2. Copyright © R.Holme 2003. All rights reserved. Steam Engine Valves The valves used for opening the steam ports or passages and closing them may be placed in three groups, viz., slide valves, piston valves, and poppet valves. In addition to these three types there is the Uniflow engine, in which special exhaust ports take the place of valves. The gear operating these valves may be the same for the slide and piston valve, but poppet valves require different gear. The type of valve and valve gear used has a decided influence on the degree to which the admission, cut off, release and compression can be adjusted. It also influences the quickness with which a valve opening or closing can be accomplished, and therefore effects the economy and flexibility of operation of the engine.

3. Copyright © R.Holme 2003. All rights reserved. Slide Valve The slide valve was the earliest form of valve and it is still used on many engines. It is not an economical valve to use. A slide valve is that part of an engine which controls the admission of steam to the cylinder, and, after the steam has completed its work on the piston, allows the exhaust steam to discharge from the cylinder. Slide valves may be of the rectangular type working on a flat Face within the steam chest, or of the piston type, working in a cylindrical steam chest.

4. Copyright © R.Holme 2003. All rights reserved. Slide Valve (2) The slide valve maybe operated by various valve gears, most of which are in turn operated by an eccentric attached to the crankshaft. The setting of the valve is most important, as the efficiency of the engine is dependent on the valve's being set correctly.

5. Copyright © R.Holme 2003. All rights reserved. Slide & Piston Valves

6. Copyright © R.Holme 2003. All rights reserved. Piston Valve The piston valve is an improvement on the "D" type slide valve and is suitable for high pressure and high temperatures, its principal advantage being a great reduction in friction. In effect, the piston valve is a slide valve so modified in form that, instead of sliding on a flat seat, it slides lengthways within a cylindrical seat. In addition to this change of form, it is common to admit the steam at the centre of the valve instead of at the ends, as in a common slide valve, and so have the advantage of exhaust steam at a low pressure on the cover joints and glands. In this case, the eccentric sheave is different in the setting, being in the position of following the crank instead of leading, as in the case of the outside admission valve.

7. Copyright © R.Holme 2003. All rights reserved. Pressure Effects – Slide Valves For large engines working at high pressures the "D" type slide valve is subjected to excessive friction between the valve and valve chest faces. This is brought about by the pressure acting on the back of the valve, and forcing it against the face of the valve chest. Effective work is lost in overcoming the friction on the valve and more wear will take place at the valve face, eccentric straps and sheaves. All work done in moving the valve will reduce the efficiency of the engine. The piston and poppet valves are not subjected to these loads and are practically frictionless; therefore for high pressure and high temperature steam these valves can be used.

8. Copyright © R.Holme 2003. All rights reserved. Steam Lap Steam lap is the amount by which the steam edge of the slide valve overlaps the steam port when the valve is in mid- position; the greater the steam lap, the earlier the cut-off of steam. Without steam lap, the steam is carried for the full length of the stroke, i.e., the engine works without expansion of the steam in the cylinder.

9. Copyright © R.Holme 2003. All rights reserved. Exhaust Lap Exhaust lap is the amount by which the exhaust edge overlaps the steam port when the valve is in mid-position. This controls the outlet of the exhaust steam after it has done its work. The greater the exhaust lap, the earlier closing of the port to exhaust. This causes what is known as the compression of the steam. Compression gives a cushioning effect to the piston and the end of the stroke, so bringing the piston to rest quietly, after completing the stroke.

10. Copyright © R.Holme 2003. All rights reserved. Minus Exhaust Lap This is sometimes spoken of as an Inside Lap and it is the amount the steam port is open to exhaust when the valve is in mid- position. The greater the minus exhaust lap, the later the closing of the port to exhaust. This has the effect of giving less compression and reducing the cushioning effect

11. Copyright © R.Holme 2003. All rights reserved. Lead Lead is the amount of steam port opening at the beginning of the stroke. This has the effect of admitting steam to the cylinder before the crank has reached the dead centre, preventing what is known as "wire-drawing" or throttling of the steam at the beginning of the stroke, and so commencing the stroke with the full pressure of the steam. Lead also has the effect of increasing the cushioning of the piston at the end of the exhaust stroke. The amount of steam lap and lead given to a slide valve determines the setting of the eccentric sheave, that is, the angle of the eccentric relative to the crank (the Angle of Advance of the eccentric).

14. Copyright © R.Holme 2003. All rights reserved. Valve Motion Animation

15. Copyright © R.Holme 2003. All rights reserved. Piston Valves Piston valves may be of the outside steam or the inside steam type. In the outside steam type the steam enters over the ends of the valve and exhausts in the centre. In the inside steam type the steam enters in the centre of the valve and exhausts over the ends. The advantage of the inside type over the outside.type is that in the former, exhaust pressure only acts on the cover joints and the glands, making it much easier to maintain steam tightness.

16. Copyright © R.Holme 2003. All rights reserved. Stephenson’s Link This system of valve gear is one of the oldest in existence and while a large number of patent gears have been designed to take its place, it is still in common use today. The gear consists of an arrangement designed to form a means of reversing steam engines and it is composed of two complete sets of eccentrics, rods, and link, the eccentric rods being connected at the top by means of a link, in which a motion block slides, this is attached to the slide valve spindle. Either eccentric may be brought into operation by sliding the link backwards or forwards by means of the reversing arm, so that either eccentric rod may be brought in line with the valve spindle, bringing the valve under the control of that particular eccentric, the other eccentric being out of action. As the eccentrics are keyed to the shaft at different angles, the direction of rotation of the engine may be reversed.

17. Copyright © R.Holme 2003. All rights reserved. Stephenson’s Link (2) Opening out the link consists of sliding the link towards the eccentric rod in use, so increasing the travel and making all events occur later. This gives greater power to the engine, using more steam with less expansion, and reducing the economy of the engine. An important advantage of this gear is that the cut-off of the steam may be varied by what is known as "linking up" or "opening out" the link. Linking up consists of sliding the link over by means of the reversing lever, until the eccentric rod is out of line with the valve spindle. This has the effect of reducing the valve travel and making all events occur earlier, cutting off the steam earlier and giving greater expansion of the steam. This gives greater economy in running the engine.

19. Copyright © R.Holme 2003. All rights reserved. Open and Crossed Rods With open rods the earlier the cut off, the greater the lead. With crossed rods the lead decreases with the shortening of the cut off. In either case the port opening is reduced but to a lesser extent with open rods