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. To contact the writer please use this
The die block works on a pin, or in a fork, at the end of the lever. Return to the GWR Information Index Page. across the frames and activates both sets of valve gear, (and there
This reversing motion, designed by Daniel Gooch in 1847, had curved links supported by a hanger. Walschaerts Valve Gear. Valve gears site, especially in this context the page on Stephenson's
HOW THE LOCOMOTIVE WORKS. This enabled them to be combined on a sleeve, which was loose on the axle. cylinders. 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. The âRocketâsâ gear worked on this principle. 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. 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. Its simply to enable the reader to identify and name the key parts. There's a link on Don Ashton's website to where you can download it from. of the expansion link and only the reverse rod contributes to
Valve gear diagram is also on front cover. GOOCHâS GEAR. 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. 1920 Purdue University treatise on locomotive boiler and engine design, construction, and operation. Slide valves largely used in the 19th century; 2. XD. (and almost everything else) on GWR Locomotives "GWR Two Cylinder
This was soon superseded by the eccentric, which is merely a flat pulley mounted on the axle; but the hole for the axle is bored out of centre - hence the name. The amount of lead remains constant over the whole range of cut offs that the gear is designed to provide. Everything in this world takes time; and it requires a certain amount of time for the steam to pass through the port and exert its full pressure on the piston head. Yahoo. DESIGN AND INVENTION - 13 THE Stephenson, Allan, and Gooch link motions required four eccentrics on the driving axle, as explained in the chapter “Locomotive Valve Gears”.With inside-cylinder engines this requirement restricted the thickness of the crank webs - a serious matter as the engines increased in size. The âaheadâ and âasternâ positions of the eccentrics were, of course, exactly opposite, and so the treadle could not be operated unless they were moved half a turn, or the engine moved to suit. Walschaerts' gear has two distinct inputs, from the eccentric & from the crosshead. 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. IN FORWARD GEAR. The former is flat, working over ports cut in a flat face on the cylinder casting. 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. 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 dispensing with eccentrics are now therefore used on all modern locomotives where the eccentrics, if used, would be âoutsizeâ. A school child may pause with awe if asked to spell âincomprehensibilityâ; but, if the word is divided into two and three letter syllables, any child will spell it without hesitation. The action of the gear is simplicity itself. Stephenson Valve Gear Diagram. Stephenson valve gear - most common valve gear in the 19th century, normally inside the locomotive frame. The Stephenson valve gear or Stephenson link or shifting link is the oldest and simplest standard design of steam locomotive valve gear.The design is normally attributed to George Stephenson.. A big eccentric running hot and beginning to bind forms a very effective band-brake. That should be where the rocker shaft would be on a locomotive with the main driver in the third position. 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. 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. Once the general principles are understood the various means of applying them will present no difficulty. when the locomotive is put into forward and reverse.The
Link (Expansion Link) 8. Animating the Valve Gear. Then there is the âlaunchâ 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. form, or if you prefer use Linked In or even Facebook. 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 The die
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. No lap or lead could be given to the valve with this gear. 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.
There may be two, three, or four of these cylinders. Connexion between die block and valve spindle is made in a variety of ways. The valves on a steam engine control the flow of steam in and out of the cylinders. Young Valve Gear . 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. This gear did not permit of variation in the point at which steam was cut off from the cylinder. 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. The rocking lever itself is pivoted at the middle, and supported by a lug on the motion plate. Gooch valve gear FULL FORWARD. the reversing shaft by the lifting link, and is moved up and down
Guys I've been re-reading Don's book on Valve Gears and, in particular, the Stephenson's Gear section.
Valve gears such as Walschaert's are said to be constant 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 had projecting pins at either end, and the eccentric rod had a double âgabâ, resembling two V-forks placed back to back, at the end. 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. Lifting Link 5. The very earliest valves were operated by a simple tappet gear. independant of each other. setting, to an amount sufficient to allow the valve to open at the right time. 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