Torsional Vibrations in Drive Lines Paul G. Jones University of Illinois Earthmoving Industry Conference Central Illinois Section Peoria, III. April 6-7, 1965 S428 650291 Downloaded from SAE International by Imperial College London, Thursday, August 09, 2018Torsional Vibrations in Drive Lines Paul G. Jones University of Illinois ABSTRACT The critical speeds for torsional vibrations of an engine and its driven system are not influenced appreciably by small amounts of inherent damping. However, if damped vibra­ tion absorbers, fluid couplings, and other components con­ taining considerable damping are present, the peak ampli­ tudes of displacement of the various masses of the system will not occur at a common frequency and the concept of resonance loses its usual meaning. It is recommended that the response of the damped system be investigated when large damping mechanisms are present. AN ANALYSIS OF THE TORSIONAL vibrations of engines and driven auxiliary machines must be made in order to in­ sure that the operating conditions are such that large ampli­ tudes of vibration will not occur. The free vibration of a multidegree of freedom system is very complex in that the motion of any mass of the system generally is not harmonic and only under certain specified conditions will all the masses perform harmonic motions of the same frequency. When such a motion does occur, the system is said to be vi­ brating in a principal or natural mode of vibration, and the corresponding frequency is called the principal or natural frequency. The lowest nonzero natural frequency is called the fundamental frequency. If there are exciting or shaking torques present, in gen­ eral they will be nonharmonic. However, a nonharmonic shaking torque can be shown to be equal to the sum of com­ ponents which are harmonic. When the frequency of any harmonic component of torque is equal to the frequency of one of the principal modes of vibration, a condition of reso­ nance exists and the engine is said to be running at a criti­ cal speed. There are many opportunities for resonance since the torque may consist of a large number of harmonics and the system may have many principal modes. If there is no damping in the system (which is necessary for the condition of resonance as used here), the amplitude of vibration would theoretically increase with time and approach infinitely large values. In practical systems, there is always some in­ herent damping which will tend to limit the vibration to a value which is just sufficient for the damping to dissipate the energy input of the shaking torque. However, operation at a critical speed can be very dangerous and may result in overstressing or actual fracture of some part, rapid wear of bearings and gears, and undesirable vibration of the engine and associated mechanisms. In order to mitigate the torsional vibration, there are several types of tuned vibration absorbers and vibration dampers that can be employed. When the shaking torque is constant or a function of the angular velocity of the ma­ chine (as is the case in multicylinder engines), it is possible to use an undamped tuned absorber very effectively. Such absorbers are designed to produce a torque which continu­ ously counteracts the shaking torque by being tuned to the same frequency and being 180 deg out of phase with it. Since they are tuned to a single frequency for a given angu­ lar velocity of the machine, they are effective in eliminat­ ing only one component of the shaking torque. The vibra­ tion damper is somewhat more general in purpose and can reduce the vibration of the machine for a wide range of speeds, but it is not as efficient as the tuned absorber since it dissipates energy by generating heat through the damping action of its frictional forces. The following paragraphs review some of the elementary aspects of the vibration of torsional systems, followed by a discussion of some types of d