To notch or not to notch

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tom_tom_go
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To notch or not to notch

Post by tom_tom_go » Fri Aug 10, 2018 2:01 pm

I seem to get into polite arguments about Slomo's these days and the latest one was about how 'notching' can achieve what an inertia device does for realistic acceleration/deceleration and therefore are not worth buying.

Mr Roundhouse advised when I got my Silver Lady that their simplified Walschaerts type valve-gear is not suitable for 'notching' so I have never done it. I have assumed given the size of locos in this scale it causes stress to the running gear..

I am interested to read what others think.

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Re: To notch or not to notch

Post by Big Jim » Fri Aug 10, 2018 3:24 pm

As far as I can understand 'notching up is not possible with the simplified walcherts valve gear as fitted to RH locos.

As for being able to replicate a slomo? You are not really comparing eggs with eggs so to speak. One is a friction device that utilises a flywheel to store energy that will either add momentum when an external force is removed or add drag and resistance when an external force is applied.

The other is a series of valve operations that may change steam consumption on a set of cylinders.

I suppose you could say that by fiddling with the reverser you can replicate the effects of a slomo. Although the same can be said about gently playing with the throttle.

An explanation of how cut off might work can be found here.
https://www.lner.info/forums/viewtopic.php?t=4368

I have been told that I can do this on my 5 inch gauge engine but whether or not it makes any practical difference I could not say.

I am no physicist (the only thing I can really remember from O level physics was getting thrown out of the class for breaking wind) perhaps some of our more learned members can help.
If at first you don't succeed, use a bigger hammer!

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Re: To notch or not to notch

Post by tom_tom_go » Fri Aug 10, 2018 4:35 pm

Big Jim wrote:
Fri Aug 10, 2018 3:24 pm

As for being able to replicate a slomo? You are not really comparing eggs with eggs so to speak. One is a friction device that utilises a flywheel to store energy that will either add momentum when an external force is removed or add drag and resistance when an external force is applied.

Whoa there Jimbo, that's not the case at all and as I get told this frequently please read the following:

http://smallsteamperformance.com.au/wear-issues/

I can push and pull both my Slomo fitted locos when one finger.
Big Jim wrote:
Fri Aug 10, 2018 3:24 pm

I suppose you could say that by fiddling with the reverser you can replicate the effects of a Slomo. Although the same can be said about gently playing with the throttle.
I have yet to see a RH loco with simplified Walschaerts type valve-gear run at a 'continuous' realistic speed and be able to stop and start smoothly. Also, without some kind of inertia device you cannot run light engine slowly.

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Re: To notch or not to notch

Post by GTB » Fri Aug 10, 2018 4:40 pm

tom_tom_go wrote:
Fri Aug 10, 2018 2:01 pm
I seem to get into polite arguments about Slomo's these days and the latest one was about how 'notching' can achieve what an inertia device does for realistic acceleration/deceleration and therefore are not worth buying.
I assume by 'notching' they mean using the reverser to change the valve cut-off, so that steam only enters the cylinder for part of the stroke......... ? Around here you notch up a diesel, not a steam loco........ :roll:

Roundhouse simplified Walschaerts valve gear can't be 'linked up' to change the cut-off. Yes, the reverser lever can be moved to an intermediate position, but the cut-off isn't changed. All that happens is that the valve travel is reduced, so the port openings are restricted.

The result is equivalent to closing the regulator a bit. Roundhouse use slide valves, which means that even with the exhaust port restricted there is little compression braking effect downhill, as the valve just lifts off the seat and the loco coasts. I had the regulator fail open on my only r/c loco once and I completed the run on the reverser. The result was nothing like a SloMo in action and I might have still been using the regulator for all any bystander could discern.

Accucraft locos fitted with their piston reverser design of valve gear can't change the cut-off either.

I've got two locos fitted with full working Stephenson gear and another with full working Walschaerts gear, all of which can be linked up to work expansively. Can't say I've seen anything remotely like the flywheel effect of a Slo-Mo when the valve gear is linked up. They certainly don't store energy on the downhill and then release it back on the uphill, or on curves.

They are sluggish if started while linked up, but run more freely once moving. They slow more uphill and tend to be faster downhill if linked up and left to themselves.

My locos with full valve gear do use noticeably less water when linked up, as the cylinders are using more of the energy in the steam before exhausting it. Which is why real valve gears with variable cut-off were invented in the first place.

The next time one of these characters expounds on his theory, tell him to demonstrate how it works using one of his locos in front of an audience.

Regards,
Graeme

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Re: To notch or not to notch

Post by tom_tom_go » Fri Aug 10, 2018 4:42 pm

Thank you Graeme for the response, that is everything I needed to know :mrgreen:

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Re: To notch or not to notch

Post by Keith S » Fri Aug 10, 2018 8:41 pm

Just to contribute to what Graeme has said, it's the "combination lever" on Walschaerts' valve gear that gives it the ability to change the valve cut-off, or "notch up". Roundhouse provides a dummy combination lever but if you look where it attaches to the valve stem it has no effect on the valve movement. If you look at a complete Walschaerts' gear you will see that the valve stem is attached to the combination lever at an intermediate point between the radius rod and the crosshead, combining their motion- the action of the crosshead provides the cut-off action. The whole point of Walschaert's valve gear, I suppose there are two main points, one of which is to provide reversing from a single eccentric, the other is to provide variable cut-off with the same single eccentric. Since Stevensons valve gear employs two eccentrics, the cut-off can be mixed in or linked from the other eccentric. Also, even though I understand the desire to have as many animated fiddly bits as possible, I find it a bit weird that people put the dummy combination lever on their Berties and Millies, without having any other visible external valve-gear components...

Interestingly, I was aboard a restored paddle-steamer with the paddlewheel at the back, river-boat fashion. This had two horizontal cylinders driving one paddle wheel via cranks and I was expecting to see Stevenson's valve gear on it, but instead it seemed to have an extremely primitive valve-gear consisting only of a crosshead-driven arm, and no eccentric action from the crank whatsoever. After staring at it for a while, I could not work out how it could possibly provide any cut-off- the valves being just sort of knocked back and forth by the crosshead link. Conversely, I was aboard a Swiss paddle-steamer with the wheels on the side, and she had a full Stevensons gear and I could see the engineer constantly fiddling with the cut-off via a reversing wheel. It was very interesting to watch because the ship was doing very short "hops" and used reverse to slow herself, so the engineer was extremely busy. Imagine driving a locomotive from a cab with no windows, recieving guidance only from a man sitting on the roof, ringing a bell.

Anyway, some Accucraft models have a full Walschaerts valve gear, I think. It would be interesting to see if anyone who has one of those, like the 7/8ths Decauville for instance, has noticed any characteristics from being able to "notch up"... well I reckon just as Graeme said his locomotives do.

I'm sorry my post is only semi-relevant to your original question. The weather here is terrible, therefore I'm quite bored and procrastinating on some housework of the "optional" and "extremely dull" variety.

I can't imagine there is any way to drive a little garden locomotive in a way that does for it what a flywheel will. It uses rotational momentum to provide inertia to a degree that a little garden train can't possibly have the mass to provide on its own. Although I suppose on an engine with piston-valves, "notching up" might provide the sort of benefit that an "exhaust throttle" does on some small models. I know a Mamod can be made to behave itself somewhat with an exhaust throttle, but nowhere near to the same degree a flywheel would.

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Re: To notch or not to notch

Post by IanC » Fri Aug 10, 2018 11:06 pm

GTB and Keith S are both correct. They sum it up nicely. Nothing further to add as far as I am concerned.
Ian

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Re: To notch or not to notch

Post by dewintondave » Sat Aug 11, 2018 6:36 am

As dear LBSC said "you can't scale nature". Small low mass models bolt off the line, that's why I've gone to larger scale. Bigger cylinders take longer to fill, and a longer stroke is just perfect
Best wishes,
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Re: To notch or not to notch

Post by JMORG » Sat Aug 11, 2018 10:21 am

I have had the pleasure of seeing a Katie with a Slomo but after the import tax they do become ruinously expensive. I have often wondered whether adding a flywheel to the axles themselves would be a good alternative to the chain drive of a slomo. Might try it!
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Re: To notch or not to notch

Post by tom_tom_go » Sat Aug 11, 2018 10:32 am

The Lady Anne Slomo does not use a chain drive, the sprocket provided attaches to the axle and then you mesh the Slomo to that sprocket:

http://smallsteamperformance.com.au/lady-anne-slomo/

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Re: To notch or not to notch

Post by TonyW » Sat Aug 11, 2018 11:58 am

tom_tom_go wrote:
Fri Aug 10, 2018 4:35 pm
I have yet to see a RH loco with simplified Walschaerts type valve-gear run at a 'continuous' realistic speed and be able to stop and start smoothly. Also, without some kind of inertia device you cannot run light engine slowly.
You are welcome to visit here any time Tom.
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Rhos Helyg Locomotive Works: http://www.rhoshelyg.me.uk
Facebook: https://www.facebook.com/RhosHelygLocoWorks

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Re: To notch or not to notch

Post by tom_tom_go » Sat Aug 11, 2018 1:13 pm

Thank you Tony, there are many of you I would like to visit one day.

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Re: To notch or not to notch

Post by Busted Bricks » Sat Aug 11, 2018 2:27 pm

dewintondave wrote:
Sat Aug 11, 2018 6:36 am
As dear LBSC said "you can't scale nature". Small low mass models bolt off the line, that's why I've gone to larger scale. Bigger cylinders take longer to fill, and a longer stroke is just perfect
I agree. I often wish it was 1/12th scale we were modelling in, not 16mm or 7/8ths.

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Re: To notch or not to notch

Post by steveh99 » Thu Aug 16, 2018 10:53 am

Only a slomo will give controlled acceleration and de-accerlaration and allow for a smooth slow running performance


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Re: To notch or not to notch

Post by tom_tom_go » Thu Aug 16, 2018 11:27 am

Steve and I both run on layouts that have at least one run round loop and this is where the Slomo makes the difference. To be able control a live steamer light engine realistically is only possible with inertia.

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