Question on steam cyl slide valves

[cessna]

Hi, I have built the steam powered donkey by William Harris, I am at the stage of checking operation using air pressure, ( up to 100psi ) when I apply air most of it blows out the cyl exhausts, I have checked the timing and operation of the slide valves and they seem to be as per the plans, the air is lifting or going under the valve and blowing out the exhaust and not leaving enough pressure to operate the engine, any suggestions would be welcome.
Thanks
Terry

 

[Philjoe5]

Terry,
I had this problem with a beam engine I built. The plans called for screwing the valve rod into a female threaded valve. The only way that works is if everything is dead nuts on, not usually likely. I fixed this by cutting a "Tee" set of grooves on the back of the valve. The valve rod goes in on branch of the T, the valve rod nut fits in the other branch.

It sounds like you valve is not being pressed onto the face of the cylinder. It needs to float, so check that the valve rod lifts easily off of the valve and that the groove for the valve rod is deep enough.

Cheers,
Phil

 

[Steve J]

I recently finished that model as well and had same problem. I also put the t in the back of the valve and made sure it was all loose and free. In other words the valve must float on the stem. In addition I end milled a small hole in the back of the valve(not a thru hole) to fit a coil spring. something like you would find om a ball point pen. put everything in place with the spring in the hole and then put the cover on. it helps. I never got mine running with the winch drums on. It runs ok but not great just driving the pinion.

 

[Gedeon Spilett]

you may add a small piece of phosphor bronze clinker (ie scrounged from an electrical contact), curved to make a very light spring to press the valve onto the face of the cylinder while putting the valve chess cover. As said above, the valve does not be firmly fixed to the stem, and must have free movements in the direction orthogonal to the cylinder port face but no play in the longitudinal dimension however, in order to not alter the steam distribution.

 

[GailInNM]

One other thing that will give the same symptoms is if there is leakage around the piston. Not likely but I have seen it happen when a packing is used on the piston as a ring and was not installed properly.
Gail in NM

 

[cessna]

Thank you for all the replies, the valve for this engine has a cross slot for the nut but I can see a slot for the rod would help, I will give it a try.
Terry