Stuart 7A crankshaft play

Hello everyone,

I own a Stuart 7A steam engine which was not built by me. The engine is not running. Thus I disassembled everything and found out that there is no axial play between the crankshaft and the two bearings.

If I place the two lower parts of the bearings and the crankshaft, the crankshaft is moving freely. If I place the two lower parts of the bearing, the crankshaft and one upper part (doesn’t matter if left or right), the crankshaft is moving freely.
But if the two lower, the two upper parts (without nuts) and the crankshaft is placed, the crankshaft stops moving freely because there is some friction between the crankshaft and the inside of both bearings.

I think there has to be removed some material. Sadly the engine came without the drawings.

Thus I‘m wondering how much play is indicated by the drawings?. What’s the dimension of crankshaft webs, upper bearing width and the needed distance between them to have the necessary play?

Can anyone help me out please?

Thanks in advance
Sebastian

I suspect the crankshaft may not be quite straight (or the bearings may not be in line). Rectification may be possible but first you need to ascertain exactly what needs altering. It may even be a case of adding material (as in shims).

Hi Sebastian,

From your description it sounds like there is not much wrong with the engine and as stated above, it may be that the bearings are not exactly in line. Also steam engines need a bit of room to expand when hot and models are often made to too tight a tolerance.

You could assemble the bearings and pass a long reamer of the correct size thru to align and enlarge the bore (if you have one of the correct size) or take a piece of round bar of similar size to the crankshaft and coat with valve grinding paste to lap the bearings for a running fit. Ensure that all paste is removed after lapping.

Paul

If the problem is in fact axial play (ie lengthwise), bearing alignment and reaming is probably not the answer.

It sounds like maybe the cheeks of the crankshaft throws are rubbing on the sides of the bearing caps. Probably a few careful strokes of a flat file to one side of the bearing cap would create the small running clearance needed for free turning.

The exact measurements are not critical. All that matters is there is at least .001" or .02mm or so of clearance there to allow free turning. A bit more will not hurt anything.

If that does not fix it, then your problem may well be radial play (up and down movement) and said reaming or lapping could fix it. Or you could dodge it by slipping a small piece of paper between the bearing cap and the lower half to pack it up and provide clearance. Preferably do it on the side away from the flywheel so it does not rattle around too much.

Edited By Hopper on 05/01/2021 01:40:34

Most Stuart drawings don't show any gap and it is upto the builder to make the required "fits" so a little off the inner edge of a bearing may be needed. As top and bottom are made separately it is quite possible the tops are longer or if you check the outer edges the cap may be sitting a bit too far inwards in which case the stud hole scan be lengthened/ enlarged

You can check it is a width issue by assembling the bearings and then seeing if you can slip the crank webs in between them. One other thing that may be causing your binding is insufficient corner clearance - if a tool with a small nose radiu has been used to turn the crankshaft it will leave a small fillet in the internal corner between shaft and web so the inner corner of teh bearing needs to be chamfered to clear this.

If the crank is tight for part of a revolution but loose in others then it's slightly bent.

Hello everyone,

first I would like to thank everyone for the fast response and your help! I really appreciate this!

I checked all the hints which everyone gave. JasonB you were right! I found out that both upper bearing parts are wider than the lower parts - this explains why the crankshaft is freely rotating without the upper or only with one upper bearing part.

As the bearing parts contains a counter-sunk which is bigger as the radius of the crankshaft this is also not the problem.

Thanks JasonB, also for the hint that the Stuart drawings do not contain any play.

Now I will remove some material from the inside of the upper bearing parts.