Piston vs. Caprotti Valves 5

Piston vs. Caprotti Valves – The Final Discussion? Part 5

See previous page for Wardale’s responses to John Duncan’s Comments 17 to 20.
See next page for Wardale’s responses to John Duncan’s Comment 25.

Note: The full text of this correspondence can be downloaded in PDF format under the title:
Wardale responses to Caprotti proposals 3 Sept 2009.


John Duncan’s Comments # 21 – quoting Wardale: “Lastly (from the view of the projects aim, not least). Walschaerts gear is aesthetically more attractive.”

Well! What can I say?  Cost and efficient use of steam from the boiler is the aim, not the looks of Walschaerts Valve gear.

Wardale’s response: “Cost and efficient use of steam from the boiler is the aim, not the looks of Walschaerts valve gear”. Firstly, the piston valves and valve gear as specified for the 5AT will give as efficient use of money and steam as Caprotti valves, if not better. Secondly, the looks of Walschaerts valve gear do matter, to me at any rate. The purpose of the 5AT is to keep the aesthetic spectacle of main line steam in the UK alive. Full stop. That was the only reason for proposing it. If members of the 5AT group seem to have lost sight of this, or perhaps never had sight of it, that is their misfortune, as it causes them to go in wrong directions. But I have always worked for steam with the sole purpose of keeping it alive as an aesthetic spectacle, and all other things (engineering, efficiency, economics etc) have only been tools to this end. So I don’t lose sight of the aesthetics, which is why, for example, the front streamlining does not extend down to near rail level like on the DB 10 class, as it should do for good aerodynamics. But it simply makes it look wrong, in my eyes, and it’s my eyes that count, up to now anyway. Walschaerts gear does look better than Caprotti, the awkwardly-angled drive shafts beings very unaesthetic. And in accordance with the raison d’être of the 5AT, this is a reason to prefer it. Fortunately the engineering, performance, and aesthetic reasons coincide.

John Duncan’s Comment 22:

I remember as a fitter on steam locomotives, the amount of wear on the die block and in the expansion link with a mixture of oil and brake block dust in the normal cut off position on Walschaerts valve gear. Not as much on Stephenson expansion links on ex-GW locomotives with the valve motion inside the frames of the locomotive.

Wardale’s response: Amount of wear of dieblock and expansion link. No longer an issue, with the application of mechanical lubrication to these parts, as per SAR locomotives.

John Duncan’s Comment 23:

I went out in 1960 as relief fitter from Chester MPD to North Wales depots on the footplate and rode on LMS and BR class 5’s, both piston valve and Caprottis. I noted on the piston valve locomotive was driven in first valve on the regulator and 30% to 35% cut-off, if notched up to less than 20% cut-off and the regulator opened to `full’ resulted in a lot of vibration from the motion. Coasting with the regulator `shut’ a knocking noise was set up in the big and little ends, so the Driver had to crack open the regulator to stop this happening. The driving technique on the Caprotti fitted locomotives was different.  Once under way it was ‘full’ regulator and a fine cut­off sometimes a low as 3% to 5%. Every exhaust beat could be heard, sharp and clear. No banging or shaking on the footplate and just the noise of the rail joints when coasting. It’s called ‘expansive working’, save coal, water and cash!

Wardale’s response: Piston valves are no barrier to short cut-offs, see (14) earlier. Do you think Gresley Pacifies were not worked at less than 20% cut-off? Please reread C. J. Allen and 0. S. Nock. Do not extrapolate a problem with the engines you rode on to include piston valve engines in general, and certainly not the 5AT. No knocking of any kind can be guaranteed from shopping to shopping when roller bearings are fitted throughout and Franklin wedges on the driving and coupled axleboxes. No loss of exhaust beats even down to the smallest cut-offs with Porta type long lap valves (experience with 3450, and even 19D 2644 which had only elementary alterations to the standard SAR piston valves). “Every exhaust beat [on a Caprotti engine] could be heard, sharp and clear” – and probably pulled the fire to pieces, on 71000 anyway (sorry for that one!) I do know what ‘expansive working’ is: even normal piston valves have delivered it in the past.  Nevertheless Porta valves with longer lap than prevalent in former times greatly increase their capacity to do so.

John Duncan’s Comment 24: Passage photo-copied from Chapelon’s Locomotive a Vapeur as follows:

Poppet Valves are necessary:

  1. On locomotives with very high superheat to avoid the necessity for lubrication of admission ports of piston valves and thereby to protect the oil from cracking effects and to allow it to retain its functional value as a lubricant. For this reason it is simpler to lubricate a poppet valve locomotive than one with piston valves.
  2. Because the weight of a double beat poppet valve of the type now used is infinitely less, for a given cross sectional area of steam passage, than that of a piston valve even when the latter is of the lightest type in welded plate.

Now, when it is required to build a large compound locomotive with very large cross sectional areas through the l.p. valves this can be done, even on the largest locomotives, with poppet valves of which the moving parts weigh only 4-5 kg. whilst comparable piston valves would weigh more than 100 kg. Furthermore, the poppet valve only needs to be raised from its seat by about 30mm, equal to one eighth of its diameter, to provide the required cross-sectional area, whilst it is necessary to provide piston valves with a travel of the order of 200mm. This makes very clear the infinitely smaller inertia forces arising with poppet valves compared with those for piston valves.

Thus, with poppet valves one can achieve the largest cross sectional areas for steam passages to l.p. cylinders, even at the highest speeds, without fatigue in the distribution mechanism which cannot be achieved with complete safety with piston valves.

This has been confirmed clearly by experience, numerous incidents having occurred in l.p. valve gear with long travel piston valves or those of very large diameter, whilst such problems have not arisen with poppet valves.

The drop piston valve has the same advantages as the poppet valve from the inertia viewpoint, but it does incorporate valve rings and in terms of wear and lubrication is in the same situation as ordinary piston valves.

All recent experience has shown. that the principal improvements to the steam distribution systems of locomotives have been in the increased cross sectional areas open to steam in its passage through the valves.

Wardale’s response: Quoted passage from Chapelon, La locomotive à Vapeur.

  1. Piston valve lubrication is normalised at high steam temperature by lubricant feed direct to the valve liners and by valve liner cooling.
  2. Weight: what counts is the inertia load produced by a valve on the valve gear: it has been shown in point (10) earlier that this is low with the piston valves of the 5AT, hence the point is irrelevant in this case.
  3. Quote: “when it is required to build a large compound locomotive…” The 5AT is neither large nor compound, so again the arguments do not apply here. They may become relevant in the case of a large high-speed compound design, but a valid decision in this matter could only be made by calculations for both types of valve. In such a case there is the option of cushioning piston valve inertia, as investigated by Porta but not required on the 5AT. Chapelon gives 100 kg weight for a piston valve: the 310 mm diameter 55 mm lap 24 ring valves for 3450 weighed 67 kg, with rings and spindle, with perfect mechanical reliability. The two such assemblies per 5AT cylinder are calculated to weight 54.48 kg (FDC 4.(442)), so Chapelon’s figure simply does not apply.
  4. Chapelon’s further comments about the ultra large valves needed for compound l.p. cylinders do not apply as the 5AT is not a compound.
  5. The last sentence in italics is agreed with, and the design of the 5AT fully complies with it, due to having a collectively large circumferential valve liner port area, very long valve lap, good flow coefficient, and minimum cross sectional area of the valve liner bridge bars. Lastly, it can be noted that the French chose piston valves instead of poppet valves for their last express steam class, the (large, compound) 241P’s of 1947.

See previous page for Wardale’s responses to John Duncan’s Comments 17 to 20.
See next page for Wardale’s responses to John Duncan’s Comment 25.