Experiences With Large Center-Crank Shafts
Authors: Illmer Louis 
Published: January 2024

This paper is descriptive of some disastrous experiences with a number of large gas-engine shafts of the center-crank type. Investigations as to the cause of the repeated shaft failures revealed inherent structural weakness due to defective design. The reinforcements undertaken to remedy this condition were based upon careful stress analysis of actual sag determinations conducted upon the wheel shaft.

The center-crank shafts in question were mounted upon three-point bearing supports and carried a heavy flywheel between the intermediate and outboard bearings. The stress diagrams show that this mode of support is likely to set up a pernicious interaction of bearing load, culminating in excessive wear in the intermediate main bearing.

When the wheel shaft lacks adequate stiffness, the appreciable lifting action at the free end of the web portion of a center-crank shaft may exert a considerable thrust against the cap of the aligned outer main bearing, which in turn reacts upon the intermediate journal, causing it to become overloaded. The resulting rapid wear in the intermediate main bearing reduces the upward cap thrust and this drop in alignment gradually relieves the intermediate bearing of overload. Further wear causes a portion of the downward load on the intermediate bearing to be transferred to the outer main bearing.

It was found that when this readjustment is complete, the downward load upon the two main bearings becomes approximately equalized. The considerable sag required to attain this state of equilibrium as to wear, involved running under stress conditions so severe as to lead to ultimate breakdown of the wheel shaft.

Starting with newly aligned bearings, the observed wear in the intermediate bearing was found to exceed ⅛ in. in less than thirty days of continuous operation, but after the intermediate bearing had dropped sufficiently to equalize the load upon the two main bearings, the wear became more nearly normal.

The stress diagrams corresponding to the estimated wheel-shaft deflection curves show the critical surface fibers to be subjected to a reversing stress ranging from 16,700 lb. per sq. in. compression to about 27,000 lb. per sq. in. tension stress, which is far in excess of the allowable range fixed by the Appendix.

The life of these shafts, as expressed in observed running time required for breakdown, appears to be in fair accord with the expected number of revolutions as determined by Stromeyer’s Law of Fatigue.

It is further shown that heavy counterweights are capable of exerting a detrimental influence upon the stress relations of a heavily loaded wheel shaft of the center-crank type. Under these conditions the best counterweight proportions are attained when its centrifugal force is made to balance that of the crank throw and the large end of the connecting rod.

The character of the reinforcements that were necessary to strengthen the defective wheel shaft is outlined, and the resulting improvements in stress and sag relations are recorded in the diagrams. In making replacements, both carbon and nickel-steel forgings were tried out, but the nickel-steel shafts did not come up to expectations.

Material specifications and acceptance tests are also tabulated, and the results of test bars taken from some critical sections of the defective nickel-steel shaft are compared with similar tests made upon a carbon-steel shaft.

In order to keep the wheel-shaft stress within desired limits under load conditions found in these engines, it finally became necessary to enlarge the intermediate main journal to 3/2 of its original diameter, thus making its dimensions fully as large as required for side-crank construction.

The conclusion arrived at is that for large center-crank shafts carrying heavy flywheels with a long span between supports, the resulting stress relations are best determined upon the modified basis advocated in this paper. The maximum stresses so found are considerably higher than would be expected from simple crank-shaft formulae, such as are commonly used to determine center-crank-shaft stresses.

Before fixing upon the final dimensions of a large shaft of this type, it is therefore expedient to make a careful analysis of the underlying stress conditions so as to insure a proper margin of safety in both the web and wheel parts after allowing for the lengthening of the span between the wheel supports due to excess wear in the intermediate main bearing.