Valve gear

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The Walschaerts valve gear on a steam locomotive (a PRR E6s). Walschearts valve gear.jpg
The Walschaerts valve gear on a steam locomotive (a PRR E6s).

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".

Contents

Purpose

In the simple case, this can be a relatively simple task as in the internal combustion engine in which the valves always open and close at the same points. This is not the ideal arrangement for a steam engine, though, because greatest power is achieved by keeping the inlet valve open throughout the power stroke (thus having full boiler pressure, minus transmission losses, against the piston throughout the stroke) while peak efficiency is achieved by only having the inlet valve open for a short time and then letting the steam expand in the cylinder (expansive working).

The point at which steam stops being admitted to the cylinder is known as the cutoff , and the optimal position for this varies depending on the work being done and the tradeoff desired between power and efficiency. Steam engines are fitted with regulators (throttles in US parlance) to vary the restriction on steam flow, but controlling the power via the cutoff setting is generally preferable since it makes for more efficient use of boiler steam.

A further benefit may be obtained by admitting the steam to the cylinder slightly before front or back dead centre. This advanced admission (also known as lead steam) assists in cushioning the inertia of the motion at high speed.

In the internal combustion engine, this task is performed by cams on a camshaft driving poppet valves, but this arrangement is not commonly used with steam engines, partly because achieving variable engine timing using cams is complicated. Instead, a system of eccentrics, cranks and levers is generally used to control a D slide valve or piston valve from the motion. Generally, two simple harmonic motions with different fixed phase angles are added in varying proportions to provide an output motion that is variable in phase and amplitude. A variety of such mechanisms have been devised over the years, with varying success.

Both slide and piston valves have the limitation that intake and exhaust events are fixed in relation to each other and cannot be independently optimised. Lap is provided on steam edges of the valve, so that although the valve stroke reduces as cutoff is advanced, the valve is always fully opened to exhaust. However, as cutoff is shortened, the exhaust events also advance. The exhaust release point occurs earlier in the power stroke and compression earlier in the exhaust stroke. Early release wastes some energy in the steam, and early closure also wastes energy in compressing an otherwise unnecessarily large quantity of steam. Another effect of early cutoff is that the valve is moving quite slowly at the cutoff point, and this creates a constriction point causes the steam to enter the cylinder at less than full boiler pressure (called 'wire drawing' of the steam, named after the process of making metal wire by drawing it through a hole), another wasteful thermodynamic effect visible on an indicator diagram.

These inefficiencies drove the widespread experimentation in poppet valve gears for locomotives. Intake and exhaust poppet valves could be moved and controlled independently of each other, allowing for better control of the cycle. In the end, not a great number of locomotives were fitted with poppet valves, but they were common in steam cars and lorries, for example virtually all Sentinel lorries, locomotives and railcars used poppet valves. A very late British design, the SR Leader class, used sleeve valves adapted from internal combustion engines, but this class was not a success.

In stationary steam engines, traction engines and marine engine practice, the shortcomings of valves and valve gears were among the factors that lead to compound expansion. In stationary engines trip valves were also extensively used.

Valve gear designs

Valve gear was a fertile field of invention, with probably several hundred variations devised over the years. However, only a small number of these saw any widespread use. They can be divided into those that drove the standard reciprocating valves (whether piston valves or slide valves), those used with poppet valves, and stationary engine trip gears used with semi-rotary Corliss valves or drop valves. [1]

Reciprocating valve gears

Early types

  • Slip-eccentric - This gear is now confined to model steam engines, and low power hobby applications such as steam launch engines, ranging to a few horsepower. The eccentric is loose on the crankshaft but there are stops to limit its rotation relative to the crankshaft. Setting the eccentric to the forward running and reverse running positions can be accomplished manually by rotating the eccentric on a stopped engine, or for many engines by simply turning the engine in the desired rotation direction, where the eccentric then positions itself automatically. The engine is pushed forwards to put the eccentric in the forward gear position and backwards to put it in the backward gear position. There is no variable control of cutoff. [2] On the London and North Western Railway, some of the three-cylinder compounds designed by Francis William Webb from 1889 used a slip eccentric to operate the valve of the single low-pressure cylinder. These included the Teutonic, Greater Britain and John Hick classes. [3]
  • Gab or hook gear - used on earliest locomotives. Allowed reversing but no control of cutoff.
Constant lead gear (Walschaerts-type gear)

One component of the motion comes from a crank or eccentric. The other component comes from a separate source, usually the crosshead.

  • Walschaerts or Heusinger valve gear - most common valve gear on later locomotives, normally externally mounted.
  • Deeley valve gear - fitted to several express locomotives on the Midland Railway. The combination levers were driven, as normal, from the crossheads. Each expansion link was driven from the crosshead on the opposite side of the engine.
  • Young valve gear - used the piston rod motion on one side of the locomotive to drive the valve gear on the other side. Similar to the Deeley gear, but with detail differences.
  • Baguley valve gear - used by W.G. Bagnall.
  • Bagnall-Price valve gear - a variation of Walschaerts used by W.G. Bagnall. This gear is fitted to Bagnall 3023 and 3050, both preserved on the Welsh Highland Railway.
  • James Thompson Marshall seems to have designed at least two different modifications of Walschaerts gear.
    • One was relatively conventional.
    • The other was very complex and drove separate valves on top of the cylinder (for admission) and underneath the cylinder (for exhaust). After the inventor's death, this gear was fitted experimentally to Southern Railway N Class locomotive number 1850, the work taking from 16 October 1933 to 3 February 1934; but it failed on 22 March 1934. Since the inventor was unable to modify the design, the valve gear was replaced by standard Walschaerts gear between 24 March and 11 April 1934. [4]
  • Isaacson's patent valve gear - a modified Walschaerts gear, patented in 1907 by Rupert John Isaacson, and others, patent no. GB190727899, published 13 August 1908. [5] It was fitted to the Garstang and Knot-End Railway's 2-6-0T Blackpool (built 1909) and to Midland Railway No. 382 during 1910–11. [6] Isaacson also has a patent (GB126203, published 8 May 1919) for an improved sight-feed lubricator. This was patented jointly with his representative, Ysabel Hart Cox. [7]
  • Soo Line 346 in 1961, showing the Kinkan-Ripken arm on the connecting rod at the right hand edge of the picture Soo Line 0-6-0 No. 346 Running Gear.jpg
    Soo Line 346 in 1961, showing the Kinkan-Ripken arm on the connecting rod at the right hand edge of the picture
    Kingan-Ripken valve gear. This is a Walschaerts-type gear in which the combination lever is linked to an arm on the connecting rod, near its small end, instead of to the crosshead. Patented in Canada by James B. Kingan and Hugo F. Ripken, patent CA 204805, issued 12 October 1920. [8] This gear was fitted to some locomotives of the Minneapolis, St. Paul and Sault Ste. Marie Railway ("Soo Line"); [9] Hugo Ripken worked as a foreman in the Soo Line's Shoreham Shops in Minneapolis. [10]
Dual eccentric gear (Stephenson-type gears)
Stephenson's Valve gear. Two eccentrics at nearly 180-degree phase difference work cranks from the main drive shaft. Either can be selected to work the valve slide by shifting the slotted expansion link. Stephenson link motion (Heat Engines, 1913).jpg
Stephenson's Valve gear. Two eccentrics at nearly 180-degree phase difference work cranks from the main drive shaft. Either can be selected to work the valve slide by shifting the slotted expansion link.

Two eccentrics joined by a curved or straight link. A simple arrangement which works well at low speed. At high speed, a Walschaerts-type gear is said to give better steam distribution and higher efficiency.

Baker valve gear assembly Baker valve gear.jpg
Baker valve gear assembly
  • Baker valve gear - fairly common in the United States, it had no sliding parts.

Radial gears

Both components of the motion come from a single crank or eccentric. A problem with this arrangement (when applied to locomotives) is that one of the components of the motion is affected by the rise and fall of the locomotive on its springs. This probably explains why radial gears were largely superseded by Walschaerts-type gears in railway practice but continued to be used in traction and marine engines.

Poppet valve gears

Conjugating gears

View of Henschel & Son conjugated valve gear mechanism used on Victorian Railways H class locomotive, driven from the outside Walschaerts valve gear H220motion3.jpg
View of Henschel & Son conjugated valve gear mechanism used on Victorian Railways H class locomotive, driven from the outside Walschaerts valve gear

These enable a 3-cylinder or 4-cylinder locomotive to be built with only two sets of valve gear. The best known is Gresley conjugated valve gear, used on 3-cylinder locomotives. Walschaerts gear is usually used for the two outside cylinders. Two levers connected to the outside cylinder valve rods drive the valve for the inside cylinder. Harold Holcroft devised a different method for conjugating valve gear by linking the middle cylinder to the combination lever assembly of an outside cylinder, creating the Holcroft valve gear derivative. On a 4-cylinder locomotive the arrangement is simpler. The valve gear may be inside or outside and only short rocking-shafts are needed to link the valves on the inside and outside cylinders.

Bulleid chain-driven valve gear

See Bulleid chain-driven valve gear

Corliss valve gear

See Corliss steam engine

Large stationary engines often used an advanced form of valve gear developed by George Henry Corliss, usually called Corliss valve gear. This gear used separate valves for inlet and exhaust so that the inlet cut-off could be controlled precisely. The use of separate valves and port passages for steam admission and exhaust significantly also reduced losses associated with cylinder condensation and re-evaporation. These features resulted in much improved efficiency.

Controls for valve gear

A locomotive's direction of travel and cut-off are set from the cab by using a reversing lever or screw reverser actuating a rod reaching to the valve gear proper. Some larger steam engines employ a power reverse, which is a servo mechanism, usually powered by steam. This makes control of the reversing gear easier for the driver.

See also

Related Research Articles

<span class="mw-page-title-main">Poppet valve</span> Type of valve

A poppet valve is a valve typically used to control the timing and quantity of gas or vapor flow into or out of an engine, but with many other applications.

<span class="mw-page-title-main">Variable valve timing</span> Process of altering the timing of a valve lift event

Variable valve timing (VVT) is the process of altering the timing of a valve lift event in an internal combustion engine, and is often used to improve performance, fuel economy or emissions. It is increasingly being used in combination with variable valve lift systems. There are many ways in which this can be achieved, ranging from mechanical devices to electro-hydraulic and camless systems. Increasingly strict emissions regulations are causing many automotive manufacturers to use VVT systems.

<span class="mw-page-title-main">Walschaerts valve gear</span> Type of valve gear

The Walschaerts valve gear is a type of valve gear used to regulate the flow of steam to the pistons in steam locomotives, invented by Belgian railway engineer Egide Walschaerts in 1844. The gear is sometimes named without the final "s", since it was incorrectly patented under that name. It was extensively used in steam locomotives from the late 19th century until the end of the steam era.

<span class="mw-page-title-main">Caprotti valve gear</span>

The Caprotti valve gear is a type of steam engine valve gear invented in the early 1920s by Italian architect and engineer Arturo Caprotti. It uses camshafts and poppet valves rather than the piston valves used in other valve gear. While basing his design on automotive valves, Caprotti made several significant departures from this design to adapt the valves for steam. Having agreed a joint-venture with Worcester-based engineering company Heenan & Froude from 1938, Heenan & Froude fully acquired Caprotti post-World War II in 1947.

<span class="mw-page-title-main">Gresley conjugated valve gear</span> Type of mechanism for controlling steam flow in a reciprocating steam engine

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.

<span class="mw-page-title-main">Baker valve gear</span>

After about 1910, the Baker valve gear was the main competitor to Walschaerts valve gear for steam locomotives in the United States. Strictly speaking it was not a valve gear but a variable expansion mechanism adapted to the Walschaerts layout replacing the expansion link and sliding die block. The Baker arrangement used more pivot bearings or pin joints, but avoided the die slip inherent to the expansion link, with the aim of lessening wear and the need for service; it could also facilitate longer valve travel.

<span class="mw-page-title-main">Stephenson valve gear</span> Simple design of valve gear

The Stephenson valve gear or Stephenson link or shifting link is a simple design of valve gear that was widely used throughout the world for various kinds of steam engines. It is named after Robert Stephenson but was invented by his employees.

<span class="mw-page-title-main">Corliss steam engine</span> Type of steam engine using rotary steam valves

A Corliss steam engine is a steam engine, fitted with rotary valves and with variable valve timing patented in 1849, invented by and named after the US engineer George Henry Corliss of Providence, Rhode Island.

<span class="mw-page-title-main">Southern valve gear</span>

Southern valve gear was briefly popular on steam locomotives in the United States. It combines elements of the Walschaerts and Baker patterns.

<span class="mw-page-title-main">Bagnall–Price valve gear</span>

Bagnall–Price valve gear is a type of steam engine valve gear developed at locomotive manufacturer W.G. Bagnall as an alternative to the more common Walschaerts valve gear and also to supersede the Baguley valve gear their designs had previously utilised. The gear was patented in 1903 by W.G. Bagnall and T. S. Price, the manager of the works.

<span class="mw-page-title-main">Uniflow steam engine</span> Type of steam engine

The uniflow type of steam engine uses steam that flows in one direction only in each half of the cylinder. Thermal efficiency is increased by having a temperature gradient along the cylinder. Steam always enters at the hot ends of the cylinder and exhausts through ports at the cooler centre. By this means, the relative heating and cooling of the cylinder walls is reduced.

<span class="mw-page-title-main">Piston valve (steam engine)</span> Form of valve within a steam engine or locomotive

Piston valves are one form of valve used to control the flow of steam within a steam engine or locomotive. They control the admission of steam into the cylinders and its subsequent exhausting, enabling a locomotive to move under its own power. The valve consists of two piston heads on a common spindle moving inside a steam chest, which is essentially a mini-cylinder located either above or below the main cylinders of the locomotive.

<span class="mw-page-title-main">Expansion valve (steam engine)</span>

An expansion valve is a device in steam engine valve gear that improves engine efficiency. It operates by closing off the supply of steam early, before the piston has travelled through its full stroke. This cut-off allows the steam to then expand within the cylinder. This expanding steam is still sufficient to drive the piston, even though its pressure decreases as it expands. As less steam is supplied in the shorter time for which the valve is open, use of the expansion valve reduces the steam consumed and thus the fuel required. The engine may deliver two-thirds of the work, for only one-third of the steam.

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).

<span class="mw-page-title-main">James Thompson Marshall</span>

James Thompson Marshall was an English railway and mechanical engineer known for inventing the 'Marshall valve gear' for steam locomotive use. James Marshall began his engineering career at the Leeds-based Steam Plough Company, and later moved to the city's Boyne Engine Works.

<span class="mw-page-title-main">Oscillating cylinder steam engine</span> Steam-engine design which does not require a valve gear

An oscillating cylinder steam engine is a simple steam-engine design that requires no valve gear. Instead the cylinder rocks, or oscillates, as the crank moves the piston, pivoting in the mounting trunnion so that ports in the cylinder line up with ports in a fixed port face alternately to direct steam into or out of the cylinder.

<span class="mw-page-title-main">South African Class 19C 4-8-2</span>

The South African Railways Class 19C 4-8-2 of 1935 was a steam locomotive.

<span class="mw-page-title-main">South African Class 10B 4-6-2</span>

The South African Railways Class 10B 4-6-2 of 1910 was a steam locomotive from the pre-Union era in Transvaal.

<span class="mw-page-title-main">South African Class 10A 4-6-2</span> South African steam locomotive

The South African Railways Class 10A 4-6-2 of 1910 was a steam locomotive from the pre-Union era in Transvaal.

<span class="mw-page-title-main">Reversing gear</span> Controller for the valves on a steam engine

On a steam locomotive, the reversing gear is used to control the direction of travel of the locomotive. It also adjusts the cutoff of the steam locomotive.

References

  1. Steam Locomotive Valve Gear Archived 5 January 2012 at the Wayback Machine Animations of Stephenson's, Walschaerts', Baker's, Southern and Young's valve gear. SteamLocomotive.com, Accessed 1 September 2014
  2. "Slip-eccentric valve gear". Roundhouse-eng.com. Archived from the original on 27 April 2012. Retrieved 2 December 2012.
  3. Van Riemsdijk, J.T. (1994). Compound Locomotives: An International Survey. Penryn: Atlantic Transport Publishers. pp. 23–24. ISBN   0-906899-61-3.
  4. Bradley, D.L. (April 1980) [1961]. The Locomotive History of the South Eastern & Chatham Railway (2nd ed.). London: RCTS. p. 93. ISBN   0-901115-49-5.
  5. "Espacenet - Bibliographic data". Worldwide.espacenet.com. Retrieved 12 January 2013.
  6. "Brief Biographies of Mechanical Engineers". Steamindex.com. Archived from the original on 13 February 2004. Retrieved 12 January 2013.
  7. "Espacenet - Bibliographic data". Worldwide.espacenet.com. Retrieved 12 January 2013.
  8. "CIPO - Patent - 204805". Patents.ic.gc.ca. 12 October 1920. Archived from the original on 8 March 2014. Retrieved 13 January 2013.
  9. "Railway age gazette". [New York, Simmons-Boardman Pub. Co.] via Internet Archive.
  10. Dorin, Patrick C. (1979). The Soo Line. Burbank, California: Superior Publishing Co. p. 25. ISBN   0-87564-712-X.
  11. "0-6-4st Badger". www.martynbane.co.uk. Archived from the original on 4 March 2016.
  12. "Correspondence 60". www.irsociety.co.uk. Archived from the original on 4 November 2011.