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It is true that the full torsional strength of a shaft is only equal to the weakest portion of it, so that three weak spots more or less can, theoretically, make no difference one way or the other. But, practically, there is the undue strain and wear of the bearings at these points, and if a pulley transmitting any considerable amount of power is situated anywhere along the length A B it is sure to be unpleasantly in evidence at all times.

Only an eighth or a quarter out, but oh, what shaft-breaking stories that fraction could tell!

The following is a simple method for testing the alinement and level of a line of shafting that is already up.

Fig. 29.

As in Fig. 29, stretch a line C so that it is exactly opposite the shafting. Set it equidistant from the shaft end centers G and F and free from all contact along its entire length except at its retaining ends A and B. Now, it is self-evident, as line C is straight and set equidistant from the shaft end centers G and F, that if you set the entire center line of the shafting at the same distance from line C, as G and F, you are bound to get your shafting into perfect alinement.