Connecting Rods and Crankshaft

     The connecting rods link the pistons with the crankshaft and transmit to it the loads arising from the combustion gas pressure taken by the pistons. In operation, the connecting rod is subjected to both gas pressure and inertia loads, and therefore, it must be adequately strong and rigid and light in weight as well. Connecting rods are generally fabricated from a high quality steel in the form of a bar with ring-shaped heads at its ends, the heads being known as the connecting rod big end and small end and serving to attach the rod to the crankpin and the gudgeon pin of the piston, respectively.

     Shank, or blade, 3 of the connecting rod is provided with an lcross section to give the rod maximum rigidity with the minimum of weight. The small end of the connecting rod is made in the form of a continuous eye into which bronze bush 2 is pressed so as to provide an interference it, whereas the big end of the rod is split into halves with the upper half integral with the rod shank and the lower half in the form of detachable cap 6 (Fig. 5).

     The bore in the big end of the connecting rod is machined after the cap is assembled on the rod. Therefore the rod caps must not be interchanged. To avoid misplacing the rod caps during assembly, the connecting rods and their mating caps are marked on one side with serial numbers starting with the first rod from the radiator, to identify their location in the engine.

Fig. 5 connecting rods
a)connecting rod components; b)cross-sections through connecting rod shanks (blade) and methods of feeding oil to piston pin; c)angled connecting rod big end; d)methods of locating connecting rod cap
1-connecting rod small end; 2-connecting rod bush; 3-connecting rod shank (blade); 4-connecting rod big end; 5-connecting rod bearing half-liner (insert); 6-connecting rod cap; 7-cotter pin; 8-horned nut; 9-locating lug; 10-connecting rod bolt; 11-oil hole; 12-oil passage; 13-serrated joint; 14-tab washer.

Both halves in the big end of the connecting rod are joined by means of special high-strength bolts 10 and nuts. The nuts on the connecting rod bolts are tightened with a torque indicating wrench and then cottered. The big end of the connecting rod houses a sliding contact bearing comprising two half-liners, or inserts 5. The half-liners are kept from shifting endwise or rotating by locating lugs’, or locking lips, 9 that nestle in special slots provided in the housing on one side of the rod. The big end of the connecting rod of automobile engines features a hole through which oil is squirted onto the cylinder walls.

     The oil necessary to lubricate the piston pin is supplied either through oil hole 11 or via oil passage 12 drilled through the connecting rod shank.

     The parting line between the connecting rod and its cap is generally arranged at right angle to the axis of the shank, but in some engines, the parting line is necessarily arranged diagonally, because the proportions of the big end of the connecting rod are such that the lower part of the rod could not otherwise be passed through the cylinder for assembly purpose. With such an angled big end, the cap is secured to the connecting rod by setscrews instead of bolts and nuts. To resist the greater tendency for the inertia forces to displace the cap sideways relative to the connecting rod, either a serrated or a stepped joint is generally preferred for their abutting faces. Hence, the retaining setscrews in their clearance holes are completely relieved of shear loads. Tab washers 14 are used under the heads of setscrews in order to prevent the latter form working loose.

     The crankshaft takes the downward thrusts of the pistons and connecting rods when the fuel-air mixture is burned in the cylinders and changes these thrusts into torque which is transferred to the drive line of the automobile; it also drives various engine mechanisms and components,. The periodic gas pressure and inertia forces taken by the crankshaft may cause it to suffer wear and bending and torsional strains. The crankshaft therefore must be adequately strong and wear-resistant. 

     The crankshaft is either forged from a high-quality steel or cast in a high-strength iron. It consists of main bearing journals (Fig.6) 1, crankpins 11, webs, or cheeks, 2 that connect the journals and crankpins together, a nose (front end), and a shank (rear end). Counterbalance weights 12 necessary for balancing the crankshaft are either formed integrally with, or attached separately to, the crank webs. The main bearing journals and crankpins are induction hardened to improve their wear resistance. Drilled diagonally through the crank webs are oil holes to supply oil to the crankpins. The crankpins are bored hollow in order to reduce the crankshaft inertia. The open ends (or end where angular blind holes are necessary to clear counterbalance weights are sealed by screw plugs 17, since the hollow interior c of each crankpin also acts as an oil supply duct for big end lubrication and as a centrifugal oil cleaner. With the crankshaft rotating, mechanical impurities *wear products) contained in the oil inside the hollow crankpins settle on the crankpin interior walls under the action of centrifugal forces. In V-type engines, each crankpin has two connecting rods assemble on it, and therefore, the crankpins here are longer than in in-line cylinder engines. The crankshaft front end carries one or two gears for driving the valve mechanism and also other engine mechanisms, fan drive pulley 16, and a starting crank jaw (ratchet) or bolt is. Mounted between the crankshaft pulley and gear is oil slinger 6 that throws oil away from the crankshaft front bearing seal. In some engines, the crankshaft gear is carried on the rear end of the shaft.

Fig. 6 crankshafts
a) of in-line tractor engine; b)of v-type automobile engine
1-main bearing journal; 2-web (dieek); 3-thrust half-washers; 4-main bearing cap insert; 5-flywheel; 6-oil slinger; 7-dowel; 8-flywheel bolt; 9-flywheel ring gear; 10-main bearing saddle insert; 11-crankpin; 12-counterbalance weights; 13-crankshaft gear; 14-oil pump drive gear; 15-bolt; 16-fan drive pulley; 17-screw plug; 18-clean oil outlet tube; 19-crankshaft flange; A-crankpin size group marking; B-oilway to crankpin hollow interior; O-crankpin hollow interior.

     Attached to the rear end of the crankshaft is flywheel 5. In some engines, the flywheel is located relative to the crankshaft by dowels 7 and clamped firmly to the rear face of the shaft by a ring of bolts 8 screwing directly into the shaft end. Other engines have their crankshafts provided with flange 19 in which holes are drilled for securing the flywheel. In front of the flange, the crankshaft is provided with an au return thread which, in conjunction with a close clearance plain bore housing, forms a labyrinth-type seat operating upon the Archimedean screw pump principle to oppose the leakage of oil into the bell housing.