This book is intended as an elementary treatise on the kinematics of reciprocating steam engines and steam turbines. Sufficient attention is given to the behavior of the steam itself to enable the student to study intelligently the machine for which the steam is the source of power. The indicator card, or pressure- volume diagram, is employed in this connection. No consideration is given to the underlying heat theory or to the details of construction of the various parts of the machines.

The book has been planned primarily to meet the needs of students who take up this subject as a part of, or immediately following, their course in the elements of mechanism, before they study the theory and practice of heat engineering or machine design.

The purpose of the authors has been to present the subject in such a way as to make clear to the beginner the mechanical principles on which the steam engine operates with special reference to the valve gear and governing devices, and the various diagrams used for studying the same. Examples are given of the different types of mechanisms, these examples being chosen merely to illustrate principles and methods, without particular reference to their relative importance.

In dealing with a subject which has been so thoroughly developed as has the steam engine, it would be useless to claim that any new principles are set forth in an elementary textbook such as the present one. The aim is to treat the subject in a logical manner, as concisely as possible, yet with sufficiently detailed explanations to make the principles easily understood.

Chapter X describes the principle of action of steam turbines in general and explains briefly the various types of turbines, giving an example of each

Chapter XI treats of the method of controlling the steam supply to turbines and describes two mechanisms which are used for this purpose.

Thanks are due to the various builders of engines and turbines for their ready response to requests for information, for the loan of cuts, and for permission to make free use of the material contained in their publications. Acknowledgment is also made of the assistance rendered by the authors' associates at the Massachusetts Institute of Technology.

The steam engine is a machine by means of which steam is enabled to do mechanical work. The steam ends of direct-acting steam pumps, steam drills, steam hammers, and the like are essentially steam engines adapted to some particular work. The various tools operated by compressed air, such as drills, riveters, pneumatic hoists, and air brakes, belong to the same general class of machines, the main difference being that the working fluid is air instead of steam. All are machines by means of which a compressible fluid does work by virtue of a change in the internal condition of the fluid.

A machine such as an air compressor is the reverse of the engine in that it works upon a compressible fluid and puts it in a condition in which it is able to do work. A pump which pumps water or other liquid is, like the compressor, a machine for doing work on a fluid but differs from the compressor in that the fluid upon which it works is practically incompressible and the pump merely changes the position of the fluid without producing any appreciable change in its internal condition.

In the design of any one of these machines four elements must be considered. First, the properties of the vapor, gas, or liquid with which the machine works; second, the kinematics of the machine itself, that is, the geometry of the machine; third, the dynamics of the machine, that is, the transmission of forces through the parts of the machine; fourth, the details of construction of the parts so that they shall be strong enough and be practical to make and use. These four elements are, of course, closely related to each other and it is impossible to study one without some reference to the others.

In the present treatise we are to deal principally with the kinematics of some of the machines already referred to, touching upon the other sides of the question only so far as is necessary in order to deal with the subject in a logical and intelligent manner.

Since the steam engine is the most common and important of these machines, and the principles involved are broader, we shall consider that first and considerably more in detail than the other machines which follow.

Steam engines may be divided into two general classes:

1. Those, known as reciprocating engines, in which the steam imparts a reciprocating motion to a piston, and this motion by means of a suitable mechanism causes rotation of a shaft or else is carried directly from the piston to the point where the work is done.

2. Those in which the steam imparts rotation to a shaft directly with-out the intervention of a reciprocating piston.


CHAPTER I - GENERAL DISCUSSION OF A RECIPROCATING STEAM ENGINE

1. There are a great many types of reciprocating steam engines, differing widely in size and general design. Certain fundamental principles of design and method of action are common to all however. The parts of a reciprocating engine may be divided into three main groups:

1. Stationary parts (frame, cylinder, bearings).

2. Piston, piston rod, crosshead, connecting rod, crank, shaft, and flywheel, to which the steam imparts motion.

3. Valve mechanism, which controls the supply of steam.

The most direct way to gain familiarity with the parts and with the principles of operation is to study in detail a simple example.

2. Description of a Simple Engine. Fig. i represents a small reciprocating engine of the type known as a plain-slide-valve engine. Directly on the concrete or masonry foundation rests the frame D, carrying, in suitable bearings near one end, the engine shaft O, while bolted to it at the other end is the cylinder E. The cylinder is closed at the ends by heads, and is covered, or lagged, with some material which is a good non-conductor of heat, to prevent too rapid radiation. In the cylinder is the piston F, which moves from one end to the other under the influence of steam pressure. There must be no leakage of steam past the piston, and it is made steam tight by two split rings in grooves around the piston, which spring outward and press against the cylinder walls. The piston is rigidly attached to the piston rod G, the latter being attached at the other end to the crosshead H. Where the piston rod passes through the cylinder head leakage of steam is prevented by packing. The crosshead slides back and forth between the guides I, which prevent any tendency to bend the piston rod.

The motion of the crosshead is carried to the crank pin A by means of the connecting rod J, the latter being attached to the crosshead by the wrist pin or crosshead pin B. The connecting rod is provided with boxes of suitable bearing metal, and provision is made for taking up wear. In this particular engine the shaft 0, crank, and crank pin are forged in one piece, called a crank shaft. The weight of the crank and crank pin and part of the weight of the connecting rod are balanced by the counterweights K, which latter are bolted to the crank. The shaft in this case carries two heavy flywheels, which serve to make the engine run steadily and provide a means of taking off power by belts. The eccentric L is fast to the shaft and is connected by the eccentric strap and eccentric rod to the valve stem guide W which in turn is connected to the valve V by the valve stem Y. The valve has a reciprocating motion on suitable guides, in a steam-tight box C, known as a steam chest, or valve chest, which is cast on the side of the cylinder. This valve controls the flow of steam to and from the cylinder. The vertical surface, against which the valve runs, and which is called the valve seat, has in it three openings M , N, and P called ports; M and N open into the cylinder near the ends while P connects to the exhaust pipe R. The metal left between the ports forms the bridges. These ports and bridges are shown in Fig. I b and Fig. I d.

3. Fundamental Definitions. The end of the cylinder which is nearer the crank is usually spoken of as the crank end while the opposite end is called the head end. The port M is called the crank-end steam port while N is called the head-end steam port; P is called the exhaust port. When the crank and connecting rod are in line, the crank being toward the cylinder and the piston at the head end, the engine is said to be on the head end dead point or dead center. After the crank has turned 180 degrees so that the piston is at the crank end of the cylinder the engine is said to be on the crank end dead point or dead center. The motion of the piston from the head end of the cylinder to the crank end is called the forward stroke, while the motion from the crank end back to the head end is called the return stroke.

4. Action of the Engine. Referring to Fig. I a, steam from the boiler enters the steam chest through the throttle valve T, surrounds the valve and enters port N as soon as the valve uncovers it, thus admitting steam on the head-end side of the piston. At the same time steam which has already done its work on the crank-end side of the piston may flow out through the port M, into the exhaust cavity of the valve, around the bridge and into the exhaust port P. Fig. i c, which is a section through cylinder, steam chest and valve, will help to make clearer how the steam enters and leaves the steam chest. The difference of pressure on the two sides of the piston drives it toward the crank end of the cylinder and its motion is transmitted through the piston rod, crosshead and connecting rod to the crank pin, thus causing the shaft to turn. At the proper time the valve moves so as to stop the flow of steam into the head end, then connects the head end with the exhaust, and finally moves far enough to admit steam through port M into the crank end, thus driving the piston back to the head end. This, in brief, is the way that steam under pressure causes a piston to have a reciprocating motion which, in turn, is transformed into a continuous rotation of the shaft.

The opening of the port to admit steam to the cylinder is called admission, cutting off the supply by closing the port is called cut-off, the opening of the exhaust for spent steam is called release and the closing of the exhaust is called compression. These are the four events of the stroke and will be discussed in detail later.  


Piston, Crosshead, Connecting Rod and Crank

5. From the preceding description it is evident that the reciprocating motion of the piston is transferred by the piston rod to the crosshead, and is transformed into rotary motion of the shaft by the connecting rod and crank. It is important to get clearly in mind the action of these parts and the effect of the connecting rod on the motion of the cross- head and therefore of the piston if, under the steadying action of the flywheel, the shaft turns with uniform angular speed. Since the motion of crosshead and piston are the same we will refer to the motion of the crosshead pin as the motion of the piston.

6. Displacement of Crosshead. In referring to the piston position at any time it is customary to describe its position by stating the linear displacement of the crosshead from either one of the extremes. This displacement is commonly given in percentage of the length of the stroke. For example, if a certain event occurs when the piston is moving toward the crank end and has moved three quarters of the distance from the head end to the crank end that event is said to occur at 75 per cent of the forward stroke. The crosshead displacement for any given crank angle, or the crank angle for any given crosshead position, may be found graphically or may be calculated. For ordinary work the graphical method is convenient and sufficiently accurate. It is well, however, to be familiar also with the analytical method and we will accordingly con- sider both.