TORSIONAL VIBRATION AND CRITICAL SPEED IN CRANKSHAFTS

It has long been recognized that, in automotive engines, particularly those of six or more cylinders, excessive vibration is apt to occur despite all precautions taken in balancing; and that this is because the engine impulses coincide at certain speeds with the torsional period of the crankshaft, or rate at which it naturally twists and untwists about some point or points as nodes. Very serious vibration occurred in the main engines for the United States submarines S 4 to S 9, which are required to complete five specified non-stop shop tests and an investigation was made of which the author reports the findings in detail, illustrated with photographs and charts.

A tuning-fork torsional-vibration study is described at length in which the engine was run at its critical speed and a vibrating tuning-fork with a thin brass reed attached to one prong drawn horizontally across a brass plate coated with lamp-black and oil and mounted upon the turning gear at the forward end of the crankshaft. A Lanchester-type vibration damper was used to remedy the difficulties encountered; and its application is discussed.