Holcroft valve gear

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South Eastern and Chatham Railway official photograph of Maunsell N1 prototype No. 822, taken at Ashford works in 1922. The lever running forward across the outside cylinder is part of the Holcroft valve gear which drives the valve of the inside cylinder SECR N1 class.jpg
South Eastern and Chatham Railway official photograph of Maunsell N1 prototype No. 822, taken at Ashford works in 1922. The lever running forward across the outside cylinder is part of the Holcroft valve gear which drives the valve of the inside cylinder

The Holcroft valve gear was a type of conjugated valve gear designed by Harold Holcroft and used on three-cylinder steam locomotives of the South Eastern and Chatham Railway (SECR). It bore many similarities to the Gresley conjugated valve gear used on the original LNER A1 design. It varied from the Gresley method of operation by using the combination lever assembly instead of the valve spindles to drive the middle cylinder of a three-cylinder design. This had operational advantages over Gresley's design.

Valve gear

The valve gear of a steam engine is the mechanism that operates the inlet and exhaust valves to admit steam into the cylinder and allow exhaust steam to escape, respectively, at the correct points in the cycle. It can also serve as a reversing gear. It is sometimes referred to as the "motion".

Harold Holcroft was an English railway and mechanical engineer who worked for the Great Western Railway (GWR), the South Eastern and Chatham Railway (SECR) and the Southern Railway (SR).

Steam locomotive railway locomotive that produces its pulling power through a steam engine

A steam locomotive is a type of railway locomotive that produces its pulling power through a steam engine. These locomotives are fueled by burning combustible material – usually coal, wood, or oil – to produce steam in a boiler. The steam moves reciprocating pistons which are mechanically connected to the locomotive's main wheels (drivers). Both fuel and water supplies are carried with the locomotive, either on the locomotive itself or in wagons (tenders) pulled behind.

Contents

Background

When working for the GWR at Swindon, Holcroft schemed out and patented (in 1909) a conjugated valve gear for three-cylinder steam locomotives, which allowed for the elimination of the valve gear for the inside cylinder by deriving the motion for the inside valve from that of the outside valves. This would be done by an assembly of levers behind the cylinder block.

Somewhat later, Nigel Gresley of the GNR had designed and patented his own application of conjugated valve gear (Late 1916) which required further development to enable it to be applied to three cylinders of differing inclination. Gresley was involved with ARLE and he and Holcroft met at the formers office at Kings Cross in January 1919. Holcroft had come up with a practical proposal to allow the Gresley valve gear to operate the inside valve and retain valve events at their correct relationship, by skewing the cylinder and valve axis.

In 1917 Holcroft produced a proposal for a 3-cylinder 4-4-0 for the SECR, which included a conjugation mechanism behind the cylinders to drive the inside valve, but this was not taken up. In February 1919 Holcroft was asked by the SECR CME Maunsell to design a 3-cylinder version of the SECR N class 2-6-0, which became the N1. It proved impractical to locate the mechanism behind the cylinders on this design due to the leading coupled wheel fouling the area required for the gear. This made it necessary to revise the design with side rods to transmit the motion forward to the lever mechanism mounted near the front of the frames. Thus, some of the advantage of 'neatness' and compactness of the original was lost. [1] The gear was also used on the sole K1 and the first of the U1 class.

In the long term, however, the Holcroft gear was not perpetuated on the Southern Railway, and further SR 3-cylinder locomotives were constructed with 3 sets of valve gear.

Design

Three-cylinder locomotives have certain advantages over two-cylinder ones, but a significant disadvantage over a two-cylinder locomotive is the need for a third set of valve gear, normally mounted between the frames where routine maintenance is inconvenient. The conjugation mechanism removes the need for the third set of valve gear, replacing it with a somewhat simpler arrangement of transverse levers.

Conjugated gear is effectively a mechanical adding machine, where the position of the valve for the inside cylinder is the sum of the positions of the two outside cylinders, but reversed in direction. It can also be thought of as a rocking lever between one outside cylinder and the inside cylinder, as is common on 4-cylinder steam locomotives, but with the pivot point being moved back and forth by a lever from the other outside cylinder. See Gresley conjugated valve gear for more detail.

Gresley conjugated valve gear

The Gresley conjugated valve gear is a valve gear for steam locomotives designed by Sir Nigel Gresley, chief mechanical engineer of the LNER, assisted by Harold Holcroft. It enables a three-cylinder locomotive to operate with only the two sets of valve gear for the outside cylinders, and derives the valve motion for the inside cylinder from them by means of levers. The gear is sometimes known as the Gresley-Holcroft gear, acknowledging Holcroft's major contributions to its development.

When it proved impractical to mount the conjugation apparatus behind the cylinders on the N1 Holcroft had to revise the design to have the apparatus in front of the cylinders. He did not wish to use an extension of the valve spindles, as on Gresley conjugated valve gear. Instead, Holcroft carried-forward the action of the combination lever assembly that controlled the fore and aft movement of the valve spindle (which admitted "live" steam into the cylinder and ejected "spent" steam out through the ports) to a pivot point connected to a lever from the "inside" valve spindle. [2] The Holcroft variant held an advantage over Gresley's "conjugated" design, as the valve timing was immune to variations brought about by heat expansion of the valve spindles and flexing of the conjugation assembly when under heavy use. [3] It also had potential maintenance benefits because most of the conjugation mechanism was located outside the locomotive frames and could thus be accessed without an inspection pit. [3]

Locomotive frame

A locomotive frame is the structure that forms the backbone of the railway locomotive, giving it strength and supporting the superstructure elements such as a cab, boiler or bodywork. The vast majority of locomotives have had a frame structure of some kind. The frame may in turn be supported by axles directly attached to it, or it may be mounted on bogies (UK) / trucks (US), or a combination of the two. The bogies in turn will have frames of their own.

Miniature Four-Cylinder Holcroft Gear

Harold Holcroft was close friend of LBSC, the pioneer designer of small live steam passenger hauling model locomotives. In the early 1940s LBSC had constructed a 4-6-2, four-cylinder locomotive in 2 1/2" gauge, Tugboat Annie, intending to use Baker valve gear. The complexity of the arrangement proved daunting, and Holcroft 'came to the rescue' with a four-cylinder conjugated valve gear, a modification of the gear used on the Gresley's 120-degree cranks for three cylinders to suite the 135-degree cranks of the four-cylinder model. Holcroft wrote up the engine for the Railway Gazette, while LBSC described the engine and later its gear in Model Engineer . [4]

Lillian "Curly" Lawrence, known as LBSC, was one of Britain's most prolific and well known model or scale-steam-locomotive designers. LBSC were the initials of Britain's London, Brighton and South Coast Railway. LBSC, “Curly” to his friends, was born 27 September 1883 and christened William Morris Benjamin later changing his surname to Mathieson when his father changed the family name. After 1902 William changed his name to Lillian Lawrence; why he chose a female name is unclear, however he was nicknamed "Dolly" at school by account of his long, blond, curly hair, and was pictured on occasion, wearing female shoes and clothing while driving his models. Despite his 'unusual make up' as described by his friend George Barlow in the foreword to Brian Hollingsworth's biography, his retiring nature, and the prejudices that may have existed in 1930's, he was readily accepted as an expert live steam model engineer. Curly had been making steam engines from tins and bits and pieces since childhood and his engineering skills were largely self taught. In 1908 he married Sarah Munt otherwise known as Mabel. Curly loved steam locomotives from the time he was a child and spent several years in the employ of the LBSC Railway, from which he later adopted his pen name.

Live steam pressured steam method of operating machinery

Live steam is steam under pressure, obtained by heating water in a boiler. The steam is used to operate stationary or moving equipment.

<i>Tugboat Annie</i> 1933 film by Mervyn LeRoy

Tugboat Annie is a 1933 American pre-Code film directed by Mervyn LeRoy, written by Norman Reilly Raine and Zelda Sears, and starring Marie Dressler and Wallace Beery as a comically quarrelsome middle-aged couple who operate a tugboat. Dressler and Beery were MGM's most popular screen team at that time, having recently made the bittersweet Min and Bill (1930) together, for which Dressler won the Academy Award for Best Actress.

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References

Notes

  1. ↑ "Locomotive Adventure" Harold Holcroft, Ian Allan. London
  2. ↑ Middlemass, (Backtrack: 4), pp. 148–154
  3. 1 2 Holcroft, (Engineer: 181, 1946), pp. 145–147
  4. ↑ LBSC (Model Engineer, Vol. 115, Issue 2883, 1956) pp. 282-284, "Facts about Tugboat Annie"

Bibliography

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