PREFACE. THE following Tables have been made with great care, in order to render them accurate, easy, and useful for practical men. In calculating the diameter of Wheels, the pitch is assumed equal to length of arc (on pitch circle) contained betwixt the centres of two contiguous cogs; or the quotient of circumference of pitch circle divided by the number of cogs. The Tables for Power are calculated for Wheels whose breadths are equal to twice their pitch; it having been found advantageous, in some instances, to make the calculated power (with this proportion) equal to the required power, and then to give additional allowance, in extra pitch, or breadth, or both, to provide for wear and tear to which the Wheels are likely to be subjected. The power is also calculated for one revolution per minute, reduced from the well-known rule established in the extensive practice of Hick of Bolton, and published in the Wheel List belonging to his celebrated establishment. The results obtained by Hick's rule do not exactly agree with the authorities quoted in "Buchanan's Practical Essay on Millwork," being rather higher than Roberton's; while again a difference exists betwixt Roberton and Carmichael, in consequence of the former neglecting, and the latter admitting, the length of cogs into the calculation. For Wheels with three inches pitch there is little difference in the results obtained, and the application of the Tables, as shewn in rules and examples, will enable practical men to readily obtain the power agreeing with any of the authorities before named. The Shafts Tables estimate the weight of one cubic foot of cast iron, 450 pounds; of wrought iron rolled bars (to 6 inches diameter, inclusive) 480 pounds; and of wrought iron hammered bars (6 inches diameter and above) one per cent. heavier than rolled bars, or nearly 485 pounds. The power of Shafts (to resist torsion) is also calculated for one revolution per minute (to correspond with the Table of Wheels), and for several divisors, to provide for the various allowances assigned by different engineers, when estimating the strengths of cast or wrought iron shafts. The Table for Bolts is adapted to the uniform system of screw threads, introduced by Whitworth, of Manchester; and the extreme strains, on section of bolts at bottom of thread, are reckoned from one ton to six tons per square inch, to give sufficient range for including the materials of which bolts are generally made. Holding-down bolts of wrought iron are often estimated for a maximum strain of three tons, and of copper two tons per square inch. The remaining Tables, with the examples shewing their application and use, will be readily under |