A Catechism of the Steam Engine

by C. E. John Bourne

626. Q.--Will you describe the construction of the cap for connecting the piston rod with the crank pin?

A.--The cap for attaching the piston rod to the crank pin, is formed altogether of brass, which brass serves to form the bearing of the crank pin. The external diameter of the socket by which this cap is attached to the piston rod is 3-5/16 inches. The diameter of the crank pin is 3 inches, and the length of the crank pin bearing 3-7/8 inches. The thickness of the brass around the crank pin bearing is 1 inch, and the upper portion of the brass is secured to the lower portion, by means of lugs, which are of such a depth that the perpendicular section through the centre of the bearing has a square outline measuring 7 inches in the horizontal direction, 3-7/8 inches from the centre of the pin to the level of the top of the lugs, and 2-1/2 inches from the centre of the pin to the level of the bottom of the lugs. The width of the lugs is 2 inches, and the bolts passing through them are 1-1/4 inch in diameter. The bolts are tapped into the lower portion of the cap, and are fitted very accurately by scraping where they pass through the upper portion, so as to act as steady pins in preventing the cover of the crank pin bearing from being worked sideways by the alternate thrust on each side. The distance between the centres of the bolts is 5 inches, and in the centre of the cover, where the lugs, continued in the form of a web, meet one another, an oil cup 1-5/8 inch in diameter, 1-1/8 inch high, and provided with an internal pipe, is cast upon the cover, to contain oil for the lubrication of the crank pin bearing. The depth of the cutter for attaching the cap to the piston rod is 1-1/4 inch and its thickness is 3/8ths of an inch.

627. Q.--Will you describe the means by which the air pump rod is connected with the crank which works the air pump?

A.--A similar cap to that of the piston rod attaches the air pump crank to the connecting rod by which the air pump rod is moved, but in this instance the diameter of the bearing is 5 inches, and the length of the bearing is about 3 inches. The air pump connecting rod and cross head are shown in perspective in fig. 50. The thickness of the brass encircling the bearing of the shaft is three fourths of an inch upon the edge, and 1-1/8 inch in the centre, the back being slightly rounded; the width of the lugs is 1-5/8 inch, and the depth of the lugs is 2 inches upon the upper brass, and 2 inches upon the lower brass, making a total depth of 4 inches. The diameter of the bolts passing through the lugs is 1 inch, and the bolts are tapped into the lower brass, and accurately fitted into the upper one, so as to act as steady pins, as in the previous instance. The lower eye of the connecting rod is forked, so as to admit the eye of the air pump rod; and the pin which connects the two together is prolonged into a cross head, as shown in fig. 50. The ends of this cross head move in guides. The forked end of the connecting rod is fixed upon the cross head by means of a feather, so that the cross head partakes of the motion of the connecting rod, and a cap, similar to that attached to the piston rod, is attached to the air pump rod, for connecting it with the cross head. The diameter of the air pump rod is 1-1/2 inch, the external diameter of the socket encircling the rod is 2-1/8 inches, and the depth of the socket 4-1/2 inches from the centre of the cross head. The depth of the cutter for attaching the socket to the rod is 1 inch, and its thickness 5/16 inch. The breadth of the lugs is 1-3/8 inch, the depth 1-1/4 inch, making a total depth of 2-1/2 inches; and the diameter of the bolts seven eighths of an inch. The diameter of the cross head at the centre is 2 inches, the thickness of each jaw around the bearing 1 inch, and the breadth of each 9/16 inch.

628. Q.--What are the dimensions of the crank shaft and cranks?

A.--The diameter of the intermediate shaft journal is 4-3/16 inches, and of the paddle shaft journal 4-3/8 inches; the length of the journal in each case is 5 inches. The diameter of the large eye of the crank is 7 inches, and the diameter of the hole through it is 4-3/8 inches; the diameter of the small eye of the crank is 5-1/4 inches, the diameter of the hole through it being 3 inches. The depth of the large eye is 4-1/4 inches, and of the small eye 3-3/4 inches; the breadth of the web is 4 inches at the shaft end, and 3 inches at the pin end, and the thickness of the web is 2-5/8 inches. The width of the notch forming the crank in the intermediate shaft for working the air pump is 3-1/2 inches, and the width of each of the arms of this crank is 3-15/16 inches. Both the outer and inner corners of the crank are chamfered away, until the square part of the crank meets the round of the shaft. The method of securing the cranks pins into the crank eyes of the intermediate shaft consists in the application of a nut to the end of each pin, where it passes through the eye, the projecting end of the pin being formed with a thread upon which the nut is screwed.

629. Q.--Will you describe the eccentric and eccentric rod?

A.--The eccentric and eccentric rod are shown in fig. 51. The eccentric is put on the crank shaft in two halves, joined in the diameter of largest eccentricity by means of a single bolt passing through lugs on the central eye, and the back balance is made in a separate piece five eighths of an inch thick, and is attached by means of two bolts, which also help to bind the halves of the eccentric together. The eccentric strap is half an inch thick, and 1-1/4 inch broad, and the flanges of the eccentric, within which the strap works, are each three eighths of an inch thick. The eccentric rod is attached to the eccentric hoop by means of two bolts passing through lugs upon the rod, and tapped into a square boss upon the hoop; and pieces of iron, of a greater or less thickness, are interposed between the surfaces in setting the valve, to make the eccentric rod of the right length. The eccentric rod is kept in gear by the push of a small horizontal rod, attached to a vertical blade spring, and it is thrown out of gear by means of the ordinary disengaging apparatus, which acts in opposition to the spring, as, in cases where the eccentric rod is not vertical, it acts in opposition to the gravity of the rod.