Piston rings

[Chickens]

I have built a Breisch duplex steam pump. Having trouble getting rings to seal. I tried making them from bronze (cylinder and piston are bronze). They slowly failed. Now I have a ptfe 1 piece ring backed with a vitron 0 ring. Works on air but not on steam. I have lapped the slide valve in. Steam seems to just blow out the exhaust with zero piston movement.

[tetramachine]

I would not use bronze for piston rings, CI, Steelwould be the material. Turn the OD to cylinder size plus .015"-.020, After OD, then offset the ring in the lathe about .015, and bore the ID, then cut the ring to create a expansion joint. Make 2 rings like this, lap the rings flat, and cut a ring groove with
the both rings in the same groove.

[Chickens]

Ci steel rings? In bronze cylinder and piston??? Might be wrong but that don't sound right to me.

[milwiron]

In the latest copy of LS&OR magazine there's a short article on using AFLAS copolymer o-rings in steam pumps. High heat, oil resistant and probably more forgiving than PTFE. They might be worth a try since you're already set up for o-rings.

[cbrew]

Now I have a ptfe 1 piece ring backed with a vitron 0 ring.

Morning,
is the ptfe ring split?
from my research, this is the preferred method.

are you sure the blowby is with the piston?

[NP317]

Ditto.
CBrew covered my questions.
RussN

[Chickens]

No I'm not sure the blowby is from the pistons. I spent quite a bit of time lapping the valves so I am now looking at the rings. It did run a little on the bronze rings on steam. Not at all on the ptfe rings (on steam). The rings aren't split. I had to make a cone to expand them then tapered (funnel) to compress them. I don't know how to test whether it's the rings or valves. At this point I (suspect) the rings but not sure. That's the reason for this post. I read an article about making them out of graphite or carbon impregnated ptfe.Thanks for the advice. I am still considering my options.

[Chickens]

I would consider O rings but would have to make a different piston and from research the clearence is a bit touchy to get the correct pressure against the cylinder wall and side clearence so they will roll. Also there is an exhaust port about 1/4" from the end of the piston travel. Not sure how an 0 ring would wear.

[RET]

Hi,

Two things.
Get some graphite packing and put it in the piston ring groove. Installed properly, this will make a temporary seal so you can see if piston blowby is your problem. The exhaust port you describe can further complicate things depending on its size and how it is shaped and positioned. If possible, the piston seal should not run over the port. If you are using slide valves, make sure that the slide can (and does) seat properly on the port face.

If you are going to try "O" rings to seal the piston, don't use the standard dimensioning given in the "O" ring catalogs. You will wind up with too much interference for a good sliding seal. I have found that .004 to .007 thousandths radial "squeeze" is all that is required and the "O" ring should just fit in the groove with no freedom to roll. Once the seal is made, the seal is "self energizing." As the pressure increases, the "O" ring will deform to keep the seal leak tight. In cast iron cylinders, you will have to replace the "O" ring after a couple of years, in bronze you should get a service life about twice as long.

Hope this helps a bit.

Richard Trounce.

[Chickens]

Ret. I tried graphite packing. Seemed to run fine on air but locked up tight on steam. But I will try it again. A good way to determine if the cylinder/piston is leaking. Slide valves are only about 7/8" square. They are bronze on bronze. I have lapped them in as best as I could with Time Saver. How do you get/muasure that .004 to .007 thousanths radial squeeze given the properties of 0 rings? Thanks you have been very helpful.

[Chickens]

The design of a duplex pump calls for the piston seal (0 ring) to run past the exhaust port which is a 16th hole about a quarter inch in from the end of the piston stroke.

[Builder01]

The design of a duplex pump calls for the piston seal (0 ring) to run past the exhaust port which is a 16th hole about a quarter inch in from the end of the piston stroke.

The piston seal runs past the exhaust port? If that is true, now the exhaust port is connected to the other side of the piston. I'm not sure how that can even work.

As for calculating the squeeze on your piston O ring, it is not difficult. I got my piston O rings from McMaster-Carr. They can supply you a drawing of the actual cross sectional dimension of any of their O rings. You must make the groove in your piston such that you get the .004 to .007 squeeze on the ring. The pistons do not touch the cylinder walls. The O rings do all the work. A little addition and subtraction will get you the correct depth for the groove in the piston. I have bronze pistons and bronze cylinders with a pair of Viton O rings in each. It works well! I used to have graphite packing, it did not last long. The Viton rings have been in for a year and a half and have very little wear.