Problem with Accucraft 3-cyl shay

O

opabert

Registered
My Accucraft 3-cyl shay has a problem with steam. She comes to a standstill after running 150 mtrs. Pressure is than 20 psi, enaugh for running I think. When the pressure raises to 40 psi she will start running very slow. When she runs on air pressure, 7 psi is enough to let her run during 15 minutes. I use the Regner steam oil (perhaps to thick for accucraft?). The firing is good, because the pressure raises very fast to 40 psi.
I thought that probably the piston o-rings were the problem. So I replaced the rubber o-rings by PTFE o-rings, but that didn't help.
When the cylinders get hot, the pistons run very stiff in 2 of the 3 cylinders.
Does anyone have heard of this problem before? Does anyone have a solution for this problem?
 
Did you make or purchase the PTFE piston rings, that would be my first area for investigation.. ? Should be no problem with the Regner steam oil, so long as you are using some.
 
opabert said:
When the cylinders get hot, the pistons run very stiff in 2 of the 3 cylinders.
Does anyone have heard of this problem before? Does anyone have a solution for this problem?
Click to expand...
I suggest a clue could lie within the cylinders. IMHO any difference between two stiff when hot cylinders and the other is worth exploring.
 
wpandyr said:
Did you make or purchase the PTFE piston rings, that would be my first area for investigation.. ? Should be no problem with the Regner steam oil, so long as you are using some.
Click to expand...
I bought them with a local supplier. I think the rings are from Roundhouse. The rings are a little bit too thick. So you have to turn off the ring grooves of the pistons to fit them. As I cannot work with a lathe, the supplier did it for me.
Regards,
Opabert
 
The 3cyl Shay is a 3/8" bore. Id start with #10 Silicone orings. When you change over to the split rings you must machine new pistons with deeper grooves as they are currently designed for a 1/16" thickness Oring.

Also if you are using the white PTFE it expands greatly when it heats up and I think they call for a undersize ring OD as it expands like .010" or more. Ive used a Graphite PTFE and it worked great but the pistons needed to be machined as the groove was too shallow for a decent ring.
 
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