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surface. Rule 2, Section 6, of the rules of the U. S. Supervising Inspectors provides as follows: “No braces or stays hereafter to be employed in the construction of boilers shall be allowed a greater strain than 6,000 lbs. per sq. in. of section." The method to be
FIGURE 101. employed in staying a boiler depends upon the type of boiler and the pressure to be carried. Formerly when comparatively low pres . sures were used (60 to 75 lbs. per sq. in.) the diagonal crow foot brace was considered amply sufficient for staying the flat heads of boilers of the cylindrical tubular type, both above and below the tubes, but in the present age, when much higher pressures are demanded, through stay rods are largely employed. These are soft steel or iron rods 14 to 2 in. in diameter, extending through from head to head, with a pull at right angles to the plate, thus having a great advantage over the diagonal stay in that the pull on the diagonal
tay per square inch of section is more than 5 per cent in excess of what through stay
would have to resist FIGURE 102.
under the same condi
tions of pressure, etc. The method of calculation for diagonal bracing is given in Chapter I and will not be discussed here.
The weakest portion of the crow foot brace when in position is at the foot end, where it is connected to the head by two rivets. With a correctly designed brace
the pull on these rivets is direct and the tensile strength of the material needs to be considered only, but if the form of the brace is such as to bring the rivet holes
above or below the center line of the brace, or if the rivets are pitched too far from the body of the brace, there will be a certain leverage exerted upon the rivets in addition to the direct pull. Fig. 101 shows a brace of incorrect design and Figs. 102 and 103 show braces designed along correct lines.
Other methods of staying, besides the crow foot brace and through stays, consist of gusset stays, and for locomo
tives and other fire box boilers screwed FIGURE 103.
stay bolts are employed to tie the fire box to the external shell. The holes for these stay bolts are punched or drilled before the fire box is put in place. After it is in and riveted along the lower edge to the foundation ring, or mud ring as it is sometimes called, a continuous thread is tapped in the holes in both the outside plate and the fire sheet
y running a long tap through both plates. The steel stay bolt is then screwed through the plates and allowed to project enough at each end to permit of its being riveted cold. Stay bolts are liable to be broken by the unequal expansion of the fire box and outer shell, and a small hole should be drilled in the center of the bolt, from the outer end nearly through to the inner end. Then in case a bolt breaks, steam or water will blow out through the small hole, and the break will be discovered at once. The problem of properly staying the flat crown sheet of a horizontal fire box boiler, especially a locomotive
boiler, is a very difficult one and has taxed the inventive genius of some of the most eminent engineers.
Before the invention of the Belpaire boiler, with its outside or shell plate flat above the fire box, the only method of staying the crown sheet was by the use of cumbersome crown bars or double girders extending across the top of the crown sheet and supported at the ends by special castings that rest on the edges of the side sheets and on the flange of the crown sheet. At intervals of 4 or 5 in. crown bolts are placed, having the head inside the fire box and the nut bearing on a plate on top of the girder. There is also a thimble for each bolt to pass through, between the top of the crown sheet and the girder. These thimbles maintain the proper distance between the crown sheet and girder and allow the water to circulate freely.
The Belpaire fire box dispenses with girders and permits the use of through stays from the top of the flat outside plate through the crown sheet and secured at each end by nuts and copper washers.
For simplicity of construction and great strength the cylindrical form of fire box known as the Morison corrugated furnace has proved to be very successful. This form of fire box was in 1899 applied to a locomotive by Mr. Cornelius Vanderbilt, at the time assistant superintendent of motive power of the New York Central and Hudson River R. R. This furnace was rolled of 34-in. steel, is 59 in. internal diameter and in ft. 24 in. in length. It was tested under an external pressure of 500 lbs. per sq. in. before being placed in the boiler. It is carried at the front end by a row of radial sling stays from the outside plate, and supported at the rear by the back head. Figs. 104 and 105 show respectively a sectional view and an end elevation of
this boiler. It will be seen at once that the question of stays for a fire box of this type becomes very simple. The boiler has proved to be so satisfactory that the company has since had five more of the same type constructed.
Gusset stays are used mainly in boilers of the Lancashire model and are triangularshaped plates sheared to the proper form and having two angle irons riveted to the edges that come against the shell and the head. The angle irons are then riveted to the shell and the flat head. This form of brace is simple and solid, but its chief defect is, that it is very rigid
and does not allow for the unequal expansion of the internal furnace flues and the shell. Fig. 106 illustrates a gusset stay and the method of applying it.
Coming now to through stay rods, it is safe to say that whenever and wherever it is possible to apply them they should be used. In all cases they should
be placed far enough apart to allow a man to pass between them for the purposes of inspection and washing out of the boiler. Through stay rods are usually spaced 14 in. apart horizontally and about the same distance vertically. The ends, as far back as the threads run, are swaged larger than the body, so that the diameter at the bottom of the thread is greater than the diameter of the body. There are several