Holcroft valve gear
Encyclopedia
The Holcroft valve gear was a type of valve gear
designed by Harold Holcroft
and used on three-cylinder steam locomotive
s 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 and resolved some of the problems associated with Gresley's design.
for three-cylinder steam locomotives. The Gresley application of conjugated valve gear required further development to increase reliability and promote ease of maintenance. The problems associated with these issues reduced efficiency when the locomotive was under way, which made LNER locomotives sound "off-beat", and increased the incidence of damage to the track. In an effort to resolve these shortcomings, Holcroft set about designing a new, more reliable version of the Gresley conjugated valve gear.
between the frames. The middle cylinder was driven from the combination lever assembly that controlled the fore and aft movement of the valve spindle on the right-hand "outside" cylinder. This admitted "live" steam and ejected "spent" steam through the ports of the middle cylinder in relation to the position of the "outside" cylinder's combination lever.
Holcroft’s conjugation mechanism was a revision of Nigel Gresley
’s method of operating the "inside" cylinder via an extension of the "outside" valve spindles on LNER locomotives. 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. 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. 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.
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...
designed by Harold Holcroft
Harold Holcroft
Harold Holcroft was a British railway and mechanical engineer who worked for the Great Western Railway , the South Eastern and Chatham Railway and the Southern Railway...
and used on three-cylinder steam locomotive
Steam locomotive
A steam locomotive is a railway locomotive that produces its power through a steam engine. These locomotives are fueled by burning some combustible material, usually coal, wood or oil, to produce steam in a boiler, which drives the steam engine...
s of the South Eastern and Chatham Railway
South Eastern and Chatham Railway
The South Eastern and Chatham Railway Companies Joint Management Committee , known by its shorter name of the South Eastern and Chatham Railway was a working union of two neighbouring rival railways, the South Eastern Railway and London, Chatham and Dover Railway , that operated services between...
(SECR). It bore many similarities to the Gresley conjugated valve gear
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...
used on the original LNER A1 design
LNER Gresley Classes A1 and A3
The London and North Eastern Railway LNER Gresley Classes A1 and A3 locomotives represented two distinct stages in the history of the British 4-6-2 "Pacific" steam locomotives designed by Nigel Gresley...
. 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 and resolved some of the problems associated with Gresley's design.
Background
When working for the GWR, Holcroft collaborated with Sir Nigel Gresley to invent and patent the Gresley conjugated valve gearGresley 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...
for three-cylinder steam locomotives. The Gresley application of conjugated valve gear required further development to increase reliability and promote ease of maintenance. The problems associated with these issues reduced efficiency when the locomotive was under way, which made LNER locomotives sound "off-beat", and increased the incidence of damage to the track. In an effort to resolve these shortcomings, Holcroft set about designing a new, more reliable version of the Gresley conjugated valve gear.
Design
Holcroft's valve gear assembly was designed to regulate the timing of valve events admitting steam into the three cylinders. This increased efficiency whilst reducing mechanical wear and the effects of hammerblow on the track. To achieve this, Holcroft introduced a method of driving a middle cylinder from the right-hand set of "outside" Walschaerts valve gear, eliminating the use of a separate set of valve gearValve 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...
between the frames. The middle cylinder was driven from the combination lever assembly that controlled the fore and aft movement of the valve spindle on the right-hand "outside" cylinder. This admitted "live" steam and ejected "spent" steam through the ports of the middle cylinder in relation to the position of the "outside" cylinder's combination lever.
Holcroft’s conjugation mechanism was a revision of Nigel Gresley
Nigel Gresley
Sir Herbert Nigel Gresley was one of Britain's most famous steam locomotive engineers, who rose to become Chief Mechanical Engineer of the London and North Eastern Railway . He was the designer of some of the most famous steam locomotives in Britain, including the LNER Class A1 and LNER Class A4...
’s method of operating the "inside" cylinder via an extension of the "outside" valve spindles on LNER locomotives. 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. 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. It also had potential maintenance benefits because most of the conjugation mechanism was located outside the locomotive frames
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...
and could thus be accessed without an inspection pit.