ENGINE VIBRATION

Evening Post, Volume CXV, Issue 105, 6 May 1933, Page 17

ENGINE VIBRATION

CRANKSHAFT DESIGN

SOME OF THE PROBLEMS

Possibly no. part of the ear engine has received more attention, recently than the crankshaft. Quite a few cars, well known for stability of design and freedom, from radical changes in engine construction, have had important changes made in this respect during the last two or three years. Engineers and designers have had to provide higher engine speeds and vibrationless performance. The average motorist cares very little about crankshaft speed, but is critical about vibrationless running. He is not at all content with a motor that ha 3 periods of vibration at low and at high speeds, or that causes the body of the car to drum. Research in crankshaft design line led engineers to approach crankshaft troubles from various angles. Not one ohanrkshaft in a thousand nowadays causes vibration through being out of balance. In one 'factory a balancing machine rejects crankshafts if they are out of balance to the extent of the equivalent of the weight of one-fifth of a postage stamp placed on a big-end journal; this, too, in a mass production factory. Crankshaft whip'a few years ago was considered a serious obstacle in the dosign of a smooth running engine. Any crankshaft when turning over at 3000 to 5000 revolutions per minute is inclined to whip like a skipping rope. The phenomenon prevails at high speeds and causes the engine to be very rough. As the'whip takes place towards the centre of the crankshaft, enormous loads, running into tons per square inch have; to be supported by the centre bearing (or central bearings) of the crankshaft. Five or six years ago engine speeds took a leap forward and whip increased. Tho ■engineers h'rst of all strengthened the crankshaft, making it heavier. They then said that such a heavy sluif t would require greater bearing support, so they fitted more boarings. In place of three or four wo got seven in six cylinder engines. However, as research has subsequently shown, length in a crankshaft is not a desirable thing, and if the crankshaft of a six cylinder engine is given seven bearings it necessarily becomes long. The extra length occasioned by fitting seven main bearings actually increases tho tendency to whip, but whip is impossible on account of tho multiplicity of bearings. All the. conditions —length, relative lightness, etc.— are, however, present. Vibrations caused by insufficient bearing support and whip were, to a large extent, overcome by multi-bearing crankshafts but in their stead vibrations and strains and stresses of a far more serious nature developed. Hesearch lias shown that the increase in length of a crankshaft occasioned by the fitting of multiple bearings gives rise to torsional strains and stresses itt

the shaft itself, which in turn produce vibrations. Take, for instance, a lons ltexiblo coil spring. By holding one end firmly and twisting the other, the spring will be seen to' bow out in the centre. A similar thing happens to a long crankshaft. The twisting throws additional loads on the centre bearings. The tovsional strains and stresses arise as the shaft tries to twist itself out of true, and consequently vibrations arc produced. Several large six cylinder cars which formerly had seven bearing shafts are now turned out with four. By reclucI ing the number of main bearings such crankshafts have become shorter (thus avoiding torsional stresses and strains and their resultant vibration), whip has been decreased (a short shaft whips less than a long one), and vibration is reduced to a minimum. Massive proportions in a shaft that is supported by few bearings are essential. A short, sturdy shaft that is absolutely rigid will not whip to any extent, so fewer bearings , hazard no risk of shaft breakage. , The impression must not be given that multi-bearing crankshafts are too I light and produce vibrations on ac-. I count of torsional strains and stresses.. ! Providing that such crankshafts are of j. substantial proportions and absolutely , rigid nothing can equal them for a smooth running. Several light cars have four cylinder . engines with only two bearing crank- , shafts, but these shafts are partieuI larly solid and rigid, the engines being • designed for anything up to 6000 rcvo- ! lutions per minute. The shaft is as • short as possible and there are no con- • trc bearings to transmit whip into ' vibrations, should there be any whip. Counterweights and vibration damp J ers lessen vibration emanating from crankshafts. When cast iron pistons I were used in engines the weight of the I reciprocating parts was necessarily high I and counterweights placed on the crank- ; shaft helped greatly to balance the mass .of tho moving parts. Now that alu- • minium pistons are the order of the day, counterweights are less essential. • Vibration dampers are fitted to many '■ cars and are of great assistance in correcting crankshaft vibration whether ' periodic or otherwise. Mathematical ; considerations enter into the question of I periodic engine vibration.