Ques. What is the most important part of a steam engine?
ANS. The cylinder, because within its compass is accomplished the change of energy of the fuel into the work the engine is designed to perform.
Ques. Why is the cylinder the most approved form for its office of transforming the energy of combustion into work?
- The circular form is the strongest.
- It is easier to make and repair
- It is best adapted to fit the round form of the piston.
Ques. Name some of the parts and uses of the steam cylinder?
ANS. Cylinder heads, with the studs or bolts to hold it to the flange, also the bolts to hold it to the frame; cylinder jackets, to prevent condensation. Fig. 13 and Fig. 14 show in outline the parts and names of the steam cylinder and the valve chest and valve.
Note—In the elementary form of the steam engine the cylinder served the triple purpose of boiler, engine and condenser. In after history the boiler and condenser were made into separate parts to form and condense the steam while the cylinder is used for its true office— the convertion of the energy in the steam into mechanical power.
Steam Cylinder and Valve
Parts of the cylinder, see Fig. 13, "a"
- bore of cylinder
- counter bore
- cylinder heads
- stuffing box
Parts of the valve chest, see Fig. 14, "b ":
- steam ports
- exhaust ports
- slide valve
- valve stem stuffing box
- valve stem gland
- cylinder head stud bolts
- valve chest cover
- valve chest flanges
- steam inlet.
Note—In spite of all improvements, the cylinder acts in nearly all engines as a steam generator and condenser—at certain parts of the stroke of the piston the steam condenses and at other parts it re-evaporates.
THE STEAM PISTON
This is a circular disc fitted to the bore of the cylinder and which receives and transmits the pressure of the steam to the other moving parts of the engine. Fig. 15 exhibits an approved form of the steam piston with the piston head or " follower plate" removed so as to show the interior construction.
Parts of the steam piston are shown and may be defined thus:
- piston web
- bull ring
- follower bolt holes
- packing ring
- packing springs
- steam-tight joints.
The forms of pistons are innumerable according to the varying size of the cylinders they are designed to fit, but there are two things necessary for all; they should be steam tight and yet so adapted to the bore of the cylinder that they move with the least possible friction. Besides, they should be strong enough to meet the unequal strains of the pushing of the steam and to hold the end of the piston rod immovable.
Connecting Rod Ends
This is the device which forms the connection between the piston rod and the connecting rod; it is similar to the joints of the human body; it guides the piston rod so as to keep it straight, in spite of the bending motion caused by the angularity of the connecting rod.
Fig. 17 exhibits a crosshead partly in section so as to show the method of adjusting the guides. Such as this are frequently used on girder frame engines.
The names of parts are as follows:
- crosshead body
- wrist pin
- piston rod socket
- set screw
- jam nuts
- slot for piston rod key.
A crank is a lever or bar movable about a centre at one end. This, in the steam engine, allows the conversion of reciprocating or sliding motion into circular motion. The connecting rod is the device used between the crank and the crosshead. Fig. 16 shows three forms of connecting rod end.
THE CONNECTING ROD
Connecting Rod Ends
Fig.18 shows the details of the two ends of a connecting rod, the names and numbers are as follows:
- crank pin key
- crank pin gib
- crank strap
- crank brasses
- connecting rod and crank pin
- wrist pin brasses
- wrist pin gib
- wrist pin strap
- set screws
- crosshead end of connecting rod.
It may be remarked that the centre of the crank pin end of the connecting rod moves in a true circle, while the centre of the opposite end has the reciprocating—back and forth—rectilinear motion.
The eccentric is a species of a crank; its peculiarity being that the " crank pin " is increased to such a size that it exceeds the diameter of the shaft. The distance between the centre of the crank pin and the centre of the shaft being " the radius of eccentricity" which is the distance between the centre of the disc and the centre of the shaft, see Fig. 19, " 1 " and " 2."
Fig. 19 is a drawing in outline of an eccentric, the parts of which may be described:
- shaft centre
- eccentric centre
- radius of eccentric
- eccentric strap
- set bolts
- eccentric rod
- eccentric rod foot.
The governor is the ruling spirit of an engine. The use of a governor of any kind is to supply more or less energy to the engine as needed. Where the changes of load are liable to be very great, or where absolute uniformity of speed of rotation is not essential, the regulation is frequently performed by hand, as in the case of locomotives, hoisting engines, etc. Where the speed remains constant, however, as in the case of factory engines, etc., the prime mover is always equipped with a governor which will automatically vary the supply of water, gas or steam, as the case may be, in proportion to the demands made upon it and in such a manner as to keep the speed of rotation constant, under varying loads.
In steam engines there are two methods of varying the supply of energy.
- By varying the pressure under which it is admitted to the cylinder.
- By varying the point of the stroke at which it is cut off and allowed to expand.
Note.—The "throw" of the eccentric is equal to twice the eccentricity.
Note.—A shaft governor regulates the speed of the engine by changing the cut-off for ordinary variations of load. An inertia governor is also a shaft governor, but uses the inertia stored in the wheel itself or another moving weight, to overcome very sudden changes on the load.
The first of these is the older and by far the simpler of the two methods. The governor which acts according to this method, or throttling governor, a simple and modern form of which is shown in Fig. 20, is too well known to require an extended description.
Referring to Fig. 20, the names of the parts of this important adjunct of the steam engine areas follows:
- governor shaft
- governor balls
- stem swivel
- oil holes
- stuffing box
- gland box
- valve discs
- valve seats
- stem guard
- throttle valve flange
- valve chest flange
The device shown in the figure is the well known "ball" or throttling governor. This operates upon the princple in physical science explained under the heading of "centrifugal force "; also under heading " elementary mechanics" in technical literature, thus:
- A body in motion tends to move in a straight line.
- A body when moving cannot stop itself or change its motion, and unless acted on by some other body will go on in the same direction and at the same speed forever.
- Inertia is that property of matter by which it is of itself incapable of changing its state whether in motion or at rest. It takes some time, however, to put a body in motion or change its direction.
The governing forces of the ball (fly-ball) governor consist of centrifugal force opposed by gravity. Familiar modifications of this construction are provided with springs as a substitute for gravity. Shaft governors or shifting eccentric governors, represent another type where springs are used to oppose centrifugal force.
Very much more upon this subject will be found in other parts of the work given in connection with description of particular engines and also under the heading of "Care and Management of the Steam Engine."
PIPING OF THE STEAM ENGINE
The two principal pipes of the ordinary engine are: first, the main steam pipe which leads from the boiler to the engine; and second, the exhaust pipe—that which wastes the steam into the atmosphere after it has done its work. There are also cylinder drain pipes to carry off the water formed by the condensation of the steam.
Points, relating to Piping an Engine.
- Run steam and exhaust pipes as straight as possible. If they must be over fifty feet long use one size larger pipe.
- It should be of such ample size as to cause as little friction as possible.
- It should be well covered to prevent condensation.
- The corners should be well rounded and be as free as possible from flanges and valves.
- The exhaust pipe should be of ample size.
- The exhaust pipe should also be covered with nonconducting material where the exhaust steam is to be used for heating purposes, at least to the point where it is drawn into the heating system.
- Support steam and exhaust pipes on suitable hangers.
- Blow the main steam pipe out thoroughly with steam before making connection to throttle, in order to clear it of any dirt or pieces of scale which might otherwise have to work out through the engine.
The use of an exhaust pipe head is obvious:
- This device deadens the noise on high speed engines.
- It preserves the roofs from the disagreeable effects of oilimpregnated water falling upon it.
Note.—It will be understood that in the marine type or condensing engine, the exhaust pipe leads to the condenser, where the steam is re-converted to water, hence there is no regular exhaust into the air in low pressure engines.
- New catechism of the steam engine 1902 pages 39-48