With fig. 72 is given a table of sizes of crank shafts, of the slab form, from actual practice of some of the best makers, and with fig. 73 a table of sizes for the · bent’ forms. When dealing with the Robey gas engines mention was made of the method by which the reciprocating parts were balanced, by means of counter weights strapped to the webs. The method of building up and fixing them is clearly shown on figs. 75 and 76. CHAPTER VII PISTONS The general practice with gas engine makers is to dispense with slide bars and crosshead as used in the steam engine, and to transmit the force from the piston direct to the connecting rod, the former being made of trunk form, very long, and usually about double its diameter. The gudgeon pin is at or about the centre of its length. The piston should be of such a length that the pressure due to the angularity of the connecting rodusually termed the pressure on the slide bars—shall not be excessive. This pressure should not exceed 20 lbs. per square inch of piston circumferential rubbing surface, which is usually taken as the diameter x its length. A simple graphic method of finding this pressure from indicator diagrams is as follows. In fig. 77, ABC is the indicator diagram taken from an engine having a cylinder 18 inches in diameter and a stroke of 24 inches, and the length of the connecting rod is 2:5 times the length of the stroke. The length of diagram EC represents the stroke of the piston. Now draw the length of connecting rod and path of crank pin to same scale, divide the crank pin circle into any number of parts, and draw the position of crank to correspond. Assume position 2 to be one of these, and with radius equal to length of connecting rod mark off position of piston on its path, as at 2'. Now if the distance 2' to D, which represents the pressure on the piston at the point 2', be marked off on horizontal line E F by drawing a vertical line from point G cutting the centre line of connecting rod at H, then G H will be the vertical pressure on the slide bar at position 2 of crank. In the same way the pressures may be found at any other position, and the results plotted to give a diagram as at fig. 78. The maximum pressure at G H = 26 lbs. per square inch of piston area, and if the diameter of the piston is 18 inches = 254 square inches, this area x 26 = 6604 lbs. maximum |