A History of Lentz Valve Gear on British Steam Locomotives

The following article is reproduced here with the kind permission of Richard Carr. It was originally written as a part of his Paxman History Pages. Formatting has been adapted slightly to match that of the LNER Encyclopedia.

Background

As speeds and boiler pressures of stationary steam engines increased at the end of the 19th century, and superheating was introduced, the limitations of slide valves and other valves then in common use became apparent. To overcome their deficiencies Dr Hugo Lenz (1859-1944) of Austria developed his poppet valve system and other innovations. The first Lentz engine was shown at the Paris Exhibition of 1900 where its inventor was awarded the Gold Medal. Lentz valves were subsequently applied to steam engines in power stations in Vienna and Rome with considerable success, achieving substantial reductions in coal consumption (1). Paxman obtained a licence to use Lenz's inventions and commenced building Paxman-Lentz steam engines at Colchester in 1907. It continued to build horizontal stationary engines of this type until 1934.

(The equipment invented by Dr Lenz does not bear his actual name: it was always called 'Lentz' as this was thought easier for English-speaking people to pronounce.)

Locomotive Valve Gear

The performance and efficiency of steam locomotives, and indeed of all reciprocating steam engines, depend heavily on the valves and valve gear controlling admission of steam into, and exhaust from, the cylinders. Early locomotives were generally fitted with slide valves. These functioned satisfactorily with saturated steam but not with highly superheated steam. The growing use of superheat during the early years of the 20th century led to widespread adoption of piston valves instead. However piston valves also have their limitations. They are costly, relatively heavy and require frequent attention. Variable cut-off results in uneven wear of the valve cylinder. Substantial mechanical force is required to operate them. Carbonisation of lubricating oil is a constant problem, particularly when high degrees of superheat are used.

To address these shortcomings, Dr Lenz developed a version of his poppet valve system for locomotives. This was first applied to an engine on the Oldenburg Railway of Germany in 1907. As first applied to locomotives, the spindles of Lentz valves were arranged vertically. In the later improved and simplified form of Lentz gear, the spindles were arranged horizontally which made the valves more accessible for maintenance. Perhaps because of the disruption caused by the First World War, few locomotive engineers seem to have given much attention to Lentz valves until the 1920s. By 1924, in Austria, horizontal Lentz valves had been adopted as a standard for all new locomotives and were being fitted rapidly to older locomotives because of the country's pressing need for fuel economy at that time. (2)

Paxman's Involvement and Overseas Orders

In about 1923 Paxman began to promote Lentz valves for locomotives. However, Paxman's licence clearly did not restrict the Company's activities in this new field to Britain. The first recorded Paxman order for Lentz valve gear for a standard gauge locomotive was for Holland. Ordered in January 1924 and despatched in February 1925, the gear was for a 4-4-0 express passenger engine of the Dutch State Railways. The locomotive's conversion from piston valves is discussed in the November 1924 issue of The Locomotive (3). The Paxman order book mentions "Three Sets of Lentz Valve Gear" so possibly three locomotives were converted.

The other overseas orders which appear in the Paxman copy order books are:

10 sets of Lentz Valve Boxes which were despatched to the North British Locomotive Company, Glasgow between April and July 1926. From other sources it has been possible to establish that these were for ten of an order for twenty 12th class 4-8-2 locomotives placed with North British for Rhodesian Railways (now Zimbabwe). The ten fitted with Lentz poppet valves were given Rhodesian Railways Nos 182 to 191. Their Lentz gear was replaced later by piston valves, but not before Paxman had received two orders for spares for them in 1928.
40 sets of Lentz Valve Gear for Spain, despatched in May and August 1929. Paxman microfilm records say that 30 sets were for Series 1400 locomotives and 10 sets for Series 1700. One wonders if this order stemmed from successful trials of an engine mentioned in the November 1924 issue of The Locomotive. Here it was reported that a powerful 4-8-0 belonging to the Madrid, Saragossa, Alicante Railway had been fitted experimentally with Lentz valves and had run for twelve months without attention. As a result "the railway company contemplates the installation of these valves to a number of their locomotives". (4)
Lentz gear for India. The order book entry refers solely to a "Cam Chamber" for Lentz Patents Ltd and the destination column is blank. However, Paxman microfilm records suggest the order was for a set or sets of Lentz valve gear, not solely a cam chamber, for an Indian State Railways 4-6-2 locomotive, XS2 type. This 1928 order ties in with an order (No 3636) received by the Vulcan Foundry, Newton-le-Willows in 1928 for two 4-6-2 XS2 type locomotives with Lentz gear for the North Western Railway (NWR) of India. These received NWR Nos 780 and 781. (At about the same time NWR ordered a pair of similar locomotives with Caprotti poppet valve gear for comparative purposes.)

In Europe, in addition to the Dutch and Spanish orders mentioned above, there was an interesting Paxman-Lentz French connection. In 1926 André Chapelon, the noted French locomotive engineer, put forward proposals for improving locomotive performance which led to a decision to completely rebuild a 3500 class Pacific of the Paris-Orléans Railway incorporating his ideas. In his book on Chapelon, Rogers writes "Because of the high superheat envisaged the locomotive chosen was one on which it had already been intended to try the Lentz poppet valve system, and for which arrangements had been made with Messrs Paxman & Co of London, licensees of the Lentz patent". (5) (The rebuild of No 3566 was highly successful and another 19 of the class were similarly rebuilt.) Elsewhere in a French language publication there is reference to Chapelon visiting England in 1926 for discussions with Paxman, possessors of the distribution licence for Lentz valves, about applying them to rebuilt P-O Pacifics. As an aside it is also interesting to note that during his visits to England Chapelon met with Nigel Gresley and Oliver Bulleid of the LNER who were themselves beginning to trial Lentz valves.

The references to the Paris-Orléans Railway's discussions with Paxman raise interesting questions about the terms of Paxman's licence to manufacture and sell Lentz valve gear for locomotives and about the actual nature of the Company's relationship with the licensor. In the Paxman copy order book the customer for the January 1924 order for Holland is entered as 'Paxman Loco Valve Gear Co'. Apart from this one brief reference we currently know nothing about this company. The customer for all subsequent Lentz locomotive equipment orders in the book is recorded as 'Lentz Patents Ltd'. We know that Lentz Patents was based in London and that it held the British patent rights to the gear. (Albert Howe, who had been in charge of Paxman's Gas Engine Drawing Office, went to work for Lentz Patents in 1925. In 1930 he returned to Colchester, becoming Paxman's Chief Diesel Engineer in 1940 and Engineering Director in 1954.) What were the realities of the relationship between Paxman and Lentz Patents Ltd? There are also other questions. The last entry in the Paxman copy order book for Lentz locomotive gear is dated 21 November 1928, but we know the LNER, for example, was fitting Lentz valves on locomotives as late as 1934. Did Paxman dispose of, or lose, its Lentz licence at the end of 1928? Alex Walford has a booklet entitled 'Poppet Valves for Locomotives' subtitled 'A General Description with Instructions for the Care and Maintenance of the Oscillating Cam Poppet Valve Gear'. The description and photographs inside are undoubtedly those of the type of equipment supplied by Paxman but the booklet makes no mention of either Lentz or Paxman. The publication, 'specially compiled for the use of Foremen and Engine Fitters', is undated but was prepared and issued by 'Associated Locomotive Equipment Ltd' of 66 Victoria Street, London SW1. The main competitor with Lentz in the locomotive poppet valve market was Caprotti. An undated handbook 'for Shed and Running Staff', which appears to have been written c.1936/37, entiled 'The Caprotti Valve Gear for Locomotives' was published by Caprotti Valve Gears Ltd whose address is also given as 66 Victoria Street, London SW1. The identical addresses suggest the possibility that Caprotti acquired the Lentz locomotive valve business, the future of which was looking decidedly uncertain by the mid-1930s as will become evident below. The links between Paxman, Lentz Patents Ltd, Associated Locomotive Equipment Ltd and Caprotti Valve Gears Ltd merit further investigation.

Oscillating Cam and Rotary Cam Variants

Before looking in detail at British locomotives fitted with Lentz valves, it should be explained that two different types of gear were used to operate the valves: oscillating cam and rotary cam. Fundamental to any Lentz poppet valve arrangement is the shape of the cam used to open and close a valve. The specially shaped profile was designed to quickly lift a valve fully open, to hold it open for the desired interval, and then speedily close it but in a controlled manner so that it was brought gently to rest on its seat without shock or hammering. The cams on Lentz stationary steam engines were of the oscillating type, operated by rods attached to eccentrics on the governor shaft. Very similar rocking or oscillating cams were used to operate the Paxman Silent Valve on the Company's heavy-fuel-oil engines of the 1920s. Initially oscillating cam arrangements were used to operate Lentz valves on locomotives. Although Paxman had developed its own patent valve gear it was soon found that, when converting piston valve engines to Lentz, the existing Walschaerts or Stephenson motion could be adapted with only minor changes to oscillate the cams.

In about 1927 Lentz Patents Ltd was asked to design a rotary cam arrangement. Driven from one of a locomotive's coupled axles through a gearbox and drive shafts, the camshaft was rotated rather than oscillated, much as in an internal combustion engine. A downside of this arrangement was problems in obtaining sufficiently variable cut-off points for steam admission to the cylinder. This topic is discussed in more detail lower down the page.

River Esk, Ravenglass & Eskdale Railway

The first British locomotive to be fitted with Lentz valves was ordered by the Ravenglass & Eskdale Railway (RER) to cope with a growing volume of mineral traffic on its line. Named 'River Esk', the miniature 15" gauge 2-8-2 goods locomotive was built by Paxman in 1923. Paxman had no aspirations to move into locomotive building and it has been suggested the Company only took on the 'River Esk' contract in response to an approach by Sir Aubrey Brocklebank. Brocklebank was a shareholder in the RER which was searching for someone to build its new locomotive. He was also better known as Chairman of the Cunard Steamship Company. According to a 1985 issue of Paxman's World "Cunard was a large customer of Davey Paxman & Co and Sir Aubrey asked them if they were interested in building a 'one off' miniature railway engine. Paxmans agreed … although it is not known if it was costed as a commercially priced contract, or whether they regarded it as a novel but useful way of keeping a large customer happy, … " . (6) There may, however, have been a more persuasive reason for Paxman accepting this unusual commission. At the time the Company was preparing to offer Lentz poppet valves for locomotives, actuated by its own patent valve gear. Here was a chance to install and demonstrate the benefits of the new equipment in a British locomotive. We don't know what inducements, if any, Paxman had to offer the RER to agree to fitment of the Lentz valve gear. The change did involve significant modification of Henry Greenly's original design which had a form of radial valve gear and balanced slide valve cylinders. (7)

Unlike the double beat type fitted to full-size locomotives, River Esk's Lentz valves were single beat. Measuring 1.3/8" in diameter, there were two steam and two exhaust valves per cylinder, actuated by cams fitted on a central transverse camshaft in the steam chest. The shaft was oscillated by a lever attached to the Paxman valve gear by a long valve rod.

For reasons which remain obscure the Lentz valve arrangement on River Esk did not prove satisfactory and was replaced by more conventional Walschaerts gear in 1928. It is not known whether the problems lay with the operation of the Lentz valves themselves or the Paxman patent valve gear, or a combination of the two.

Nigel Gresley, Oliver Bulleid and the LNER

The two British locomotive engineers who showed the greatest interest in Lentz valves were the LNER's Chief Mechanical Engineer, Nigel (later Sir Nigel) Gresley and his Principal Assistant, Oliver Bulleid. (Bulleid left the LNER in autumn 1937 to become CME of Southern Railways.) These two can't be accused of not giving Lentz valves a fair trial, having experimented with them on no less than five classes of locomotives. Before looking in more detail at these, it is worth noting that Gresley was on friendly terms with Henry Greenly who, as noted above, designed the first British locomotive to be fitted with Lentz valves. Five of the Paxman-built locomotives designed by Greenly for the Romney Hythe & Dymchurch Railway were based on Gresley's famous LNER A1 (later A10) 4-6-2 Pacifics. One wonders what discussions Greenly may have had with Gresley about his experience of Lentz valves and Paxman with 'River Esk' and whether such exchanges played any part in encouraging Gresley to experiment with Lentz valve gear. According to Bulleid, it was in 1923 that Gresley decided to investigate the possibilities of Lentz gear.

LNER J20 Class 0-6-0 Goods Locomotive, No 8280

LNER No 8280 The first standard gauge British locomotive to be fitted with Lentz valves was a J.20 Class 0-6-0 goods locomotive, No 8280, originally built for the Great Eastern Railway (GER). Parts for the Lentz conversion were ordered from Paxman (Order No 15571) in November 1924 and despatched to LNER's Stratford Works in east London in March 1925. The order is described as being for 'One Lentz Valve Box complete'. No 8280 had two inside cylinders, both served by the one box which was fitted in June 1925. A picture of the mono-bloc valve box appeared in a detailed article about the conversion in the February 1926 issue of 'The Locomotive'. There it was reported that No 8280 "has now been in service for a period exceeding nine months and has shown most satisfactory results, both in ordinary running and during the formal tests". (8)

Mono-bloc valve box for No 8280
Mono-bloc valve box as fitted to No 8280

The September 1927 issue of 'The Locomotive' reported that "The goods engine which we fully described in our issue of February 1926, has been in continuous service for upwards of two years, and the poppet valve gear has not, we are informed, given the slightest trouble, nor have any repairs or renewals to any part of it been required". (9)

No 8280 was converted back to conventional piston valves in September 1937.

LNER B12 Class 4-6-0 Passenger Locomotives with Lentz Valves (B12/2s)

Following the success of trials with the 0-6-0 goods locomotive, generally working heavy coal traffic, the LNER decided to fit Lentz valves to a B12 Class 4-6-0 express locomotive. Originally constructed for the GER in 1913, No 8516 was one of the former S69 or 1500 Class. Conversion to Lentz valve gear was carried out at LNER's Stratford Works in December 1926.

An article in the September 1927 issue of 'The Locomotive' reported that this engine had "been fitted with, in general, the same type of poppet valve equipment (as No 8280), but certain modifications were introduced by way of improvement". A major difference was that the valve chests were not a separate 'bolt on' unit but were made complete with the two cylinders in one casting. Like the 0-6-0 goods locomotive, oscillating cam valve gear was again used. The article concluded: "We are given to understand that this poppet valve engine has been in service on main line passenger trains since the end of last year and that the working results have been so satisfactory that the fitting of further engines of the same type is contemplated". (10)

In his 1929 paper to the Institution of Locomotive Engineers, Bulleid noted that the behaviour of the poppet valve gear on Nos 8280 and 8516 "has been consistently good throughout their respective periods of service: allowance must of course be made for the fact that, being the first engines so fitted, they were well looked after and were worked by the same drivers as far as possible". (11)

LNER converted another five of their existing B12s to Lentz valves. These were No 8525 in September 1928, Nos 8519, 8540 and 8533 in June, July and September 1929 respectively, and No 8532 in April 1930.

A motive power crisis arose in East Anglia in 1927. As an interim solution to a locomotive shortage the LNER placed an order for ten new B12s with Beyer Peacock of Manchester. This was the last batch of B12s to be built and they were ordered with piston valves as normally fitted to the class. As an afterthought Gresley required them to be fitted with Lentz poppet valves. The makers had not included this in their contract price and an unpleasant dispute followed. As Hughes commented "Harsh words were said, and strong letters written. Indeed, court proceedings were threatened, but in the end settlement was reached by the LNER paying £1,500, half the Beyer Peacock claim". (12) The Paxman copy order book shows that the order (No 16889) for ten sets of Lentz equipment for these locomotives was placed on 16th April 1928 and despatched to Manchester on 27th June 1928. The locomotives themselves, Nos 8571-80, were all built in 1928 and delivered to the LNER between August and October that year.

The B12s with Lentz valve gear, classified as B12/2, were not a success. The improvement in economy on coal and water, compared with piston valve engines, was marginal. All but one of the 1928 batch soon gave trouble with twisted camshafts. An even more serious problem was that the monobloc cylinder castings were prone to cracking, leading to the need for early and expensive replacements. Alex Walford thinks this problem was probably due to the fact that the inlet and exhaust ports were close together and the temperature difference between inlet and exhaust steam led to excessive stresses in the casting. It is a measure of the seriousness of the problem that the LNER started converting some of the 1928 batch locomotives to piston valves in November 1931, barely three years after delivery. All sixteen of the B12s fitted with Lentz valves, conversions and new builds, had either been converted back to the original type of piston valves and valve gear, or been rebuilt as B12/3s with long-travel piston valves, by January 1934. In fairness, the problems on the B12s were perhaps attributable more to the casting cracking problem than to the operation of the poppet valves or their valve gear.

LNER D49 Shire/Hunt Class 4-4-0 Express Passenger Locomotives

At about the same time as it was fitting Lentz valves to some B12 locomotives, the LNER also fitted them to a number of the new D49 'Shire' class 4-4-0s being built at Darlington. Eventually 48 of this class of 76 three cylinder simple expansion passenger engines were built with Lentz valves.

The first six 'Shire' class locomotives with Lentz valves were built in 1928. These were the only ones of the class to have oscillating cam operated valves and received the classification D49/3. Like all the piston valve engines of the class (D49/1), they were named after Shire counties. Their poppet valves were housed in individual valve boxes attached by bolts to the top of each cylinder. Each cylinder had four valves, with an admission and exhaust valve at each end. As with the B12s, the oscillating cam arrangement does not appear to have been a great success. When these six engines needed new cylinders in 1938 they were all rebuilt as D49/1s with conventional piston valves.

Throughout the time of the LNER's early experiments with oscillating cam valve gear, Gresley felt that to gain the full advantage of Lentz valves some simpler form of operating mechanism was required than the usual Walschaerts' or Stephenson's motion. The ideal was a rotary motion. The first step was to develop a simple form of driving gear, driven from a coupled axle of the locomotive through a gearbox, which was successfully trialled on J20 No 8280 in July 1927. Gresley then invited Lentz Patents Ltd to design a rotary valve gear for a three cylinder engine, which it did in collaboration with the LNER. (13)

Rotary cam gear for Hunt Class
Rotary cam Lentz gear as fitted to the D49 Hunt Class

The arrangement consisted of a revolving camshaft running right across the block of three cylinders cast in line, operating all the inlet and exhaust valves and giving any valve event desired. The last feature was achieved by having a series of ring cams (initially five, which proved insufficient, and later replaced by a seven step cam) to operate each steam valve, each cam having a different profile to give a pre-arranged admission point, valve opening and cut-off point. The whole valve gear mechanism was totally enclosed and ran in an oil bath.

The first two 'Shire' class engines to be fitted with this rotary cam Lentz valve gear, No 336 Buckinghamshire and No 352 Leicestershire, were completed in early 1929. Early reports from the Running Department indicated that they were performing much better than the six with oscillating cams and all future D49s were built with the rotary cam gear. The engines with rotary cam gear were classified as D49/2 and a total of 42 were built between 1929 and 1935. These were not named after Shires but after fox hunts. To bring the first two into line they were renamed in 1932; No 336 as 'The Quorn' and No 352 as 'The Meynell'. The D49/2s retained their rotary cam Lentz gear to the end of their working lives.

Rotary cam gear on No 352
Rotary cam Lentz gear on No 352 Leicestershire, later The Meynell

LNER C7 Class 4-4-2, Nos 732 and 2212

Two LNER C7 Atlantics, former North Eastern Railway Class Z, were used by Gresley to test a number of pieces of auxiliary equipment. No. 732 was fitted with rotary cam Lentz valve gear in December 1933. In 1934 No 2212 was rebuilt and as it needed new cylinders the opportunity was taken to fit Lentz valves. No 2212 did not re-enter service until January 1936. It is suggested these two Lentz C7s ran well, with a crisp exhaust beat, but no more of the class were converted to poppet valves. Nos 732 and 2212 were classified as C7/2. There remains some question as to whether these engines kept their Lentz gear until being withdrawn from service.

LNER P2 Class 2-8-2, No 2001 Cock o' the North

Among the best known of Gresley's large express locomotives was No 2001 Cock o' the North, built in 1934. The first of the P2 class of six engines designed for working heavy passenger trains on the Edinburgh-Aberdeen route, it was the only P2 to be fitted with Lentz valve gear (rotary cam). A three cylinder simple expansion engine, like most of Gresley's designs, Cock o' the North was the most powerful passenger steam locomotive to be built for service in Britain.

Gresley and Bulleid took a considerable interest in André Chapelon's rebuilt Pacifics of the Paris-Orléans Railway. Cock o' the North incorporated a number of Chapelon's ideas, such as a double Kylchap exhaust and wide steam passages as well as Lentz valve gear. In December 1934 the locomotive was taken to the Vitry-sur-Seine test plant in Paris for trials. Gresley and Bulleid spent three weeks in France observing the trials. Those on the test bank were disappointing because, like many other engines, Cock o' the North could not be driven hard on the test bank without the axle boxes heating. After being consulted by Gresley about the problem, Chapelon proposed that the locomotive be tried out on the line with a test load. Here performance was satisfactory but Chapelon's figures showed it was not as economical in coal and water as his rebuilt P-O Pacifics. (14)

The second P2 class locomotive, No 2002 Earl Marischal, also built in 1934, was fitted with piston valves so that a direct comparison could be made with No 2001 and its poppet valve gear. No 2002 proved to be lighter on coal than No 2001 and the four later P.2 class locomotives built in 1936 were all fitted with piston valves. When Cock o' the North was partially rebuilt in April 1938 with an A4 style smokebox, the Lentz valve gear was replaced with piston valves as fitted to the rest of the class. (15)

In his 1947 paper to the Institution of Locomotive Engineers, Spencer noted that "As originally turned into traffic No 2001 had continuous cams, but, after approximately 10,000 miles' service, trouble was experienced owing to point contact of the follower rollers breaking down the case-hardened surface of the inlet cams. Stepped cams were therefore fitted, but the valve events were consequently restricted to six ranges of cut-off in fore gear. … On an engine of such high tractive effort the large difference in power between each cut-off position proved inconvenient and was not conducive to economical working". (15)

For various reasons Cock o' the North was not a success on the route it was designed for. A major criticism was the high coal and water consumption. Bulleid defended the engine in the discussion which followed Spencer's 1947 paper, saying "the fundamental reason for this was that she was not properly used. Instead of working trains well up to her capacity over long runs she was employed in a service such as Edinburgh to Dundee, went to Aberdeen and hung about there, and did very poor mileage per day, with the result that she showed a heavy coal consumption, most of the coal being burnt through misuse rather than in working trains". (16)

LMS and GWR Lentz Locomotives

The extensive experiments of the LNER with Lentz valve gear were not matched by other members of the 'big four' British railways. Southern Railway does not appear to have tried it all. The Great Western, under Charles Collett, rebuilt a 29XX 'Saint' class 4-6-0 passenger locomotive, No 2935 Caynham Court, with rotary cam Lentz valves in 1931. In the same year the LMS modified five 2-6-0 mixed traffic locomotives with Lentz rotary cam poppet valves. Formerly class 5 engines of the Lancashire & Yorkshire Railway, Nos 42825, 42818, 42822, 42824 and 42829 ran with this gear until they were rebuilt in 1953 with Reidinger poppet valve gear. The GWR's 'Caynham Court' retained her Lentz gear until the engine was scrapped in 1948.

That the LMS and GWR did not proceed further with Lentz valves is not surprising. Senior locomotive engineers of the big four moved in a small world. They knew each other well and openly shared information and experience. They were all members of the Institution of Locomotive Engineers and presented detailed technical papers on current issues which were published in the Institution's Journal. By the mid-1930s it would have been clear to Stanier and Collett from Gresley's experiences, and perhaps also their own, that the future of Lentz gear was not looking as promising as it had a few years before.

A Mixed British Experience

Despite the persistent and courageous efforts of Gresley and Bulleid over ten years or so, the application of Lentz valves to British locomotives cannot be described as a success. After 1934 no more LNER engines were fitted with them. More telling were the many cases where Lentz valve gear was removed and replaced by piston valves. River Esk, the first British locomotive to be fitted with Lentz gear was converted to piston valves within five years of being built. Scarcely three years after the 1928 batch of B12s had been delivered, work started on replacing their valve gear in November 1931. All sixteen of the B12s fitted with Lentz gear had been converted to piston valves by January 1934. The J20 No 8280, the first standard gauge British locomotive to be fitted with Lentz valves, was converted back to piston valves in September 1937. The six Shire class locomotives built in 1928 with oscillating cam Lentz valve gear had this removed in 1938. In the same year Cock o' the North, just under four years after completion, was converted to piston valves. Considering the commitment, time, money and effort Gresley had put into experimenting with Lentz gear there must have been compelling reasons for him to decide not to proceed further and to sanction all these conversions back to piston valves.

The question naturally arises as to why Lentz valve gear on locomotives did not enjoy in England the success it appears to have found in, say, Austria or France. All the British locomotives fitted with Lentz gear were simple expansion types. Under operating conditions on the LNER, extensive trials showed that the improved economy in coal and water achieved with Lentz valves, as compared with piston valves, was generally small and in one or two cases negative. Where there was an improvement, presumably this was too small to justify the cost of further Lentz conversions or applications. One wonders also if some British railway workshops struggled with the demands of setting up and properly maintaining poppet valves. Although piston valves require frequent attention to maintain steamtightness, by the mid-1920s they were almost universal and workshop personnel were fully familiar with them.

An important difference between British and French locomotive practice was in the use of compounding. In Britain the only truly successful compounds appear to have been the three cylinder 4-4-0s of the Midland/LMS Railways. Most British locomotive engineers continued to favour the simple expansion type as being the most practical overall for operating conditions in the UK. By contrast, in France the four cylinder compound was highly successful and built in large numbers. André Chapelon took a very scientific approach to improving the performance of locomotives on the Paris-Orléans Railway. During some of his early trials he measured the temperature in the intermediate receiver of one of the superheated locomotives and found that the degree of superheat in the low pressure cylinder was practically nil. This being so, the LP cylinders were not contributing a proper share to the engine's power output. His proposed solution was to increase the temperature of the superheat by up to 100° C, to 350° or 400° C, at admission to the high pressure cylinder. Such high degrees of superheat made it all the more advisable to use poppet valves rather than piston valves, which perhaps explains their wider use in France. However the potential advantages of poppet valves were not restricted to compounds. In the conclusion to his 1947 paper Spencer comments "It is clear that independently controlled inlet and exhaust events are essential if substantial improvement in cylinder performance is to be realised from high superheat and pressures of 250 pounds per square inch and upwards on single expansion engines". (15)

In most cases where locomotives were fitted with oscillating cam Lentz valves, these were operated by Walschaerts or Stephenson's motion with minor adaptations. The fact that oscillating cam Lentz gear was removed relatively quickly from most British locomotives equipped with it suggests there may have been particular problems with this type of arrangement. On the other hand it is no secret that Gresley clearly believed rotary motion was the ideal. Most of the locomotives fitted with rotary cams retained their Lentz gear for a longer period. A major shortcoming of the rotary cam type was the limited cut-off positions available for controlling steam admission to the cylinders. As previously mentioned, an infinitely variable cam was tried on Cock o' the North but its surface broke down under the load imposed by the followers on the ends of valve spindles. The chosen solution was to use a series of stepped cams but, as explained earlier, the limited number of fixed cut-offs was restrictive and did not allow an engine to be run at its most economical. In the conclusion to his 1947 paper Spencer noted that the results of the LNER's experience with poppet valves "have emphasised … the advisability of providing infinitely variable cut-off". (15)

Caprotti Valve Gear

Shortly before Paxman Lentz valves for locomotives appeared, Signor A Caprotti of Milan developed his poppet valve system. This was first fitted to a locomotive of the Italian State Railways in 1921 and then to a number of other engines in Italy. In Britain, the LNER converted two of its B3 class 4-6-0 four cylinder express locomotives, Nos 6166 and 6168, to Caprotti valve gear in 1929. These showed an average saving on coal of 16%. Two further B3 class engines, Nos 6167 and 6164, were fitted with a later design of Caprotti gear in June 1938 and June 1939 respectively. In his 1947 paper, Spencer noted that these engines gave consistently good results and that, but for the war, further experiments would have been made. He added later that "Opinions differ as to the comparative advantages of the horizontal Lentz valve and the vertical Caprotti valve : the former is easily accessible but the latter has the advantage that its weight is not carried by the spindle and this may have some effect on the even seating of the valve". (15)

The LMS fitted Caprotti valve gear to a four cylinder 4-6-0, No 5927, and later to twenty 'Black Fives'. On the London Midland Region of British Railways the Class 8 4-6-2 Duke of Gloucester, No 71000, was built with Caprotti gear in 1954 and a further thirty 'Black Fives', Nos 73125-54, were built with it in 1956.

During the latter part of the 1930s the Caprotti gear manufactured in Britain was designed in the Drawing Offices of Caprotti Valve Gear Ltd, 66 Victoria Street, London SW1. At this time, probably since the late 1920s, the actual manufacture of their valve boxes was carried out by the well-known engineering concern of William Beardmore & Co Ltd of Dalmuir, Glasgow.

Paxman Order Book Entries for Lentz Locomotive Gear

The following table reproduces the entries for each Lentz locomotive gear order in the relevant surviving Paxman copy order book. Additional notes appear under the details of each order.

Order No	Date	Description	Reg No	Who for	Sent to	Date sent
15307	07/01/24	Three sets of Lentz Valve Gear 12x16	21337	Paxman Loco Valve Gear Co	Holland	25-02-25
For the Dutch State Railways 4-4-0 locomotive referred to in The Locomotive, Nov 15, 1924 (pp 338-9) where the locomotives are described as Series 836-935. Paxman microfilm records say it was Class 1700.
15571	14/11/24	One Lentz Valve Box complete	21601	Lentz Patents Ltd	LNER Stratford	19-03-25
For the conversion of LNER Class J20 0-6-0 goods loco No 8280.
16086	15/01/26	10 Sets of Lentz Valve Boxes	22116	Lentz Patents Ltd	N B Loco Co, Glasgow	April - July '26
Paxman microfilm records say these were for Rhodesian Railways. See also Orders 16959 and 17085 below. The North British Locomotive Co received an order (L817) for twenty 12th class 4-8-2 locomotives for Rhodesian Railways in 1925. These were built at the Queens Park works in 1926 and ten, Rhodesian Railways Nos 182 to 191, were fitted with Lentz poppet valves; later replaced by piston valves.
16831	14/02/28	Rotary Cam Poppet Valves	22861	Lentz Patents Ltd	Darlington	24/08/28, 31/08/28
Thought to be for the first two D49/2 Hunt/Shire Class locos fitted with rotary cam poppet valves, 336 Buckinghamshire and 352 Leicestershire, built in 1929 and renamed in 1932 as The Quorn and The Meynell respectively. Paxman microfilm records note the order was for two three-cylinder locomotives.
16889	16/04/28	10 Sets of Equipment	22919	Lentz Patents Ltd	Manchester	27/06/28
Believed to be for the ten B12 Class locos built by Beyer Peacock at Manchester in 1928 for the GE section of the LNER.
16959	05/07/28	Lentz Valve Engine Spares	22989	Lentz Patents Ltd	Rhodesia	12/10/28
Paxman microfilm records confirm the order is for spares for Rhodesia for locomotives built by the North British Locomotive Company. See order no 16086 above.
17046	22/09/28	40 Sets of Lentz Valve Gear	23076	Lentz Patents Ltd	Spain	02/05/29
Paxman microfilm records say 30 sets for Series 1400 locomotives and 10 sets for Series 1700 locomotives.
17050	29/09/28	Cam Chamber	23080	Lentz Patents Ltd	(blank)	18/06/29
Paxman microfilm records suggest the order was for a set of Lentz valve gear, not solely a cam chamber. These records also say it was for an Indian State Railways 4-6-2 locomotive, XS2 Type. The Vulcan Foundry at Newton-le-Willows received an order in 1928 for two XS2 type engines with Lentz valve gear for the North Western Railway of India. These were built in 1930 and received NWR Nos 780 and 781.
17085	21/11/28	Lentz Valve Spares 12th Class	23115	Lentz Patents Ltd	Rhodesia	03/04/29
Microfilm records confirm order is for spares for Rhodesian Railways, "for 12th Class Engines". See order no 16086 above.

References

The two key documents are papers read to the Institution of Locomotive Engineers by Oliver Bulleid in 1929 and Bert Spencer in 1947, see 11 and 15 below. An enjoyable and interesting book with useful information is Geoffrey Hughes's book on Gresley, see 12 below.

1. Paxman catalogue No 723 'Paxman-Lentz Steam Engines', dated May 1915, p 3.
2. THE LOCOMOTIVE Magazine and Railway Carriage and Wagon Review, November 15, 1924, p 338.
3. ibid, pp 338-339.
4. ibid, p 340.
5. Chapelon : Genius of French Steam by Col H C B Rogers, Ian Allen, London 1972 p 25.
6. Paxman's World, Issue 11, Christmas 1985.
7. THE LOCOMOTIVE, January 15, 1924, p 5.
8. ibid February 15, 1926, pps 48-51.
9. ibid September 15, 1927, p 273.
10. ibid pp 273, 275.
11. Poppet Valves on Locomotives, O Bulleid, Paper 248 presented to the Institution of Locomotive Engineers 28th February 1929, printed in the Journal of the ILE, 1929, p.594.
12. The Gresley Influence, Geoffrey Hughes, Ian Allen, London 1983. p 62.
13. Poppet Valves on Locomotives, O Bulleid, 1929 Paper (see 11 above), pp 588, 590.
14. Chapelon, Col H C B Rogers, (see 5 above) pps 42-44.
15. The Development of LNER Locomotive Design 1923-1941, paper by B Spencer read to the Institution of Locomotive Engineers 19th March 1947, printed in the Journal of the ILE, 1947, pps 164-210, discussion pps 210-243, 524-541.
16. Bulleid - Last Giant of Steam, Sean Day-Lewis, George Allen & Unwin Ltd 1964. p 90.

Acknowledgements: My thanks to Alex Walford who encouraged me to write this page, provided valuable source material and advised on several points relating to Lentz valves and valve gear; to Mike Gipson for retrieving information from Paxman's microfilm records; and to Chris West who provided information on Lentz locomotive built for Rhodesia and the North Western Railway of India.

LNER Acknowledgements

Thank you to Richard Carr for permission to reproduce this article from his Paxman History Pages.