5at Train

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New Generation Steam -- The Ultimate 4-6-0 -- 380 mile range -- An Engineering Legacy 

Valve Gear and Valve Events

Valve Types

The valves on a steam engine control the flow of steam in and out of the cylinders. Several types of valves were developed over the years, but most fall within three main catagories:

  • Slide valves largely used in the 19th century;
  • Piston valves which superseded slide valves in the 20th century;
  • Poppet valves similar in principle to those in an internal combustion engine.

Valve Gear

"Valve gear" is the mechanism used to move the valve system that opens and closes the inlet and exhaust ports that let steam into and out of a locomotive's cylinders.

There are many different designs of valve gear, including the following more commonly used types:

  • Stephenson's valve gear was not invented by George Stephenson but by two of Robert Stephenson's employees, William Howe and William Williams (see http://www.timewarp.demon.co.uk/ned/howebiog.html).  It was first applied by Robert Stephenson & Sons in 1842 and took its name from the company and not the man.  Stephenson's valve gear was extensively used worldwide throughout the 19th century and through the first half of the 20th Century on Great Western Railway 2-cylinder locomotives.  It is normally driven from "eccentrics" mounted on one of the drive axles and located inside the loco's frames.  One advantage of Stephenson's valve gear is its variable lead which reduces at longer cut-offs and vice versa.  See Wikipedia article for more details.
  • Walschaerts (no apostophe) valve gear invented by Belgian railway engineer Egide Walschaerts in 1844, gained near-universal acceptance in the early 20th century.  It is commonly externally mounted and driven by an eccentric crank mounted on the end of the locomotive crank-pin. Walshaerts valve gear gives constrant lead.  See Wikipedia article for more details and an excellent animation.
  • Baker and Southern valve gear are US designs based on Walschaerts principles, but using a complex arrangement of levers and rocking shafts to replace the Walschaerts expansion link.
  • Caprotti valve gear invented by Arturo Caprotti, an Italian engineer, and gained some popularity in mid-20th century European locomotive designs.  Caprotti valve gear uses a rotary-drive from a gearbox mounted at the end of an eccentric crank on the end of the locomotive crank-pin. (See Wikipedia article for more information).

Valve Events

Six "valve events" may be defined as follows:

  • The point of Admission when the port opens allowing "live" (high pressure) steam from the steam chest to be admitted into the cylinder;
  • The point of Cut-off when the valve closes the port, cutting off the steam supply to the cylinder;
  • The period of Expansion as the steam expands against the moving piston and its pressure reduces;
  • The point of Release when the valve opens the port allowing the expanded steam to be released through the exhaust passages and chimney (or to the "receiver" in the case of steam flow from high to low pressure cylinder in a compound locomotive);
  • The point of Exhaust Closure when the valve closes the port to prevent further release of exhaust steam;
  • The period of Compression as the piston continues to move towards the cylinder end, causing compression of any remaining steam until the point of Admission is reached and the port reopens to live steam.

The six events are illustrated on the Indicator Diagram below (see also the Locomotive Power page of this website).

Lap and Lead

The terms "Lap" and "Lead" are often used together as if they are closely associated.  However they describe very different phenomena as described below:

  • lap is the distance (measured in inches or millimeters) that a locomotive's valve overlaps its port when the valve is in its central position. Steam lap is the amount (or distance) that the steam side of the valve overlaps the port, while exhaust lap is the amount (or distance) that the exhaust side of the valve overlaps the port.  Lap is a function of valve and port geometry;
  • lead is the distance (measured in inches or millimeters) that a locomotive's port is open when the piston is at dead centre. Lead is governed by the valve gear and its setting.  Walschaerts valve gear is normally gives a fixed lead whereas Stephenson's valve gear gives a variable lead that increases as the cut-off is shortened.

The illustrations below are taken from "Locomotive Valves and Valve Gear" by Yoder and Warren first published in 1921 and republished by Camden Miniature Steam.

Valve Lap (taken from "Locomotive Valves and Valve Gear" by Yoder and Warren page 21, published by Camden Miniature Steam)

Valve Lead (taken from "Locomotive Valves and Valve Gear" by Yoder and Warren page 149, published by Camden Miniature Steam)

Note: (quoting from Wardale's FDC 5 line 34):  "Variable lead can be applied to Walchaerts valve gear by (i) shortening the eccentric crank length or (ii) slotting the combination lever top and suspending the front of the radius rod from a link held in one arm of a crank, and by raising or lowering it varying the effective length of the combination lever dimension between the radius rod and valve spindle joints."

In the US, the Combination Lever in Walschaerts valve gear is usually called the "Lap and Lead Lever" since its geometry defines the amount of lead that is given to the valve.  The amount of valve movement that is derived from the Combination Lever equates to 2 x (lap + lead).

Purpose of Lead: Increasing lead is comparable to advancing the ignition on a petrol engine.  With positive lead, the admission occurs before the piston reaches dead centre, thus ensuring that the steam has time to begin applying pressure to the piston as it begins it "power" stroke.  High speed steam engines require a long lead whereas low speed freight engines require short or even negative lead.  The analogy with petrol engines remains the same.

In the 5AT FDCs, Wardale describes lead as follows:

In practice the criterion to be satisfied by lead steam is to aid the obtaining of the least pressure drop between the steam chests and cylinders during the admission phase, and this is generally coincident with obtaining full steam chest pressure in the cylinder at dead centre. This criterion should be satisfied over the widest possible range of speeds and cut-offs. Factors on the 5AT favouring limited lead are as follows.

  • Extremely good internal streamlining at the valves and valve liner ports.
  • High superheat.
  • Minimal heat transfer from the inlet steam to the cylinder and piston surfaces.
  • Minimal steam leakage past the piston rings and piston rod and tail rod packings.
  • Low clearance volumes for the given level of internal streamlining.
  • Generally low cut-off working (which increases the crank rotation during which the valve is open to lead steam).
  • Minimal slackness in the valve gear, due to the extensive use of roller bearing joints, a very important factor in the ability of the valve gear to deliver the correct motion over a long period of time.

Factors on the 5AT favouring high lead are as follows.

  • The necessity to get steam chest pressure in the cylinders at the start of each stroke over the widest range of speeds and cut-offs, which includes at relatively high cut-offs when maximum cylinder power is required for acceleration.
  • The very high coupled wheel rotational speed at maximum train speed, limiting the time for lead steam to enter the cylinders.

In the design of the 5AT, Wardale has adopted a lead of 7.0 mm compared to 6.35 mm on the BR 5MT.

Porta also explains the purpose of lead in his "Compounding" paper as described in the "Triangular Losses" page of this website.

Purpose of Steam Lap: The longer the steam lap, the greater the distance that the valve has to travel in each direction to open the ports at each end of valve chamber.  Long lap valves therefore require "long-travel" valves, which Gresley famously applied to his original A1 Pacific locomotives after the 1925 comparative trials with a GW Castle.

The advantages of long steam lap (and therefore long valve travel) derives from the fact that the valve must travel further and therefore faster over the port, thereby delivering:

  • "sharper" events during which the period of partial valve opening is shorter, thereby reducing the period of choking or "wire drawing";
  • for any given cut-off, the length (and area) of port that is opened will be longer thereby increasing steam flow into and out of the cylinder.

In the design of the 5AT, Wardale has adopted a steam lap of 65mm thus giving the valve a mid-gear movement of 2 x (lap + lead) = 2 x (65 + 7) = 144mm compared to 98.4 mm on the BR 5MT giving a 46% increase.

Purpose of Exhaust Lap: Exhaust lap is not so commonly used as steam lap, the majority of locomotives having zero exhaust lap.

The effect of exhaust lap is to:

  • delay the point of steam release thereby extending the period of expansion and getting more "work" from the steam
  • reduce the exhaust steam pressure (because of the extra expansion);
  • advance the start of compression and thus the amount of compression;
  • reduce the port opening available for the passage of exhaust steam;
  • shorten the period of exhaust steam flow and thus reducing the draft on the fire.

The latter two effects can be seen as having "negative" effects on loco performance, however they can be counteracted by (a) the provision of large port opening (which are in any case necessitated by the provision of a long steam lap), and (b) a good exhaust system that generates high smokebox vacuum from the available exhaust steam.

The advanced compression resulting from the provision of exhaust lap is not necessarily a negative effect, since on a SGS design it compensates for the reduced pressure at which compression starts when the exhaust system is improved.  Locomotives with improved exhausts may otherwise suffer inadequate compression and consequently require too much lead steam thereby showing significant indicator "triangular losses" at the start of the stroke.

A supplementary benefit of exhaust lap is that when combined with valve exhaust diffusers and a Kordina at the exhaust passage junction below the blast nozzles, it helps to reduce the exhaust pressure peak at the blast nozzles during release and the consequent draught peak on the fire.

In the design of the 5AT, Wardale has adopted an exhaust lap of 18 mm compared to zero on the BR 5MT.

Valve Ports

Valve ports are slotted openings cast or cut into the circumference of each valve liner or sleeve at each end of the steam chest, though which high pressure steam passes on its way to the cylinder, and through which low pressure steam passes from the cylinder to the blast (exhaust) pipe.  At each end of the steam chest, a single port consisting of several adjacent slots (marked A in the illustration below) carries both admission steam from the steam chest to the cylinder and exhaust steam from the cylinder to atmosphere.  Note - the valve slides back and forth along the machined surface between tapered edges C and D. The slots marked B are not valve ports but openings that allow exhaust steam to pass through the sleeve and into the exhaust passages leading to the blastpipe.]

Port openings in a Valve Liner (from "Locomotive Valves and Valve Gear" by Yoder and Warren first published in 1921 and republished by Camden Miniature Steam)

Note: in the case of a Uniflow engines, separate admission and exhaust ports are fitted.  The advantage gained is that the ports and cylinder surfaces are not subject to cyclical temperature fluctuations (and thus heat losses) caused by admission of hot high-pressure steam and the exhausting of cold low-pressure steam through the same openings.