STRUCTURAL DETAILS. Structurally the steam engine consists of the working parts and the fixed structure or frame by which the working parts are supported and maintained in their relative positions. The supporting frame is usually a structure of cast or forged iron or steel varying in shape to suit the form of engine of which it is a part. The working parts of an engine comprise the cylinder and piston, the valves and valve gear. the governor, the piston rod, crosshead, and connecting rod, and the crank shaft and fly wheel. The function of the engine frame is to constitute a fixed link in a kinetic chain, and to perform this function it must be essentially rigid and strong. In a horizontal engine, as shown in Fig. 2, it will be evident that the duties of the frame are to support the cylinder C at one end, and the fly wheel W at the other end, and to steady and align the trans forming Mechanism connecting these two parts. The frame of a vertical engine performs an ex actly similar duty, but of course differs in con struction to suit the different arrangement of the moving parts. Thus in an inverted vertieal engine there is a broad base plate, on which the crank shaft is directly supported. From this base plate rises a columnar structure support ing the cylinders and guiding the reciprocating parts. Cast iron and cast steel are particularly adapted to furnish the rigidity and massiveness desired, and are, therefore, the materials com monly employed for engine frames. In certain classes of engines, such as the locomotive and the marine engine, where lightness combined with great strength is an essential, forged steel frames are used.
Turning now to the working parts. the cyl inder and piston as the motor elements naturally call first for consideration. The engine cylinder is a barrel-shaped or box-shaped casting, with its interim• bored out into the form of a perfect cylinder. The ends of this cylinder are closed by cover-like castings fastened by studs or bolts to the body. The front head has a circular ori fice at the centre to allow- for the exit of the piston rod, and where a tail rod is used a simi lar orifice is necessary in the back head. The barrel or body of the cylinder has one or more orifices for the admission and exhaust of the steam, and these orifices are covered by a box like chamber called the steam, chest, inside of which work the valres, as will be described immediately. The body and often the ends of the cylinder casting are usually covered with a lagging or envelope of some material whieh is a poor conductor of heat and whose purpose is to prevent the condensation of steam in the cyl inder by the loss of heat due to radiation from its outer walls. The piston is a circular disk of
metal of sufficient strength to resist distortion by the steam pressure. In the edge of this disk are set rings of metal so arranged that they are constantly pressed outward against the walls of the cylinder, either by their own elasticity or by the elasticity of springs pressing against them from behind. These rings are called packing rings, and their function is to make the piston fit steam-tight in the cylinder and at the same time render its movement as free from friction as possible.
The function of the valves and valve gear is to control the admission and exhaust of the steam. The functions of the valve are both primary and secondary. The primary function is to admit the steam from the boiler to one side of the piston, while the exhaust steam filling the other end of the cylinder is permitted to escape with the least possible resistance. The secondary functions are to close the admission of steam at the point necessary to give the expansion de sired and to close the exhaust orifice at such a point in the return stroke that a certain vol ume of steam shall be caught and compressed behind the piston to serve as an elastic cushion. It is necessary also that the opening of the in lets and outlets of the cylinder should be so timed with respect to the stroke of the piston that pressure may not be brought too soon against the piston head, nor the exhaust opened until the expanding steam has done its entire work for that stroke. The valves for admitting and distributing steam in an engine cylinder may open the ports which they control, either by lifting from their seats or by sliding upon their seats. When the engine is a double-acting one there must be provision to connect each end of the cylinder with the boiler and each end with the exhaust pipe. When the engine is single acting it is only necessary to connect one end to the boiler and exhaust pipe alternately. Ap parently the simplest arrangement would be to have four valves, as shown by Fig. 2, one at each end for steam, and one at each end for ex haust. This is the arrangement in the ('orliss engine. An important type of engines makes use of a separate valve for admitting steam at each end, while the exhaust is controlled by a single valve for both ends. Such engines are called Mice-rally. engines. Another arrangement is to have one valve for admitting steam to both ends and another controlling the exhaust for both ends. Such engines are called tico-ralre en gines. A fourth class has one single colic, so designed as to perform the inlet and outlet func tions for both ends. Such are the great majority of stationary engines and all locomotive and marine engines.