury and freedom from vibration. An analysis of vibration shows that it is caused by forces which can be calculated with considerable accuracy. Vibration itself is very complex, due to the inter-relation of forces, deflection and periodicity in the parts of the engine. In this paper a number of indicating and recording instruments devised for recording the actual resultant vibration and determining its exact character are described and their operation explained. Vibration due to unbalance of rotating parts, piston unbalance inherent in four-cylinder engines, bending of the crankshaft, centrifugal force, and torsional periods are discussed. Indicator-diagrams of the various kinds of vibration are shown.
Unbalanced force and elastic reaction are the two general causes of vibration. The former includes static and dynamic unbalance of reciprocating and rotating parts, while elastic reaction includes bending and twisting of the crankshaft and crankcase caused by centrifugal forces of unopposed masses and uneven turning-effort. The amplitude and frequency of torsional vibrations can be calculated from indicator-diagrams made by the new instruments. Half of all vibration in motor cars is due to simple unbalance of rotating and reciprocating parts and as much care should be given to balancing pistons, connecting-rods, flywheel, and clutch as to balancing the crankshaft. Four-cylinder engines are inherently unbalanced and when the vibration impulses come into step with the natural period of the chassis on its springs, a period of large amplitude results. In an engine operated at full load and slow speed, explosion pressure causes downward deflection of the crankshaft, and this effect is practically doubled when detonation occurs. Centrifugal forces, although balancing one another, are in different planes and introduce bending stresses in the shaft which are transmitted to the crankcase. Deflection caused by the dynamic centrifugal force is twice that caused by equal static force. The crankcase has its own period of vibration and bending increases as the square of the speed, so that it becomes significant only at the higher speeds.
The crankshaft tends to wind and unwind under intermittent turning-efforts, the degree varying with the stiffness of the shaft, the inertia of the pistons and the mass of the flywheel. It therefore vibrates torsionally at a definite frequency which, at some definite speed, falls into step with the natural rate of vibration of the shaft assembly. When the period is determined for one rate of rotation, it can be calculated for all other speeds. The frequency of vibration ranges from 180 to 250 cycles per sec. and the amplitude ranges from 0.01 to 0.03 in. at crankpin radius at normal speeds.
It is not desired to emphasize the importance of vibration unduly, as the most smooth running engine may not be entirely free from vibration in some of its forms and even a rough engine may not be seriously objectionable to its owner. However, undue vibration must be overcome and progress is being made by a better understanding of its causes, so that eventually an engine that is free from vibration may be more cheaply built than one in which vibration occurs.


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