valve settings

[steamjohn248]

Valve settings Walchaerts.

We are currently completing a Hercules 7 1/4" started about forty years ago

I got my head round Stephensons valve gears and how to set them, both marine and loco but we are having trouble with the Herc.

All the Dimensions are as per the drawings within a few thou. we are playing around with dummy valves at the moment before machining the bronze castings.

With the steam ports milled it seems we have two variables, the stroke of the valve rod (and therefor valve) governed by the return crank, (set for outside admission) and the lap, i.e. the overall length of the valve compared with the outside dims. of the steam ports.

By .....ering about with these, the only two variables, we seem to be able to achieve a reasonable lead, (we are making the exhaust line for line, its only a shunter) and a valve travel that will uncover one port fully but the other only about 3/4. Have set front and back dead centres in accordance with full size practice. mid gear the die block does not move and so on. Read all the books but we are obviously missing something. Doubtless there are those on this site who are well versed in this sort of thing. any advice would be gratefully received.

Steamjohn

[JohnP]

Steamjohn,
If you've mastered Stephenson's valvegear, Walschearts' should be a doddle. First suggestion is to download Charlie Dockstader's valvegear simulation software, then you can play around with any dimensions without cutting metal!
I'm a bit concerned about your comment saying thaat the valve doesn't move in mid gear - it should! Specifically, that's what the combination lever is for, to provide the twice lap plus lead movement of the valve in mid gear. It is my belief that lead is NOT required in our sizes, and particularly with plain bearings it can hinder starting when close to a dead centre. All that the lead steam does is squeeze the oil film in the bearings, increasing friction.
Usually, the length of the rod from the return crank to the expansion link is measured from the job, to account for any "tolerance stackup".
With the piston on dead centre, moving the reverser should result in there being no movement of the valve - the problem here is that with the return crank being about 90° ahead or behind the crank, the expansion link is swinging at its fastest when the piston is moving slowest, so you need to set the dead centres with care.
The fact that you are seeing uneven openings of the ports suggests that there is something wrong with the "backset" - the position of the pin driving the expansion link, leading to uneven swings of the expansion link about its "neutral" position (which corresponds to the dead centres).

I would again advise getting Charlie Dockstaders software. You can get it from
www.tcsn.net/charlied/
Hope this helps,

JohnP

[steamjohn248]

Hello JohnP

Thanks for that. I looked at Dockstaders stuff ages ago but never really thought I could make any use of it. Having looked again I think I'll try and see if the Dims on the job are as they should be. I simply made all the valve gear to the drawings, even the backset seems about right but I'll check it all against the file info. Take your point about lead, filling the dead space before the piston reaches dead centre and cushioning the motion probably is'nt very important in a shunter. I've only ever helped set the valves on a full size job for main line running, horses for courses.

Steamjohn

[JohnP]

Steamjohn,
Good luck with the valvegear program. If you need any help, send a personal message and I'll try to assist. The point about lead is that our cylinders are much smaller in relation to port size than full size - the "square - cube" law. The disadvantage is that it's easier to see a gap when the port is open to lead steam than when there isn't, if you see what I mean.

Cheers,

JohnP

[chris vine]

Hi Steamjohn,

I am sure John P is correct in saying that you need to have a look at the valve gear when the piston is at dead centres. The difficult bit is setting the motion on a dead centre.

A good way is to set some sort of dial gauge on the piston or crosshead and to turn the wheels so that the piston is, say 20thou, from top dead centre, on either side. Mark the wheel with a pencil next to a suitable fixed point on the frames and the dead centre position will be half way between the marks. You must make sure to take up back lash by always moving the piston towards its centre position, IE end of cylinder.

This method will give you a more accurate centre position than just using a dial gauge to watch when the piston seems to stop.

My suspicion is that the return crank is not set quite right on the crank pin. When it is, as John P says, with the piston on a dead centre, the valve will not move when you shift the reverser. (once you have found the centre positions for the piston you can put the dial gauge on the valve spindle/crosshead.

Have fun and don't expect all the events to be perfect!!......

Chris.

[steamjohn248]

Thanks for all the advice guys. I "inputted" all the dimensions from the drawings into the valve gear programme and when I tried to run it the expansion link turned over onto its back and the valve left the cylinder block altogether!!! Guess I hit a wrong key somewhere.
Anyway, set the return cranks as advised and put it all together as per Dick Simmonds drawings, fed it the air and away it went, sounds pretty much like most of the 2 cylinder engines I remember as a kid lugging goods trains around. we will see how it likes steam and how it pulls.

Once again thanks fior the words of wisdom.

Steamjohn

[JohnP]

Glad to help.

Keep us informed of progress.

JohnP