Tight running mill engine

I have made a Potty mill engine and I am pleased with the results, it runs well on low air pressure. I gave it away to a relative a few months ago so I decided to make another.Its now finished but will not run as well as the first being rather tight around the crank.

As the crank is made of five parts which is pinned, I have clocked it up on the lathe and has its a bit out of true by .005 . So when its together the discrepancy is tight on the bearings

My question is how best is it to run in, let it run for a few hours or place some lapping paste in the oil holes. When the bearing supports are not tightened down it quite free, any advice will be appreciated.

My son and I are in the process of making the same engine. Had a similar problem with the crankshaft being out of whack because we drilled and reamed the holes in the drill press -- which turned out to be twenty thou out of alignment across the table surface. In that case we set the crank up in the four jaw in a holding block and used a parting tool to go in and machine the crank pin true to the main shafts. The pin and shafts are now true but the two crankshaft webs sit at a definite angle if you look closely!

If you are meaning the mainshafts are out of line by five thou, you might chuck it up, put a centre in the end and take a very light cut over the mainshaft. Then make an undersize bearing and eccentric or flywheel to fit.

As far as "running in" goes, you might be better off to just ream the bearings a bit larger, or even emery paper then out a bit.

I would not put grinding paste in the bearings. You will never get it all out of the soft brass bushings and it will sit there and grind away forever.

Our effort so far:

Did you make the crank as 5 parts or 4 parts and then cut away the middle? if done the latter way you should have a straight crank.

Another option before motoring the engine is to have a go at straightening the crank, just put one end in the lathe and turning by hand clock to see which way it is bent and apply pressure to push it straight. I got a part made engine with about 40 thou runout on the one piece crank and managed to save it with this method though you should not need as much force.

Jason

That's a proper bit of John Stevenson fixing!

This is what I put up if someone want's to build a crankshaft.

Ian S C

Thanks for these replies.

I will adopted the four part system and remake it .

Ian's method worked a treat for me. Just make sure your Loctite isn't 30 years old as it will grab very quickly if it is (don't ask how I know)

I also find it less of a panic if you first glue the pin into the two webs then when that has set do the joints onto the main shaft. When doing the pin joint have the main shaft in place to keep things lined up.

On a small engine like this you can get away without pinning the joint, the loctie will be enough but if you do want to pin then do that once all the Loctited joints have set off nice and hard.

I'v made a crankshaft like this for my wee beam GAMMA Stirling Engine, the shaft and pin are 3 mm diameter, and the web is 2 mm thick. I can't remember which high strength locking compound I used.

Oops tried inserting a different pic, and ended 2 the same.

Ian S C

Edited By Ian S C on 07/03/2018 10:37:07

My first Polly engine that I gave away works well on the recipients nebulizer, I'm not sure what pressure it runs at. Since I started making small engines around two years ago I've made two of of each engine now numbering fourteen in total, so my relative has seven, five work on air and two Stirling engines. The nebulizer seems a good little compressor to use on small engines.

Question on Loctite. If the shafts are to be a tight fit then how will the Loctite be applied as it will be pushed out when fitting the shaft.

Picture of the second one, I hope to get this one running as well as the first.

Enough Loctite remains in the surface imperfections and scratches to provide a grip. You can always rough the surface up a little with coarse emery paper before pressing them together. The purists will be at the gate shortly with pitchforks and torches but that's the way I've always done it and never had a problem. If it holds Harley-Fergussons together, it's good.

We drilled and pinned our Potty crankshaft as well, me being a belt and braces kind of guy. Drilled 2mm and used 2mm diam. steel nails as pins, with Loctite. Probably overkill for an engine that is driving nowt but its own flywheel but at least it will never move. As mister Harley said to Mr Fergusson, nothing made too strong ever broke. (The corollory being: A little extra clearance never got in the way.)

Your engine looks very nice. Either different drawings or interpretation thereof from ours as we have the inlet steam going in the top of the valve chest and exhaust coming out the bottom. I like the way you have set up the knuckle on the eccentric rod to valve piston connection. Looks a bit more prototypical than the adjustable sleeve thingy shown on our drawings. Might have to plagiarize your set up when we get to it.

Fourteen engines in two years! That's a cracking pace. Well done. Wish I could get out into the shed for that sort of time.

Where did you get your flywheel from?

Nebuliser compressor? That's certainly something different. Who would have thought it would have enough grunt to run an engine? I'll have to keep an eye out for one at the garage sales.

The flywheel is from RDG Tools in the UK.

As for fourteen engines was off sick from work for nine months so the shed occupied my time, without it I would of been really fed up.

I'm actually in the UK on the night shift, if you need the drawings I've used send me a message and I will e-mail them to you.