two pinions of each pair, on account of their being placed so close together.

With the equalizing arrangement just specified, it is legitimate to assume an equal division of work among all the pinions that engage the rack.

No matter what mechanical power be used, all spans must be provided also with hand-operating machinery.

METHOD OF DETERMINING POWER REQUIRED FOR
OPERATING THE SPAN AND LIFTING THE ENDS.

The power required for turning any span is to be determined by the following formula:

where W = total load on rollers in pounds, and v = velocity on pitch-circle of rack in feet per second. The value of v is to be determined by the formula

where D = diameter of pitch-circle of rack, and t = assumed time in seconds for turning the draw through one fourth (1/4) of a revolution. This method gives the power required tinder ordinary conditions; but it is always necessary to figure also the power required to open the span against an assumed unbalanced wind-pressure. This is to be determined as follows:

The unbalanced wind-pressure on one arm is to be taken at five (5) pounds per square foot of the exposed surface of the floor and both trusses.

Let P = total unbalanced wind load on one arm in pounds, and v = velocity of travel of its centre of pressure in feet per second; then