Baker Valve Gear. Reverse Arm & Shaft 7. forward and back. Allan valve gear, a straight-link valve gear. Yet another arrangement is needed when the cylinders are outside and the valves on top of them. This diagram shows the Stephenson link motion reversing gear, with the reverse lever set for running forward. A similar arrangement was widely used in America when the Stephenson link motion was in vogue, before the advent of the radial gears. This particular form survives to this day, and is still fitted to many modern locomotives. One collar was set to drive the eccentrics for forward, and the other for backward motion, the sleeve carrying the eccentrics being capable of sideways movement, sufficient to allow either pin to engage, by a treadle on the footplate. This direct connexion is also used for inside cylinder valves, when the steam chest is above or below the cylinders but inclined towards the centre of the driving axle, instead of being parallel to the cylinder bores. Valve gears such as Walschaert's are said to be constant lead valve gears. [From parts 26-27, published 26 July-2 August 1935], [Read the previous article in part 26] [Read the next article in part 27]. In the above diagram the gab of the forward eccentric is engaged with a pin on the valve spindle. Experience shows that Bulleid's valve gear was very free running when well maintained. There's a link on Don Ashton's website to where you can download it from. Larry also points out the fact that Cut-Off is always 75% when Lap equals Port Width, for all values of Port Width. The farther out it is the more the eccentric will âwobbleâ or âtumbleâ. STEPHENSON VALVE GEAR, the joint invention of Williams and Howe in 1842, was first applied to locomotives by Robert Stephenson & Co, and is still in use throughout the world. ... Stephenson’s gears way back in the Acorn computer days. Component names used do vary, especially in US practice, but those Stephenson valve gear: | | | British LMS Stanier Class 5 4-6-0 Locomotive no. "linked up",and the movement of the extension rod is eccentrics cause the expansion link to rock to and fro. This arrangement answered perfectly for a time, but soon another trouble arose - the need for expansive working. It is apparent that if a âstrapâ 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. Valve gear diagram is also on front cover. 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. Slide valves largely used in the 19th century; 2. There is the regular âopenâ locomotive link, which has an open slot, and the fork-ended eccentric rods are coupled above and below it. When the engine is moving slowly, the port need not open so soon; but when it is running quickly it is obvious that an early opening of the steam ports will help to ensure full pressure on the pistons at the beginning of the stroke, not only maintaining the speed, but also enabling an early cut-off to be made, thus utilizing the steam's expansive properties to the utmost. At the same time, if the rod were not exactly opposite, but a little towards the middle, the travel of the valve would be shortened, and expansive working obtained, due to the steam being cut off earlier. That should be where the rocker shaft would be on a locomotive with the main driver in the third position. Download Windows Valve Gear Programs File 10A, 1.57 MB, Version 2.7.3 Download Windows Valve Gear Programs File 10B, 1.26 MB, Version 2.7.3 Download Windows Valve Gear Programs File 10C, 1.25 MB, Version 2.7.3 Download Windows Valve Gear Programs File 10D, 1.28 MB, Version 2.7.3 reduced and affected by both eccentrics. This fine model in the Science Museum, South Kensington, can be operated by the public and clearly shows the application and working principles of the Gooch gear. These could be raised or lowered to engage with pins on a rocking lever attached to the valve spindle, so producing a backward or forward movement of the engine as required. 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. A plate with a hole in it was fixed to the outside of each eccentric, and a collar on the axle outside of this carried a peg which fitted the hole. Lifting Arm 6. The locomotive invariably starts with the gear in this position. The amount of lead remains constant over the whole range of cut offs that the gear is designed to provide. To move the engine ahead, steam must be admitted in front of the piston; to move it backwards, steam must enter the cylinder behind it; and to enable the valve to âkeep its relationshipâ, as one might say, with the piston at every part of the stroke, a different setting is necessary for either direction of movement of the engine. gear, nor the implications of the relationships of the components. The Walschaert gear also transmits the moving force of the valve in practically a straight line; consequently there are less springing and yielding of the parts than in the Stephenson gear. July 24, 2009 Previous Next Go to Decoders for HO Atlas Genset: November 1, 2010 Wishlist Data. When the die block is in the middle of the link it escapes the âwobbleâ, but is moved by the bodily swing of the link; and, this being just sufficient to move the valve and open the port at either end, steam enters the cylinders and the engine keeps going. 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. Southern Valve Gear. Link (Expansion Link) 8. IN FORWARD GEAR. Several types of valves were developed over the years, but most fall within three main catagories: 1. The eccentric rods were connected to the top and bottom of the link, but the link itself was placed the other way round, with the concave side towards the steam chest. across the frames and activates both sets of valve gear, (and there GOOCHâS GEAR. By Steammechanic Watch. transferred to the extension rod to open the valves to admit steam.In It is based on the eccentric rod valve gear developed for stationary steam engines, but with the added complexity that unlike a stationary steam engine, a steam locomotive needs to reverse. This arrangement reduces the vertical height required to accommodate the gear on the locomotive. form, or if you prefer use Linked In or even Facebook. CARMICHAELâS VALVE GEAR made use of a single eccentric and a pair of V-shaped gabs. Many of the Brighton tank engines designed by Stroudley would run at quite a good speed, in either direction, in mid-gear. Reach Rod: 4. 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. and rock arm assemblies are on both sides of the locomotive and Stephenson valve gear - most common valve gear in the 19th century, normally inside the locomotive frame. Stephenson's Link Motion. All motorists know the advantage of advancing the ignition as far as possible when a car is running at high speed. #267B in the International Library of Technology. It is fitted to 0-4-0WT "Dolgoch", preserved on the Talyllyn Railway. arm. and from the components inside the frames. rod movement, and in reverse gear the die block is at the lower end layout is fundamentally similar on all the classes with two outside There were several variations of the gab motion. The easiest one to use for Stephenson's is Bill Hall's. This lay-out of the Stephenson link motion is used largely on the Great Western locomotives, and gives every satisfaction. Stephenson Valve Gear Diagram. Lifting Link 5. HOW THE LOCOMOTIVE WORKS. Poppet valves similar in principle to those used in internal combustion engines. Walschaerts Valve Gear. If the eccentric were 90 deg. 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. This lifts the link; and the die block, instead of being right at the end of the slot, is now a little way from it. Reply. Bing (M'soft) block and the extension rod are hung from the link hanger, and move 4767 sho... World Heritage Encyclopedia, the aggregation of the largest online encyclopedias available, and the most definitive collection ever assembled. The Gooch valve motion set for running ahead. 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. He connected the valve spindle to one end of a centrally-pivoted rocking lever. Steam is admitted and exhausted from either end of the cylinder by a valve, which may be of the slide or piston type. XD. William T. James valve gear 1832, used first by Baltimore and Ohio Railroad, United States. Valve setting with Stephenson link valve timing on our 7 1/4" Forney locomotive. The locomotive is a parallel instance; taken as a whole it appears formidable and complicated, yet any average person can easily understand it if the component parts are separately considered. A typical example of this was to be found in the old Stroudley âGladstonesâ 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. Daniel Gooch, the first locomotive superintendent of the Great Western, devised a link motion in which the link itself was not raised or lowered for reversing, but swung from stationary hangers in what would be mid-position in the Stephenson motion. Once the general principles are understood the various means of applying them will present no difficulty. In this position, the maximum movement is transmitted to the valve spindle. This picture shows a working demonstration model of the gear in the Science Museum, South Kensington. If, however, the valve spindles and piston rods are parallel, this is not possible, as the drive would then become what the railwaymen call âskew-whiffâ; so a rocking lever is introduced. The die block works on a swinging arm pivoted to the inside of the frame, and movement is transmitted from this to the inner pendulum lever by an inclined coupling-rod. 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. To reverse the engine the driver had to move the treadle half-way, lift the pins out of engagement, move the valve spindles by another lever, until the engine began to move backward, and force the treadle right home as the holes in the eccentric plates came opposite the pins in the crank collarsâa truly cumbersome way of reversing. Walschaerts' gear has two distinct inputs, from the eccentric & from the crosshead. The opening time, however, still had to be the same - that is, with the crank on dead centre; and therefore, the eccentric had to be moved on ahead of the 90 deg. It is obvious that there is less to-and-fro movement at this point of the link than at the ends. It was obvious that a quick reversing gear was essential, and so the single-lever-operated gear came into being. Early locomotives, including the famous âRocketâ, had a single eccentric for each valve, but it was not a fixture on the axle; otherwise the engine could not have been reversed. Thus, the link motion came into being. ... especially in this context the page on Stephenson's 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 all kinds of steam engines.It is named after Robert Stephenson but was actually invented by his employees. Guys I've been re-reading Don's book on Valve Gears and, in particular, the Stephenson's Gear section. Slide valves largely used in the 19th century; 2. For reasons of practicality two significant deviations from the prototype have been made. Where the cylinders are inside the frames, with valves between, or with outside cylinders having valves inside the frame, direct connexion is possible. This is really the better plan, as it compensates for any slight misalignment of valve spindle and extension, and prevents undue friction in the motion and excessive wear on the glands. Nutty is a valuable text and It is, for example, extensively used on the Great Western for two-cylinder tender and tank engines, the general efficiency of which is above question. The piston moves up and down the cylinder, in exactly the same way, whichever way the engine runs. Piston valves which superseded slide valves in the 20th century; 3. The net result is that the whole link swings bodily back and forth, in addition to the movement at the ends given by the âwobblingâ of the eccentrics themselves. From Larry Koehl, Live Steam Magazine, June 1976 The Throw of Eccentric formula, below, applies only to locomotives using Stephenson valve gear. We must, however, consider what happens when the piston is half way along the stroke, and the crank on bottom centre, at its lowest position. As the two eccentrics are, of course, firmly fixed to the axle, it naturally follows that as the eccentrics revolve, this eccentric centre will sometimes be in front of and sometimes behind the true centre of the axle. when the locomotive is put into forward and reverse.The In mid gear the die block is in the centre of the expansion link, and 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. 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. The final arrangement was a double gab on the valve spindle itself, the ends of the two eccentric rods being connected by a bar. This reversing motion, designed by Daniel Gooch in 1847, had curved links supported by a hanger. As stated earlier, the eccentrics do not come exactly opposite, each one being advanced beyond the âright angleâ, to compensate for the lap and lead of the valve; and as this advance is in the direction of motion the angles will be on the same side of the crank. Now the fun starts; it's time to animate the valve gear. For adhesives, read manufacturer's instructions regarding vapour … The die Piston Valve Steam Locomotives" by E.J. (A Walschaert Gear section starts on page 87.) The valves on a steam engine control the flow of steam in and out of the cylinders. This minimizes âlost motionâ due to any wear or slackness in the parts, and also gives better valve movements when the engine is running notched up, as there are no âoffsetâ connexions for which allowance would have to be made. Its simply to enable the reader to identify and name the key parts. While all these arrangements worked well as reversers pure and simple, the fixed cut-off point still remained; but in 1842 a draughtsman named Williams, employed by Robert Stephenson & Co, conceived the idea of uniting the two eccentrics by means of a link, and connecting the valve rod to a die-block working in it, so that, by moving the rod opposite one or other of the eccentrics, the engine could be reversed. 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. In other instances, the valve spindle extensions do not work in guides at all, but are suspended from swinging levers pivoted to the frames. There may be two, three, or four of these cylinders. For some more sophisticated understanding of valve gears I suggest The ordinary type of motion, in which the fore-gear eccentric rod is connected to the top of the link and the back-gear rod to the bottom, âadvances the ignitionâ, as it were, as the die block approaches the centre of the link, and the bodily swing of the link mentioned above takes effect. (and almost everything else) on GWR Locomotives "GWR Two Cylinder As soon as it gets under way the driver ânotches upâ by bringing the reverse lever a little towards the middle. To contact the writer please use this The Walschaerts valve gear was slow to gain popularity. Yahoo. Return to the GWR Information Index Page. Despite its advantages, the Stephenson link motion has fallen from favour during recent years; this, however, is not due to any inherent disadvantages in the gear itself, but rather to mechanical drawbacks which make it unsuitable for very large engines. Stephenson Valve Gear. Valve gears dispensing with eccentrics are now therefore used on all modern locomotives where the eccentrics, if used, would be âoutsizeâ. Connexion between die block and valve spindle is made in a variety of ways. ahead of the crank, the engine ran forward; if 90 deg. 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. 1.1 Indicator diagram of steam cycle 4 2.1 Newcoman atmospheric engine 9 2.2 Green steam engine 10 2.3 Jensen steam engine 12 2,4 Watt steam engine 13 2.5 Stuart steam engine 14 2.6 Stephenson Valve Gear 16 2.7 Walschaert Valve Gear 17 2.8 Flywheel 18 2.9 Sliding valve 18 2.10 Piston and piston rod 19 2.11 Crosshead 20 3.1 Project flow chart 23 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. At first sight, a non-technical or lay reader might wonder why a reversing gear is necessary. 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 âfull gearâ; 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. The missing parts appear to be the valve stem and rocker arm. Inside Walschaerts' isn't common, the Great Western Castles & Kings used it; the geometry is exactly the same as outside gear. Image size. This valve gear fulfils a double purpose. Valve gears such as Stephenson's for example are variable lead valve gears. Larry suggests adding up to 0.002 inches to the Throw of Eccentric value for each pin in the valve gear (including the eccentric, too) because, even when new, there will be lost motion, possibly enough to eliminate the lead. This setting information is the same for all Stephenson link valve gear. It not only allows for reversing the engine, but the driver can also set it to cut off steam at any part of the stroke of the piston. Links themselves are of several kinds. full forward gear only the forward eccentric rod controls extension Williams's idea in its original form could not be carried out, as it was mechanically impossible to couple the eccentrics themselves by a link; but âtwo heads are better than oneâ, and a Stephenson pattern-maker named Howe made the idea a working pro position by shifting the link to the ends of the eccentric rods. There's a good diagram of the Stephenson valve gear on page 24 of MR Cyclopedia Volume 1 - Steam Locomotives. Allan valve gear, a straight-link valve gear. The die block works on a pin, or in a fork, at the end of the lever. 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