First, find the twisting moment and the bending moment (including that caused by the weight of the shaft itself) by computing the tooth-pressure, which is the force producing directly these moments, calling the twisting moment T and the bending moment M. The equivalent twisting moment for a combination of these two moments is given by the equation

where T ' is the equivalent twisting moment.

The corresponding extreme fibre-stress is to be found by the equation

where d is the diameter of the shaft, and f is the extreme fibre-stress. This should never exceed twelve thousand (12,000) pounds per square inch for all ordinary conditions of operation, or twenty-four thousand (24,000) pounds per square inch for the unusual conditions of the machinery stalled by the unbalanced wind-pressure when working at its utmost capacity, or for operation by man-power.

In no case is any shaft of less than two and one-quarter (2 1/4) inches in diameter to be used for any part of the machinery of draw-spans.

Suitable cast-iron boxes are to be provided for all bearings. All boxes, bearings, couplings, collars, etc., are to be made in accordance with the best machine-shop practice. The boxes for the line of shafting running to ends of span are to have wooden shims beneath them so that the shaft can be aligned perfectly after the span is swung.

The hand-power turning-machinery is to be so arranged that the levers can be applied conveniently to shafts near the centre of span for both the turning and the end-lifting machinery. Shafts must also be provided for applying the hand-power levers to the end-lifting machinery at each end of the span. Suitable hand-levers are to be provided for as many men as are required for operating the draw. These  levers  are to  be constructed entirely of steel, excepting only