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Connecting Rod Design and Machining
The connecting rods for any non-combustion engine are relatively simple to design. The forces generated (117 lbs) are so low that you dont' have to worry about buckling or bending the rods. Just to prove this, a quick Finite Element Analysis (shown below) of the part gives us the max stress at being roughly 3,000 psi which is well within a safe limit.
 Finite Element Analysis - click for larger image |
We decided early on in our planning stages to make the crank out of 1/4" rod. And from our piston design, we already know that the pin holding the piston to the connecting rod will be 1/8" diameter and that the connecting rod should fit inside of a 1/4" slot. We also decided early on to make the connecting rods out of aluminum since we knew strength wasn't an issue.
This left us several things to decide. The first decision was how long the rod needed to be. The rod extends 1/2" into the piston, and needs 1/2" clearance for the crank since we have a 1/2" stroke. This leaves us with a minimum of 1" in length. Something else that needs to be considered is the clearance between the rod and the walls of the piston's pocket, and the cylinder walls. This isn't as much of an issue for this designsince we decided to go with a 1" diameter piston and a stroke of 1/2". However, the length still affects the angular range across which the rod must rotate. This leads us to our other considerations.
The next consideration is the bearing surface on which the pins are going to ride. Should we use bearings to reduce friction? Ball bearings are too expensive for us college kids, so they are out. We could easily go with brass or bronze bushings. To get a proper fit we could need to press them in and then drill the through hole to size afterwards (or else the hole size will be slightly undersized due to compression of the bearing). Another option would be to design a split bearing for the connecting rods, which would mean we have to worry about very small fasteners. Our initial decision was to not worry about bearings because the friction wasn't too bad. If later we find out that the friction is really slowing down the engine, we can always go back and drill the holes out slightly larger in order to press in brass bushings.
We did however decide to make the connecting rods slightly longer than 1" to reduce the angular rotation. Going with a 1" rod would have given us a rotation of:
θ = sin ֿ¹( stroke / rod length) = sin ֿ¹ (.5/1) = 30°
Going with a 1.5" rod would in turn give us an angular rotation of:
θ = sin ֿ¹ (.5/1.5) = 19.5" °
This reduces the energy lost to friction of the connecting rod rotating inside the piston by 35%. It also keeps the overall height of the engine reasonable. Lastly, we put a couple extra holes in the connecting rod to ligthen it up a bit and make it look cooler. The final design including dimensions is in the pdf file below:
To make the connecting rods, we took the easy way out. Having access to a waterjet has slightly spoiled us, so that's what we used to cut the rods. It took a total of ~30 seconds per rod, leaving the holes undersized slightly so that we could follow them with a drill bit.
 Connecting Rods in mill, ready to be drilled out to final size/finish. Click picture for larger view. |
This would be impossible to match time-wise doing it by hand. However, the rods are designed so that they can be cut fairly easily using a rotary table to do the outer corner radii and slanted walls. There's no need to make the rods this fancy however and you can make them however you would like, as long as you leave enough clearance so that the rods don't hit the walls inside of the piston. The finished product can be seen below:
 Finished connecting rod - click for larger image |