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The first combustion engines needed
no connecting rods.

Historically seen, the con-rod is a typical element of the steam engine. It does not appear in the atmospheric gas engine. Only in the four-stroke Otto engine was a crank mechanism necessary where the con-rod converted the stroke movement of the piston into the rotary movement of the crankshaft.

The descriptions are topsy-turvy, the same as
with the pistons.

Just like the floor of the piston is at the top, one describes the con-rod connection to the piston as con-rod foot and to the crankshaft, as con-rod head. It's much more simple to use the expressions 'big-end' and 'small-end'. The two ends are connected by the con-rod shaft, which mostly has a double-T profile.

A lot of relative movement in the big-end.

Of the two ends, only the big-end can be opened. In rare cases, e.g., in small two-strokes, it may be one piece, indeed, the crankshaft must then be made up of individual parts. Con-rods nearly always have friction bearings. The small end is either float-mounted onto the shrunken gudgeon pin or the piston is connected to the con-rod by means of a clamp-pin. In addition, the gudgeon pin is also prevented from making contact with the cylinder sleeve. The small-end, in relation to the piston, only has a small amount of movement. The big-end, on the other hand, has a great deal of movement, to reduce the friction, which is why roller bearings are more worthwhile here.

Lubrication through horizontal drillings or oil-splash.

The con-rod plays an important role in the lubrication of the engine. In the simplest case oil passes through the big-end bearing and through the eccentric movement, is thrown against the cylinder wall. This process can be influenced by grooves in the frontal surface facing the crank webs (see fig. 1). The last picture shows a possible drilling for the lubrication of the gudgeon pin as well as additional jets where the big-end meets the shaft. Drillings may also be found in the small-end which also serve to supply oil.

Tempered steel and double-T cross-section.

In the case of the con-rod, the important features are the light weight and the high degree of firmness. Under the force applied through the combustion gases it is compressed, i.e. it is placed under high pressure force. If the engine revs higher, the occurring centrifugal pulling forces on the con-rod are very high. Apart from this, there is also the possible buckling of the con-rod shaft. To sum up, one can speak of the relative quick change over from pushing- to pulling strain, thereby, engines with a high gas pressure, e.g., Diesel engines, increase the pushing- and high revving petrol engines increase the pulling strain.

Very different materials according to the stress factor.

On the one hand, con-rods are placed under high warping and buckling strain, on the other hand, they should be lightweight because of the inertia forces. This why, e.g., in moped engines, they can be manufactured from an aluminium alloy, otherwise they are always made of spheroidal graphite cast iron, tempered steel, or sinter materials. When the demands on them are higher, they may be forged . In racing- and only a few standard engines, there are also con-rods made of titanium or even carbon fibre. In these engines, at high revs, the movement of the con-rod is so fast that even their aerodynamics play a role. This then means a special shaping and a smooth surface.

Clear classification through predetermined
breaking points.

In the case of friction bearings always, and also more and more often with roller bearings, the big-end is always two-piece. In recent times, the con-rods are manufactured as one-piece, then specifically broken again (cracked) (see fig. 3.). By doing this it is ensured that, without pins or any other centering means, the two halves cannot be moved out of position against each other. However, to be able to do this, special materials with a different yield/break limit are necessary. Before the actual rupture (cracking) takes place, the intended cracking points have small holes drilled into them by using fine laser beams. Previously, the seats had to be secured, e.g., in a complicated process of inserting fitting-pins, which indeed, did not prevent the mixing up of the bearing-cups.

Sloping division with large diameter bearings.

The two bearing halves of the big-end are held together by expansion bolts. In the case of very large sized crank pins, e.g., in Diesel engines, they may be divided slantingly, to enable them, despite the big size of the con-rod bearing, to fit inside the cylinder diameter. As far as the firmness is concerned, the slanting division is not as favourable.

Con-rod bearings become oval in time.

By the way, by altering the con-rod, neither the bore nor the stroke can be changed. If the distance between the small- and big end is increased, then the compression area is decreased and thus, the compression ratio is higher. Under normal driving conditions the round shape of the big-end changes somwhat. The diameter is stretched upwards and downwards to the tune of 1 or 2 tenths. Their size also increases from the center to the sides.

Con-rods for racing purposes should be balanced out.

For use in racing engines, the weight of the con-rod(s) can also be modified. Similar to the piston, care must be taken that the weights in the individual cylinders do not deviate too much. In the case of the con-rod, because of the second order inertia forces, a lot depends on the weight distribution between the big- and the small end. Thus, the weight of the big- and small end can be determined separately by placing the two ends horizontally on two precision balances, then grinding off material from uncritical points and attempting to harmonize the weight distribution in all the cylinders. To the latter, there is an even more elegant method, thereby, the two ends are each placed on a blade and then brought into oscillation. Using the suitable formula, the number of oscillations will allow the weight distribution to be determined. 12/11

Balancing the con-rod