It is apparent that if a “strap” of metal is fixed on the eccentric pulley, and the strap is connected to the valve spindle by a suitable rod, the movement of the eccentric will cause the spindle to move to and fro. The reversing shaft goes The die block works on a pin, or in a fork, at the end of the lever. Alexander Allan, of Crewe works, combined the ideas of the Stephenson and Gooch gears and used them with a straight link, which he claimed was easier to machine up than a curved one. Don quotes the formula for eccentric rod length to give equal lead as t+X^2/2t (Where t is the distance from the axel centre to a defined point relative to the centre of the link). Both the B class and E class Double Fairlies wer… There, in a nutshell, is the whole explanation of the “works” of the engine. One end of this is parallel with the centre line of the valve gear, and the other lines up with the valve spindle, to which it is connected by a short rod. "linked up",and the movement of the extension rod is Some had the gabs pointing downwards towards the valve spindle; some had them pointing upwards; and some had one above and one below, so that they faced one another, and one engaged directly the other disconnected. Link (Expansion Link) 8. Lifting Arm 6. Early locomotives, including the famous “Rocket”, had a single eccentric for each valve, but it was not a fixture on the axle; otherwise the engine could not have been reversed. This page is not intended to discuss the workings of the Experience shows that Bulleid's valve gear was very free running when well maintained. includes an excellent isometric drawing showing the parts in detail. There is the regular “open” locomotive link, which has an open slot, and the fork-ended eccentric rods are coupled above and below it. STEPHENSON’S LINK VALVE GEAR – GWR HALL 4-6-0 This kit is designed to accurately produce in 4mm scale a working model of the Stephenson’s link valve gear. Allan valve gear, a straight-link valve gear. Animating the Valve Gear. This gear did not permit of variation in the point at which steam was cut off from the cylinder. Therefore the valve travel is shortened, steam being cut off sooner in the stroke by the earlier closing of the port; and as the die block approaches the middle of the link so the steam is cut off earlier as the travel of the valve becomes less and less. The “box” link has no open slot, but a curved groove machined out on one side of it, in which the die block works; the eccentric rods, which are not forked, work on pins fixed in the plain side. 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 all kinds of steam engines.It is named after Robert Stephenson but was actually invented by his employees. Where the cylinders are inside the frames, with valves between, or with outside cylinders having valves inside the frame, direct connexion is possible. Because Stephenson’s gear can achieve near perfection in enabling a one-piece valve to supply both ends of a double-acting cylinder equally its design in any one particular case ought to follow a complete and correct procedure, in the knowledge that straying from its basic simplicity will impair its excellence. The amount of lead remains constant over the whole range of cut offs that the gear is designed to provide. This piston-rod is moved back and forth by a piston, which is a disk moving up and down in a circular chamber closed at the ends and called a cylinder. On moving the reverse lever the forward gab is disengaged and the back gear rod comes into operation. Further movement of the piston would result in the valve moving to the right across the port faces in the steam chest, passing steam to the back end of the cylinder and opening the front end to the exhaust - ready for the return stroke. Image size. This upset the Carmichael gear because, if the eccentric were advanced to give early opening when driving the valve direct, it would be correspondingly late when driving through the rocker. The Principles of the Joy and Walschaerts Motions. This arrangement answered perfectly for a time, but soon another trouble arose - the need for expansive working. Valve gears dispensing with eccentrics are now therefore used on all modern locomotives where the eccentrics, if used, would be “outsize”. This shows the reach rod and the piston valve and external rock He connected the valve spindle to one end of a centrally-pivoted rocking lever. Although the joint invention of Williams and Howe, it is known as the “Stephenson” link motion, partly because the firm of Robert Stephenson first applied it, and partly to distinguish it from other forms of link motion, such as Gooch and Allan, which will be mentioned later. A short shaft with two pendulum levers is carried in a substantial bearing on top of the frame; the outside lever is connected to the valve spindle by a short link. To enable the ports to be opened and closed at the right time for the steam to enter either end of the cylinder, push the piston to the other end, and then escape into the air through passageways and blast-pipe, the valves are moved by a simple system of rods and links called a valve gear. A RAILWAY traveller about to start a long journey often strolls up to take a look at the engine, and if he is non-technical he usually regards the locomotive as a wonderful and complicated piece of mechanism. Piston valves which superseded slide valves in the 20th century; 3. Links themselves are of several kinds. Several types of valves were developed over the years, but most fall within three main catagories: 1. Component names used do vary, especially in US practice, but those eccentrics cause the expansion link to rock to and fro. This lay-out of the Stephenson link motion is used largely on the Great Western locomotives, and gives every satisfaction. The locomotive invariably starts with the gear in this position. Already a deviant? THE STRAIGHT LINK of Allan’s valve gear, here shown set for forward running, is moved upwards to reverse the direction of travel, and at the same time the valve-rod die block is moved downwards in the link. Check between the first and second drivers for another hole. For adhesives, read manufacturer's instructions regarding vapour … Now the fun starts; it's time to animate the valve gear. Then there is the “launch” link; this also has the open slot, but the eccentric rods are coupled to lugs on the concave side of the link, opposite the ends of the slot. XD. ahead of the crank, the engine ran forward; if 90 deg. cylinders. Stephenson's Valve Gear: 7mm cast white-metal kit 19 th Century swing-link version - non-working, cosmetic only Instructions HEALTH & SAFETY: The castings contain some lead. is intermediate between mid gear and full gear then the locomotive is Each eccentric is set so that when the crank is on dead centre the valve just begins to open the steam port, with the motion in “full gear”; that is, with the eccentric rod lining up as near as possible with the valve-rod connexion, the die block being at the end of the link. is no reach rod on the left hand side of course) but the rock shafts Wonderful it certainly is; but complicated - certainly not. Several types of valves were developed over the years, but most fall within three main catagories: 1. forward and back. The steam reverser in the diagram above is in fact the early version, use only on the first ten Merchant Navys, with the lever to the valve gear attached to the rod between the two cylinders. Slide valves largely used in the 19th century; 2. Gooch valve gear transferred to the extension rod to open the valves to admit steam.In It is fitted to 0-4-0WT "Dolgoch", preserved on the Talyllyn Railway. The reversing shaft goes across the frames and activates both sets of valve gear, (and there is no reach rod on the left hand side of course) but the rock shafts and rock arm assemblies are on both sides of the locomotive and independant of each other. Stephenson valve gear: | | | British LMS Stanier Class 5 4-6-0 Locomotive no. Stephenson Valve Gear Diagram. Although a brief glance into history (except on the Continent) might place Stephenson’s gear as being largely replaced by Walschaerts’ gear around the turn of the 20 th century, both gears were invented almost simultaneously. IMPROVED GAB GEAR, with two eccentrics, so arranged that lap and lead could be given to the valve to enable the steam in the cylinder to be used expansively. 561x289px 57.58 KB. The steam entering the inlet has pushed the piston from the front (right-hand) end of the cylinder to a point half-way along the bore. This enabled them to be combined on a sleeve, which was loose on the axle. Many of the Brighton tank engines designed by Stroudley would run at quite a good speed, in either direction, in mid-gear. experiences much less horizontal movement so insufficient motion is AN EARLY VALVE GEAR, in which a loose eccentric on the axle is driven by a collar and driving pin. 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