[Fig. 3254] represents a second example of joint with rivets unevenly pitched, this form finding much favor in recent practice. The four inner rows of rivets are spaced at narrow pitch and the two outer rows are wide pitched.
[55]“The strength percentage of this joint is calculated from three points of view, as follows:
[55] From “Steam Boilers.”
“First. The plate section at the wide pitched rivets.
“Second. The rivet section in one pitch.
“Third. The plate section at the narrow pitch plus half the double shear of the outer or wide pitched rivet.”
The steam pressures generally employed in the boilers of stationary engines range from about 60 to 100 lbs. per square inch, and as a result of these comparatively low pressures less perfect forms of construction are employed than would be permissible if higher pressures were used.
The strength of the shell plate of boilers of small diameter is always largely in excess of the requirements, and as a result the strength of the joints may bear a very low percentage to that of the solid plate, and yet give a sufficient factor of safety for the working pressure.
Take, for example, a boiler shell of 36 inches internal diameter with a shell plate 1⁄4 inch thick, and allowing the strength of the material to be 48,000 lbs. per inch of section, and with a factor of safety of 4, the working pressure will be 166 lbs. per square inch, thus:
| Strength of the material. | Plate thickness × 2. | ||
| 48000 | × | (.25 × 2) | = 6662⁄3 lbs. = bursting pressure. |
| 36 | |||
| Diameter of boiler. | |||