Connecting Rod

From a historical point of view, the connecting rod was necessary, basically, in the steam engine, and not with the first internal combustion engine. It only appears again in the Otto engine. In this motor the connecting rod should also transfer the stroke movement of the piston to the rotary movement of the crankshaft. Whereby, the emphasis is very much on light weight with extreme hardness. In addition, the connecting rod makes the lubrication possible.

On the one hand, demands are made on the connecting rods by high tractive-and pushing forces as well as bending or buckling, on the other hand, they should be light-weight because of the mass-forces. For this reason, they can be produced, e.g., for motorbike engines, from an aluminium alloy, otherwise always from nodular graphite casting, quenched and tempered steel or sinter materials. For use in very high stress situations they are forged. Connecting rods have a shaft with a double-T cross section. In racing engines and some (very few) standard engines there are also connecting rods made of titanium or carbon fibre. In such engines the movement of the connecting rod can be so fast through higher RPMs in the crankcase, that their aerodynamics also plays a role. This then means special styling and a smooth surface.

The larger connecting rod "eye", (crankshaft-side) is always divided with sliding bearings, although increasingly nowadays, with roller bearings. Lately, connecting rods are also produced in one-piece and are then systematically broken (figure 3).This method ensures that even without special pins or other centring means, both of the two halves are not shifted or misplaced. Here however, special materials with a different yield-point-behaviour are necessary. Before the rupture separation, (cracking) the intended rupture point is drilled into using fine laser beams. Previously the seats had to be carefully held in place by, e.g., locating pins which did not cancel out the danger of the mixing-up of bearing caps.

Both bearing halves of the larger connecting rod "eye" are held together by anti-fatigue bolts. With very large dimensioned crankpins, e.g., for diesel engines they can be diagonally divided so that in spite of the large diameter of the big-end bearing they can be inserted from above through the cylinder. The diagonal division however, does not do much for the hardness.

The lubrication of the large connecting rod "eye" occurs through drillings in the crankshaft from the main bearings. The smaller big-end bearing receives its lubrication by spray-oil and/or by a drilling in the shaft. The oil coming from the big-end bearings of the crankshaft is also thrown up onto the walls of the cylinders by centrifugal force.

By the way, through the connecting rod neither the bore nor the stroke can be altered. If one increases the distance between the large and the small connecting rod "eye", only the compression area becomes smaller increasing only the compression ratio. In the course of normal use, and after longer service, the large connecting rod "eye" becomes slightly oval. It is stretched upwards and downwards causing a larger top-to-bottom diameter of about 1 or 2 tenths. From the centre outwards to both sides it also becomes larger.

For the application in racing engines, connecting rods can also be processed to optimise their weight. As with the pistons, the individual cylinders should not deviate too strongly from one another. With the connecting rod, apart from the second degree mass forces, also the weight distribution between small and large connecting rod "eye" play an important role. In the horizontal position, using two precision scales, one is able to determine the weight proportions of each connecting rod section and can thus attempt to evenly distribute the weight to all the cylinders by the grinding off of material from non-critical places. To the latter there is a more elegant method still, whereby the large- and the small sections of the connecting rod are mounted in each case on a blade and then caused to oscillate. Using suitable formulae, one can determine the weight distribution from the number of oscillations. 09/08