No go Scotty

[SignalFailure]

I made a simplified version of Elmer's 'Scotty' a month or so back but it refuses to run on air (aquarium pump) or steam (tiny pot boiler giving about 30 psi). The whole thing gets very very hot on steam and seizes up, I made the cylider of cast iron and the piston brass.

On air it "almost goes" - maybe a revolution or two but doesn't seem keen on making the return stroke. I took care to make this one as accurately as possible (I think it's my most accurate yet) and it turns over silky smooth when tweaked.

When part dis-assembled I can tell that air is getting to the cylinder via the rotary valve/crank fine and air is exhausing properly via the hole at the end of the crank.

When I apply liberal quantities of oil I can see it bubbling around the junction of the flywheel/bearing and crank-web/bearing but sufficient air seems to make it to the cylinder.

The piston rod runs very freely through the bracket (excuse unfinished state!).

My question (finally!!), has anyone who has built one of these got any tips to get me running? Is there any way to form a better seal at each end of the bearing? Might slimming down the yoke or thinning the centre of the flywheel help?

Any suggestions greatfully received - I'm eager to build a scaled up version if I can get this one going!

Here's the engine for reference (hope posting of images is ok in this section!)

 

[Bogstandard]

SF,

To me, the aquarium pump just hasn't got the umph to keep the engine running, especially as it isn't run in yet. If you can get it onto a standard shop compressor, you might find it will start to go like stink. Some of these little engines need at least 20 psi constant pressure, and the Scotty isn't the most efficient of engines.

With regards to running the engine on steam. Unless it has been made with compatible materials, you might find it actually locking up as one part expands thru heat more than another. But reading what you have said, brass in a C.I. cylinder is ok. It just might need an hours running on air to smooth everything off, then you can retry on steam.
Steam is very unforgiving when it encounters a tight engine, they usually lock up solid.
What you deem as smooth running, might just be a tight engine to someone who has a lot more experience.

But don't take it all to heart, you have made your engine, it looks good, and it looks like it should run. So if you can somehow get it onto a good constant air supply, you might find yourself grinning all over your face.

John

 

[mklotz]

Does an aquarium pump provide enough pressure? Try something larger, like an airbrush or shop compressor or tire inflator. Compressed air will also eliminate the possibility of heat-induced binding.

Check the fit of the yoke to the slider attached to the crankshaft. It may be a hair too tight in one spot or, alternately, loose enough to 'cock' slightly at some point.
A little bit of cocking can make the piston rod bind, especially if it's a good close fit in the support bracket.

Check the 'phasing' of the flats on the crankshaft. Something may have shifted since you assembled it.

If the assembled engine turns over smoothly by hand, try 'running it in' a bit with your lathe. Set the lathe to its slowest speed, grasp the flywheel in the chuck and let the lathe turn it over for a while. The idea here is to wear away any slight imperfections that you may not be able to identify visually.

My "Scotty", made very early in my career, is probably much looser than yours appears to be but it still runs nicely on an airbrush compressor. (Never tried an aquarium pump.)

 

[SignalFailure]

John: I've had it lying on it's side with the crankshaft in the drill press chuck for quite a while to run it in. I usually apply some cutting oil during this phase to assist with knocking off the sharp bits and it seems to have passed the stage where particles of metal appear in the oil. Maybe it is still too tight - I'll run it on the lathe or drill some more tomorrow.

mklotz: The yoke does cock i.e. rock backwards and forwards - I figured more clearance over the web was better than less but maybe I've allowed too much? Do you think it likely to run better if it remains upright i.e. lies flush on the web at all times?

Maybe I'm expecting too much from the aquarium pump as it only puts out about 5psi but Elmer does say it should run ok on 5 to 10 psi so I was shooting for 5

Thanks both for the wise words and encouragement

 

[mklotz]

mklotz: The yoke does cock i.e. rock backwards and forwards - I figured more clearance over the web was better than less but maybe I've allowed too much? Do you think it likely to run better if it remains upright i.e. lies flush on the web at all times?

Maybe I'm expecting too much from the aquarium pump as it only puts out about 5psi but Elmer does say it should run ok on 5 to 10 psi so I was shooting for 5

Thanks both for the wise words and encouragement

While the aquarium pump may produce 5 psi, can it sustain that pressure at whatever flow is demanded by the engine? I agree with John. Put a serious compressed air source to it and see how it does.

If that fails, consider making a new slider with a better fit to the yoke. As I remember, that's a very simple part so making a new one should not be that onerous. I don't think engine orientation matters (on this engine, on my Ringbom it surely matters). Mine runs the same in any orientation.

 

[Philjoe5]

SF,
I completed a horizontal mill steam engine recently, my first steam engine build. When I put compressed air in the first time I had to push the regulator up to 40 - 50 psi. Within a few hours of running and tweaking the timing I had it down to less than 10 psi. Now with 20 hours of run time on it I can run it on 3-5 psi. So keep at it for sure. Put some real pressure on it and assuming you've got the timing worked out, see what happens. BTW, when I first put 40-50 psi air on this engine I had the timing off so that I was admitting air to the top of the cylinder just shy of TDC. The engine would go a few revolutions and stop. By adjusting the timing, I got it to run consistently. ;D

Cheers,
Phil

 

[Bogstandard]

SF,

Putting it onto a drill won't really solve the problem.

It might make it a bit freer, and I do it just to 'knock off' the high spots. If you think about it, it is being run totally the opposite of how it was designed to run, and if carried out too much, can induce wear into the engine in positions it shouldn't have wear. A fellow modeller I know, ruined a lovely four cylinder wobbler engine by running it from the outshaft with a drill. I actually have it in my workshop in the hope that one day I can restore it, but it looks like it might be easier to recycle it.

John

 

[SignalFailure]

I usually put a rubber band on the lathe chuck and hold the flywheel up to it and rely on friction - maybe I'll go back to that for a time!

Thanks John and thanks Phil

 

[Bogstandard]

SF,

I think you have misunderstood me slightly. By turning over the engine as you are doing now, you can damage the engine beyond repair.
I have just been to my shop and taken these two pics of what I am on about.


This is the engine I was on about, sitting on my shelf hasn't done it any favours.
Anyway, when it was built it would not run, so my mate decided to wack it over with a drill, supposedly to free it off. It ended up so freed off it would have been unusable anyway. So I took it under my wing. The problem was caused by one of the inlet pipes being blocked with silver solder, so it was never going to run right in the first place. The damage caused was major, he has forced it over by using the drill, against the internal pressure caused by the blocked pipe.

This is the sort of damage caused, this is only minor compared to bent rods and oval worn bottom bearings on the cylinders.
Look at the crankpin hole, that was a perfect fit half an hour before it was run by driving the engine from the crankshaft.

One perfectly good engine ruined by bad practices. It is OK to do it for a short time, with everything open to atmosphere and no real tight spots. Otherwise you could end up with an engine like this.

BTW, if anyone would like to build one of these, I do have the full set of articles, complete with boiler build.

John

 

[SignalFailure]

Seems that pressure is the problem - hooked it up to a footpump and it spins nicely...

[youtube=425,350]NlahK0GLucQ[/youtube]

EDITED: Put better video on youtube instead!

 

[Bogstandard]

SF,

Very well done.
I bet you are smiling now.

John

 

[SignalFailure]

;D

Must get a beefy compressor!

 

[mklotz]

Nice going, SF, and a lesson well-learned. I'm pleased that you got it running and I'm betting you're ecstatic.

If you're looking for a small air source, consider one of the inexpensive airbrush compressors sold by the import companies. They're easy to transport, quiet and will easily drive even my two cylinder (3/4") engines. Just the thing for table top demos to your friends and neighbors. (I've even taken mine to the local pub.)

 

[DickDastardly40]

I've never tried what I'm about to suggest but wonder if the idea has any merit.....

If you want an even pressure source for fault finding purposes, how about an inflated car inner tube? More pressure required just rest your foot on it. Not sure what pressure you can get on a tube that isn't inside a tyre but you can perhaps get enough to look at the engine while it's running rather than be pumping on the foot pump and perhaps you'll get a longer run.

Devils advocacy as usual.

Oh forgot to say congratulations on the running so far.

 

[SignalFailure]

I've never tried what I'm about to suggest but wonder if the idea has any merit.....

Ah, so you want me to look a complete fool by hugging an inner-tube for hours eh? I'm wise to your 'dastardly' plans! ;D

 

[DickDastardly40]

I'm wise to your 'dastardly' plans! ;D

Curses, foiled again! ;D