Member postings for John Baguley

Hi John,

Here are a pair I made for a chap at our club. They are for a 3½" gauge 75000 class loco which originally had plain stainless bobbins that leaked badly:

The valves consist of a central bobbin made of bronze, stainless, or even brass. The heads are rings of PTFE held onto the bobbin by threaded caps. You machine the PTFE heads to a push fit in the liners and the fit sorts itself out as soon as you steam the loco.

The bottom photo shows that I altered the method of valve adjustment to a much better method. The originals were adjusted by altering the nuts on the threaded valve rod which is a right pain to do. In my method the valve floats on a central 'bush' threaded onto the valve rod. To adjust the valve you just rotate the bush and secure it with a locknut. Think I got the idea from an article in an old ME

There's a bit of a description of these valves here:

**LINK**

Unforunately, I didn't take any photos of the bobbins before I assembled them but hopefully you get the idea. I might have done a CAD drawing before I made them, I'll have a look.

It might be an idea to see if you can check the valve liner bores to make sure they are truly round and parallel. It's possible they may have distorted when they were fitted to the cylinder block? PTFE heads will accommodate slight ovality etc. in the bores though but better if they are true.

John

Edited By John Baguley on 07/09/2015 22:14:16

Hi John,

There's a very good chance it's the piston valves leaking. It's very difficult to get plain metal bobbins to seal well enough. Even if you machine them to as good a finish as you can and a good fit, the high spots soon wear off and the valves soon start to leak. Don's method is to make them a very tight drive fit, coat them with Molybdenum Disulphide greae, force them into the liners and keep knocking the valve from one end of the liner to the other until they become free. The idea is to remove all the high spots and leave the liner bore and the valve bobbin with a very smooth surface. I have to say I tried that some years ago but didn't have a lot of success with it!

I've given up with solid bobbins now and use PTFE heads with complete success. The only problem is that once you run on steam the valves will leak if you try and run the loco on air cold. No problem on steam though.

There are another couple of possible causes. The piston packing may be leaking allowing steam to blow past or the valve liners may not be sealing into the cylinder block and allowing the steam to pass straight from the inlet to the exhaust.

John

Hi Bazyle,

Think it's the Tin Turtle built by Des Adeley that's hopefully going to be 'serialised'.

John (that JB)

I hope we are not divided, just catering for the different aspects of 2½" gauge

John

Just to add to Ian's post here's the wheel standards used by the N25GA:

As we run mostly on passenger hauling raised tracks our standard is a bit coarser than that required by the G3 Society as we don't need to run through points etc. The width of the wheel tread is wider than the G3 standard, which can be useful when running on some raised tracks which are a bit over gauge in places! Some people with G3 standard wheels do seem to have problems running on some raised tracks as the wheels sometimes tend to fall between the rails due to the narrower treads.

John

I give up! I'll go and do some gardening instead

John

Just had a go at drawing it out and it seems impossible. To me, there seem to be some vital dimensions missing. You really need the position of the cylinder bolting face from the datum point. You can work it out from the drawing for the base. I reckon it should be 60mm. That makes the length of the slope at the datum end to be 32.45mm.

Can't understand why it's been dimensioned like it has. Don't like to be critical but It's a dreadful drawing.

John

Edited By John Baguley on 22/07/2015 16:23:51

The cylinder centreline should be 62mm - 20 + 17 + 40/2. That is also the dimension given for the height of the crankshaft centre.

The co-ordinates do give X and Y - the decimal points should be commas! Yes, C1, C2, C3 are the centres for R1, R2, R3 and C4 is the crankshaft centre.

The 62mm length for the cylinder also seems wrong as it's much longer than the height of the cylinder centreline. Maybe the drawing has been distorted in printing?

John

David,

Just a thought - have you got the eccentric rods connected to the expansion link the right way round. You may have got the forward and reverse rods crossed over on one side.

Out of interest, which loco is it?

John

Fizzy,

Don't forget that the pressure the pump actually needs to produce is proportional to the area of the ram. A 1/8" diameter ram has a cross sectional area of only 0.0123 sq. inches so to produce 100psi the pressure on the ram only has to be 1.23psi so the pressure trying to lift the cylinder off the port face is very low. Think that's right anyway!

Most good lubricators will easily produce several hundred psi. The biggest problem is getting the ram to be a good enough fit in the cylinder bore. I like the Jim Ewins type where the ram goes through two O rings which seal perfectly.

The LBSC oscillating type usually have a packed gland on the cylinder to seal the ram.

Whilst the ram may seem to seal perfectly when the oil is cold, as soon as it gets hot they may leak like a sieve!

John

Edited By John Baguley on 21/07/2015 18:26:30

Edited By John Baguley on 21/07/2015 18:27:57

Ron,

Yes, one side of the superheater comes out of the header on the front tubeplate and the return goes into the dry header that feeds the cylinders.

If you can, TIG weld the joint in the firebox, but if not, something like Silverflo20 or 24 will be ok. Silverflo55 is fine for the smokebox ends.

John

Hi Ron,

The B5 superheaters are what is known as coaxial type. They consist of a 1/2" diameter stainless outer tube with a 5/16" diameter copper tube running down the middle. The outer tube is blanked off at the firebox end and the copper tube doesn't quite reach the end. The steam from the wet header travels down the stainless outer tube and returns up the inner copper tube to the dry header.

Frankly, the coaxial type of superheaters are not very efficient and you would do better replacing them with the conventional type with two separate tubes jojned at the firebox end, the sort you are used to. Extend them over the fire right to the back of the firebox and you will get superheat worth having.

Here's a cross section of a coaxial type that I drew up in 3D for Neville Evans's Didcot that I've got all the castings for.

This is slightly different in that the end of the inner copper tube is supported by the blanking plug in the end of the outer stainless tube and the steam enters via a slot in the end. I won't be fitting one of these but the normal two element type instead.

John

Edited By John Baguley on 20/07/2015 18:20:29

Ron,

Have sent you a pm.

John

Hi John,

Both are 80psi. LBSC mentions testing the boilers to 160psi in the final part on construction of the boilers. I think the majority of his boilers were 80psi WP.

John

Vic,

That sounds like Altro flooring. It has carborundum embedded in it to make it non slip. It's used behind pub bars etc.

I used it for my workshop floor. Managed to get a big enough offcut from Ebay for a very good price. Only problem is it's light blue but most of it's now covered by machines and rubber mats so it's not too bad

It's best glued down if possible to stop it creeping which is what I did.

John

Edited By John Baguley on 08/06/2015 19:03:47

Hi David,

It sounds more like underexposure of the resist. Try exposing it for longer? Does the developer come ready mixed or do you dilute it yourself? Might not be strong enough. When I last used the photo resist method the developer I used was ordinary sodium hydroxide solution.

John

Hi Ray,

Ok will pm you.

Just to wet your appetite, here's a photo of a very nice chassis being built by David Williams taken last Sunday at our 40th Anniversary Rally at Cheltenham.

John

Hi Ray,

The valve gear as drawn by LBSC is very poor and I would recommend using the updated design by Don Ashton which was described in Engineering in Miniature January 2008. I can let you have a copy if you don't have access to it. I also redesigned the valve gear independantly of Don as I didn't realise he had already done it! Both new designs give much improved valve events.

You will need to update the boiler design by fitting bushes for all the fittings. On the drawings the blower valve and water gauge screw directly into the backhead and the top feeds screw directly into the boiler barrel which is not allowed now.

I started drawing PG in 3D CAD and found a few errors so far. The hornstays are shown as 3/8" wide but should be 5/16" to match the horns. Also if the front bogie stretcher is fabricated using brass angle, the angle obscures the bottom cylinder mounting holes in the frames.

John

Hi Fizzy,

Have a look at the Carrs range of solders as used for building etched brass kits. Some of those are low melting point and I've used them in the past for various jobs including soldering tanks and tenders. I got my stock from C&L Finescale.

**LINK**

John

It sounds like the one described in an article in ME Volume 103 Issue 2586 (December 12th 1950). I've got that issue if you can't get hold of it.

John