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63-535: The Walschaerts valve gear was slow to gain popularity. The Stephenson valve gear remained the most commonly used valve gear on 19th-century locomotives. 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 and allowing easy access for service and adjustment, which resulted in it being adopted in some articulated locomotives . The first locomotive fitted with

126-484: A double thread (aka 2-start) and a coarse pitch to move the mechanism as quickly as possible. The wheel is fitted with a locking lever to prevent creep and there is an indicator to show the percentage of cutoff in use. This method of altering the cutoff offers finer control than the sector lever, but it has the disadvantage of slow operation. It is most suitable for long-distance passenger engines where frequent changes of cutoff are not required and where fine adjustments offer

189-428: A full choice of cut-off positions between maximum and mid-gear, but only those which correspond with the notches. The position of the notches is chosen by the locomotive designer or constructor with a view to the locomotive's intended purpose. In general engines designed for freight will have fewer notches with a 'longer' minimum cut-off (providing high tractive effort at low speeds but poor efficiency at high speeds) while

252-423: A gradient, the Walschaerts valve gear enables the engine driver to set the cutoff point near the end of the stroke, so that the full pressure of the boiler is exerted on the piston for almost the entire stroke. With such a setting, when the exhaust opens, the steam in the cylinder is near full boiler pressure. The pressure in the steam at that moment serves no useful purpose; it drives a sudden pulse of pressure into

315-550: A harmonic valve gear, the Stephenson arrangement may be considered as optimum. Nevertheless, the fact the link needed to be bodily displaced in order to reverse meant that it required considerable vertical clearance. At the time of its introduction, it was deemed important in the locomotive world to keep the centre of gravity, and therefore the boiler centre line as low as possible. Because valve gears in Britain were generally placed between

378-514: A matter of great skill to reduce the regulator opening by enough to safely unlatch the Johnson Bar while maintaining sufficient steam pressure to the cylinders. Each time the regulator was re-opened was a chance to encounter wheel slip and in loose coupled trains each closure and opening of the regulator set up dynamic forces throughout the length of the train which risked broken couplings. The screw reverser overcame all these issues. The dangers of

441-404: A nearly constant sound. The valve gear operation combines two motions; one is the primary lead motion which is imparted at the bottom of the combination lever (12). The secondary is the directional/amplitude motion which is imparted at the top. Consider that the driver has adjusted the reversing lever such that the die block is at mid-gear. In this position the secondary motion is eliminated and

504-480: A passenger locomotive will have more notches and a shorter minimum cut-off (allowing efficiency at high speeds at the expense of tractive effort). If the minimum cut-off provided for by the notches was too high, it would not be possible to run the locomotive in the efficient way described above (with a fully open regulator) without leading to steam wastage or 'choking' of the steam passages, so the regulator would have to be closed. That limits efficiency. The Johnson Bar

567-419: A radius rod which connected the valve-rod to a "stationary" link pivoting around a fixed point. The advantages sought were reduced height for the gear and lighter action as the reversing lever was only required to lift the weight of the radius rod. This meant that the link was convex (in relation to the eccentrics) instead of concave. Gooch valve gear had the disadvantage of angularity between the valve spindle and

630-480: A wide margin. 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. Heusinger's gear was closer to the form generally adopted, but most authorities accept Walschaerts' invention as sufficiently close to the final form. The Walschaerts valve gear

693-414: Is an improvement on the earlier Stephenson valve gear in that it enables the driver to operate the steam engine in a continuous range of settings from maximum economy to maximum power. At any setting, the valve gear satisfies the following two conditions: In an economical setting, steam is admitted to the expanding space for only part of the stroke; at a point set by the driver, the intake is cut off. Since

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756-436: Is at the top of the expansion link (7), maximal power in reverse is obtained. (On some engines the die block was in the top of the link in forward gear. This type was generally used on tank engines, which worked in forward and reverse equally.) Once the locomotive has accelerated the driver can adjust the reverser toward the mid-gear position, decreasing cut-off to give a more economical use of steam. The engine's tractive effort

819-422: Is effectively part of the entire valve gear , being connected to the various linkages and arms in order to serve its function in adjusting them. This means that the forces in the valve gear can be transmitted to the lever. This is especially the case if the engine has unbalanced slide valves , which have a high operating friction and are subject to steam forces on both sides of the valve. This friction meant that if

882-466: Is provided with Fletcher's patent arrangement of Allan straight link gear. Reversing lever 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. This is the most common form of reverser. It is also known as a Johnson bar in the United States. It consists of a long lever mounted parallel to

945-411: Is rigidly attached to the con-rod pin connected to the main drive wheel. Note that this is the only suitable attachment point on any of the drive wheels that is not fouled by the passage of the coupling rod or the connecting rod . The eccentric crank is of a length such that the pin attachment to the eccentric rod (2) is 90 degrees out of phase with the lead motion. The eccentric rod provides motion to

1008-411: Is then less than it was at starting, but its power is greater. The primary lead motion is provided by the crosshead arm (9) and the union link (11). This pivoting bar gives the in phase component of motion to the bottom of the combination lever (12). The secondary directional/amplitude motion is derived from a mechanical linkage made up of several components. The eccentric crank (UK: return crank) (1)

1071-661: The North Eastern Railway , but they found them little to their liking, mainly because of maintenance difficulties: any oil leakage from the locking cylinder, either through the piston gland or the cock, allowed the mechanism to creep, or worse “nose-dive”, into full forward gear while running. Stirling moved to the South Eastern Railway and Harry Smith Wainwright , his successor at that company, incorporated them into most of his designs, which were in production about thirty years after Stirling’s innovation. Later still

1134-533: The 1830s, the most popular valve drive for steam locomotives was known as gab motion in the United Kingdom and V-hook motion in the United States . The gab motion incorporated two sets of eccentrics and rods for each cylinder; one eccentric was set to give forward and the other backwards motion to the engine and one or the other could accordingly engage with a pin driving the distribution valve by means of

1197-545: The 1850s but, in Europe, although occurring as early as 1846, they did not become widespread until around 1900. Larger marine engines generally used the bulkier and more expensive marine double-bar link, which has greater wearing surfaces and which improved valve events by minimising geometric compromises inherent in the launch link. In the United Kingdom, locomotives having Stephenson valve gear normally had this mounted in between

1260-414: The Johnson Bar is unlatched while the engine is operating under high steam pressure (wide regulator openings and high cut-off) or at high speeds, the forces that are supposed to act on the slide valves can instead be transmitted back through the linkage to the now-free reversing lever. This will suddenly and violently throw the lever into the full cut-off position, carrying with it the real danger of injury to

1323-489: The Johnson Bar was favored because the change could be made quickly in a single motion instead of the multiple turns of the handle of a low-geared screw reverser. In the screw reverser mechanism (sometimes called a bacon slicer in the UK), the reversing rod is controlled by a screw and nut, worked by a wheel in the cab. The nut either operates on the reversing rod directly or through a lever, as above. The screw and nut may be cut with

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1386-647: The Walschaerts valve gear was built in 1849 at the Belgian Tubize workshops. Egide Walschaerts was awarded a gold medal, honorable mentions and a diploma at the 1878 Universal Exhibition in Paris , and also in Antwerp in 1883. In 1874, New Zealand Railways ordered two NZR B class locomotives. They were Double Fairlie locomotives, supplied by Avonside ; the first use in New Zealand of Walschaerts valve gear and probably

1449-428: The atmosphere and is wasted . This sudden pulse of pressure causes the loud puffing sound that members of the public associate with steam engines, because they mostly encounter engines at stations, where efficiency is sacrificed as trains pull away. A steam engine well adjusted for efficiency makes a soft hissing sound that lasts throughout the exhaust stroke, with the sounds from the two cylinders overlapping to produce

1512-401: The direction of travel, on the driver’s side of the cab. It has a handle and sprung trigger at the top and is pivoted at the bottom to pass between two notched sector plates. The reversing rod, which connects to the valve gear , is attached to this (handle) lever , either above or below the pivot, in such a position as to give good leverage. A square pin is arranged to engage with the notches in

1575-410: 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. When the piston is at either dead centre movement of the radius rod should not move the valve rod. Therefore the expansion link die slot should be an arc of a circle of radius equal to the length of the radius rod. The throw of the return crank must give

1638-400: The driver, damage to the valve gear and triggering wheel slip in the locomotive. The only way to prevent this is to close the regulator and allow the steam pressure in the valve chest to drop. The reversing lever can then be unlatched and set to a new cut-off position and then the regulator could be opened again. During this process the locomotive is not under power. On ascending gradients it was

1701-400: The eccentric rod in full gear, whereas the best forms of the Stephenson gear, the thrust was in a straight line. The Gooch gear gave constant lead at whatever cutoff. This was observed to be a disadvantage when similar locomotives fitted with either Gooch or Stephenson gear were compared in service Gooch gear was never popular in Britain except with one or two engineers down to the 1860s, but it

1764-482: The eccentric rod pivots were set behind the link slot (or below on a vertical engine). These became known respectively as the 'locomotive link' and the 'launch link'. The launch link superseded the locomotive type as it allows more direct linear drive to the piston rod in full gear and permits a longer valve travel within a given space by reducing the size of eccentric required for a given travel. Launch-type links were pretty well universal for American locomotives right from

1827-438: The effect of increased lead and higher compression at the end of each stroke. This process was popularly known as "linking up" or “notching up” , the latter because the reversing lever could be held in precise positions by means of a catch on the lever engaging notches in a quadrant; the term stuck even after the introduction of the screw reverser. A further intrinsic advantage of the Stephenson gear not found in most other types

1890-402: The exhaust is also shut, during the rest of the stroke the steam that has entered the cylinder expands in isolation, and so its pressure decreases. Thus, the most energy available from the steam (in the absence of a condenser ) is used. The Walschaerts valve gear enables the engine driver to change the cutoff point without changing the points at which intake starts. Economy also requires that

1953-417: The expansion link (7) which is pivoted in a central location back to the body of the locomotive. The expansion link holds the radius bar (8), captive by a die block which is integral with the radius bar but is free to move vertically in a constrained curved path along the expansion link. The vertical position of the radius bar is controlled in the cab by the driver adjusting the reverser which in turn controls

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2016-611: The first time that a British manufacturer had supplied it. They were Cape gauge . The Mason Bogie , a modified Fairlie locomotive of 1874, was the first to use the Walschaerts gear in North America . The first application in Britain was on a Single Fairlie 0-4-4T, exhibited in Paris in 1878 and purchased by the Swindon, Marlborough and Andover Railway in March 1882. According to Ernest Ahrons ,

2079-497: The forward-looking Southern Railway engineer Oliver Bulleid fitted them to his famous Merchant Navy Class of locomotives, but they were mostly removed at rebuild. Patented in 1882, the Henszey's reversing gear illustrates a typical early solution. Henszey's device consists of two pistons mounted on a single piston rod. Both pistons are double-ended. One is a steam piston to move the rod as required. The other, containing oil, holds

2142-483: The frames beneath the boiler, the extremely cramped conditions made the valve gear inaccessible for servicing. Also reversing could be a strenuous occupation as it entailed lifting the weight of the link plus eccentric rod ends. In order to address these problems two main variants were developed: In the Gooch valve gear (invented by Daniel Gooch in 1843) the reversing and cut-off functions were achieved by raising or lowering

2205-419: The fresh charge of incoming admission steam. American locomotives universally employed inside Stephenson valve gear placed between the frames until around 1900 when it quickly gave way to outside Walschaerts motion. In Europe, Stephenson gear could be placed either outside the driving wheels and driven by either eccentrics or return cranks or else between the frames driven from the axle through eccentrics, as

2268-435: The gabs: - vee-shaped ends to the eccentric rods supposed to catch the rocker driving the valve rod whatever its position. It was a clumsy mechanism, difficult to operate, and only gave fixed valve events. In 1841, two employees of Robert Stephenson and Company , draughtsman William Howe and pattern-maker William Williams, suggested the simple expedient of replacing the gabs with a vertical slotted link, pivoted at both ends to

2331-510: The indicator showed the intended position. A second mechanism—usually a piston in an oil-filled cylinder held in position by closing a control cock—was required to keep the linkages in place. The first locomotive engineer to fit such a device was James Stirling of the Glasgow and South Western Railway in 1873. Several engineers then tried them, including William Dean of the GWR and Vincent Raven of

2394-399: The linkages involved in controlling cutoff and direction grew progressively heavier and there was a need for power assistance in adjusting them. Steam (later, compressed air) powered reversing gears were developed in the late 19th and early 20th centuries. Typically, the operator worked a valve that admitted steam to one side or the other of a cylinder connected to the reversing mechanism until

2457-471: The locomotive frames. In 1947, the London, Midland and Scottish Railway built a series of their Stanier Class 5 4-6-0 locomotives, most of which had the Walschaerts' valve gear that was normal for this class, but one of them, no. 4767 , had Stephenson valve gear mounted outside the wheels and frames. Instead of eccentrics, double return cranks were used to drive the eccentric rods, and a launch-type expansion link

2520-466: The locomotive saw very little service as nobody seems to have known how to set the valves and this led to enormous coal consumption. In the 20th century, the Walschaerts valve gear was the most commonly used type, especially on larger locomotives. In Europe , its use was almost universal, whilst in North America, the Walschaerts gear outnumbered its closest competitor, the derived Baker valve gear , by

2583-439: The mechanical linkage; reach rod (3), lifting link (4), lifting arm (5) and reverse arm and shaft (6). In this way the secondary, out of phase, driver controlled component of motion is imparted to the top of the combination lever (12) by the radius bar (8). The combination lever combines these two motions with the resultant acting upon the valve stem (13), suitably restrained by the valve stem guide (10), which in turn acts upon

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2646-424: The most benefit. On locomotives fitted with Westinghouse air brake equipment and Stephenson valve gear , it was common to use the screw housing as an air cylinder, with the nut extended to form a piston. Compressed air from the brake reservoirs was applied to one side of the piston to reduce the effort required to lift the heavy expansion link, with gravity assisting in the opposite direction. With larger engines,

2709-408: The motion of the valve. Its use on engines in which all the cylinders lie in one plane, represents, in the belief of the writer, the best choice." Another benefit of the Stephenson gear, intrinsic to the system, is variable lead: usually zero in full gear and increasing as cutoff is shortened. One consequent disadvantage of the Stephenson gear is that it has a tendency to over-compression at the end of

2772-403: The orientation of the eccentric crank is changed so that it leads, rather than trails, the main crank pin. To lay out Walschaerts gear, the proportions of the combination lever must be chosen. 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. The ratio of distance from union link end to pivot with radius rod to

2835-406: The piston valve (14). The Walschaerts gear can be linked to inside or outside admission valves. This article has only considered inside-admission piston valves until now, but outside-admission valves (slide valves and some piston valves) can use Walschaerts valve gear. If the valves have outside admission the radius bar connects to the combination lever below the valve stem rather than above and

2898-400: The piston valve travel is shortest, giving minimal injection and exhaust of steam. The travel of the piston valve is twice the total of lap plus lead. Contrast this to when the die block is at the bottom of the expansion link (7), giving maximum steam injection and exhaust. This is the most powerful forward setting and is used in accelerating forward from rest. Conversely when the die block

2961-418: The plates and hold the lever in the desired position when the trigger is released. The advantages of this design are that change between forward and reverse gear can be made very quickly (as is needed in, for example, a shunting engine). The reversing lever has a catch mechanism which engages with a series of notches to hold the lever at the desired cut-off position. This means that the operator does not have

3024-419: The required oscillation of the expansion link. There have been many variants of Walschaerts valve gear, including: Stephenson 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. During

3087-418: The reversing gear, which operates in the usual way, according to the type of valve gear in use. The Ragonnet power reverse, patented in 1909, was a true feedback controlled servomechanism . The power reverse amplified small motions of the reversing lever made in the locomotive cab with modest force into much larger and more forceful motions of the reach rod that controlled the engine cutoff and direction. It

3150-417: The rod in a fixed position when the steam is turned off. Control is by a small three-way steam valve (“forward”, “stop”, “back”) and a separate indicator showing the position of the rod and thus the percentage of cutoff in use. When the steam valve is at “stop”, an oil cock connecting the two ends of the locking piston is also closed, thus holding the mechanism in position. The piston rod connects by levers to

3213-651: The straight expansion link simplified manufacture. Once again, the Allan gear was not often used in the UK but fairly common on the Continent. Notable UK examples are the Great Western Railway 's 1361 and 1366 classes, and the narrow-gauge Ffestiniog Railway 's 0-4-0TT class (which were produced by George England and Co. ). Talyllyn Railway 's second locomotive, Dolgoch (which was produced by Fletcher, Jennings & Co. )

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3276-458: The stroke when very short cut-offs are used, and therefore the minimum cut-off cannot be as low as on a locomotive with Walschaerts gear. Longer eccentric rods and a shorter link reduce this effect. Stephenson valve gear is a convenient arrangement for any engine that needs to reverse and was widely applied to railway locomotives, traction engines , steam car engines and to stationary engines that needed to reverse, such as rolling-mill engines. It

3339-425: The throttle be wide open and that the boiler pressure is at the maximum safe level to maximise thermal efficiency . For economy, a steam engine is used of a size such that the most economical settings yield the right amount of power most of the time, such as when a train is running at steady speed on level track. When greater power is necessary, e.g. when gaining speed when pulling out of a station and when ascending

3402-456: The tips of the eccentric rods. To change direction, the link and rod ends were bodily raised or lowered by means of a counterbalanced bell crank worked by a reach rod that connected it to the reversing lever. This not only simplified reversing but it was realised that the gear could be raised or lowered in small increments, and thus the combined motion from the “forward” and “back” eccentrics in differing proportions would impart shorter travel to

3465-717: The traditional Johnson Bar (which grew as locomotive power, weight and operating steam pressures increased through the first half of the 20th century) led to it being banned in the USA by the Interstate Commerce Commission . From 1939 all new-build steam locomotives had to be fitted with power reversers and from 1942 Johnson Bar-fitted engines undergoing heavy overhaul or rebuilding had to be retro-fitted with power reverse. Exceptions existed for light, low-powered locomotives and switchers . For switching, which required frequent changes of direction from full-ahead to full-reverse gear,

3528-422: The valve, cutting off admission steam earlier in the stroke and using a smaller amount steam expansively in the cylinder, using its own energy rather than continuing to draw from the boiler. It became the practice to start the engine or climb gradients at long cutoff, usually about 70-80% maximum of the power stroke and to shorten the cutoff as momentum was gained to benefit from the economy of expansive working and

3591-527: Was a major impetus to the development of power reverse systems, because these typically had two or even three sets of reverse gear, instead of just one on a simple locomotive. The Baldwin Locomotive Works used the Ragonnet reversing gear, and other US builders generally abandoned positive locking features sooner than later. Many American locomotives were built, or retro-fitted, with power reversers, including

3654-540: Was mostly the case in Great Britain. Abner Doble considered Stephenson valve gear: "(...) the most universally suitable valve gear of all, for it can be worked out for a long engine structure or a short one. It can be a very simple valve gear and still be very accurate, but its great advantage is that its accuracy is self-contained, for the exact relationship between its points of support (eccentrics on shaft, valve crosshead, and link hanger arm) have but little effect on

3717-461: Was quite common in France. The Allan straight link valve gear (invented by Alexander Allan in 1855) combined the features of the Stephenson and Gooch gears. The reversing and cut-off functions were achieved by simultaneously raising the radius rod and lowering the link or vice versa. As with the Gooch gear, this saved space but the Allan gear gave performance closer to that of the Stephenson. Moreover,

3780-404: Was used on the overwhelming majority of marine engines. The Great Western Railway used Stephenson gear on most of its locomotives, although the later four-cylinder engines used inside Walschaerts gear. Details of the gear differ principally in the arrangement of the expansion link. In early locomotive practice, the eccentric rod ends were pivoted at the ends of the link while, in marine engines,

3843-422: Was used. This one cost £13,278, which was about £600 more than those built at the same time with Walschaerts' valve gear. The aim of the experiment was to find out if a valve gear having variable lead (as opposed to the constant lead of the Walschaerts' motion) would affect performance. On trial, it proved to have no advantage, although in normal service it did gain a reputation as a good performer on banks. As

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3906-481: Was usually air powered, but could also be steam powered. The term servomotor was explicitly used by the developers of some later power reverse mechanisms. The use of feedback control in these later power reverse mechanisms eliminated the need for a second cylinder for a hydraulic locking mechanism, and it restored the simplicity of a single operating lever that both controlled the reversing linkage and indicated its position. The development of articulated locomotives

3969-444: Was variable lead. Depending on how the gear was laid out, it was possible to considerably reduce compression and back pressure at the end of each piston stroke when working at low speed in full gear; once again as momentum was gained and cutoff shortened, so lead was automatically advanced and compression increased, cushioning the piston at the end of each stroke and heating the remaining trapped steam in order to avoid temperature drop in

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