THE STEAM CYLINDER, ITS PARTS AND CONNECTIONS.
The cylinder proper is constructed of a single piece of cast iron bored out smooth.
The cylinder heads are the flat discs or caps bolted to the ends of the cylinder itself. Sometimes one cylinder head is cast in the same piece with the engine frame.
The piston is a circular disc working back and forth in the cylinder. It is usually a hollow casting, and to make it fit the cylinder steam tight, it is supplied on its circumference with piston rings. These are made of slightly larger diameter than the piston, and serve as springs against the sides of the cylinder. The follower plate and bolts cover the piston rings on the piston head and hold them in place.
CONNECTING ROD AND CROSS-HEAD.
(J. I. Case Threshing Machine Co.)
The piston rod is of wrought iron or steel, and is fitted firmly and rigidly into the piston at one end. It runs from the piston through one head of the cylinder, passing through a steam-tight “stuffing box.” One end of the piston rod is attached to the cross-head.
The cross-head works between guides, and has shoes above and below. It is practically a joint, necessary in converting straight back and forth motion into rotary. The cross-head itself works straight back and forth, just as the piston does, which is fastened firmly to one end. At the other end is attached the connecting rod, which works on a bearing in the cross-head, called the wrist pin, or cross-head pin.
The connecting rod is wrought iron or steel, working at one end on the bearing known as the wrist pin, and on the other on a bearing called the crank pin.
The crank is a short lever which transmits the power from the connecting rod to the crank shaft. It may also be a disc, called the crank disc.
CROSS-HEAD.
(J. I. Case Threshing Machine Co.)
Let us now return to the steam cylinder itself.
The steam leaves the boiler through a pipe leading from the top of the steam dome, and is let on or cut off by the throttle valve, which is usually opened and closed by some sort of lever handle. It passes on to the
Steam-chest, usually a part of the same casting as the cylinder. It has a cover called the steam-chest cover, which is securely bolted in place.
The steam valve, usually spoken of simply as the valve, serves to admit the steam alternately to each end of the cylinder in such a manner that it works the piston back and forth.
There are many kinds of valves, the simplest (shown in the diagram) being the D-valve. It slides back and forth on the bottom of the steam-chest, which is called the valve seat, and alternately opens and closes the two steam ports, which are long, narrow passages through which the steam enters the cylinder, first through one port to one end, then through the other port to the other end. The exhaust steam also passes out at these same ports.
The exhaust chamber in the type of engine now under consideration is an opening on the lower side of the valve, and is always open into the exhaust port, which connects with the exhaust pipe, which finally discharges itself through the exhaust nozzle into the smoke stack of a locomotive or traction engine, or in other types of engines, into the condenser.
The valve is worked by the valve stem, which works through the valve stem stuffing-box.
Of course the piston does not work quite the full length of the cylinder, else it would pound against the cylinder heads.
The clearance is the distance between the cylinder head at either end and the piston when the piston has reached the limit of its stroke in that direction.
In most engines the valve is so set that it opens a trifle just before the piston reaches the limit of its movement in either direction, thus letting some steam in before the piston is ready to move back. This opening, which usually amounts to 1-32 to 3-16 of an inch, is called the lead. The steam thus let in before the piston reaches the limit of its stroke forms cushion, and helps the piston to reverse its motion without any jar, in an easy and silent manner. Of course the cushion must be as slight as possible and serve its purpose, else it will tend to stop the engine, and result in loss of energy. Some engines have no lead.
Setting a valve is adjusting it on its seat so that the lead will be equal at both ends and sufficient for the needs of the engine. By shortening the movement of the valve back and forth, the lead can be increased or diminished. This is usually effected by changing the eccentric or valve gear.
The lap of a slide valve is the distance it extends over the edges of the ports when it is at the middle of its travel.
Lap on the steam side is called outside lap; lap on the exhaust side is called inside lap. The object of lap is to secure the benefit of working steam expansively. Having lap, the valve closes one steam port before the other is opened, and before the piston has reached the end of its stroke; also of course before the exhaust is opened. Thus for a short time the steam that has been let into the cylinder to drive the piston is shut up with neither inlet nor outlet, and it drives the piston by its own expansive force. When it passes out at the exhaust it has a considerably reduced pressure, and less of its force is wasted.
Let us now consider the