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The weighbar or reverse shaft had two arms for each set of gear; one arm had a “drop” or lifting-link connected to the straight expansion link, and the other arm was similarly connected to the valve rod, so that when the driver operated his reversing lever the link would move up or down as required, and at the same time the valve rod would move in the opposite direction to meet it. The steam ports are opened a little sooner than they are when in full gear. First Edition. A similar arrangement was widely used in America when the Stephenson link motion was in vogue, before the advent of the radial gears. The valves on a steam engine control the flow of steam in and out of the cylinders. Dispose of filings safely and wash hands after handling. extension rod movement. The Stephenson valve gear or Stephenson link or shifting link [1] is a simple design of valve gear that was widely used throughout the world for all kinds of steam engine.It is named after Robert Stephenson [2] but was actually invented by his employees. Some engines will run in either direction, with the lever in the middle, and die blocks in the middle of the link; the advance of the eccentrics, due to the valve lap, is responsible for this apparent paradox. Yahoo. AN EARLY VALVE GEAR, in which a loose eccentric on the axle is driven by a collar and driving pin. When a train is running it does not need much steam to keep it going, and a mere puff in each cylinder at the beginning of the stroke is often sufficient. Eccentric Crank 2. There are numerous sources for descriptions of the functioning of DOUBLE GABS on the valve spindle were used in the final development of the gab gear, and the two eccentric rods were connected by a bar as shown in this diagram. The idea and principles of the Stephenson link motion have been applied to other forms of valve gear. The reversing lever shifts the link, which is straight and not curved as in the Stephenson and Gooch gears, and at the same time moves the die block attached to the valve rod seen at the right of the picture. Spend a bit more time with the valve gear program. when the locomotive is put into forward and reverse.The It is fitted to 0-4-0WT "Dolgoch", preserved on the Talyllyn Railway. 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. 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. 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). The Walschaerts valve gear was slow to gain popularity. When the position of the expansion link For the smaller types of locomotives, however, the gear continues to hold its own. This arrangement answered perfectly for a time, but soon another trouble arose - the need for expansive working. independant of each other. The eccentric rods were not therefore directly connected to the valve spindles, but drove them by a hook and pin, which could be lifted out of engagement by a handle on the footplate. This had projecting pins at either end, and the eccentric rod had a double “gab”, resembling two V-forks placed back to back, at the end. full forward gear only the forward eccentric rod controls extension This reversing motion, designed by Daniel Gooch in 1847, had curved links supported by a hanger. Stephenson valve gear: | | | British LMS Stanier Class 5 4-6-0 Locomotive no. Share: Go to Turbine Lift Off! Stephenson's Link Motion. To contact the writer please use this Valve gear diagram is also on front cover. Reverse Arm & Shaft 7. Piston Valve Steam Locomotives" by E.J. ... Stephenson’s gears way back in the Acorn computer days. A typical example of this was to be found in the old Stroudley “Gladstones” of the London, Brighton and South Coast Railway, where the piston rods passed over the leading coupled axle, and the valve spindles below it, the former inclining downward and the latter upward, the centre lines meeting at the middle of the crank axle. Piston valves which superseded slide valves in the 20th century; 3. (and almost everything else) on GWR Locomotives "GWR Two Cylinder Its simply to enable the reader to identify and name the key parts. ALLAN’S STRAIGHT LINK MOTION, designed in 1855, made use of two eccentrics for forward and reverse gear. There are various forms of the Stephenson link motion itself, but all of them have the two eccentrics, one set for forward and one for backward gear, the ends being coupled to a curved slotted link. This diagram shows the Stephenson link motion reversing gear, with the reverse lever set for running forward. Baker Valve Gear. with This enabled them to be combined on a sleeve, which was loose on the axle. 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. Slide valves largely used in the 19th century; 2. Link (Expansion Link) 8. Valve gears dispensing with eccentrics are now therefore used on all modern locomotives where the eccentrics, if used, would be “outsize”. By Steammechanic Watch. The die block works on a pin, or in a fork, at the end of the lever. A big eccentric running hot and beginning to bind forms a very effective band-brake. 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