Stuart Beam Engine Piston Rings

[comstock-friend]

It's time to put the piston in my beam engine cylinder. My casting kit is the old style matching the construction book by Andrew Smith; in it he refers to a parting tool to 'machine the (piston) packing groove'. However, I have 1" cast iron rings in my hardware kit, and the current Stuart design uses an O'ring in an 1/8" wide x 3/32" deep groove.

In the "How Slow Can You Go" thread: https://www.homemodelenginemachinist.com/threads/how-slow-can-you-go.2050/page-2#post-50628
there seems to be a running comment that iron piston rings provide more drag and thus prohibit running the engine at a slow rate. This being a beam engine, slow is the desired speed.

What's the consensus on piston sealing: packing, cast iron ring or O'ring? I plan to run this on steam as well as on air...

John

 

[Charles Lamont]

Purely my choice, but I would use the rings supplied. I think when run-in the friction would be less than an o-ring.

 

[comstock-friend]

Anybody have a drawing of the Stuart Beam Engine piston with cast iron rings? The Andrew Smith book (and I assume the original Stuart plans) used packing. The current Stuart plans call for using an O-ring. My casting kit came with two 1" OD rings, 0.0625" wide and 0.0440" wall. Thinking my kit was somewhere in between. Would rather use iron rings than the O-rings. Were both rings used or one with a spare?

Should I use a two piece piston to avoid expanding the ring over the OD?

Any thoughts or insights?

John

 

[propclock]

I have a vintage Stuart Beam engine I run it many many hours at shows on air ~3psi slow.
I have also run it on steam for short periods . I use 1 VITON O ring. adjust your piston
groove depth until you get a very low friction , but sealing fit. Nicely polished cylinder.
I also use Viton for some I.C. engines. Work fine . The vintage plans show 1 ring.

 

[jcreasey]

I have been told that the very best thing for piston rings is a plastic called peek.
I haven't tried it because it is insanely expensive but it is reportedly very low friction and very good resistance to heat.

 

[comstock-friend]

Yes, I see a 1" bar of the stuff is about $ 60 per foot ( McMaster-Carr )

For now I've worked out the groove dimensions for the iron rings (which I have). I've gapped them in the bore about 0.006", they are 0.0625" wide and 0.045" thick. I've got a 0.006" groove side clearance and about 0.005" back clearance. Now if I can refrain from breaking them getting them in the grooves!

John