Valve events from Walschaerts valve gear are indeterminate because of the angularity effect of both connecting and eccentric rods, but they can be approximated by geometric construction using Zeuner, Reuleaux or Bilgram diagrams.Â Valve events can be more accurately determined by iterative computer programs such as: Charles Dockstader’s website also includes a program for constructing a Zeuner diagram which provides insights into the variables and how they affect valve events. It can only be changed by changing the proportions of these components. 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. Close coupling between loco and tender. The Walschaert valve gear, when correctly designed, provides constant valve lead irrespective of the chosen cutoff in both forward and reverse gear. The gear is sometimes named without the final "s", since it was incorrectly patented under that name. Much has been written about the comparative trials conducted in 1925 between a GWR Castle 4-6-0 and the much larger LNER A1 Pacific 4-6-2 when the diminutive Castle walked away with all the honours, bettering the Pacific in time-keeping and (by a long margin) fuel consumption.Â The immediate result was the start of a programme of rebulding the A1s into A3s, having higher boiler pressure and long-travel valves which transformed their performance.Â Yet, the precise advantages that the use of long-travel valves confers has seldom been clearly explained. In the US, the Combination Lever in Walschaerts valve gear isÂ usually called the “Lap and Lead Lever” since its geometry defines the amount of lead that is given to the valve.Â The amount of valve movement that is derived from the Combination LeverÂ equates to 2 x (lapÂ + lead). This moves the valve in each direction by a distance which should be set equal to lap plus lead of the valve. Heusinger's gear was closer to the form generally adopted, but most authorities accept Walschaerts' invention as sufficiently close to â¦ Each expansion link was driven from the crosshead on the opposite side of the engine. Slide valves largely used in the 19th century; Piston valves which superseded slide valves in the 20th century; Poppet valves similar in principle to those used in internal combustionÂ engines. To layout the Walschaert gear the proportions of the combination lever must be chosen. The reversing rod, which connects to the valve gear, is attached to this lever, either above or below the pivot, in such a position as to give good leverage. THE ORIGINAL VALVE GEAR designed by the Belgian engineer, Egide Walschaerts, in 1844, employed an eccentric in conjunction with the crosshead to operate the valve. The total movement of the valve rod in mid gear should be symmetric between front and back dead center. http://www.timewarp.demon.co.uk/ned/howebiog.html, “WValve” created by Dr. Allan Wallace and downloadable from his own, Two spreadsheets (one for Walschaerts simulation, the other for Stephensons) by Don Ashton and downloadable from, Several programs created by Charles Dockstader and downloadable from his, delay theÂ point of steam release thereby extending the period of expansionÂ and getting more “work” from the steam. Low clearance volumes for the given level of internal streamlining. The amount of lead provided is determined by the proportions of the combination lever and the lap designed into the valve. Edmund Heusinger von Waldegg: independently designed the Walschaerts valve gear (hence aka the Heusinger valve gear) Hermann Kemper: patent for maglev train technology Georg Knorr: considerably improved compressed air brake Georg Krauss: founder of Lokomotivfabrik Krauss & Co. in Munich, later part of Krauss-Maffei: Johann Friedrich Krigar The valve gear operation combines two motions; one is the lead motion and the other is the directional motion required in full gear. The combination levers were driven, as normal, from the crossheads. 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: 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. Double headlights at each end coordinated with the direction of travel on tender. Because the die block does not move, the end of the radius rod is in effect a fixed point about which the combination lever pivots, so the valve motion is that caused directly by the cross head motion acting through the union link and combination lever. Lengthening valve travel while keeping lap and lead unchanged, allows the port openings to be lengthened thereby increasing the area through which the steam passes on its way into and out of the cylinder and reducing the pressure drops in both directions.Â Or, if the port length is unchanged, longer valve travel requires the use of longer lap. In the design of the 5AT, Wardale has adopted a lead of 7.0 mm compared to 6.35 mm on the BR 5MT. Consider the case of outside admission valves such as slide valves. 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. Purpose of Steam Lap:Â The longer theÂ steam lap, the greater the distance that the valve has to travel in each direction to open the ports at each end of the valve chamber.Â Long lap valves therefore require “long-travel” valves,Â which Gresley famously applied to his original A1 Pacific locomotives after the 1925 comparative trials with a GW Castle. It was extensively used in steam locomotives from the late 19th century until the end of the steam era. In Germany and some neighbouring countries, like Poland and Czechoslovakia, the Walschaerts gear is generally named the Heusinger valve gear after Edmund Heusinger von Waldegg, who invented the mechanism independently in 1849. This page was last edited on 14 February 2007, at 01:03. Minimal heat transfer from the inlet steam to the cylinder and piston surfaces. Piston valves which superseded slide valves in the 20th century; 3. Cast metal chassis. A simple Stephenson gear in partial cutoff 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 can also serve as a reversing gear.It is sometimes referred to as the "motion". For internal admission valves (most piston valves, but there are some That is, at the neutral position, the valve ports should not open at all. Factors on the 5AT favouring long lead are as follows. 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. PLEASE NOTE - this model and explanation contain a glaring error. The ratio of distance from union link end to pivot with radius rod to the distance between the valve rod end to the pivot with the radius rod should be in the same proportion as half piston travel to valve lap plus lead. Learn More. and the valve heads were altered to give 5mm exhaust lap with a less rounded exhaust edge.Â This was motivated principally by the need to reduce the intensity of the draught peaks as a further aid towards improving firebed stability.Â However a little thought showed that there were other reasons why exhaust lap should be used.Â 2644 was generally worked at short cut-offs – 25% down to 15% and sometimes even less – at which the release was early, therefore exhaust lap was needed to lengthen the expansion period.Â During the return stroke compression started from a low pressure due to the improved exhaust, which meant that too little steam was compressed.Â Advancing the start of compression by means of exhaust lap was therefore also beneficial and it could be stated as a general rule that whenever the exhaust of a locomotive was improved there was a need far increase exhaust lap. With the piston in the front or rear dead centre positions, the expansion link should be vertical and moving the die block up and down should not alter the valve spindle position. The terms “Lap” and “Lead” are often used together as if they are closely associated.Â However theyÂ describe very different phenomena as described below: 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. In practise the valve spindle length is adjusted to ensure this as part of setting the timing. Heusinger's gear was closer to the form generally adopted, but most authorities accept Walschaerts' invention as sufficiently close to the final form. It was extensively used in steam locomotives from the late 19th century until the end of the steam era. In the 20th century, the Walschaert valve gear was the most commonly used gear, especially on larger locomotives. In Germany and some neighbouring countries, like Poland and Czechoslovakia, the Walschaerts gear is generally named the Heusinger valve gear after Edmund Heusinger von Waldegg, who invented the mechanism independently in 1849. Porta also explains the purpose of lead in his “Compounding” paper as described in the “Triangular Losses” page of this website. (SeeÂ. The actual throw required from the crank is determined from the dimensions of the expansion link. If this were the only motion made by the valve then the engine would only start if the piston were close to dead centre, and even then the direction in which the engined moved would depend upon whether it was fractionally before or after the dead centre point. However, only a small number of these saw any widespread use. Methods of constructing of Zeuner, Reuleaux and Bilgram diagrams is outlined at the SE Lounge website, and a step-by-step procedure for constructing a Reuleaux diagram (as used by Porta for preliminary estimates of valve events) is provided on the next page of this website. Each expansion link was driven from the crosshead on the opposite side of the engine. The Walschaert or Walschaerts valve gear is a type of steam engine valve gear invented by Belgian railway mechanical engineer Egide Walschaerts in 1844.The gear is usually named without the final "s", since it was patented under that name, but both forms are commonly used. The Walschaerts valve gear is a type of valve gear invented by Belgian railway mechanical engineer Egide Walschaerts in 1844 used to regulate the flow of steam to the pistons in steam engines. Drive on tender on 4 wheels with 4 traction tires. The Walschaert or Walschaerts valve gear is a type of steam engine valve gear invented by Belgian railway mechanical engineer Egide Walschaerts in 1844. The Walschaerts valve gear is a type of valve gear invented by Belgian railway mechanical engineer Egide Walschaerts in 1844 used to regulate the flow of steam to the pistons in steam engines. Gab valve gear was an early form of valve gear used on steam engines. There are many different designs of valve gear, including the following more commonly usedÂ types: Valve ports are the slotted openings cast or cut into theÂ 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 on its way to the blast pipe.Â Â At each end of the steam chest, a single port consisting ofÂ several adjacentÂ slots (marked A in theÂ illustration below) carries both admission steam from the steam chest to the cylinder and exhaust steam from the cylinder to atmosphere. The Mason Bogie locomotive type was the first to use the Walschaert gear in North America. A displacement of the union link end by half the piston travel must cause a valve rod displacement of the valve lead plus the valve lap. Next the fact that in midgear, the valve opening should be the same for the piston in both front and rear dead center positions is used to find the mid gear position of the pivot between the combination rod and reach rod. Brake blocks between the wheels. “sharper” eventsÂ during whichÂ the period of partial valve opening is shorter, therebyÂ reducing the period of choking or “wire drawing”; for any given cut-off, theÂ length (and area) of port that is opened will be longer thereby increasing steam flow into and out of the cylinder. of them on a great website, www.steamlocomotive.com, showing in motion details of the differing styles of valve gear used in the US, including Stevens, Baker, Southern, Young and Walschaerts. This page covers (very briefly) a number of topics related to valves.Â These come under the following headings: Attention is also drawn to separate pages as follows: The valves on aÂ steam engine control the flow of steam in and out of the cylinders.Â Several types of valvesÂ were developed over the years, but most fall within three main catagories: “Valve gear” is the mechanism used to move the valve system thatÂ opens and closes theÂ inlet and exhaust ports that let steam into and out of a locomotive’s cylinders. Minimal steam leakage past the piston rings and piston rodÂ and tail rod packings. In Germany, the Walschaert gear is often named the Heusinger valve gear after Edmund Heusinger von Waldegg, who invented the mechanism independently in 1849. En mecánica, la distribución de válvula de Walshaerts (distribución de válvula de Heusinger) de una máquina de vapor, es un tipo de mecanismo que acciona la válvula que regulan el paso del vapor de agua a los cilindros. However, the Walschaerts valve gear had the advantage that it could be mounted entirely on the outside of the locomotives, leaving the space between the frames clear; which resulted in it being adopted in some articulated locomotives. The combination levers were driven, as normal, from the crossheads. It was slow to gain popularity. The Walschaerts valve gear is a type of valve gear invented by Belgian railway mechanical engineer Egide Walschaerts in 1844 used to regulate the flow of steam to the pistons in steam engines. In Germany and some neighbouring countries, like Poland and Czechoslovakia, the Walschaerts gear is generally named the Heusinger valve gear after Edmund Heusinger von Waldegg, who invented the mechanism independently in 1849. This condition precludes adjustment of the valve gear by altering the length of the radius rod. Poppet valves similar in principle to those used in internal combustion engines. The combination levers were driven, as normal, from the crossheads. Extremely good internal streamlining at the valves and valve liner ports. Valve gear was a fertile field of invention, with probably several hundred variations devised over the years. Generally low cut-off working (which increases the crank rotation during which the valve is open to lead steam). Purpose of Lead: Increasing lead isÂ comparable to advancing theÂ ignition on a petrolÂ engine.Â With positive lead, theÂ admission occurs before the piston reaches dead centre, thusÂ ensuring thatÂ the steam has time to begin applying pressure to the piston as it begins it “power” stroke.Â High speed steamÂ engines requireÂ a longÂ lead whereas low speed freight engines require short or even negative lead.Â The analogy with petrol engines remains the same. Valve Events may be defined as the four defining points in the cylinder power cycle – viz: These four points, in turn, define the four intervening periods as shown on the idealised Indicator Diagram (below) –Â viz: The timing of each valve event can be defined either by the percentage of piston travel or by the crank rotation angle at which they occur. For example, 'Southern' valve gear was found on Southern Railroad locomotives. The valves on a steam engine control the flow of steam in and out of the cylinders. Deeley valve gear - fitted to several express locomotives on the Midland Railway. In the design of the 5AT, Wardale has adopted an exhaust lap of 18 mm compared to zero on the BR 5MT. A supplementary benefit of exhaust lap is that when combined with valve exhaust diffusers andÂ a Kordina at the exhaust passage junction below the blast nozzles,Â it helps to reduce the exhaust pressure peak at the blast nozzles during release and the consequent draught peak on the fire.Â Wardale made this point in his book where he described his decision to introduce 5mm of exhaust lap on his modified 19D No 2644.Â He wrote: During late May 1980 2644 was given an M repair â¦. This manipulates the valves. In Germany, the Walschaert gear is often named the Heusinger valve gear after Edmund Heusinger von Waldegg, who invented the mechanism independently in 1849. Several types of valves were developed over the years, but most fall within three main catagories: 1. It is named after Robert Stephenson but was invented by his employees. Factors on the 5AT favouring limited lead are as follows. Finally, some mathematical algorithms for estimating valve movements (ignoring angularity distortions) is presented in Appendix 1 of an instructive paper titled Predicting Locomotive Performance written by Prof. Bill Hall.Â The algorithms can be used for predicing valve openings to steam and exhaust at any position (angle) of the main crank. The Walschaert gear had the advantage that it could be mounted entirely on the outside of the locomotives, leaving the space between the wheels and frames clear; this caused adoption first among some articulated locomotives. The throw of the return crank must now be determined; it should be such as in full forward and reverse gear to cause a total movement of the valve rod equal to two times (valve lap plus port opening). The Walschaerts valve gear is an improvement on the earlier Stephenson valve gear in that it enables the engineer to operate the steam engine in a continuous range of settings from maximum economy to maximum power. The valve slides back and forth along the machined surface between taperedÂ edges C and D.Â The slots marked B are openingsÂ that allow exhaust steam to pass through theÂ sleeveÂ and into the exhaust passages leading to the blastpipe. Purpose ofÂ Exhaust Lap:Â ExhaustÂ lap is not so commonly used as steam lap, the majority of locomotives having zero exhaust lap. The pressure of this steam provides the driving foâ¦ Note: Jamie Keyte of the 5AT Group is in the process of developing a new steam loco performance simulation package which he has appropriately named “STEAM” (see separate page of this website). The advanced compression resulting from the provision of exhaust lap is not necessarily a negative effect, since on aÂ SGS design it compensates for the reduced pressure at which compression starts when the exhaust system is improved.Â Locomotives with improved exhaustsÂ may otherwiseÂ suffer inadequate compression and consequently require too much lead steamÂ thereby showing significant indicator “triangular losses” at the start of the stroke. Although a brief glance into history (except on the Continent) might place Stephensonâs gear as being largely replaced by Walschaertsâ gear around the turn of the 20 th century, both gears were invented almost simultaneously. Walschaerts or Heusinger valve gear - most common valve gear on later locomotives, normally externally mounted. A second motion is superimposed on the crosshead motion to the valve; the return crank provides a motion which lags the crankpin motion by 90 degrees. At the instant when the space on one side of the piston starts to expand, i.e. Deeley valve gear - fitted to several express locomotives on the Midland Railway. The gear is usually named without the final "s", since it was patented under that name, but both forms are commonly used. Lengthening valve travel also increases the speed that the valve travels, which aids lubrication (even though it is unlikely to achieve hydrodynamic conditions) and also increases the speed with which the valve opens and closes, thereby minimising the amount of “wiredrawing” (and triangular losses) that occur just before the point of closure, and giving more clearly defined valve events. In 1844 Egide Walschaerts invented a valve gear that by 1848 appeared very much as it does today. 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 periodÂ of exhaust steam flowÂ and thusÂ reducing the draught on the fire. Heusinger's gear was closer to the form generally adopted, but most authorities accept Walschaerts' invention as sufficiently close to â¦ The Stephenson valve gear remained the most commonly used valve gear on 19th-century locomotives. Greenly, H., "Waelscharts Valve Gear", Marshall, From TrainSpottingWorld, for Rail fans everywhere, Animation of forwards motion of the gear. The Kuhn slide is part of a modified Walschaerts (German:Heusinger) valve gear on steam locomotives and is named after its inventor, Michael Kuhn (1851â1903). Purpose. You can unlock your login by sending yourself a special link via email. Motor with flywheel. Tutorial on setting Walchaert Valve gear on a live steam locomotive. In Germany, the Walschaerts gear is often named the Heusinger valve gear after Edmund Heusinger von Waldegg, who invented the mechanism independently in 1849. The gear is sometimes named without the final "s", since it was incorrectly patented under that name. Your IP address 22.214.171.124 has been flagged for potential security violations. But what were the advantages and disadvantages of each compared â¦ The T end of the eccentric rod carries two pins; the lower pin is shown engaged with its slot in the reversing link. The term is also used to refer to this particular type of Walschaerts valve gear system as a whole. at the very start of a stroke, the valve opens to admit steam from the boiler into that space. But not answered there is the "why" of it. By choice of position of the reach rod the die block may be moved from top to bottom which superimposes a varying portion of this motion ranging from either 90 degrees in lag behind the crank (full forward gear) through none in midgear to 90 degrees leading the crank (full reverse gear). 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. This is the case shown in the diagrams above. 6060 drive rod and valve gear (3812841990).jpg 2,398 × 1,503; 820 KB A steam tank locomotive from The Powerhouse Museum.jpg 1,024 × 783; 588 KB Ab class locomotive (New Zealand Railways, number 658, 4-6-2), closeup view of the link motion. Walschaerts or Heusinger valve gear - most common valve gear on later locomotives, normally externally mounted. The term is also used to refer to this particular type of Walschaerts valve gear system as a whole. At any setting, the valve gear satisfies the following two conditions: 1. The gear is sometimes named without the final "s", since it was incorrectly patented under that name. 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. Stephenson’s valve gearÂ was not invented by George Stephenson butÂ by two of Robert Stephenson’s employees, William Howe and William Williams (see, Walschaerts (no apostophe) valve gear invented by Belgian railway engineer Egide WalschaertsÂ in 1844,Â gainedÂ near-universalÂ acceptance in the early 20th century.Â It isÂ commonly externally mounted and driven by an eccentric crank mounted on the end of the locomotive crank-pin. Lag and lead of these motions are always described relative to the engine moving forwards. [Note: in the case of a Uniflow engines, separate admission and exhaust ports are fitted.Â The advantageÂ gained is that the ports and cylinder surfaces are not subject to cyclical temperature fluctuations (and thus heat losses)Â caused by admission of hot high-pressure steam and the exhausting of cold low-pressure steam through the same openings.Â One consequential disadvantage is that the greater temperature differential between the steam inlets (at the cylinder end) and exhaust outlet (at the centre centre) results in differential expansion which can cause excess ring wear and (in the extreme) siezing of the piston in the cylinder.]. In Europe, its use was almost universal, while in North America, the Walschaerts gear outnumbered its closest competitor, the Baker valve gear, by a wide margin. In this, Animation showing changes occurring to reverse, http://train.spottingworld.com/wiki/index.php?title=Walschaert_valve_gear&oldid=865, Articles lacking sources from August 2006. This criterion should be satisfied over the widest possible range of speeds and cut-offs. The Walschaerts valve gear was slow to gain popularity. The latter two effectsÂ can be seen asÂ 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. With the reverser in mid gear position since there should be no superimposed return crank motion, the expansion link die slot should be an arc of a circle centered on the pivot in mid gear and of radius equal to the length of the radius rod. Deeley valve gear - fitted to several express locomotives on the Midland Railway. â¢ The valve operating mechanism (often known as the valve gear). exceptions on rebuilt locomotives) the lag and lead motions must be reversed usually by placing the return crank 90 degrees in advance of the crank pin and the combination rod pivots on the radius rod above the linkage to the valve rods instead of below. Slide valves largely used in the 19th century; 2. The Kuhn slide (German: Kuhnsche Schleife) is part of a modified Walschaerts (German:Heusinger) valve gear on steam locomotives and is named after its inventor, Michael Kuhn (1851â1903). Walschaert's valve gear: lt;p|>The |Walschaerts valve gear| is a type of |valve gear| invented by |Belgian| railway |mecha... World Heritage Encyclopedia, the aggregation of the largest online encyclopedias available, and the most definitive collection ever assembled.
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