Walscharts Valve Gear help

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k36no4862002
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Location: Surrey, UK

Walscharts Valve Gear help

Post by k36no4862002 »

Hello,
I need help as I'm baffled by this problem. Im trying to time the engine units for my Challenger. For some reason i get too much port opening when in full gear on a dead centre. This is where I am so far. (Note this is not built to the Goldman drawings but my own design!)

I have port openings on the top of the cylinder of 10mm steam slot, 10mm bridge and a 25mm exhaust hole. The opening (exhaust cavity) in the valve is 45mm and the total valve length 75mm.
I have checked that the radius of the radius rod and the curve of the motion plate are the same. When I put the locomotive into a top or bottom dead centre and I move the reversing lever there is no movement to the valve. I have looked this up and it seems to be the correct procedure. I if I then push the locomotive forwards I can see the valve open to the steam port fully and then close again as it should.

My problem seems to be that when on a dead center (top and bottom) the valve is open to steam by about 3 to 4mm. My understanding is that the valve should just be cracked at this point??

So does anybody have an pointers of things to check? (I have a solution in the back of my mind which would be to make the valves longer and increase the throw of the eccentric crank a little and adjust the eccentric rod to suit, OR just finish the loco and see how it goes!)

I have built loco's and rebuilt locos before so I have a pretty good idea of things but on this one I have run out!!

Thanks
Paul
Paul Edmonds,
Surrey, United Kingdom.
2.5" Scale D&RGW K36, 1/6" Scale Challenger 4-6-6-4 (nearly done!!)
VGC
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Re: Walscharts Valve Gear help

Post by VGC »

Could you provide an elliptic valve diagram or an x-y table with piston positions (x) and valve positions (y) for one wheel turn cycle?
Ideally this at two or more cylinder fillings, or one backward and one forward.
Perhaps there is something I (or we) can see in these diagram data.
David Powell
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Re: Walscharts Valve Gear help

Post by David Powell »

I have only built one set of Walscharts valve gear to my own design, so I am no expert.
However, my usual procedure with any engine, and any valve gear is to make the slide valves overlong initially, set everything up as best as I can and see what I get.
Often there is no need for the valve to fully open to steam, it is exhaust freedom and point of admission that will determine whether you get a lively, powerful and economic engine.
Before frantically chopping or changing your valve gear parts I would suggest you make a " dummy " valve. at least a little longer by the amount the valve is open at dead centre and just turn the wheels by hand lots of times, see what you get, think hard, invite friends over for their opinions. and then take whatever action you figure out.
Anyway it is all good fun and better than not having a hobby..
Regards. David Powell
k36no4862002
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Re: Walscharts Valve Gear help

Post by k36no4862002 »

Thanks David,
I'll probably load it onto a trailer and take to my local club and see what the collective view is over a cup of coffee or a beer.
It's just weird that it all works fine except for the 'advance' on the ports, I have one person saying it's fine, but everything I read says it's not, I know that if I just build it and see is an option. Hopefully someone will come up with a "did you check" idea and the penny will drop.
Thanks
Paul Edmonds,
Surrey, United Kingdom.
2.5" Scale D&RGW K36, 1/6" Scale Challenger 4-6-6-4 (nearly done!!)
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baggo
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Re: Walscharts Valve Gear help

Post by baggo »

Just a thought. Are you sure that the combination levers are the correct dimensions? The dimensions of the levers should give an initial valve travel of 2 x (Lap + Lead)

What happens when you put the reverser in mid gear and push the loco? The ports should just crack open by the amount of lead that you have designed into the valve gear. If the ports are opening by more than the amount of lead with the reverser in mid gear then your combination levers are wrong.

If that's correct then it's probably the return crank throw that is too great.

John
Secretary of The National 2½" Gauge Association
Member of North West Liecestershire SME

http://www.modeng.johnbaguley.info
k36no4862002
Posts: 151
Joined: Sun Feb 08, 2009 4:27 pm
Location: Surrey, UK

Re: Walscharts Valve Gear help

Post by k36no4862002 »

John,
I'll have a double check of the combination lever this weekend. Due to the size of the valve gear rods the 2 pins on the combination lever are as close as I could get them, which as you say may be more than the 2 X (Lap + lead) so this is a possibility and yes that would stack up as 1 idea I had to fix the problem was to make the valves longer to overcome the 'advance' error in the timing.
I'll have a look and report back.
Thanks
Paul
Paul Edmonds,
Surrey, United Kingdom.
2.5" Scale D&RGW K36, 1/6" Scale Challenger 4-6-6-4 (nearly done!!)
David Powell
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Re: Walscharts Valve Gear help

Post by David Powell »

Two cylinder( or more) engines benefit from a lot of advance in the valve timing.However, having too much advance can give rise to lumpy running at slow speeds. As with most things the usual answer is a happy compromise. I get good results with from a " thin black line" to " a thick black line "of opening at dead centres. regards David Powell.
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Bill Shields
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Re: Walscharts Valve Gear help

Post by Bill Shields »

k36no4862002 wrote: Wed Jun 30, 2021 9:19 am Thanks David,
I'll probably load it onto a trailer and take to my local club and see what the collective view is over a cup of coffee or a beer.
It's just weird that it all works fine except for the 'advance' on the ports, I have one person saying it's fine, but everything I read says it's not, I know that if I just build it and see is an option. Hopefully someone will come up with a "did you check" idea and the penny will drop.
Thanks
considering that this is a 1.6 scale CHALLENGER -> just getting it onto a trailer and out to the club is an undertaking in itself!
Too many things going on to bother listing them.
k36no4862002
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Re: Walscharts Valve Gear help

Post by k36no4862002 »

David,
When you say a thick black line . . how thick, couple of mm?

Bill - When I get it to my club track I'll be able to get some pics and will post here, being in a workshop means it's not that easy to get decent pics of the entire loco.
It will go onto my trailer OK, I have a pretty decent setup - kind of a roll on roll off process. My bigger problem is finding another loco to tow it round the track, as I'm not pushing it!!
Paul Edmonds,
Surrey, United Kingdom.
2.5" Scale D&RGW K36, 1/6" Scale Challenger 4-6-6-4 (nearly done!!)
David Powell
Posts: 523
Joined: Thu Mar 13, 2003 10:38 pm
Location: Pickering Ontario Canada.

Re: Walscharts Valve Gear help

Post by David Powell »

Hi, I have tried measuring with shim stock or feeler gauges, but the slack in most of the valve gears tends to make this a fairly useless occupation. Just turning the wheels gives a general indication of where things are, but when the valve rod does not have a " tail" that goes through the valve chest and out through a gland you need to load the valve towards the crank to simulate the effect of steam chest pressure on the end of the rod.
I reckon a thin black line as you look into the port to be from 2 to about 10 thous, and a thick black line from about 10 to 25 thous,
Hope this rather vague and subjective rmble helps , David Powell.
k36no4862002
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Joined: Sun Feb 08, 2009 4:27 pm
Location: Surrey, UK

Re: Walscharts Valve Gear help

Post by k36no4862002 »

David, thanks.
In short my 'line' is about 3mm so I'm way off and I dont have much play in the valve gear. Given the actual 'gear' works well then I think I need to make longer valves.
Thanks
Paul Edmonds,
Surrey, United Kingdom.
2.5" Scale D&RGW K36, 1/6" Scale Challenger 4-6-6-4 (nearly done!!)
David Powell
Posts: 523
Joined: Thu Mar 13, 2003 10:38 pm
Location: Pickering Ontario Canada.

Re: Walscharts Valve Gear help

Post by David Powell »

While my experience is mainly with Stephensons one point I would make is that the valves need not ,and perhaps were not, intended to actually open the ports fully to steam
. If the ports stay open to about 80 0r so percent of the stroke after you make new longer valves you should be able to get good running
. If you make a dummy sectioned valve where you can see the exhaust port you should be able to assess the exhaust openings as well.
Regards David Powell
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