Britannia 70047 "Derek Stokes"

[Roger]

Jan 24, 2019 12:22:46 GMT terrier060 said:

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.

If you look in this steam table you can see that the temperature at 100psi is nearer 170C, so it wouldn't take much superheating to reach 230C. In reality, I think superheaters take time to heat up, there probably aren't enough of them, and the cylinders take ages to warm up too.

Also, the pressure in the valve chest is rarely going to be anywhere near boiler pressure to for a sustained period, and that will only happen if the locomotive can be notched well back. During most of a run, the pressure will be very much less, with an associated drop in temperature.

For most club locomotives doing a single lap and then stopping, I doubt if you ever get even close to the terminal temperatures that the design could potentially reach in continuous full throttle running with a big load.

I agree, you'll almost certainly get away with it because of these things.

[JonL]

So I have followed the advice of the forum and machined liners which just slip into the bore with an overhang each end (as per Roger's advice about being able to set the liner in place before the adhesive sets). I have kept one end solid and full diameter in a pathetic attempt to keep the whole thing a little more rigid. I've used Meehanite (as per Malcolms advice).

Next stage is to set it in place, but I've just remembered that I think LBSC says that the Valve liners need to be set in place with solder, so I will fit the cylinder liner after that is done so I don't blast the adhesive with soldering heat.

Thanks to everyone for their kind advice.

[JonL]

I've had a look at the Permabond adhesive they specified but sadly I think they have suggested something not quite suitable. It says it breaks down to 20% of its normal strength at 150 degrees C. It was F202 for anyone interested. I made sure to give them all the info, I think they just overlooked that.

[mbrown]

In this situation, the Permabond is little more than sealant. There is little or no force trying to separate the liner from the cylinder block as it is "trapped" in place by the covers. There are no differential expansion issues with a meehanite liner in an iron block. Personally, I would regard the loss of grip as insufficient to cause problems in practice.

My LYN has a false top fixed to the cylinder block with the ports cut in it enabling me to mill the passageways in the top of the block. That plate, on which the valve works, is secured with 6 x 6BA CSK screws and sealed with one of the Loctite grades (can't remember which). There is only Loctite sealing around the port bars and there has been no leakage.

It's not a precisely comparable situation, but I don't think you should worry. Soldering cast iron isn't that easy - LBSC was writing about bronze castings.

If course it's your call as you've got to be happy with it.

Good luck.

Malcolm

[RGR 60130]

If you keep to the 'as designed' pitch circle diameter (PCD) of the cylinder end cover bolts, will the bolts intrude into the sleeves? If so this can be used to your advantage as the end cover bolts will help hold the sleeves in place. By overlapping the body of the cylinder casting and the sleeve they will stop the sleeves rotating. The thread form will also secure the sleeves longitudinally. The sealant's primary function in this scenario would be to prevent steam travelling down the outside of the liner.

Reg

[JonL]

Good point Reg. I had intended to move the PCD out but I'd rather not. I'll look at where the studs will fall.

Malcolm, I am always interested in the simplest route to a good solution! I've emailed Permabond and await their reply. I don't really want to spend £25 on a bottle of something which doesn't help,hopefully they either have a better solution or their solution would be ok as you suggest.

EDIT: Also I was wrong about LBSC, he says to make the valve liners a push fit and press them in.

[JonL]

Permabond have responded with the suggestion of using HM162, which is much more temperature tolerant. It does have a maximum fill gap of 0.2mm, which I hope I've achieved!

HM162 Datasheet

At £25 a bottle its not unreasonable I don't think, I'll let you know how it goes. In the mean time I need to make a guide tube to allow me to wind the cylinder sleeves in to the correct place once they are complete.

[JonL]

I spoke too soon. £40 once shipping and VAT applied! I didn't realise they applied VAT after postage costs either. I wish websites were more up-front about their pricing. The postage cost only came up once I went to pay (understandable I guess) and the VAT was only added on after card details entered.

[JonL]

More work on the piston valve sleeves tonight.







Next job is to tidy up the exhaust ports with a file, but I won't go too mad until the reaming has been completed.

It only occurred to me tonight that before I fit the sleeves I probably should drill and tap the holes for the cylinder covers.... that would have been a lot more difficult had I forgotten!

[David]

Looking good! What's the flat on the outside of the ring of holes in the first photo? Does it allow the steam to get out to the exhaust circuit?

[JonL]

Almost, the ring of holes is the port to the cylinder, and the flat is as you say, to allow the steam to get between the valve and the cylinder

[JonL]

Tonight I've set my cylinder liner in place using the Permabond. Waiting now to see how that pans out, but I've followed the instructions to the letter, we shall see. I've got high hopes.

Before I did that I test mounted the Cylinders in their final position, drilled and tapped the mounting holes. I had intended to nut and bolt the cylinders but looking at the thickness of the frames it was easily meaty enough to take a 6BA tap. I managed to snap my tap; my fault this time, I can't blame the previous owner! Fumbled whilst withdrawing the tap. Luckily it left enough meat to spin out, and the thread remained undamaged. Mocked up in this photo, almost looks like a locomotive with the Smokebox laying in place!

[JonL]

Made a dummy piston cover as a template for drilling the bolt holes for the cylinder end caps tonight. Trying to pick a PCD which suited both (with and without liner) was fun, but I think I'm sorted now.

[JonL]

Gently filed the liner flush with the cylinder tonight, very happy with the tolerances I managed to achieve. Apparently the adhesive only fills up to 0.2mm, I think I managed that.



Used my dummy cylinder cover to spot the holes for the end cap bolts, using the dummy cover to carefully maximise the space between bolt holes and steam passages etc. 



I've started making the end caps, I've machined them all to the correct diameter before getting summoned from the workshop to watch undateables... I don't think LBSC had such distractions!

We edge ever closer.

[Jim]

Just as a suggestion I always discretely mark the cylinder covers with a matching mark on the body of the cylinder so that when replacing the covers the holes match up as there does tend to be very small variations that creep in as you machine and drill the covers.

As I say it's just a thought.

Jim

[JonL]

It's a good thought,thanks. I tend to centre punch parts with a number where there are more than one. In this case there needs to be a small area relieved with a file to clear the valve chest; this should give me indexing.

[racinjason]

I would have made the drill jig a bit larger in diameter the milled a flat on it that can locate against the cylinder mounting flanges as well as the shoulder in the bore that would make it easier to get the holes all with the same orientation.
Cheers Jason

[JonL]

The intention was that it could be turned to give me the best clearance to the steam passages, something which isn't consistent.

[timb]

Hi Nobby,

Just read through the whole of this for the first time and I have to say this is some good work, you should be well pleased with what you ahve achieved so far!

Looking forward to the continuing reports.

Tim

[JonL]

Thank you Tim