Q. What are the principal defects found in boiler iron?
A. Imperfect welding, brittleness, low ductility.
Q. What is the advantage of steel as a material for boiler plate?
A. Tensile strength, ductility, homogeneity, malleability and freedom from laminations and blisters.
Q. What are the disadvantages of steel as a material for boiler plate?
A. It requires greater care in working than iron and is subject to flaws induced by the pressure of gas bubbles in the ingots from which the plates are made.
Q. How far apart should stay bolts be put in a boiler?
A. They vary from 4 to 6 inches apart, depending on thickness of plates, size of stay bolts and amount of steam pressure to be carried.
[Illustration: J. I. Case Portable Skid Engine.]
The engine frame is of the girder pattern, cast in one piece, and contains the guides for cross-head and pillow block bearing. It also forms front head of cylinder to which it is firmly bolted. The cylinder is supported and firmly attached to the large feed water heater, and both cylinder and frame are entirely disconnected from the boiler. The heater forms a support for front end of boiler. The engine is placed diagonally with the boiler, allowing the crank shaft to pass over the fire box end. This permits the use of a very large band wheel. The outer end of shaft is supported by a pillow block attached to a large bracket bolted to the boiler.
The locomotive style of boiler has a large steam dome in center and an ash pan under fire box. It is supported upon the long wooden skids by brackets bolted to the sides of boiler, the heater being also bolted to the skids.
The independent pump is connected to the heater, and bolted to skid.
The engine has a Plain Slide Valve, which receives its motion from the rocker arm, operated by the eccentric and rod. It has Governor, Steam Gauge, Pop Valve and all necessary fittings. These engines range in size from 20 to 30 horse-power, and are extensively used for driving portable saw mills.
_To find the steam pressure_ on slide valve, multiply the unbalanced area of valve in inches by pounds pressure of steam per square inch, add weight of valve in pounds, and multiply the sum by 0.15.
_Safety boiler pressure_ according to the United States Government rule is as follows: Multiply ⅛ of the lowest tensile strength found on any plate in the cylindrical shell by the thickness expressed in inches or part of an inch of the thinnest plate in the same cylindrical shell, and divide by the radius or half the diameter, also expressed in inches, and the sum will be the pressure allowable per square inch of surface for single riveting, to which add 20 per centum for double riveting.