Page:Popular Science Monthly Volume 92.djvu/444

 Balancing Crankshafts With Air-Turbines

��IN the early days, when the problem of the automobile manufacturer was to make a car run at all, rather than run economically and smoothly, balanced crankshafts were unthought of. But as the buying public began to demand cars with smooth- running engines, in order to reduce the unpleasant effects of excessive vibra- tion, the automo- bile engineer had to devise some method of equalizing the power impulses transmitted to the driving shaft of the automobile at each cylinder explosion. And so he hit on the method of weighing all the pis- tons and connecting rods, and classifying them according to their weights, in or- der to be sure that the reciprocating mass of each cylin- der was equal to that of any other cylinder in the same engine.

But balancing the crankshaft was a far more diffi- cult problem, since it is in one integral piece which serves all the cylinders of the en- gine. In the old days, the only way to dis- tribute the weight of the crankshaft prop- erly on all the cylin- ders of the engine was to get it in static balance, which means in such a position that when placed on two knife edges, one at either end, the shaft

���The Air-Turbine In Action

The crankshaft is carried on two pivoted stirrups, thus leaving it entirely free to turn about its axis without being affected by its mechanical means of rotation. The turbine wheel, which has six spoke- like arms, each with a flat blade at the end, is turned by a jet of compressed air issuing from a pipe on a level with the highest position of the blades. Two micrometers, in contact with the ends of the shaft, show the vibrations of the shaft if it is out of balance, and small weights are then attached to the shaft by spring clips as shown. These indicate at exactly what points metal must be cut off in order that the shaft may be balanced perfectly

���The turbine when not in motion. Its mechanism is clearly indicated

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��would remain in that position without revolving.

While this kind of balance was easily obtained by the trial and error method of turning the shaft and then cutting off portions of the crank-arms to make the shaft balance, it did not neces- sarily follow that either the engine or the shaft would be balanced when it was rotated, as when turned in the en- gine* itself. Two equal weights on either side of the center will balance well even though one weight is all on one piston and the other all on an- other. Unequal distribution makes no difference. But, nevertheless, the piston to which the heavy weight is at- tached will push down with a harder force at each revo- lution than the one next to it. Hence the engine runs un- evenly, even though the weights are bal- anced. Excessive vibration is set [up, so that the engine wears out quickly, and the passengers are con- stantly jarred and shaken.

The demands of the American automobile- buying public for smoothly-running cars has necessitated the development of some simple method of bal- ancing the crankshafts of such engines at a rate which will not

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