Advice on choice of material for a steamer piston

[toad281]

I am in the beginning fazes of building another model steam engine. I have the cylinders fabricated, they are made of cast iron, 1-1/4 bore, and it will have about 2-inch stroke. Given the choice of; aluminum, brass, cast iron, or cold rolled steel. What do you think would be the best choice and why? I do not intend to make ringed pistons. Happy holidays to all.

 

[Jasonb]

I tend to use aluminium for mine as it reduces the weight which helps with smooth running if there is no counterbalance on the crankshaft. Many a full size would have been iron but hollow so using a light material is the easiest way to mimic that on smaller engines where a hollow piston is not practical. The very small ones just have oil grooves cut into the piston, larger have Viton O rings if they are doing work.

 

[toad281]

Thank you for the reply. I will do some research on Viton O-rings. As I think this over another piston questions came to me. Is there a formula, or ratio for the thickness of a piston? My last engine was a smaller wobbler type, and I did not give this topic much thought. This engine will have crossheads so not a wobble type. Should the overall thickness, or height if you like, be a certain dimension vs the bore or stroke? Or perhaps a minimum?

 

[Jasonb]

I've never really worked to a formula but on the smaller size engines something around 1/3rd the diameter allows enough length to get a length of unthreaded piston rod, some threaded rod and a recess for a lock nut into the piston as well as room for whatever rings you may want to fit be that single O ring or two iron rings.

So for your 1" dia 5/16" to 3/8" would do but as you have already fabricated your cylinder that will limit your length if crank throw is not to change. I would allow 1/32" for the spigots on the cylinder covers and 1/32" between covers and piston at TDC and BDC.

So for 2" stroke and a 3/8" piston you want the cylinder flanges to be 2 1/2" face to face.

 

[toad281]

Thanks again, I bought some Vitron O-rings, but now I am looking for a chart or calculator to determine the groove dimensions in the pistons. Any leads on one?

 

[Asm109]

Use google fu to find Parker Oring handbook.

 

[Jasonb]

I would definately not suggest using any of the commercial groove data as they squash the ring far too much as they are for much higher pressure than a steam engine will be run at which increases the drag a lot.

I have tables for model applications in a couple of books, post what the section of the O ring is and I'll give you groove size.

 

[toad281]

The rings I ordered are 1-1/4" OD x 1" ID x 1/8th diameter. These are nominal measurements.

 

[Jasonb]

So for an 1/8" nominal section which is actually 0.139"

Groove depth of 0.132" and Groove width of 0.160"

 

[AndrewW]

Thanks again, I bought some Vitron O-rings, but now I am looking for a chart or calculator to determine the groove dimensions in the pistons. Any leads on one?

Hi
Check out my Jerry Howell Farm Boy build series. In particular Part6 when I discuss the piston and groove:



Also, be aware that viton cannot tolerate some fuels. Check out one of the many chemical compatibility charts available on-line. I use camping fuel which works very well.
Good luck!
Andrew

 

[toad281]

Thanks for the info. I will need to grind a tool, [like a parting tool] to cut this. Would there be any advantage to making the groove with a rounded bottom to match the O-ring? I think I remember reading somewhere that a O-ring has to have some lateral movement to work properly. I do not think that the fuel compatibility would be any issue as this is a steam engine build. As long as the steam oil is ok to use with Vitron.

 

[Jasonb]

Flat bottomed groove is fine, the ring won't be round anyway when compressed. The width does allow a bit of movement.

You can use a narrower tool and just move it over

 

[AndrewW]

Thanks for the info. I will need to grind a tool, [like a parting tool] to cut this. Would there be any advantage to making the groove with a rounded bottom to match the O-ring? I think I remember reading somewhere that a O-ring has to have some lateral movement to work properly. I do not think that the fuel compatibility would be any issue as this is a steam engine build. As long as the steam oil is ok to use with Vitron.

Ah sorry I misread your message. My brain is in internal combustion mode. I hope your engine turns out great.
Cheers
Andrew

 

[Lloyd-ss]

So for an 1/8" nominal section which is actually 0.139"

Groove depth of 0.132" and Groove width of 0.160"

Jasonb has given you groove dimensions based on his experience and they sound good to me also. Just remember that the piston radial clearance measurement is also part of the groove depth. So if you have .002" piston clearance per side, the actual groove depth in the piston should be .130", which gives a 5% crush.
O-rings are fairly forgiving. nominal compression of a size 214 o-ring for up to 1,000 psi would give it about a 12% crush for dynamic applications (.122 deep x .190 wide), but as Jasonb said, that is too much for your project and would just make the piston too tight and would wear the o-ring out faster. The shallower the groove, the wider the groove needs to be so that the squished o-ring has somewhere to go and doesn't just bind everything up. Also remember that you can always make the groove deeper, but you can't make it shallower, LOL. The piston and o-ring combo should move with very little resistance in a polished bore.

You probably got the usual 70 duro Viton. If you have problems you do have the option of going to a harder (90 duro) or softer (50 duro) Viton. like I said, o-rings are fairly forgiving.
Lloyd

P.S. There is always the option of back-up rings for very difficult applications, but that makes the groove much wider.