Piston vs. Caprotti Valves 3
Piston vs. Caprotti Valves – The Final Discussion? Part 3
Note: The full text of this correspondence can be downloaded in PDF format under the title:
Wardale responses to Caprotti proposals 3 Sept 2009.
Note: John Duncan’s comments 11 to 15 relate to a statement in Wardale’s Fundamental Design Calculations for the 5AT, saying:
“Because bending stress due to inertia load at maximum speed is greater than maximum allowable crankpin fibre stress, cushioning must be provided to relieve the inertia load on the main crankpin. Note that this is one reason to reject Caprotti valve gear as finally applied to BR locomotives, as the valves were arranged to drop from their seats during drifting thus providing a full by-pass from one end of the cylinder to the other, precluding the build-up of cushioning steam pressure. The required cushioning steam pressure at dead centre is now calculated for the worst case condition, i.e. maximum speed with minimum coupled wheel tyre thickness, this pressure being therefore suitable for all lower coupled wheel rotational speeds …..” [see FDC 3 line 137].
John Duncan’s Comment 11:
Ans 1: Reference “Locomotive Management from Cleaning to Driving” by Jas. T. Hodgson M.I.Mech.E. sixth edition. Page 221. Quote:
“PISTON VALVES are almost invariably adopted by superheated steam locomotives, and particularly when these are of the non-collapsible pattern, an objectionable knocking is set up at the crosshead pins and the small and large connecting rod bearings when running without steam.
Knocking at the crosshead pins is caused by the inertia forces of the reciprocating parts being taken up by a cushion of air compressed behind the piston towards the end of the stroke, which cushion of air is suddenly released when the port opens to admission, thus throwing the inertia forces on the crank and crosshead pins on the opposite side to that on which they had before been applied when the moving parts against the air cushion.”
It goes on to explain that to overcome this by the following:
By-pass valves, The LM&SR (Midland Division) fitted by-pass valves to their class 2P and 3P, 4-4-0’s for the long downhill runs on the Settle and Carlisle route. On the LM&SR (Northern Division) the ex. Highland Railway fitted by-pass valves under the cylinders of their locomotives for free downhill running on the Inverness to Perth mainline.
Automatic By-pass Piston Valves. Five locomotives of the ex. GCR class 11E after they became LNER class D 10 4-4-0s, were converted to Trofinoff automatic by-pass (TAB) piston valves, (outside admission and short travel) between 1935 and 1938. The valves were 10″ in diameter.
In November 1948 during my engineering apprenticeship on the valve section of the machine shop in Gorton Locomotive Works BR(E), I worked on overhauling a pair of TAB piston valves. The piston rods had a constraint in the middle and the piston valve-heads slid on to the spindle from each side. When steam was applied to the cylinders the two heads were pushed on to the centre constraint. When the regulator was shut the heads moved to each end of the steam chest allowing each side of the piston to be connected, giving full bypass with free running. At that time 62653 was being fitted with new cylinders and I had the job of trying the piston valve heads with rings fitted in the new valve liners in the Erecting Shop.
British Caprotti poppet valves operate as by-pass valves when the regulator is shut.
All above is the opposite of Wardale’s statement in FDC 3, item 137, on piston valve locomotives without a by-pass, the regulator has to be cracked open when running downhill, and they are not ‘free running’.
Wardale’s Response: Drifting: Your quote from ‘Locomotive Management’ (a book not anticipating 200 km/h locomotives): ” … a cushion of air…”. Air must definitely not be present in the cylinders as it oxidizes cylinder oil. It is one function of drifting steam to ensure this. It is claimed that knocking is overcome by by-pass valves, of which there are a great many designs, most of them ineffective (including Trofinoff valves, used on Chinese locos and a very poor design.) But on a locomotive with roller bearings on the crossheads and crankpins, as the 5AT would be, there is no knocking. 3450, with Porta-type piston valves and no by-pass valves, gave no knocking at the crossheads, crankpins or axleboxes, from one overhaul to the next. So this point does not apply. Caprotti valves as finally used on BR do operate as by-pass valves with the throttle shut: but (1) you yourself have told me that big-end wear was greater on Class 5’s with Caprotti gear than with Walschaerts gear, suggesting the benefit of compression in relieving the inertia load on the crankpins, and (2) as FDC 3(137) correctly says, cushioning must be provided on the 5AT, and the drifting steam is necessary to achieve this (FDC 3 (148)). Put another way, if Caprotti gear gave a full bypass, the inertia load at maximum speed on the 5AT would overstress the main crankpin and probably lead to rapid fatigue failure. The criteria for running at 200 km/h are much more severe than those for the examples which you give, and for the 5AT cushioning is not an option, it is a necessity. Drifting steam and the absence of a by-pass are therefore also necessities. The comment about the design not being `free-running’ is irrelevant in this context, as even if Caprotti valves were to be used (1) they would have to be arranged to provide no by-pass and (2) drifting steam would have to be supplied to give cushioning, i.e. the same as with piston valves.
John Duncan’s Comment 12: Wardale goes on to state …….. other reasons are:-
Cost; (See previous answer). Initial first cost is 10% more than the piston valve B.R. Std. Class 5 for 30 locomotives out of the remaining 142 piston valve locomotives, a small cost for the substantial savings made on the 30,000 to 35,000 miles piston and piston valve examinations.
With British Caprotti poppet valve gear the inspections were done at major overhauls in the main workshops – at 180,000 to 201,000 miles in the case of the BR Std. Class 5 73125 to 73154.
Wardale’s Response: ‘Higher initial cost of Caprotti gear is a small price to pay for the savings made on valve examinations at 30,000 – 35,000 mile intervals. Caprotti inspection was done at major overhauls at 180,000 – 201,000 miles.’ Your argument is not valid because it is based on BR piston valve performance, not that which will be achieved by the 5AT. Point (2) earlier shows that the Caprotti inspection period you give can be achieved with piston valves, so if any extra cost were to be incurred for the Caprotti gear it would bring no benefit in this regard.
John Duncan’s Comment 13:
Savings were made operating in ‘free running’ downhill.
Wardale’s Response: Savings due to ‘free running downhill’. Point (11) above shows that drifting steam would be mandatory on the 5AT, whatever type of valve were used. ‘Free running’ will have placed the full reciprocating mass inertia load on the main crankpins, and this will have been at least a contributory factor in the higher wear of the big end bearings that you observed on the Caprotti Class 5s, thereby diminishing by increased maintenance cost any saving made on the road.
John Duncan’s Comment 14:
Savings were made with full regulator at cut-offs as low as 3% to 5%.
Wardale’s Response: ‘Full regulator at cut-offs as low as 3% to 5%’. Full regulator, very short cut-off working was also used with (first generation) piston valves, e.g. A4s did much of their work at 10% cut-off or less. Porta has shown the features necessary of a piston valve to obtain good results at short cut-offs, features not found in BR valves, so ultra short cut-off working is practical with such valves – I have driven 3450 myself in mid gear (5%) on a 570 ton passenger train. Due to unwanted heat transfer effects (i.e. condensation) such very short cut-offs did not produce the most economical working with FGS. However the 5AT cylinders as a whole will have design features to maximise the benefit of short cut-offs. Ultra-short cut-off (say < 10%) inevitably produces relatively little power, and as the 5AT would be expected to operate mostly at high power to maintain high speed, such cut-offs would probably be used for only a small fraction of the operating time.
John Duncan’s Comment 15:
Savings on lubricating oil costs were considerable compared to the piston valve locomotives of the same class. I did a study on this subject for Mr. Rhyll, the District Motive Power Superintendent of the 6A District in May 1960. The piston valve locomotives were 28% higher than the Caprotti locomotives in cylinder oil consumption.
Wardale’s Response: Lubricating oil saving. This is accepted, but even here there is a reservation, in that oil fed directly to the cylinders tends to be ineffective, much of it going straight to exhaust, the cylinders relying a good deal on valve oil carried in inlet steam for their lubrication. Therefore a low valve oil feed may result in higher cylinder liner and piston ring wear.
John Duncan’s Comment 16:
Quoting Wardale: “It is specialist equipment”; Not on British Railways. If steam locomotives had continued, the British Caprotti valve gear would have become standard for most large locomotives, with standard cam boxes & poppet valves.
Wardale’s Response: ‘Caprotti valves not specialist equipment on BR.’ Answered in point (3) earlier: your point is based on misunderstanding the use of the word ‘specialist’.