3.1.5. Connecting rod
The function of the rod is to transfer the power to the piston in a way that causes the piston to move up and down within the cylinder thus requiring the connecting rod to be moderately strong. During its cycles it also has to flick oil up from the reservoir effectively lubricating all the other moving parts in the compressor.
The connecting rod is very light, has a silvery colour and also does not react to magnetic fields suggesting that it is made of a type of aluminium alloy. From the microstructure of the material it’s clear that dendrites of aluminium and an aluminium-silicon eutectic are present, showing that it is a hypo-eutectic aluminium silicon alloy. There are also many needle-like impurities in the microstructure which are thought to be inter-metallic impurities, probably iron, which introduce stress concentrations in the microstructure and reduce properties such as the strength of the alloy. Another beneficial property of aluminium alloy is its light weight and this is helpful because if a heavier material was used, it would take a lot more work to move the connecting rod.
The material is obviously made by a method of casting as the parting line and where the riser was is still visible. The surfaces that don’t contact any other part haven’t been machined and are fairly rough. The rod would have been made by die casting because of the fairly low porosity of the microstructure and then the smooth surfaces where it contacts the wrist pin and the crank shaft would be produced by boring out the existing holes from the die casting. The grooves and holes in the rod which allow the oil to lubricate the contact surfaces would be made by cutting and drilling.
The little steel rod on the end of the connecting rod that flicks the oil up would have been drawn, evident by the elongated grains in the microstructure, placed in the die and then the connecting rod would have been die cast with the rod inside it. This method casting with the...