To notch or not to notch

Discussion of Live Steam locomotives should be located here
User avatar
tom_tom_go
Driver
Driver
Posts: 4824
Joined: Wed Feb 23, 2011 3:08 am
Location: Kent, UK
Contact:

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.

Big Jim
Moderator
Moderator
Posts: 2694
Joined: Sat Jul 06, 2013 5:28 pm
Location: Near Llanelli

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!

User avatar
tom_tom_go
Driver
Driver
Posts: 4824
Joined: Wed Feb 23, 2011 3:08 am
Location: Kent, UK
Contact:

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.

User avatar
GTB
Driver
Driver
Posts: 1550
Joined: Sat Mar 05, 2011 2:46 pm
Location: Australia

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

User avatar
tom_tom_go
Driver
Driver
Posts: 4824
Joined: Wed Feb 23, 2011 3:08 am
Location: Kent, UK
Contact:

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:

User avatar
Keith S
Driver
Driver
Posts: 1624
Joined: Thu Feb 24, 2011 11:44 pm
Location: Canada

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.

User avatar
IanC
Trainee Driver
Trainee Driver
Posts: 798
Joined: Mon Feb 15, 2016 9:15 am
Location: Nr. Warrington, Cheshire

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

User avatar
dewintondave
Trainee Driver
Trainee Driver
Posts: 697
Joined: Mon Mar 07, 2011 8:52 am
Location: New Zealand

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,
Dave

User avatar
JMORG
Trainee Driver
Trainee Driver
Posts: 940
Joined: Fri Dec 09, 2011 1:16 pm
Location: Gloucester

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!

User avatar
tom_tom_go
Driver
Driver
Posts: 4824
Joined: Wed Feb 23, 2011 3:08 am
Location: Kent, UK
Contact:

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/

User avatar
TonyW
Driver
Driver
Posts: 1357
Joined: Fri May 16, 2008 9:25 am
Location: North Wales
Contact:

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 pmI 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.
Tony Willmore
Rhos Helyg Locomotive Works: http://www.rhoshelyg.me.uk
Facebook: https://www.facebook.com/RhosHelygLocoWorks

User avatar
tom_tom_go
Driver
Driver
Posts: 4824
Joined: Wed Feb 23, 2011 3:08 am
Location: Kent, UK
Contact:

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.

User avatar
Busted Bricks
Trainee Driver
Trainee Driver
Posts: 750
Joined: Sat Jan 22, 2011 9:16 pm
Location: Denmark
Contact:

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.

steveh99
Cleaner
Cleaner
Posts: 91
Joined: Tue Jan 31, 2017 5:06 pm
Location: Oxfordshire

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


User avatar
tom_tom_go
Driver
Driver
Posts: 4824
Joined: Wed Feb 23, 2011 3:08 am
Location: Kent, UK
Contact:

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.

User avatar
Hydrostatic Dazza
Driver
Driver
Posts: 1167
Joined: Tue Sep 19, 2017 4:17 am
Location: Brisbane
Contact:

Re: To notch or not to notch

Post by Hydrostatic Dazza » Thu Aug 16, 2018 10:15 pm

Not much I can add to this as others have told all, but a Slomo in my mind is a must for enjoyable slow running and sweet shunting. Garden choo choos have buggar all mass and the mass of it all is the physics of this gig. A Slomo adds a wee bit more mass but by using the well known use of a fly wheel the mass (spun up) is amplified many times. It does not add any measurable friction! The bearings and gears are free running. The only way I can see that you can flog out your loco's motion when using a Slomo is if you try to accelerate the loco like a F1 off the starting grid and stopping like one, thus you are fighting to spin up and slow down the fly wheel. The flywheel sort of thing has been in HO model railways for 50 years in the USA. (that I can recall reading about it in Model Railroad Craftsman magazines) I discovered Slomo for my choo choo build after I had fitted my pump eccentric and wheels and cranks and all is pinned.
Sigh!
So the loco is now to be a tender loco with the Slomo fitted and the tender Slomo is chain driven. (which takes up more space)
As for notching up (cut off), as said by others. I am not proclaiming to be an expert, however I did spend considerable time in the evenings with books, CAD and the simulators to get a better understanding of it all. Simply it is about greater expansive working of the steam in the cylinder by cutting off the admission of the steam via the variable valve events. Referred to as "Notching up" A cab fitted pole reverser has a pawl to engage the notches in the sector plate to give the driver various selections. A screw reverser does not have notches (but for locking purposes) but the process is still called notching up. For starting one wants lots of torque with the slow piston speed and then as the loco moves one notches up, to increase the expansive working and due to other factors of what is going on in the cylinders and valves notching up will allow greater speed. There is a lot of other things going on in cylinders and steam and radial valve gears, such as Stephenson's (he ripped it off some one else) and Walschaert's which mean there has to be compromises and the knack is designing with all of these in mind. (not always done very well in Model Engineering designs and even the full size loco designers had problems understanding valve gears) There is lots going on such as compression, lead, wire drawing at speed etc etc. I can happily report my Llewellyn # 1 when running on air, notches up nicely and the speed of the unloaded motion speeds up (as it should) and when getting closer to mid gear the motion gets a wee bit jerky due to compression along with the truncated exhaust note. That all pleased me.
Cheers from Dazza, The Hydrostatic Lubricator 8)
The chances of finding out what’s really going on in the universe are so remote, the only thing to do is hang the sense of it and keep yourself occupied. Douglas Adams

User avatar
Keith S
Driver
Driver
Posts: 1624
Joined: Thu Feb 24, 2011 11:44 pm
Location: Canada

Re: To notch or not to notch

Post by Keith S » Fri Aug 17, 2018 8:02 am

I suspect some people refer to "friction" with respect to the "Slomo" because flywheel mechanisms in toys are sometimes called "friction motors".

https://en.m.wikipedia.org/wiki/Friction_motor

One proposed method of "taming" small steam locomotives DOES involve friction, and that is the centrifugal brake, which works quite well. The mechanism from an old rotary-dial telephone has been successfully deployed as a centrifugal brake in some live-steam models.

For those who are allergic to radio-controllled locomotives, the ideal combination, it would seem to me, would be a flywheel van (or SLOMO) to control accelleration, as well as a centrifugal brake wagon to limit top speed.

User avatar
TonyW
Driver
Driver
Posts: 1357
Joined: Fri May 16, 2008 9:25 am
Location: North Wales
Contact:

Re: To notch or not to notch

Post by TonyW » Fri Aug 17, 2018 10:26 am

steveh99 wrote: Thu Aug 16, 2018 10:53 amOnly a slomo will give controlled acceleration and de-accerlaration and allow for a smooth slow running performance
Only? I don't think so. A properly set up loco fitted with radio control and with a boiler pressure commensurate with what it is being asked to do will also give smooth slow running performance. Performance is certainly good enough for garden railway operation, but I would agree that the Slomo is essential for the exhibition-style running that Steve does.

Last year I fitted a Slomo to a 20-year old Roundhouse Jack that the owner bought new. He is delighted with the performance but reports that he needs a higher boiler pressure and more regulator to "unstick" it, and that has reduced his length of run per gas fill from 45 minutes to 25 minutes.

The video is interesting. A shame we cannot see the pressure gauge, but enough evidence is there of what is happening.

One of the advantages of a stick-type controller over a knob-type is that it is possible to use the loco's regulator as a full-size one would be used, i.e. open a bit, back off, open a bit more, back off, etc. What we see in the video is the valve chests being filled with 40psi (safety valves lifts at 01:40) and the thing taking off at speed. That does not surprise me. If you did similar with a full-size loco it would just go in to a monster wheelslip.

We'll ignore the hotly-debated "does a chuffer pipe create back-pressure" issue, but there is something odd going on with the loco's chassis. In non-slomo mode every time it stops, in both forward and reverse, the eccentric on the back axle is pointing to the 8 o'clock position. Every time. This indicates to me that there is some kind of mechanical binding issue at that point in the rotation cycle, so the loco has to overcome that bind *before* it does anything else and needs steam pressure in excess of what is normally needed to do so.

Note that the regulator opens wide - very wide - so filling valve chests and superheater with full pressure steam on a mechanically unsound chassis, so the non-slomo performance was bound to fail. It could not do anything else. I think all the video demonstrates is how NOT to drive a non-slomo loco! Heavy-handed regulator control and excess steam pressure (turn the gas down, save money and increase run time) are never going to be good. Perhaps then another advantage of the slomo is that it copes with drivers who don't know how to drive...?

The other component missing is momentum. The rolling road is a different situation compared to using a loco on track.
Tony Willmore
Rhos Helyg Locomotive Works: http://www.rhoshelyg.me.uk
Facebook: https://www.facebook.com/RhosHelygLocoWorks

User avatar
tom_tom_go
Driver
Driver
Posts: 4824
Joined: Wed Feb 23, 2011 3:08 am
Location: Kent, UK
Contact:

Re: To notch or not to notch

Post by tom_tom_go » Fri Aug 17, 2018 11:57 am

I agree with Tony that with a well laid level track, long sweeping curves, appropriately weighted rolling stock and a correctly set up live steamer with R/C then you can achieve realistic running providing you have a continuous circuit.

However, I am unlikely to ever own a property that can accommodate such a railway so the Slomo allows me to run realistically on a small line with tight curves and run around light engine at a speed that's not trying to get me into orbit.

If the SSP Slomo had not been brought to market I would of probably sold up years ago.

User avatar
TonyW
Driver
Driver
Posts: 1357
Joined: Fri May 16, 2008 9:25 am
Location: North Wales
Contact:

Re: To notch or not to notch

Post by TonyW » Fri Aug 17, 2018 12:17 pm

tom_tom_go wrote: Fri Aug 17, 2018 11:57 amIf the SSP Slomo had not been brought to market I would of probably sold up years ago.
I am a big believer in the phrase "horses for courses".
Tony Willmore
Rhos Helyg Locomotive Works: http://www.rhoshelyg.me.uk
Facebook: https://www.facebook.com/RhosHelygLocoWorks

Post Reply

Who is online

Users browsing this forum: No registered users and 6 guests