The factor two (2) is used to allow one hundred (100) per cent for friction.

Case II. —

P = 60MN

where M = the number of pounds pressure the screw will exert for one pound applied on the lever, and N = number of men on said lever. By using eighty (80) instead of one hundred and twenty (120) in the above formula, there is made an allowance of fifty (50) per cent for friction, which is certainly lower than it will ever be under ordinary working conditions.

Assuming the coefficient of friction low in this case makes an error on the side of safety.

For all ordinary conditions, Case II will give the greater value for P. The threads are to be standard square thread, and the nuts which work on them are to be made long enough to keep the greatest working unit pressure on said threads down to five hundred (500) pounds per square inch.

All links used in the toggle mechanism are to be proportioned by the formula

where l = greatest unsupported distance between fillers, except in links in which only one filler is used between two flats, when it is to be taken as the entire distance from centre to centre of end-pins, t = thickness of each link, and p = the intensity of working compressive stress.

In no case is the diameter of any pin used in a toggle to be less than two and a half (2 1/2) inches.

Rail lifts are to be provided in connection with the end-lifting toggle, and the mechanism therefor is to be so designed that the rails will not begin to rise until the end rollers have been drawn from their bearings on the end shoes. The rails shall be lifted so as to clear by one (1) inch all parts over which they must pass in turning, under the assumption that the temperature of the top chords is higher by thirty (30) degrees Fahrenheit than that of the bottom chords.