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 rod– usually 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, A B C 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 E C 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 × 26 = 6604 lbs. maximum square inch pressure on slide bars due to thrust of connecting CHAPTER VIII CONNECTING RODS . THE connecting rods, subjected as they are to a compressive stress, are usually made of mild steel, and the distance between centres varies from 2:25 to 3 times the length of the stroke. The strength of any given rod may be found from the following formulae : Where W = breaking load in tons per square inch. Example 13 inches x 21 inches stroke engine, centres of connecting rod 63 inches, diameter at the middle of rod 3 inches, giving a ratio of 21 to 1. - = 23 tons per square inch of sectional area. The area of the diameter in the centre = 7 square inches, the breaking load for the rod will be 23 x 7 = 161 tons. A factor of safety of from 10 to 12 should be allowed. Fig. 83 is a type of connecting rod very largely used on all sizes of engines with satisfactory results. The large end of the rod proper is carried beyond its centre line, bored to receive a | isl |