Bronze Bearing to Shaft Clearances

[mjonkman]

I'm building Rudy Kouhoupt's walking-beam engine and I've hit a bit of a dilemma. I'm not sure what the clearance between the crankshaft and bronze bearings should be. I have Rudy's original article from Popular Mechanics 1969 (original pages from the magazine) and I also recently bought Working Steam Engines by Rudy Kouhoupt from village press since it included more detailed drawings on the engine.

The plans just state make the bronze bushings/bearings - bore 1/4", the eccentric, crank, and flywheel all say to ream 1/4". The crankshaft is also listed as 1/4". I reamed the bushings 1/4" without thinking. Obviously everything made at exactly 1/4" will probably not spin to well. Should I somehow hone these bushings out and use 1/4" shaft? What would a good clearance value be between the shaft and the bearings.

The shaft is another question - I have a piece of 1/4" drill rod, some 1/4" 416 stainless rod and some 1/4 12L14. I originally planned on using the 416 Stainless but its actually pretty rough finish and only about a 1/2 thou oversized. The more I think about it I should probably just use drill rod for shafting. In general what is the best material to use for shafts such as this?

This is my first engine, I started to build it once before when I was 14 or 15yrs old. I had found the original Popular Mechanics article in a bunch of articles my dad had saved in an old filing cabinet. Fast forward another 30 some years, I've resurrected the project - though I am starting over making it from brass and aluminum vs the steel I started with years ago.

My dad wanted me to be a machinist / mechanic way back (so I took a lot of tech courses back in high school) and had access to a lathe and drill press but never a mill. I chose computers as my career.. now I want to play machinist at night

 

[BaronJ]

Basically if the bushes and shafts are intended to run or move against each other there needs to be enough clearance for them to move smoothly without binding. Its a common sense thing really.

Often you will find on a drawing that tolerances are not specified because "fits" are determined at the point of assembly. Don't forget that lubrication, oil, or what ever may need to be able to get into the bush/shaft bearing surfaces.

 

[Philjoe5]

Welcome to HMEM.

Generally, I make a reamed hole first, then I turn the shaft on the lathe to the nominal diameter of the reamed hole minus a 0.001" and check it for fit while still on the lathe if I can. Then I may take another 0.001" off the shaft if needed.

Sometimes I need to make the shaft first, so I may turn it to the plan size minus a 0.001". Then I ream the hole to plan size. If the shaft won't go, I use one of the best tool(s) I ever bought, a set of over/under reamers. I use the over reamer and by hand ream it through the hole, and that usually does the trick.

Long story short on shafts up to about 1/2" allow 0.001" difference, above that go for a little more. Air or steam driven engines will run quite well at these tolerances.

Cheers,
Phil

 

[kjk]

I struggled with just this sort of thing when I started. The solution for me was to use 303 stainless shafting whenever possible in standard sizes and ream +.001 over when creating the bores. A set of over/under reamers is a great thing to have in your kit.

 

[mjonkman]

Just ordered a set of over under reamers from Grizzly via Amazon Prime so I should have em by Friday. Thanks for the advise. Knowing that the tolerance should be in the range of +.001 is extremely helpful. That was the information I needed.