Lady Stephanie Pumping Engine

[crueby]

Lotsa parts, great progress! Those linkages will be mesmerizing to  watch.   

[Jasonb]

Looking at your photo the tightness of your rings in the bore may be due to them being a firm fit in the groove. It is usual to make the groove wider than the rings actual cross sectional thickness but a bit shallower so they have room to spread when compressed. The ring holding itself in the groove suggests no clearance on the width.

[springcrocus]

I think you are right, Jason. I will widen the grooves a little before trying anything else.
Regard, Steve

[Jasonb]

If you let me know what the rings measure I can give you some numbers that work

[crueby]

Also be sure to test with a little oil on the rings, that can make a big difference to the 'stiction' they have. Jason is right about having the groove be a little wider than the ring, that helps a lot. I don't measure mine, just make them so the ring will drop in/out from the side, and for depth I sneak up on them till I can slide it through by hand using the piston rod. For final trimming, a fine file can be good with the piston spinning in the lathe, I find that using the parting tool for final trimming can remove too much very easily.
 

[springcrocus]

If you let me know what the rings measure I can give you some numbers that work

Jason, thanks for the offer but I'm fine. I'm using imperial "O" rings in a metric bore so a bit non-standard. I'm a bit like Chris, offer it up, add more oil, shave a bit more here, there, wherever. I'll make it fit. We're engineers, after all, it's what we do and part of the fun.
And thanks, Dave, for your interest a few posts back.
Regards, Steve

[Jasonb]

Well your starter for ten if it helps. They look to be nominal 1/8" section rings that are actually 0.139". groove depth of 0.132" and width of 0.160". may need a bit of a fudge if it is a larger imperial 1" ring in the 25mm bore.

You should really only need the one single ring if working the engine under load. For just slow display running the ring could be left out. Most of my engines are grooved for a ring but seldom fitted.

[astroud]

Take out the guess work and use the manufacturer's technical design notes

[Jasonb]

I would strongly advise anyone using O rings for models ( steam & IC) NOT to use the published makers charts.

These are based on very high pressures (1500psi) and the amount of compression of the rings to hold that back will add so much unwanted friction that you won't get a nice smooth running model and even to get it to turn over will need to be using a high pressure to overcome the friction.

The sizes I gave can be found in "Model Engineers Handbook" and are based on testing done by well respected model engineers such as Arnold Trrop and suit model use wher far less compression of the ring is needed when typically only working with 100psi.

[crueby]

I would strongly advise anyone using O rings for models ( steam & IC) NOT to use the published makers charts.

These are based on very high pressures (1500psi) and the amount of compression of the rings to hold that back will add so much unwanted friction that you won't get a nice smooth running model and even to get it to turn over will need to be using a high pressure to overcome the friction.

The sizes I gave can be found in "Model Engineers Handbook" and are based on testing done by well respected model engineers such as Arnold Trrop and suit model use wher far less compression of the ring is needed when typically only working with 100psi.

 

[astroud]

"These are based on very high pressures (1500psi)"
I am not sure what you base your statement on, O rings are used in many applications and pressure ranges, high and low. Look around your workshop or garage, likely to be several applications where the pressure difference is small but rings are used to provide a seal eg a decent oil can. The designer of the equipment would not just arbitrarily pick some groove dimensions but consult the supplier for guidance, hence the design notes that are published.
It is a misconception that the higher the 'squish' of a ring = the higher the pressure sealing available. The correct groove dimensions allow the ring to move and deform to provide the seal meaning the smallest squish being able to seal both high and low pressures.
Of course the 'if it looks right...' process is likely to be OK too for model engineering but why not take advantage of the manufacturer's know how and use good engineering.

[crueby]

Agreed on the variety of uses, but my experience  with small to middling model steam engines, 3/8" to 4" pistons, is that the manufacturers  charts give too much friction. A good starting point to fiddle from though.

[Jasonb]

Same here.

Infact some people who use O rings on IC engines don't compress the ring at all, they just rely on the gas getting behind the ring to do it's job.

[paul gough]

The following will be decried as heresy by many, but it works , so in my view justifiable. To get that “just right feel” wth a model piston using an O ring without altering piston grooves place the O ring on an arbor,  eg a piece of reasonably accurate rod, such that it is firm. Put it in the lathe and rotate at moderate speed and gently abrade/cut the surface with 800 or 1000 wet and dry. Go lightly and reproduce a radius on the surface, it does not need to be as small as the O rings. You may need to trial the O ring on the piston a couple of times to get to know what degree of removal that is required for similar treatments in the future. Regards, Paul Gough.

[springcrocus]

Base Panels

I made the base panels some while back but I needed to get the decorative scrollwork cut into them so I wandered over to my mate Wilf's workshop with panels in hand. He has a CNC mill and able to do this work much easier than I could manually.

The drawing shows a backing frame and separate panels which screw to them for access to the underside but I have chosen to make them as slot-in panels with perspex viewing windows inset into the front two only.

The panels were set against an end stop and the scrollwork cut in with the 2mm slot drill. It was slow because we only had long-series, neutral-rake slot drills available and we snapped the first one before getting half way round the first panel.



Then two of the panels had the viewing window cut out with a decent 4mm slot drill, which sped thing up somewhat.



And this is what they look like when assembled. I will glue the perspex windows to the rear faces at a later time.





Regards, Steve