piston rings

[fred v]

i have a problem. 3" bore. 4" stroke.
with the given stroke and distance between the port openings in the cyl. there is only .300" left in the piston for the rings and a web between them. using 1/8" wide rings that means the rings are separated by a .050" web.

1. is that enough or would it try to break?
2. is it possible to put both rings in one groove?
thanks,
fred v

[GWRdriver]

Hi Fred,
I would say the .050" "fin" might be susceptable to bending or breaking depending upon what your piston material is. Just as the most perilous time in a piston ring's life is when it's being installed, I think once in safely in place the probability of damage from internal forces is extremely low.

Most (model loco) piston ring configurations I've seen (I've just checked the drawings of the five or six locos I have) typically call for a 1/8" land between rings, but then I've also seen stationary engine designs with rings together.

If it was me, in steel or bronze piston I'd go ahead and go with the .050" gap.

[fred v]

thanks, Harry. i was studying the problem yesterday and thought of another solution. if i move the rings out by .025" on each side this gives me a land of .100", the rings will overlap the port edge by .025" but still have .100" riding on the cylinder.
sound OK to you?
fred v

[GWRdriver]

Fred,
Once that little critter is at cruising speed, who'd know it.

[Fred Rosse]

These are very tight dimensions with a four inch stroke. Setup of the engine must be checked to make sure the piston axial position is where you plan for it to be. Re-checking will be required, especially when the rod bearings wear a bit, and tend to shift the piston position.

[GWRdriver]

I think Fred is building to an existing design so connecting rod length has been determined and proofed (or should have been) by the designer and previous builders. The dimensions he quotes aren't a function of rod length, either piston or connecting, and in any case the reality is that on steam cylinders of this size the piston stroke path could vary by as much as 1/16" at either end, provided head clearance and minimum steam admission port area is mantained, and never be seen or heard in the operation of the loco.

If one is building to an new (not previously built) design, or an existing design where the cylinder or axle locations have been changed, the usual practice is to make up an adjustable dummy connecting rod, from bar or sheet metal, which is installed and adjusted in-place to give the desired piston stroke path (Piston path is adjustable/correctable, stroke is not). The finished rod is then made to match the bearing centers of the dummy rod.


[This message has been edited by GWRdriver (edited 10-12-2005).]

[Unka Jesse]

Heck Harry, I just rely on the threads on the crosshead end of the piston rod to adjust length. If she whops the back head, I lets the rod out a bit and vice-versa if it hits the front head. Works for me, but then I just sort of blunder along anyway.

Unka Jesse

[fred v]

thanks Fred and Harry,
i feel really stupid. i re-measured the port spread and found i had misread the caliper. i have .100" more room than first thought. no problem now; i can put 1/8" spread between the rings.

this is a new design that hasn't been built, that i know of, and i've changed the design radically from the prints which are just a bunch of separate drawings for each part. the designer died before finishing it. i've de-drawn it in Autocad so i can get things to line up and find errors.
fred v

<font face="Verdana, Arial" size="2">Originally posted by Fred Rosse:
These are very tight dimensions with a four inch stroke. Setup of the engine must be checked to make sure the piston axial position is where you plan for it to be. Re-checking will be required, especially when the rod bearings wear a bit, and tend to shift the piston position.</font>

[GWRdriver]

I use Autocad in my business, but I also use it constantly for live steam work and it's a great tool. I don't think animation is available in AutoCad, certainly not in anything other than the latest and greatest (which I don't and won't have), but still I regularly create blocks of components and place and rotate them to set locations, clearances, etc. It saves hours of tedious pencil and calculator work.

I've done the same thing on my 2X Tich, I revised practically everything, so it all had to be redrawn.

[fred v]

that's what i planned to do, sort of. my drawings show what the dimensions should be, that is if i measure right.
fred v

<font face="Verdana, Arial" size="2">Originally posted by Unka Jesse:
Heck Harry, I just rely on the threads on the crosshead end of the piston rod to adjust length. If she whops the back head, I lets the rod out a bit and vice-versa if it hits the front head. Works for me, but then I just sort of blunder along anyway.

Unka Jesse</font>

[webowser]

Say Fred,

I've got considerable AutoCAD experience and time available to help check your design if you'd like another pair of eyes to look at it.

If you're interested e-mail the dwg(s).

Bill [webowser(at)fuse.net]

[tel]

'but then I just sort of blunder along anyway.
Unka Jesse '

We knew that