valve events

hi in posetion of a 5 inch king is there any body out there who can explain

or give me any tips on checking the valve gear and fine tuning

it does run on steam but should i be able to here beats up the chimney

Hi Robin,

As you are probably aware the GWR Stars, Castles, and Kings have inside Walschaerts valve gear operating the inside cylinders and their piston valves.

The outside cylinders have their valves driven by cranked rocker arms worked off the valves to the inside cylinders.

The expansion links are very difficult to access, as is most of the internal valve gear without a major strip down of the whole loco. There is a substantial horizontal frame stretcher on the top of the frames which contains the expansion links and the intermediate weighshaft (which is split into 2 sections).

I presume your acquisition has piston valves, which are not easy to set accurately, and in any event there are peculiarities with the gear geometry that require subtle non centralising of the valves to get all the cut off and exhaust positions the same, compromising the admission points somewhat.

Any slack in the inside gear makes setting the outside piston valves difficult to achieve with accuracy.

Checking the valve settings the proper way is very involved on a GWR King, but essentially the same process is used as you would on a simple 2 outside cylinder loco with Walschaerts.

The complications arise because the expansion links are pretty much hidden, even with the boiler, smokebox, and front bogie off, and checking the correct setting of the 2 eccentrics and the correct length of the eccentric rods is similarly inaccessible.

If you say the loco runs on steam, I would give it a decent load on the track and listen.

You ought to have 4 distinct beats from the exhaust, each side of the loco exhausting at the same time, and you ought to be able to notch up the gear on the reverser to at least 25 % cut off with the beats still being loud even and clear.

If the beats are ok and you can notch up the gear then I think I would leave well alone. If however you are getting a 'whoosh whoosh' then there is probably something wrong with the piston valves leaking. (There is a further fullsize proceedure that can be used in miniature to check this further).

The exhaust pipes and passages on the GWR Kings are rather complicated and in miniature I have yet to see a 5"g example that really sparkled as it should, and with the distinctive GWR 'bark'.

Do not start to dismantle the loco unless you are absolutely sure what you are doing, and are absolutely sure you will be able to put it all back together again!

Cheers,

Julian

thanks so much Julian very informative and you have confirmed my suspicions we are getting woosh woosh

the valves are p t f e on gun metal bobins which i have copied on the advice of a well respected model engineer

from the club but made a questionable better fit this engine is a known good runner but needs some t l c

we have inspected the motion work and all appears correct nothing has come louse apart from one of the wheels which was soon sorted when i got it just wouldn't run on air but since the valve replacement she runs on air for about ten strokes before stoping (never thought this would be a walk in the park)

robin

“the valves are ptfe on gun metal bobins which i have copied on the advice of a well respected model engineer from the club but made a questionable better fit this engine is a known good runner but needs some tlc”

'have hesitated some while b4 posting, not wishing to bring down opprobrium from the experts and anyway it will contribute nothing directly to your plea for help in checking the valve timing.

Ah well, flak jacket on………. I’d now advise anyone with CI cylinder castings not to use PTFE for piston valve “rings”. The misfortune of my having GM castings precluded CI rings/bobbins and the significant thermal expansion of even the mica-filled PTFE rings caused major problems. If they are turned to give a sliding fit at room temp, they will tighten up markedly at steam temps and can creep laterally.

A recommended remedy is to turn them to a sliding fit at expected (?) steam temp by appropriating the domestic oven when SWMBO is out for the day. Consequence of so doing is that they won’t seal effectively at room temp. You then (probably) can’t run the new build on air and the loco’s performance with steam is atrocious until thoroughly hot. Altogether an imprecise solution and not just my experience, notables such as Doug Hewson report similarly.

A solution for the main cylinder piston rings has been published by Artisan on David Carpenter’s website (http://modelengineeringwebsite.com/) necessitating some complex calculations and machining. With the author’s help, I’ve remade the piston rings and will attempt the same for the valve rings, using PEEK CA30 instead of loaded PTFE.

Though there are undeniably many builders who are very happy with PTFE & PTFE/composite rings, there are some salutary statements about the materials use in DuPont’s booklet “Filled Compounds of PTFE” especially concerning wear rates, abrasion of the contact surface and lubrication.

**LINK**

You sound like a clever bloke Mr Fowler just a point this engine was a known Good runner hauling passengers regularly

With p t f e valve bobbins with o rings inside them perhaps to compensate for the expansion clever?

But that’s why we are on this forum to gleen answers from them that might no better I will take your comments on board

Many thanks for your time most us full

Robin

Hi Robin,

Fowlers Fury is quite correct that ordinary PTFE piston valves will leak on air, but should be ok on steam when the cylinders have warmed up.

Note also that I do not use steam oil when running on air, and use Singer sewing machine oil to lubricate the valves and pistons.

On the modeleng.proboards.com forum site, Roger Froud has developed a way of making piston valves with PTFE and 'O' rings underneath that seal on air and steam.

Cheers,

Julian

Thanks for your comment about the o ring system it makes sense to me been making them boar size less 5 thou what do you think ?thats at a room temp of around 18 oc would welcome your advice

Been making a revised set for one of the outside cylinders today still over size at the moment just wondering if I should make them mor of a snug fit welcom you advice

Robin

Julian; not been on modeleng.proboards.com for a while, but, if it matters, I have a locomotive with CI cylinders, and piston valves with teflon rings, "O" rings as backers.

Works well, except that there is a timing issue on one side so can't notch up - *that* was scheduled to be fixed in 2017... (best laid plans) It has been running on teflon/"O" rings for 20 or more years now.

Anyway, not that it matters much, as the choice is yours, but another check mark for "like" the idea.

Not sure if what I have matches what Roger did, but it must be fairly similar.

JohnS.

"Been making a revised set for one of the outside cylinders today still over size at the moment just wondering if I should make them mor of a snug fit welcom you advice"

Robin ~ Are you referring above to making piston valve rings or piston rings?
I'm probably the last one to be offering advice, given the mistakes & failures during my (3^rd) loco build. But when I went through the tedious, ⁴/₃𝛑r³s-aching process of heating the block in the oven; shaving a thou off the piston rings, reassembling, reheating, re-turning ad nauseam, I found I'd had to take 11 thou off the full, room temp diameter of the ring. That was for 1.5" ID cylinders estimated to be at 170C when running in order to get a sliding fit. However, as I posted above, I was so dubious about it all I scrapped those rings and instead used "Artisan's" method (modified for PEEK CA30) and hopefully his technique will provide running on air and seal at operating temp. Again, I'm going to adopt the same procedure for the piston valve 'rings'.

Julian has made reference to "Roger Froud's" approach. I have watched and read the 13 pages of postings on piston rings over the 12+ years:-
**LINK**
and have admiration for the contributors' wisdom therein. But there are so many postings offering different experiences and recommendations, that I just end up wiser but confused. You may not be, so do have a read.

If you do go for complete rebuilding of the piston valves, it might be worth considering the advantages of the design published by the respected Geo Thomas. This greatly facilitates adjusting the valves in situ and the construction is quite straightforward. (Model Engineer, 16 May 1975, page 503).

Greetings, I have just taken out a set of PTFE piston valves from my A3, which was starting to "wheeze" quite badly after 8 years.

These were made as per the Geoff King method described in ME about 10 years ago. A spring pushes a tapered brass plug into the solid PTFE bobbin. They were badly scored, and I have for the moment replaced them with iron rings.

I think that if I used them again, I would use carbon filled PTFE.

Regards

Brian

I have seen many reports that carbon-filled plastics wear 3D printer nozzles out very quickly. I would be wary that carbon loaded PTFE could shift the wear to the valve cylinder.

Neil

You are probably referring to carbon fibre loaded plastics where the fibres improve the tensile strength and wear resistance, eg for gears where you may have both carbon fibre (for strength etc) and PTFE (for lubricity). You can also load plastic with carbon black etc which isn't mechanically corrosive, generally for colour and opacity.

Murray

Message to all many thanks for your input put the king back together tonight going to steam again

Saturday and see what we’ve got👍🙏