CRANK AND CONNECTING-ROD.] 



APPLIED MECHANICS. 



859 



half a revolution, developed into a straight line. If at 

 any point, such aa 2, we draw a line 2 C representing 

 the crank, and a line 2 P representing the connecting- 

 rod, and draw Q 2 touching the circle at 2, and there- 

 fore representing the direction in which the crank-pin 

 Fig. 175. 



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is moving at the point 2, while the line P C represents 

 the direction of the piston, we may take any length P D 

 in that line representing the force of the piston (as, for 

 instance, if the pressure on the piston were 5 tons, we 

 might take P D = 5 inches), draw D E perpendicular to 

 P D, P F parallel to Q 2, and E F perpendicular to P F, 

 or parallel to C 2. Then, on the principle of resolved 

 forces, the length of P E represents the force transmitted 

 through the connecting-rod, and P F the force tending 

 to turn the crank, while D E measures the side thrust 

 on the piston-rod, and E F the longitudinal strain on 

 the crank, pushing the shaft against its bearings. If, 

 now, at the point 2 in the line A B we erect a perpen- 

 dicular 2 R 2 equal in length to P F, and at the points 

 I, 3, 4, dx., erect others deduced by the same kind of 

 construction, we can trace a curve through their sum- 

 mits, th height or ordinate of which at any point 

 measures the force turning the crank-pin at the cor- 

 responding point of its circumference ; and therefore the 

 area or space enclosed between the curve and its base 

 A B, which may be considered to bo made up of an in- 

 definite number of these ordinates, measures the total 

 force expended on the crank during a half-revolution. 

 If, now, we take a line G H equal to the diameter of the 

 crank-circle or the stroke of the piston, and, dividing it 

 into any number of equal parts, erect ordinates each 

 equal to P D, and therefore representing the constant 

 force of the piston, the area of the rectangular figure 

 G H K L thus formed will, in like manner, measure the 

 total force of the piston during one stroke. It will be 

 found that the area of this figure is exactly equal to that 

 of the curvilinear figure, as might be predicted from the 

 knowledge of the universal mechanical principle, that by 

 DO combination of machinery can we create or annihilate 

 force ; and that consequently, whatever power the piston 

 during its straight stroke impressed upon the crank, is 

 found in the crank during ita circular movement. By 

 altering the length of the connecting-rod, as compared 

 with that of the crank, we alter the figure of the force- 

 curve, but we do not change its area ; and whether the 

 Connecting-rod be long or short, the power conveyed 

 through it during the half-revolution is the same. The 

 other half-revolution being effected under similar cir- 

 cumstances, would give a force-curve precisely like that 

 of the former. 



In this investigation we have observed that the oblique 

 action of the coi>:iecting-rod causes a lateral thrust on 

 the piston-rod, measured by the line D E. It will be 

 'omul, that the longer the connecting-rod, the less will 



this lateral thrust be ; and as it is a force not only use- 

 less to the machinery, but positively prejudicial, tending 

 to bend the piston-rod or force it out of its straight 

 path, it is advantageous to reduce it to as small a quan- 

 tity as possible, and to provide means for counteracting 

 its influence. For this reason, a long connecting-rod, 

 three or four times the length of the crank radius at 

 least, should be employed, and the end of the piston-rod 

 should be made to move in guides so as to prevent it 

 from being deflected. 



As the piston at the dead centres has no influence in 

 causing the crank to revolve, it is necessary to provide 

 some means of making up for this deficiency. On the 

 crank shaft there is fixed a large heavy wheel, which 

 revolves with it, and acts as a reservoir of force to carry 

 the crank round the dead centres, and otherwise to 

 equalise the movement. A large mass of matter, such 

 as the fly-wheel, weighing sometimes many tons, cannot 

 be put in motion at a high velocity without the expendi- 

 ture of great force ; but when it is in motion it requires 

 as great force to arrest it. In the motion of a crank 

 connected with a fly-wheel, while the crank is receiving 

 its most advantageous impulse from the piston at such 

 points as 2, 3 or 4, it communicates some of its overplus 

 to the fly-wheel, which is there stored up in the form of 

 momentum or active power, to be given out upon the 

 crank and any machinery connected with it, when, at 

 the points 1 and 6, it is receiving no power from the 

 piston. 



In cases where a fly-wheel, on account of its weight 

 and bulk, cannot be applied as in marine and locomo- 

 tive engines the engine is made in duplicate, with two 

 cylinders, pistons, connecting-rods, and cranks, both of 

 the cranks being fixed on one shaft at right angles to 

 each other. While the one crank is on its dead centre, 

 Fig. 176. as C A (Fig. 176), and receiving no 

 ^ -^ " rotatory impulse ; the other crank 

 ,' \ C B is nearly at its best position for 



receiving the force of its piston. 



As the whole power of an engine 

 has to pass through its connecting-rod, 

 the joints which connect it with the 

 piston-rod and the crank require to be 

 made of great strength, and with pre- 

 cautions against friction and wear. 

 The pins of the crank and piston-rod, 

 on which these joints work, are made 

 of wrought- iron or steel, for the sake 

 of strength ; and as the friction of 

 like metals upon each other is found 

 to exceed that of different metals, the 

 eyes at the ends of the connecting-rod 

 are lined or bushed with brass, gun- 

 metal, or some soft metal, such as tin 

 alloyed with copper. The special con. 

 struction of those eyes depends upon circumstances, 

 different engineers having preferences for different 

 forms. 



In Fig. 177 is represented what is called the strap-eye. 

 The end A of the connecting-rod is squared, so that a 

 wrought-iron strap B, bent to horse-shoe form, can slide 

 on to it. Between the arch of the strap and the flat 

 end of the rod are inserted the upper brass C and the 

 lower brass D, generally made of gun-metal, sometimes 

 lined with soft metal . The brasses have projecting lips 

 or flanges at each side to prevent them from moving 

 sideways within the eye. Through the sides of the strap 

 and the head of the connecting-rod is cut a slot, into 

 which are fitted two pieces of iron, the key E and the 

 gib F, each slightly tapered, so that when the key is 

 driven gently into its place by a hammer, it acts ag a 

 wedge, pulling the strap down, and thereby tightening 

 the brasses on the pin to which they are fitted. A little 

 space is left bet wee*, the edges of the brasses to permit 

 their closer approach as they or the pin become worn in 

 the hole ; and the slot for receiving the gib and the key 

 is extended upwards in the strap and downwards in the 

 connecting-rod, to permit the key to be driven. Lest the 

 key should be shaken loose by the motion of the 



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