Further Methods of Distributing Steam to the Cylinders
WE have already seen in previous chapters that a slotted link, with a die-
Valve gears have therefore been evolved which either use a stationary link, with a die-
The Baker Valve Gear
The inventor of the gear, Mr. Abner D. Baker, an American engineer, first applied it to a traction engine in 1903. It proved satisfactory even in its original form, and in 1908 it was re-
At the first glance the action of the gear is not apparent to the casual observer, but a study of the gear in detail reveals that it is exceedingly simple. The principle on which it works may be easily understood by comparing it with the Joy gear described in the chapter beginning on page 1072. Instead of the radius rod being attached to a die-
NO SLOTTED LINK IS NEEDED by radial valve gears working on the Baker principle shown left. The valve rod is connected to the end of a radius bar swinging on a pivot, which is moved by the reverse lever as required. In mid-
FOR FORWARD RUNNING the pivot is dropped below the centre line of the valve spindle, and the arc in which the end of the bar swings is inclined in the opposite direction. The highest point of swing is now nearest to the valve spindle; thus the opposite port is opened, and the engine will go ahead.
TO REVERSE THE ENGINE the pivot carrying the end of the swinging radius bar is lifted out of line with the valve spindle. As the bar swings, the end of the valve rod attached to it follows its movement and actuates the valve, the highest point of swing being farthest from the valve spindle.
The free end of the bar, to which the radius rod is connected, will describe an arc, which corresponds to the curved guide of the Joy gear. If the fixed end of the bar is below the centre line of motion, the upper end of the arc will be nearest to the cylinder, and the lower end farthest away from it; this corresponds to the forward tilt of the Joy shaft, and the engine will go ahead. If, however, the fixed end of the bar be raised above the centre line of motion, inclination of the arc is completely reversed; the upper end is now farthest from the cylinder, and the lower end is the nearest. The movement of the rod is now changed; the valve is reversed, and the engine will move backward.
In the Baker valve gear, the swinging bar -
The Baker valve gear is “self-
The reverse yoke is double, either side being pivoted at the bottom to a point in the frame, so that the yoke can be swung back and forth by a reach rod connected at one end to the top of the yoke, and at the other to the cab lever. The radius bars swing from pivots in the upper part of the reverse yokes, so that by inclining the reverse yokes either forward or backward, these pivots are set either in front or behind the centre line of the gear -
The working is as follows. When the cab lever, and consequently the reverse yoke, is in mid-
THE BAKER VALVE GEAR shown with the rods separated to illustrate the action. The gear connecting rod, being fixed by its middle to the bottom end of the radius bar, follows the swing of the latter, and pushes the bell-
As soon as the reverse yoke is thrown forward, the radius bar pivots move to a point ahead of the centre line of the gear; the bottoms of the radius bars swing in an inclined arc instead of a horizontal one; consequently the gear connecting rod, attached to them by its middle, follows their swing and moves bodily up and down. The top of the gear connecting rod, being attached to the bell-
The Southern Valve Gear
Everyone conversant with the history of the steam engine knows the story of the lazy boy who became tired of operating the valve cocks of a Newcomen pumping engine, connected up the cocks by pieces of string to the engine beam, made it self-
The Southern valve gear belongs to the “radial” group, and although it employs a link and die-
The designer of the gear considered that there were too many rods, levers, pins, and joints in the ordinary gears. He set out to cut them down to the minimum in his own gear, and he certainly achieved no small amount of success. The few moving parts are easily maintained, and renewed as required with very little trouble.
HOW IT WORKS. The Southern valve gear employs a stationary link. When the reverse lever is pulled back, the die-
The action of the gear is as follows. In mid-
When the die-
When the reverse lever is pulled back, the die-
THE SOUTHERN VALVE GEAR applied to a Midland Terminal eight-
The Young and Deeley Gears
Although the Young and Deeley gears have not had the extended application of those described above, they are worthy of note because of their incorporation of an ingenious idea. In all the preceding gears, the primary movement has been derived either from eccentrics or from return cranks, set at (or near) right angles to the main cranks. The designers of the Young and Deeley gears do not use separate return cranks to operate their expansion links, but take advantage of the fact that the main cranks of a normal two-
The only trouble is a minor one. When separate return cranks are used, they are both set to "lead" or "follow" the main crank, and both die-
THE YOUNG VALVE GEAR derives its motion from the crank on the other side of the engine, instead of from a separate return crank set at right angles to the main crank. This diagram shows how the movement is transmitted across the frames.
The action of both gears is very much the same as the Walschaerts gear. In the Young gear the expansion link is mounted in the same way, but has a long tail, which is directly connected to the crosshead by a union link. The radius rod works on a short lever attached to a cross-
The inverted head of the combination lever is connected to the valve spindle in the usual way. The reverse shaft is double, and has two pairs of reverse arms, one being connected to a solid inner shaft, and the other to a tubular outer shaft. A double reach rod is used; and, whichever way the cab lever is moved, the die-
In the Deeley gear, as fitted to the "990" series of inside-
The Young Valve gear can be arranged to give a quicker opening and closing of the ports than the Walschaerts gear, with normal setting; but as each side of the engine is dependent on the other side for its primary movement, it means that, should any defect develop in one side only, the engine is rendered helpless, and cannot be operated on one cylinder only, as on an engine with independent gears. Though breakdowns are infrequent nowadays, nevertheless some prejudice exists, and the interdependence factor has been put forward as one reason why the gears have not been more extensively used.
Miscellaneous Valve Gears
In addition to the valve gears already described, there are a few miscellaneous gears, examples of which are found mostly on industrial locomotives engaged on short private lines such as colliery railways, and on engines used for works, depot, and dock shunting. The Hackworth gear, for example, designed by a relative of the famous Timothy Hackworth, of Stockton and Darlington Railway fame, employs a slide shaft and radius rods similar to the Joy gear, except that they are straight instead of curved. The die-
When a locomotive with slide or piston valves actuated by a valve gear of ordinary type is running well notched up, cutting off steam quite early in the stroke, an unwelcome factor is introduced, inasmuch as the exhaust ports also close early, and the trapped steam causes excessive compression in the cylinders. An engine would often run with a much earlier cut-
CYLINDERS for a Great Indian Peninsula Railway locomotive. Steam to these cylinders is controlled by an oscillating cam valve gear.
Piston and slide valves being "one-
In one form of the Lentz poppet valve gear, the inlet and exhaust valves are set horizontally at different levels. Each valve stem is actuated by a short vertical lever, the admission valve levers being pivoted at the top, and the exhaust at the bottom. Rollers are arranged in the centre of each lever; these come opposite one another, and the oscillating camshaft works between them. The end of the shaft projects through the steam chest, and is worked by an ordinary Walschaerts gear. By careful planning of the cam contours, the opening and closing of both inlet and exhaust valves can be arranged so that a close cut-
A SOUTH AFRICAN RAILWAY ENGINE of the
The Caprotti gear more closely follows automobile practice, as it uses a rotary camshaft and vertical valves. The valves are double-
Reversing and notching up are effected by altering the position of the cams, which are made to slide on the shaft, and are connected by an ingenious arrangement of rods and levers to a wheel-
Because a locomotive may have three or four cylinders, it does not follow that it must have the same number of sets of valve gear. In the GWR "Stars", "Castles", and "Kings", the inside cranks are directly opposed to the outside cranks, and therefore all that is needed to operate the valves of the outside cylinders is a simple centrally-
ON A LNER 4-
[From part 40, published 1 November 1935]