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Alternative valve mechanism

Comments sought on simple valve mechanism idea.

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John Bramwell09/02/2021 12:40:53
24 forum posts
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Hello. Just joined and looking for some comments on a valve actuating mechanism idea. I want to make an O gauge live steam loco that does not have to resemble any actual loco but can pull a train. I want the piston valve directly over the top of the drive cylinder. But that requires an elaborate linkage system. The idea is to have a pendulum lever which drives the valve shaft from the drive shaft. I graphed it out and with a drive pistion stroke of 190mm and a piston stroke of 100mm and an axis difference of 95mm there is a pendulum length of some 280mm . The pendulum arm would have pin holes elonged 12mm to allow it to rock. Maybe this has been tried before but i cannot find any examples. So please advise. I could send CAD if that would explain it better. cheers Jack

JasonB09/02/2021 13:06:28
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John, a picture may help, have a read of this to see how to add one to a post, just a screen shot of the basic concept should do.

I think I get the idea of what you are suggesting but at the moment the valve and piston will move in unison but you want the valve to lead the crank by approx 120deg to get the timing right.

Also are those sizes right, they sound big for Gauge O

Edited By JasonB on 09/02/2021 13:07:00

Brian G09/02/2021 13:56:08
797 forum posts
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Dou you mean something like the radial gear used on John Turner's Caledonia. a simplified version of Bremme/Klug ,where the vertical motion of the crank is used to swing a link? The close up of the valve gear is missing from the Sidestreet Bannerworks webpage, but is still available on the Wayback machine.

Brian G

Sidestreet Bannerworks article on Caledonia

John Turner Valve Gear

JasonB09/02/2021 14:08:49
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This is the picture that came to my mind from the description. Green pendulum pivoted at the top has it's other end moved 190mm by piston rod. The valve rod being nearer the pivot will not move as far.

pendulum.jpg

Howard Lewis09/02/2021 14:21:46
5562 forum posts
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One surprise!

A 100 mm piston stroke on an O gauge loco?

If O gauge is 45 mm , full scale the loco would have a stroke of TEN FEET! (Larger than the driving wheels, unless possibly a Crampton. )

As Jason says, your figures are much too big for an O gauge loco.

Perhaps you need to revise them?

Howard

JasonB09/02/2021 14:36:53
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Figures are probably what John has used to model the layout on paper.

There is also no real need for an elaborate linkage system to get the valve above the cylinder, just mount a traditional eccentric on outside of wheel and then have the crank pin go right through that. Then simply angle the piston valve hole so it aims at the ctr line of the axle then all you need is a simple clevis to join valve rod to eccentric rod.

If you were really clever then have a slot curved slot in the eccentric where the crank pin comes through and it could be made into a slip eccentric so engine will run either way

Edited By JasonB on 09/02/2021 14:39:54

John Bramwell09/02/2021 14:45:45
24 forum posts
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Sorry guys the CAD dimensions are x10 what i want to build. But you advice is very helpful. In order of posts': yes I have the valve piston moving in sync with the power piston I have them both down the same end at the same time both fully open, But that means it would start to open a lot earlier wouldnt it?...I realise now that you can have two cranks off the same side of the flywheel by extending the pin of the bigger one then having an arm on it. The angle of this arm would determine the timing. I like the Caladonian, will work out how to limh to my screen shot but its a bit obsolite now. Thanks again. PS how does a10mm stroke sound for an O gauge?

John Bramwell10/02/2021 01:05:40
24 forum posts
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Jasons diagram has it exactly. Except for some f!exibility in the joints to allow for the arc (11 units at the p shaft). I was thinking slots or !inear beatings. You see how simlple it is for a novice, one piece. As for the timing, if the stroke is 99units and the port is 33 units and the piston just goes from one side of the port to over the port then to the other side of the port the opening stats at 1/3 of 180 =60 degrees. A wobbler engine has !ess i think.

JasonB10/02/2021 07:15:22
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It is not so much the amount of movement but the relation of the movement of the piston in relation to the valve. As a good starting point when setting the timing the valve eccentric or in this case your pendulum needed to lead the crank by 120degrees as it stands yours has no lead so you won;t get steam entering the cylinder at the right time.

The double crank is one method that would work as is the eccentric and angled valve that I suggested above.

Example of the double crank arrangement.

Redsetter10/02/2021 09:22:10
192 forum posts
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The most difficult part of building the loco in question will be making the cylinder and piston. In comparison a single eccentric or slip eccentric valve gear is an easy job and will work well.

You could consider using an oscillating cylinder engine with a geared drive but this is not really much easier. Oscillating cylinders are not quite as simple as they look, and have to be accurately made to work properly - perhaps more so than the slide valve equivalent.

I would suggest that if a good "alternative" valve gear for small models existed, then it would have emerged already in the 100 or so years that we have been playing with steam engines.

Nick Clarke 310/02/2021 11:51:54
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Posted by Redsetter on 10/02/2021 09:22:10:

The most difficult part of building the loco in question will be making the cylinder and piston. In comparison a single eccentric or slip eccentric valve gear is an easy job and will work well.

Or as an alternative what about the slip return crank as used by the Hackfly locomotive?

Redsetter10/02/2021 12:06:15
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Posted by Nick Clarke 3 on 10/02/2021 11:51:54:
Posted by Redsetter on 10/02/2021 09:22:10:

The most difficult part of building the loco in question will be making the cylinder and piston. In comparison a single eccentric or slip eccentric valve gear is an easy job and will work well.

Or as an alternative what about the slip return crank as used by the Hackfly locomotive?

No reason why not, though perhaps a bit fiddly in 0 gauge and of course only suitable for outside cylinders. It depends how well it fits in with the overall design. It is sometimes overlooked that valve gear is three-dimensional, and has to fit in the space available.

Redsetter10/02/2021 12:26:38
192 forum posts
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The OP should read Norman Dewhurst's "A steam locomotive for 0 gauge" if he hasn't already. Dewhurst was a clever man, and I believe a self- taught engineer. He built some very interesting locomotives.

He pointed out that most 0 gauge steamers needed a push to start, so you didn't really need two cylinders, and he mounted his single cylinder outside, to avoid a crank axle. He did not find that the unequal thrusts were a problem in practice. Food for thought.

Nigel Graham 210/02/2021 13:12:18
1786 forum posts
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Good to see that people are building live-steam locos from scratch in these smaller scales.

To be honest though, unless you intend the engine to run only ever forwards, any gear more elaborate than a slip-eccentric will need about as many separate parts as any other; but is not straightforward to design..

As Redsetter points out, an oscillating cylinder is not the answer it may seem, and also needs a fair of work to disguise the fact if outside the frames - though some owners of commercially-built oscillator models have done that successfully.

My own society has quite a number of members who own Gauge 0 and 45mm -gauge locos, admittedly mostly commercially built, and the club facilities include a raised track for them. Although I am not such an owner I have noticed that none of their engines normally needs a helping hand to start, but they are all two-cylinder machines and seem to be double-acting, many with a simplified Walschaert's Gear radio-operated along with the regulator. (I don't think they can be notched-up, nor need it.)

So I am a bit puzzled by Norman Dewhurst's observation, and whilst I'm sure he was a good craftsman and his point about crank-axles is fair, I suggest his engines need the Maradonna touch because they rely on a single cylinder - not vice-versa!

.

John Bramwell16/02/2021 12:21:17
24 forum posts
6 photos

img_20210216_130034.jpg

John Bramwell16/02/2021 12:30:34
24 forum posts
6 photos

Ok. Here is the basic design of the pendulum vale actuator. Some of you thought that there was no lead but as you can see it has a modified valve piston and cylinder which is full open at TDC. So cracks open at 90 deg BTDC. The valve chamber bolts onto the power chamber. The shafts are conneted to the pendulum by slots to allow for vertiacal displacement. Will it work? John

Redsetter16/02/2021 13:42:29
192 forum posts
3 photos

John,

I am not knocking what you are trying to do, but I don't think you quite understand how the valve works, and you need to go back to first principles.

It isn't simple, and there is too much to go into here, but you have to consider both the opening and closing points, and what the exhaust is doing, as well as the steam. 

It is well established that for the basic form of valves without lap or lead, the valve and piston need to be 90 degrees out of phase.

The linkage you have shown will match the stroke of the piston to the stroke of the valve, but they are in phase with each other.

 

Edited By Redsetter on 16/02/2021 13:43:08

Mr C16/02/2021 14:14:39
6 forum posts

Just to add my 2p worth, have a look at the 16mm soc site. there are plans on there for a quarry hunslet (wild rose) which is slide valve slip eccentric. I used the dimensions to make a slip crank version with outside valve chests like you want. It took a few goes to get it to work but it did and it is a lot easier than making eccentrics and rocking lever to bring the motion outside . Not much metal is wasted if you get it wrong either. It doesn't look too bad visually either. I am sure that it will work for piston valves. Sadly I do not have pictures and the loco was made for a friend so not in my posession anymore. I do have a video of it running somewhere. Don't reinvent the wheel!

Mr C16/02/2021 14:23:20
6 forum posts

would have editted previous post but stil**LINK**l being moderated. A chap called Mike Gaskin built a rather nice model of a darjeeling pacific which looks like it has walschwarts but is slip crank. museum section of anything narrow gauge website will provid pics.

I know 16mm is bigger than O gauge but from the locos I have built I am sure it would not be too big to fit.

Brian G16/02/2021 14:45:14
797 forum posts
34 photos

Hi John

It might be worth drawing your valve gear at different positions of the crank and comparing the valve openings to the Dockstader simulations. Several of them use an oscillating lever similar to yours, but derive their action from the vertical movement of the crank which is automatically 90 degrees out of phase with the piston.

Brian G

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