Fig. 18.—Connecting Rod in the form of a Phosphor Bronze Stamping.
The Connecting Rod is another very important detail of the engine mechanism, its function being to transmit the force of the explosion from the piston to the crankshaft.
Fig. 19.—Steel Connecting
Rod turned out of the Solid Bar.
One end of the connecting rod moves up and down with the piston and oscillates (or swings to and fro) on the gudgeon pin, while the other end of the connecting rod travels in a circle, being pivoted at the crankpin and rotating in a circle which has for its centre the centre line of the engine crankshaft. This is clearly indicated in Fig. [18]. On the suction stroke of the engine the piston has to be pulled down, as we have already seen; on the explosion stroke the greatest pressure acts on the piston and pushes the connecting rod down. Thus sometimes the connecting rod is being pulled and at other times it is being pushed; in each case it has to overcome the resistance of the engine and drive the car. It is evident, therefore, that the character of the load carried by a connecting rod is just about as complex and dangerous as it is possible for a system of loading to be, and great care has to be taken in the design of such rods to ensure adequate strength without undue weight, as this would tend to keep down the maximum speed of the engine. Another important consideration is the cost of production, and for this reason one often finds it in the form of a phosphor bronze stamping of I section, although the ideal form is a round section of steel with a straight taper from gudgeon pin to crankpin end, and having a hole bored right up the centre to reduce the weight without sacrificing much strength. When the rod is made in the form of a stamping between dies it is possible to turn out great quantities at very low cost and at a very rapid rate, whereas the round steel rods would require to be machined from the solid bar to compete in price with the others. When phosphor bronze is used it is only necessary to bore out carefully and face the bearings at the two ends for the gudgeon pin and crankpin; the bearing at the crankpin end is always formed with a removable cap to facilitate fitting it nicely to the crankpin, journal and also to allow for adjustment as the bearing wears. With steel rods it is necessary to cast a white-metal lining in the crankpin end and then bore it out to form the bearing, but the crosshead bearing is usually formed by a phosphor bronze bush. It is evident, therefore, that the steel rods are more expensive, but they make a splendid mechanical job. A steel connecting rod is shown complete in Fig. [19]. Stamped steel rods of I section are also commonly used and are much better and stronger than those made entirely of phosphor bronze.
Fig. 20.—Crankpin and Crankwebs.