Before making a comment I would like to know the length and diameter of the piston rod. I assume the rod is steel (or stainless steel).

Really I am bumping the topic up the list because it is an interesting question.

JA

Is the "error" 0.19mm overall or plus/minus?

Edited By JA on 06/02/2023 16:31:24

Mr Watt has 2 straight line linkages, one as used typically on beam engines, the other on verticals (cylinder at bottom, crank overhead). As you have found they both produce a shallow S shape, but if you frig the position of the fixed points you can get top middle and bottom on a straight line. Is this what you have done? I ftf so the maximum error will be at 1/4 and 3/4 stroke, so there will be a bit of rod sticking out to accommodate bending. Ideally you'd make the gland float, but difficult in small sizes

7 thou is not much. Just make the packing gland a loose fit and let the packing fill in the gap. Teflon can do wonderful things. And make the piston a loosish fit, maybe even a little barrel shaped to allow a little angular clearance. Use a viton o-ring as piston ring and let that do the sealing. James Watt did not run one thou clearances in his engines. Some of his engines ran 1/8" piston to bore clearance and more.

Plus a bit of clearance on all the pins and links and piston rod bearing will all add up to provide some relief as Jason says.

As Mr Harley famously said to Mr Davidson: A little extra clearance never got in the way.

Edited By Hopper on 06/02/2023 23:26:45

You could host the file on something like Dropbox and then post the link to that in your thread then people could open it.

You said 60mm stroke in your original post, I see you now have a crank throw of 26mm = 52mm stroke.

5mm dia would be more than enough for the piston rod on an engine with that size bore, it's what I tend to use on engines upto 25mm bore and similar strokes to what you now have.

Edited By JasonB on 07/02/2023 07:03:04

I just plotted out the geometry of the Stuart Beam engine in Alibre by way of comparisson as there are plenty of thos ethat work. With no play in the pivots

Piston mid stroke 0.00mm offset

Piston top stroke +0.14mm offset

Piston bottom stroke  -0.14mm offset

This is 51.8mm stroke, 25.4 bore

If I allow for an H7 tolerance on all the pivots which is 0 to + 0.01mm and what your average reamer will give there is 0.07mm of sideways play at the pivot on the end of the piston rod so those offset figures could be halved allowing for play in the links.

Edited By JasonB on 07/02/2023 09:50:38

Do be careful with some of the drawings you find on the net, some have errors due to converting from imperial to metric often compounded by a scaling factor and some pivot pin and hole sizes have excess clearance to make design work on a computer screen but have not physically been built by the designer.

Having built the Stuart Beam many years ago I'm unlikely to do a scratch built metric one like I have with their other engines like the Real and James Coombes but the beam would make an idea candidate for conversion if you are not too far on with the BBB

JasonB has beaten me about pivot clearances and moving the cylinder.

There was a mantra at work – "fit, form and function". In the case a such a model, fit has to be good, form (the way it looks) is all important and, usually, function is a poor third. The model should work quite happily with your Watt linkage offset and Jason's quoted clearances. If it is tight due to the offset open up the holes for the piston rod in the cylinder cover and gland slightly. This should free up the movement and not effect the gland seal or anything else.

This has been a nice little exercise but I doubt if Watt and his engineers would have done it.

I find that I do not give enough clearance on pivots etc. This then becomes a problem on assembly.

JA

So what I'd suggest is rotate the piston centreline so it passes through the 3 points Jason has identified, then rotare the whole thing back again to get the piston CL vertical. This will mean the beam doesn't go equally up and down, but not noticeably, but the deviation from a straight line will be reduced, and the max deviation won't be at the bottom of the stroke, so the piston rod will be further extended and more accommodating.

You could even do a least squares fit to the locus of the pin, but that might be OTT, and it wouldn't be on centre at the bottom which feels like a good thing.

As regards this 'if it looks right it is right' idea, in reality that means it looks like the last one, no idea whether it will fulfill new requirements, or you could make it cheaper. Making things last for 100 years is probably false economy, it will almost certainly be obsolete long before that as technology advances.

Just spotted this thread, wish I'd seen it earlier.

Having completed my first simple beam engine recently I found it most frustrating trying to set it up to run smoothly.

I couldn't understand why the piston rod was binding at around a 1/4 and 3/4 of its travel. Of course I now know why!!

It didn't help that I reamed the holes (3/16&quot in the cylinder and gland cap so the piston rod was a precise fit. I decided to enlarge them to 4.9mm and rely on the ptfe plumbers tape between the caps to give a good seal.

It certainly did the job, much smoother, still a little binding but nowhere near as bad as it was. Engine now ticks over really slowly, so I'm really happy about it.

This may be the last post in this thread, if so thanks to all I really enjoyed reading all your posts.

Geoff