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vacuum brakes (again)

completed piping, non working!

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David Bothwell 124/06/2020 16:59:03
155 forum posts

Just completed all the piping and fittings for "Maisie" but cant get it to work" I have checked there is no blockage in the piping. I have used a Reeves (commercial) ejector, used compressed air through ejector but no movement on the brake cylinder, a bit leaky through the valve (two discs with groove as per LBSC instructions, any suggestions most welcome, especially how much air (pressure) to get a vacuum?

roy entwistle24/06/2020 18:06:36
1269 forum posts

Ejector with compressed air ?

Nick Clarke 324/06/2020 18:36:55
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Posted by roy entwistle on 24/06/2020 18:06:36:

Ejector with compressed air ?

It ought to work - it is basically a scent spray.

Regarding the leaks from the valve have you lapped the two surfaces together?

David Bothwell 124/06/2020 18:56:49
155 forum posts

Hello Roy, I was told (by someone on the forum) it should work on air, Hello Nick, used lathe bed and smooth emery, good shine on both! Not sure what you mean by "scent spray" do you mean it doesn't need much air? My thanks for the replies

Andrew Tinsley24/06/2020 18:57:05
1211 forum posts

Ejectors will work on compressed air, but injectors won't.

Andrew.

roy entwistle24/06/2020 21:33:56
1269 forum posts

Sorry blush I had an idea that one of them wouldn't work

Paul Kemp24/06/2020 22:41:33
580 forum posts
18 photos

Ok, well I am using a lot of assumptions here and because of that may be talking complete sphericals but here goes!

I think most of LBSC brake designs were steam brakes where steam was taken from the boiler and through the brake valve fed to the cylinder to apply the brakes. Have you redesigned the system with a chamber and reservoir side so that when you have no vacuum the brakes are on (normal convention) and when you apply a vacuum to the train pipe the brakes are pulled off? I suspect (but may be completely wrong) that if you haven't redesigned LBSC's set up then you may be applying a vacuum to the steam side of a steam brake cylinder - which will do nothing more than try and pull the brakes off further - if you get my drift?

Never been on a full size Atlantic but I would suppose in common with a lot of other engines that the loco brakes were steam and a combination brake valve was used to apply the engine steam brake and drop the vacuum proportionally on the train pipe to apply the train (and possibly tender brakes) at the same time. It is also common to have two sizes of ejector, small and big! Small running continuously to maintain the train pipe vacuum and big to develop the vacuum "quickly" although quick can be a relative term, to release the brakes.

It shouldn't take a lot of pressure to get your ejector to work, 60 psi should be fine, but you do need the volume. Remember as full size you don't want the pressure limit on your ejector to be too high, on a big train the ejector stops working at a pressure higher than the loss of traction effort so the brakes come on and stop you when the pressure drops before the pressure is too low to actually pull the train - trust me most firemen will know that!

It would be really good if you have made a combination brake valve in 3 1/2" gauge, that would be a thing of beauty indeed.

Apologies if I am teaching granny to suck eggs and have completely missed the point!

Paul.

David Bothwell 125/06/2020 08:09:50
155 forum posts

Thanks Paul, No you did not miss the point. I have not re-designed Lbsc's brake valve/system, I wish I had the knowledge (or nerve) to do such a thing. I will keep looking on the net and see if I can find where to insert a vacuum "tank". I bought the Maisie as a part built project and have managed to get it running. The brake system was partly made. I have just done some pipework and tidying. There is also a large cylinder and parts in the tender, but this needs some further work. Thanks once again, still pondering!

Did not quite understand your post Roy?

Nick Clarke 325/06/2020 09:18:49
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A few points from the discussion:

Not sure what you mean by "scent spray" do you mean it doesn't need much air?

No it works like one of those old fashioned sprays with a rubber bulb which you squeeze and it blows air through a venturi.

I think most of LBSC brake designs were steam brakes

Maisie was designed from the start with vacuum brakes in 1936/37

Like many designers LBSC often produced designs 'on paper' and so errors could creep in, however in the case of Maisie it was built to order and pictures of the original appear in the magazine text so if it is in there it at least worked for LBSC.

Looking at the drawing from the magazine in December 1936 I think that if you still have a leak from the control valve that could be destroying the vacuum and would be the first this I should try to eradicate.

maisie valve.jpg

(Apologies for the picture quality - a mobile phone image of a page in a bound volume 75 of ME)

SillyOldDuffer25/06/2020 09:29:31
Moderator
6447 forum posts
1421 photos
Posted by Nick Clarke 3 on 24/06/2020 18:36:55:
Posted by roy entwistle on 24/06/2020 18:06:36:

Ejector with compressed air ?

It ought to work - it is basically a scent spray.

Regarding the leaks from the valve have you lapped the two surfaces together?

Zero experience myself, but I wonder how well an ejector designed to work on steam would perform on air? Steam contains far more energy than compressed air and I'd guess the vacuum in a steam ejector owes much to cooling. Air ejectors certainly exist, but maybe the nozzles are designed differently?

As there's very little to go wrong on a ejector, maybe David is getting a vacuum that's too weak to work the brakes, especially if they're a bit leaky. David asked how much compressed air would be needed to drive his steam ejector; my guess is much more air than steam.

An alternative might be to make a simple mechanical vacuum pump to test the brakes, and - having made sure they're OK - test the ejector on steam later. David's ejector might be fine and it's the way it's being tested that's bust!

How to make a simple vacuum without going to the expense of a proper pump? Suggestions welcome! I'd look at cooling a sealed container pre-heated with boiling water, or pulling a greasy home-made piston through a length of copper water pipe with a length of string. Plastic drainpipe if a bigger volume of vacuum is needed.

Dave

 

Edited By SillyOldDuffer on 25/06/2020 09:32:08

roy entwistle25/06/2020 09:32:40
1269 forum posts

David I meant that I had an idea that ether an injector or an ejector would not work on air, wasn't sure which

Roy

Andrew Tinsley25/06/2020 10:39:08
1211 forum posts

I believe that "steam having more energy than compressed air" is somewhat misleading. Ejectors work on Bernoulli's principle. Nothing to do with steam condensing and liberating energy.

My ejectors work on air and steam. In fact a little better on air, which I find surprising. Although this is probably due to gauge calibration.

Andrew.

Nick Clarke 325/06/2020 11:01:56
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936 forum posts
33 photos

The simplest way to check if the ejector is working is to take the pipes and valve off and connect to the compressor.

Put your thumb over the vacuum pipe and you will detect the vacuum being formed or not.

Before anyone comes up with safety issues remember that 20 inHg which is what I would expect a reasonable ejector to pull is less than 10psi - it is far less than putting your finger on a bike pump and pulling the handle.

David Bothwell 125/06/2020 11:45:47
155 forum posts

Many thanks for all the responses, I will make further investigations in the morning. My ejector is a commercial one from Reeves. I appreciate you posting the drawing (which is what I have been working to (( I took it from the Maisie construction book which came with the loco, Any chance somebody could suggest a reasonable size vacuum reservoir and where it should be placed in the piping, thought of hiding it on the tender?? would it need a valve of some kind? Excuse my ignorance

Paul Kemp25/06/2020 12:25:05
580 forum posts
18 photos

Nick,

Thanks for that, like I said I was making some assumptions!

I am interested in this design, can you give any more detail on the overall system? Is the vacuum used to pull the brake off as per full size or in this case is it used to apply the brake?

Thanks,

Paul.

Nick Clarke 325/06/2020 12:35:27
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936 forum posts
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The original design was for simple working where applying steam to the ejector evacuates the train pipe and applies the brakes. It is not automatic where a break in the train pipe applies the brakes so a reservoir is not part of the design and would in fact worsen its performance.

Or to quote LBSC "Steam blows out of the steam cone into the choke, extracting the air from the train pipe and 'plonking 'em on' with great alacrity"

He could be argumentative and he might have had his failings as an engineer at times, but his writings were always readable!

Nick Clarke 325/06/2020 12:42:49
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936 forum posts
33 photos

Just had a thought (only the one!) If the system were to be set up with a reservoir and valves for automatic working the ejector evacuates the reservoir(s) releasing the brakes and you proceed. When you want to stop you allow atmospheric air into the system which applies the brakes. When you go again you empty the system - This is not by any means instant on our club's 7¼" vacuum braked locos and rolling stock.

With a simple vacuum brake you need a vacuum there instantly and hard to apply the brakes. I wonder if your commercial ejector is designed for an automatic system where the vacuum is built up over a few moments but it cannot provide an instant vacuum on demand - ie it may just be too small.

It might be interesting to compare your commercial ejector which may well be adequate for an automatic system with the one LBSC drew?

Just a thought

Nick

Edited By Nick Clarke 3 on 25/06/2020 12:43:39

Paul Kemp25/06/2020 13:25:43
580 forum posts
18 photos

Nick,

Thanks for that, confirms my thoughts from looking at the valve drawing. Need to be careful "plonking" em on hard and fast with a Maisie though, not a massive amount of adhesion so with a train a recipe for flats on the wheels unless the stock is similarly fitted - in which case there would likely be a significant lag in application.

Paul.

SillyOldDuffer25/06/2020 15:18:18
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6447 forum posts
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Posted by Andrew Tinsley on 25/06/2020 10:39:08:

I believe that "steam having more energy than compressed air" is somewhat misleading. Ejectors work on Bernoulli's principle. Nothing to do with steam condensing and liberating energy.

My ejectors work on air and steam. In fact a little better on air, which I find surprising. Although this is probably due to gauge calibration.

Andrew.

'Ejectors work on Bernoulli's principle. Nothing to do with steam condensing and liberating energy.' Interesting point. I'm not even sure Bernoulli's Principle applies to ejectors - is an an ejector one flow field or two?

My books don't help much. This from 'Steam Locomotion', Poulton, Caxton, London 1951 being typical:

dsc06278.jpg

But it does say, 'This action has the effect of creating in the first place, a partial vacuum in the ejector fitting...'

As to what it means by 'draws and entrains the air behind the nozzle', I see that as a sort of friction effect much as a moving nylon fishing line pulls water after it. The friction is due to the high-speed jet creating turbulence as it jumps the gap. In my view steam would do more work faster than air because it contains more energy, hence higher velocity at the nozzle for a given input pressure.

I suspect the sizes and angles of an ejector's nozzle, expansion chamber and outlet cone are all interrelated and depend on the pressure of the gas, which comes from the energy in the system. My suspicion - no more - is that air ejectors might work best at a particular pressure, while a steam ejector might struggle to perform with compressed air, they're not interchangeable with no loss of performance.

One of my other books mentions air ejectors are valued for efficiency (best use of energy), whilst steam ejectors are chosen for volume (max quantity pumped in a given time).

Very educational this forum. I knew full size vacuum brakes work by pulling the brake blocks off and fail safe if there's any kind of fault. I didn't know model size vacuum brakes worked the other way round, positively pushing brake blocks on to the wheel. Very sensible - a model brake doesn't need to be fail safe. Full size vacuum brake systems have reservoirs so the train doesn't have to repump from scratch each time it stops. (I wonder how long it takes a steam loco to release all the brakes on a long train after the vacuum has completely failed?) A positive action brake system likely doesn't need reservoirs because the vacuum should go straight to the wheels.

Dave

Edited By SillyOldDuffer on 25/06/2020 15:20:07

stephen goodbody25/06/2020 15:55:43
56 forum posts
40 photos

Hi David,

A vacuum ejector will work just fine on air, provided the design and manufacture of the ejector is okay.

I've built a few of these now to different designs and have found the design described by DAG Brown in his injector book to be one of the better ones. Other designs (including designs by supposedly reputable designers and published in ME) have been less good.

In my experience you should see some level of vacuum even at low air pressure (10psi), maximum vacuum somewhere between 40psi and 60psi, and a tail-off above 60 psi. On air, I'm seeing a peak vacuum of around 20 inches Hg at 60 psi air with the DAG Brown design.

That said, vacuum brake systems are very susceptible to even small leaks in the vacuum piping and equipment. Do you have a vacuum gauge available to you for testing? That's a big help and plastic-bodied gauges are very cheap.

I would start with the ejector itself - disconnect it from the vacuum piping and blow air through the inlet - if it sucks at the vacuum piping connection then that's a big plus! If it doesn't (and it may not) then you've identified the first big issue. Ideally mount the gauge to the vacuum inlet connection to see exactly what vacuum you're starting with as a baseline.

Once you've confirmed the ejector is good then pipe it back in, preferably with a gauge also connected somewhere in the vacuum piping, and see what you're getting. If less vacuum than the standalone ejector test, start tightening and checking all your joints and fittings one at a time until you see a difference in the reading.

If you do all this and you are now sure you are pulling a good vacuum at the piping to the brake cylinder, but the piston either isn't moving or is not significantly moving, then something's binding in the cylinder or brake rigging itself and preventing decent brake application force at the blocks,

Best of luck,

Steve

 

Edited By stephen goodbody on 25/06/2020 16:11:41

Edited By stephen goodbody on 25/06/2020 16:16:08

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