Britannia 70047 "Derek Stokes"

[Deleted]

I think most loctite's need 250c for disassembly, including 603. Personally, I'd buy something new, (check out their website for what's most suitable), loctite can last for a long time but it's not infinite.

Regards

Pete

[Roger]

Jan 22, 2019 11:25:20 GMT JonL said:

150 degrees C. Could be a bit borderline?

I think so. Look up the temperature of steam at various temperatures here, it's surprisingly hot at even modest pressures.

[JonL]

Jan 21, 2019 22:32:57 GMT terrier060 said:

...The Tech guy at Permabond is worth ringing. He is really helpful and has suggested a couple of adhesives to use on my Terrier cab...

You were not wrong Ed, they really were helpful. Based on what I told him he suggested ET5401 but says he will email me with more information once he has gone through the specs to make sure its the best product for my application.

Thats how to get repeat business. I even started off by saying "I'm a model engineer, I'll be buying the smallest tube of this product I can" but they treated me like a valued customer.

EDIT I've looked at the spec for the ET5401 and I don't think it will be suitable as it breaks down at too low a temperature, but we shall see what he comes back with.

[JonL]

I can't quite afford the adhesive until after pay day, so I moved on to using a boring bar to cut the chambers for the valve sleeve inserts. I need to make my judgement call on what to make the Piston valve sleeves from. I can't see LBSC recommending any material in the words and music as by the look of things he uses a casting for this, which I am not going to do. Obviously this dictates how I do the piston valve too, which I will decide once I've fully absorbed Baggo's writings on the subject, among others.

I hate machining stainless, so I am hoping to avoid it for the sleeve of the valve.

This has been a mine of info: LINK

[Roger]

Stainless isn't suitable, it's a hopeless bearing material, so you won't have to machine that. Personally I don't mind machining it, unless it's sheet material which can be a challenge.

[JonL]

I originally intended to use mild steel with PTFE lined bobbins as described elsewhere. I'll be perfectly honest I don't know what the usual material of choice is in this scenario; I'm assuming Brass? I think its the route I'll go at the moment.

[mbrown]

Cast iron liners in cast iron cylinders is the usual combination. You can get continuous cast meehanite bar which machines beautifully. Try College Engineering or Noggin End Metals.

Cast iron against cast iron works well in terms of bearings and wear, and you don't get any differential expansion problems.

Malcolm

[JonL]

That makes sense Malcolm. Noggin is my usual source. Is it still a good choice if I intend to use the PTFE glands?

[mbrown]

The gland packing is stationary within the gland housing and bears on the valve rod and piston rod - which are usually stainless steel. So the type of gland packing isn't affected by the cylinder material.

Or are you meaning the seals on the piston valves and piston? I don't have any experience of piston valves (one reason to choose a prototype for my next model which does have them!) and my cast iron cylinders have iron pistons with iron rings. But there is lots of information on this site - and lots of experience from its members - which could help there.

I like your cylinders, by the way. I have a virtually identical set of castings in iron (ex-Doris) which are earmarked for my next project.

Malcolm

[JonL]

Sorry yes, I'm referring to the bobbins that are a key part of the piston valve itself. The castings were quite nice visually, I've no idea who supplied them though. The mystery previous owner (whom I never met) bought those, but the sand inclusions make me think they should have kept the receipt.

[Jim]

I made my piston valve liners in three parts and from Bronze, as Malcolm says it's best to make the liners from the same material as the main casting.

The three parts consisted of the front and rear ends incorporating the inlet and exhaust ports with the third part being a central 'spacer'. All three parts were machined to a light press fit in the cylinder casting.

The photos probably explain it more clearly:

I machined the inlet ports as square openings to give a clean sharp closure

The completed end sections with the central 'spacer' shown in the drawing. When all is in place the spacer also closes off the open ends of ports to form the square inlet.

I machined a stop to slip into the valve casting so the spacer could be pressed down to accurately align with the inner edges of the inlet passages.

With the stop still in place to prevent any movement I then pressed the first of the outer ends in before reversing the job to press in the other end. I should add I made the two ends long enough to include the valve cylinder's projection out from the main casting. The final task was to run the cylinder hone through to ensure that the cylinder was uniformly smooth through out. Apart from all else I wanted to make sure no damage could be done to the PTFE rings I intended to use.

All this was done some years back now but I'm happy with the way it has all worked when I've had the loco in steam.

Hope this helps

Jim

[JonL]

Thats very helpful, thank you. I think I'll still use a one piece liner, but seeing yours has really helped me visualise it. Filing the ports is the bit I'm not lookinf forward to, but at least with cast iron it shouldn't be too onerous.

[JonL]

I've ordered some Meehanite bar; looking forward to machining that.

[Roger]

Jan 22, 2019 21:18:47 GMT JonL said:

I originally intended to use mild steel with PTFE lined bobbins as described elsewhere. I'll be perfectly honest I don't know what the usual material of choice is in this scenario; I'm assuming Brass? I think its the route I'll go at the moment.

I would take Mike's advice, Mild Steel or Brass aren't ideal either. I've used Leaded Bronze for all of mine with PTFE piston rings, but those are likely to end up as Fluorosint for the valves since it has a much lower rater of thermal expansion.

[JonL]

I can't quite stretch to fluorosint! However I've ordered meehanite for the piston valve cylinders (valve chests?). I suppose the material used for the bobbin isn't too much of a worry as its not in contact with anything, if I'm using PTFE rings. The pistons (actual pistons not valve chest pistons!) are my next consideration; based on what I think people are saying I suspect I'll use cast iron with PTFE rings.

I really need to work on my terminology; I keep confusing the issue!

[Roger]

Jan 23, 2019 11:38:51 GMT JonL said:

I can't quite stretch to fluorosint! However I've ordered meehanite for the piston valve cylinders (valve chests?). I suppose the material used for the bobbin isn't too much of a worry as its not in contact with anything, if I'm using PTFE rings. The pistons (actual pistons not valve chest pistons!) are my next consideration; based on what I think people are saying I suspect I'll use cast iron with PTFE rings.

I really need to work on my terminology; I keep confusing the issue!

Be aware of how much PTFE expands, you're probably going to need at least a 1mm gap if not more. You might do better using CI rings.

[terrier060]

Jan 22, 2019 14:39:02 GMT JonL said:

Jan 21, 2019 22:32:57 GMT terrier060 said:

...The Tech guy at Permabond is worth ringing. He is really helpful and has suggested a couple of adhesives to use on my Terrier cab...

You were not wrong Ed, they really were helpful. Based on what I told him he suggested ET5401 but says he will email me with more information once he has gone through the specs to make sure its the best product for my application.

Thats how to get repeat business. I even started off by saying "I'm a model engineer, I'll be buying the smallest tube of this product I can" but they treated me like a valued customer.

EDIT I've looked at the spec for the ET5401 and I don't think it will be suitable as it breaks down at too low a temperature, but we shall see what he comes back with.

Glad he was able to be of help.  I am about to try gluing my cab - watch this space.  There could be a lot of "told you so's" forthcoming if it falls apart and I have to solder it!

Regarding the piston rings - you could always use 'O' rings.  I have them in three of my locos and they are simple to fit with no blow-by.  Don't know why they are not used more - there seems to be some objection to them.  Maybe it is just too simple?

I only use a single ring in one groove.  After a lot of use there was a flat on the ring where it was in constant contact with the bore, but there was still no steam leaking past.  They need to be a good fit in the groove fore/aft, but floating with clearance on the inner diameter.  I thought that there may be frictional loss when pulling a heavy load or going up an incline, but never noticed it in practise.

[Roger]

I think the only real objection to silicone O-ring rings is they have limitations when it comes to superheating.

[terrier060]

Jan 23, 2019 19:16:26 GMT Roger said:

I think the only real objection to silicone O-ring rings is they have limitations when it comes to superheating.

Well my mine was super-heated but I cannot say for larger locomotives what the result would be - suck it and see would be my reasoning.  Rings could always be fitted at a later stage if necessary.  Would be interesting to know how many model engineers have used 'O' rings on pistons.

Standard silicone 'O' rings will stand temps of 230deg C some types go higher.  Temp of water at 100psi is about 150deg C.  I doubt if super-heat on models will reach 250.

[JonL]

As you say, it's not like they are buried and hard to remove; I could change them to whatever suits later. I'll look into those and cast rings.