Setting Piston Valves

[Speedy Builder5Participant @speedybuilder5]

I am building a 5″ SPEEDY, and have LBSC’s book and plans etc, however there is nothing that I can see about setting the valves themselves.  The engine has outside cylinders and conventional piston valves on top of them.

 

The inconvenient problem could be that the piston valve covers are held on with three screws, one of which you have to remove the cylinder itself to do the screw up.  Adjustment could be made without the covers being fully screwed in place, and finally assembled once the adjustment had been made.

 

Any ideas of how I should go about it, or web site reference etc. ?

[Speedy Builder5Participant @speedybuilder5]

[Sub MandrelParticipant @submandrel]

I recall LBSC explaining how to set the ‘invisible’ valves on one of his engines. As I recall it involved an extension rod attached to the valve or valve rod of known length so you could judge the position of the valve by the position of the rod relative to the end of the valve chest.

 

Neil

[John OlsenParticipant @johnolsen79199]

I haven’t done a locomotive, but had to set two piston valves on my Leak compound launch engine, with the same difficulty, eg there is no way to see what is happening down inside. You can use an air supply at low pressure, with the drain cocks open. That lets you tell when the valve is opening so you can get things set up symmetrical by adjusting the rod.

 

regards

 

John

[DMBParticipant @dmb]

LBSC once recommended the compressed air escape from drain valves method, I think it was a loco. with `tween frames steam chests.

[Speedy Builder5Participant @speedybuilder5]

Thanks for the input.  I guess you have to leave both valve end caps off (No support / guidance for the valve pushrod), as there would be no way to get to the adjustment nuts.

[John OlsenParticipant @johnolsen79199]

Although it is less than ten years ago, I am struggling to remember the details of how I did mine…being a marine engine of course things are a bit different anyway. There is no adjustment for the valve position on the rod on mine. The valve rod has a shoulder and the valve is held against that with a nut. I established the correct length, I think by the following process:

 

Make the rod with the shoulder a whisker too long. Set it all up in the engine. The top cover is left off so that a dial gauge or Vernier caliper can be used down the bore. I used the latter since the dial gauge would not have enough travel. Put low pressure air on the input with the drain cocks open. Turn the engine over by hand and establish the two points at which the inlets open. Half way between should be the mid position of the valve. However that is a hard position to find for the eccentric. But you can find the total travel of the eccentric by measuring the total travel of the valve. So you can some up with a set of measurements that tell you by how much the valve is too high (in the marine case) or too far forward, for a locomotive. That is the amount that has to come off the shoulder on the valve rod. (Plan B was that if I overdid things I could put a little shim in the shoulder.)

 

For example, the bottom of the valve travel is found to be 1.9  inches down. The top is found to be 0.9  inches down.

 

The valve opens to the top end of the cylinder when it is 1.4 inches down. It opens to the  bottom of the cylinder when it is 1.6 inches down. So a bit of thought tells us that the mid position of the valve on the ports is when it is 1.5 inches down from the top.

 

But the mid position of the actual travel we have at present is 1.4 inches down from the top. So the valve rod is .1 of an inch too long. We extract the rod and turn 0.1 inch off the shoulder. Mine was set up to let me do that without taking out the valve. I could undo the nut on the top of the valve, and unscrew the rod from the fork at the bottom.

 

(These dimensions are all for example only, I can’t remember the actual figures.)

 

I also set the eccentric positions by measurement, by finding the exact top dead centre for each piston in turn and then setting each eccentric to the required advance. This was all done purely by measurement, so I was rather pleased when it all worked. She will tick over very nicely with the gear notched up to a very early cutoff, so things can’t be too far out.

 

Note this method does not make any allowance for rod angularity, which with a short engine like my Leak is actually reasonably significant.  It didn’t seem to matter though, and will matter less on a locomotive where the rods are likely to be proportionately longer.

 

Also in case those who are familiar with the Leak engine are wondering…. I changed the vlave on the LP end from a balanced slide valve to a piston valve. I am not sure that this is really necessary but it is different. You would be surprised how many people think it has four cylinders at first glance!

 

regards

John

[Keith Wardill 1Participant @keithwardill1]

Hi Robert,

Several people have mentioned LBSC’s pressure setting method – this is described in ME for August 1950, Volume 103 number 2568, in an article called LBSCs Beginners Corner – he described how to set the valves for ‘Pamela’.  (the article is actually about building a boiler for Tich, and he describes the valve setting for Pamela more or less in passing).

Basically, he attached a small low pressure container (he describes a soldered-up cocoa or coffee can to a Tyre pump. The can is then attached to each steam chest with rubber pipe. Open the cylinder drain cocks, set the die-blocks in the middle of their travel in the expansion links, then turn the wheels by hand. If the gear is set up correctly, then neither die block should move (If they do, then, as LBSC says, ‘you have apple-pied the job ….. of fixing the weighbar shaft’).

 

If all isOK so far, pump up the canister and turn the wheels again – in LBSCs words ‘ Air should hiss from the front draincock as the crank arrives at front dead-centre, and from the back cock when the crank arrives at back dead-centre’. If one end blows before the other, the valve needs adjusting on the spindle. As he says, ‘it is a matter of trial and error. If you get a hiss at one end at dead centre, and nothing at the other end until the crank has passed dead centre, then the bobbin is too long, and must be shortened, keeping both bobbins the same length.

 

Hope this is some help.

[Speedy Builder5Participant @speedybuilder5]

All good information,  and thanks for taking the time everyone.

[Richard ParsonsParticipant @richardparsons61721]

 

I am revisiting the Late Mr Mason’s Minnie. I cannot get bronze over here so I am using Aluminium for the cylinder with PTFE piston. That is no problem as so far all the numbers are coming out ’right’. The valve system however is a problem so rather than re-invent the wheel I have decided to again use a piston valve with PTFE pistons. Can anyone tell me where I can find a drawing of such a valve. 

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