The side rod (or connecting rod) of a real steam engine connects the piston to the engine’s drive wheels. It works like the connecting rods of a standard gasoline engine that connects the piston to the crankshaft. The steam pressure pushes the piston, which is connected to the side rod. The other end of the side rod is attached to the drive wheel and causes it to rotate.
Since this engine is driven by electric DC motors with a chain drive, the side rods are entirely superfluous and decorative. But it makes a nice visual to help complete the effect.
Front End of Side Rod
For the piston rod, I have a half-inch steel rod that sits in five-eights inch diameter holes drilled into the wooden end caps of the cylinder. The rod can slide freely. I threaded the rear end of the rod with a die cutter so that I can attach some sort of fitting to attach to the side rod. My original plan was to fabricate a bracket by bending and drilling steel bar stock and attach to the piston rod with nuts. But when browsing the hardware store for something to use I found a galvanized pipe fitting Tee that I thought I might be able to use.
The inside of the pipe fitting was already threaded. But it turns out that the thread pitch of the pipe fitting was not the same as the thread of the steel rod that I cut. Not a problem – I just re-threaded the pipe fitting to match my steel rod with a thread cutting die. I screwed the rod into the fitting and locked it with a nut.
The fittings were also threaded on the sides, so I drilled them out. I found some clevis pins at the hardware store of the proper diameter and length and used those to create the wrist pins that attach the side rod to the piston rod. Here is a picture of the final assembly of the front end of the side rod. Note that the pipe fitting is just a little too wide for the side rod, which was cut weeks ago. I added a second layer of bar stock to the back side of the side rod to use as a spacer to prevent binding.
Back END OF SIDE ROD
The back end of the side rod attaches to the drive wheel. The drive wheel has a half-inch steel bolt protruding from the side of the wheel, to which the rod is attached. I drilled a hole in the rod and inserted a bronze bushing that I lubricated with a little axle grease.
A real steam engine also has additional rods that are used to control the valves that allow steam into the cylinders at the proper time. The control of these valves is determined by the position of the wheel during the rotation cycle. So the wheel typically will have additional linkages for this purpose. These linkages are included in this design and create another nice visual effect.
The fittings for the linkages were fabricated from steel bar stock that I cut with a jigsaw. Then I drilled and ground the pieces to get the desired shapes. The fabricated pieces are attached to the steel bolt with locking nuts. The other end includes a bolt to hold the actuating rod that gets pushed and pulled during the wheel rotation. You can see a picture of the assembly here.
Here’s what the final side rod assembly looks like in operation: