Unequal connecting rods

[deeferdog]

Thanks to all who helped me with the last question I had regarding the Bolton 12 Beam engine I am building, but now I have another. This time it is to do with the basic design of the engine. The piston rod is 1/4" mild steel yet the crank rod is 7/16" at its thinnest and 15/16" through its centre section. I assume that the beam simply transmits the power generated in the cylinder to the crank which turns the flywheel. Shouldn't both the piston rod and the connecting rod be roughly the same size? I understand that the connecting rod might be subject to flexing because the bottom end connection describes a rotary motion but are the forces on this side of the beam so much more that it requires a much heftier dimension? Did the old time steam engine engineers do much in the way of mathematics? Sorry if this a boring question but I have spent so much time on this engine I really feel that I should know a little about its design. Once again thanks to all, I have included the facing page of the plans ti illustrate my question.

 

[gbritnell]

I can only give my assumption but I'm assuming with the combined length and lateral forces that are applied to the connecting rod it has to be heavier. The piston rod is shorter and only travels in a straight line, up and down. The piston rod is also supported by the top head and packing gland. I'm sure that when engines were made at that time there was a lot of experimenting with parts that couldn't take the stress and were therefore strengthened along the way.
gbritnell

 

[Niels Abildgaard]

When both piston rod ad connecting rod are in tension crosssection shall be the same for same material.
Problem is compression where there is a strong tendency to buckle.This bucling tendency grows very fast with lenght relative to biggest cross dimension.
I think the solid piston rod and hollow and/or profiled connecting rod have same cross section area,but as connecting rod is much longer it is fluted.
A fluted piston rod will not be easy to make steam tigth either.

 

[Blogwitch]

As far as I can remember from discussions I had many years ago, the connecting rod is fluted for a reason.

The piston rod is supported by the cylinder stuffing gland which has a dampening effect built in, as it slides in and out, whereas the connecting rod hasn't any support at all, and if just made of the same material as the piston rod it would be flexing all over the place.

So what they do is make the connecting rod much thicker, especially where it would flex the most, in the middle. But this would cause major imbalance between the two rods due to massive weight difference, so they reduce the weight by cutting away at the rod and giving it deep flutes. This actually not only reduces the weight but makes the rod stronger by having a much larger surface area plus lots more supporting edges, just like a square tube, double the surface area and double the corners in the same outside dimension.

Plus like most Victorian pieces of engineering, it had to look nice as well, whereas with modern day engineering, they would most probably make it from tubes welded together to form a lattice.

John