Brass, Bronze Piston valve clearance

[Odyknuck]

The previous owner made the pistons on my Logger Mikado out of either brass or bronze. Not sure of the material however the fit clearance is only .002". Understanding the material expands more than the steel, I would think it needs more than .002" to keep them from seizing at steam temperatures however not sure by how much? Also what clearance should they have for ring gland O.D.? Please advise.

[ALCOSTEAM]

Given the expansion of brass and bronze at the temperature of steam at 125 psi your valve at .940 " will expand roughly .0034 ". That said there are alot of variables but if you retain the current valve I would certainly give it .0035 or .004 cold clearance.

just for reference here are some basic expansion figures of materials this is per degree per inch of material

brass is .0000104

cast iron .0000061

aluminum .0000128

steel .000006

[Bill Shields]

Since you have rings...why worry about being close?

I typically allow .010" and do not worry about it.

There are times to be a horologist and times to be realistic (blacksmith)

[NP317]

Your drawing shows the valve piston "ridges" should be 0.940" diameter. That gives 0.030" clearance on each side of the 1.000" bore.
I machined my valves for that same locomotive per that drawing and they work well, with cast iron spindles.
The rings do the sealing, not the valve body ridges. That valve body should be allowed to float in the bore.
I recommend you follow the drawings.

I forgot to answer your PM about where to find more valve spindle rings. Sorry about that. (Public shame...)
ANSWER: I no longer know! I may have machined my own rings, having done that in the past, but my notes aren't telling me useful info.
Do others know of a contemporary ring source for Odyknuck, now that past supplies like Cole's Model Supply are gone?

RussN

[AMc]

Hercules Seals has iron rings, go to their website and do a search for SPR-this will bring up what they carry.

[Odyknuck]

Your drawing shows the valve piston "ridges" should be 0.940" diameter. That gives 0.030" clearance on each side of the 1.000" bore.
I machined my valves for that same locomotive per that drawing and they work well, with cast iron spindles.
The rings do the sealing, not the valve body ridges. That valve body should be allowed to float in the bore.
I recommend you follow the drawings.

I forgot to answer your PM about where to find more valve spindle rings. Sorry about that. (Public shame...)
ANSWER: I no longer know! I may have machined my own rings, having done that in the past, but my notes aren't telling me useful info.
Do others know of a contemporary ring source for Odyknuck, now that past supplies like Cole's Model Supply are gone?

RussN

As I do agree with most of what your saying however the plans also show cutting the Bull ring in between the steel rings at .9995" diameter. cutting them to that dimension would seize these at temperature. So at minimum I would need to cut that section at .995" (.007" clearance) to be on the safe side for expansion. The .093" thick ring will be a special order, so I plan on using the .054 rings I bought from Ottos as they are a stock item.
Thanks
Steve

[Bill Shields]

Think that .9995 dimension is a typo...or inko as the case may be.

No need for that tight a fit there

[Odyknuck]

Don't know if it was a drawing typo or what however I plan on giving it way more clearance than a 1/2 a thou . Thanks everyone for their input.

[NP317]

Interesting.
On my Mikado I machined those 4 bull rings per the drawings, from cast iron, with their O.D = 0.9995".
They have worked without binding in the 1.000 valve cylinder inserts.

Their design has three annular slots in them (oil grooves?), and the inside diameter of the ring gives tens of thousandths " of clearance to the valve body, allowing the valve to float inside the rings.
I suggest that the mass of the ring body expands INWARD, and not outward to the valve cylinder wall. Otherwise they would bind, which they have not done so far in my locomotive. In practice they work as drawn.
Other suggestions?
RussN

[Bill Shields]

Inward expansion is an oxymoron

[tetramachine]

The expansion of the piston, is offset by the expansion of the cylinder. So .003" expansion of the piston, is partly offset by expansion of the bore, .002 ÷/-.

[Bill Shields]

You are assuming everything expands at the same time.

Pistons expand before cylinders.

What is the point of making things so tight a fit when it will run just fine loose?