A Catechism of the Steam Engine

by C. E. John Bourne

630. Q.--Will you explain in detail the construction of the valve gearing, or such parts of it as are peculiar to the oscillating engine?

A.--The eccentric rod is attached by a pin, 1 inch in diameter, to an open curved link or sector with a tail projecting upward and passing through an eye to guide the link in a vertical motion. The link is formed of iron case-hardened, and is 2-3/4 inches deep at the middle, and 2-3/8 inches deep at the ends, and 1 inch broad. The opening in the link, which extends nearly its entire length, is 1-5/16 inch broad; and into this opening a brass block 2 inches long is truly fitted, there being a hole through the block 3/4 inch diameter, for the reception of the pin of the valve shaft lever. The valve shaft is 1-3/4 inch diameter at the end next the link or segment, and diminishes regularly to the other end, but its cross section assumes the form of an octagon in its passage round the cylinder, measuring mid-way 1-1/4 inch deep, by about 3/4 inch thick, and the greatest depth of the finger for moving the valve is about 1 inch. The depth of the lever for moving the valve shaft is 2 inches at the broad, and 1-1/4 inch at the narrow end. The internal breadth of the mortice in which the valve finger moves is 5/16 inch, and its external depth is 1-3/4 inch, which leaves three eighths of an inch as the thickness of metal round the hole; and the breadth, measuring in the direction of the hole, is 1-1/2 inch. The valve rod is three fourths of an inch in diameter, and the mortice is connected to the valve rod by a socket 1 inch long, and 1-1/8 inch diameter, through which a small cutter passes. A continuation of the rod, eleven sixteenths of an inch diameter, passes upward from the mortice, and works through an eye, which serves the purpose of a guide. In addition to the guide afforded to the segment by the ascending tail, it is guided at the ends upon the columns of the framing by means of thin semicircular brasses, 4 inches deep, passing round the columns, and attached to the segment by two 3/8 inch bolts at each end, passing through projecting feathers upon the brasses and segment, three eighths of an inch in thickness. The curvature of the segment is such as to correspond with the arc swept from the centre of the trunnion to the centre of the valve lever pin when the valve is at half stroke as a radius; and the operation of the segment is to prevent the valve from being affected by the oscillation of the cylinder, but the same action, would be obtained by the employment of a smaller eccentric with more lead. In some engines the segment is not formed in a single piece, but of two curved blades, with blocks interposed at the ends, which may be filed down a little, to enable the sides of the slot to be brought nearer, as the metal wears away.

631. Q.--What kind of plummer blocks are used for the paddle shaft bearings?

A.--The paddle shaft plummer blocks are altogether of brass, and are formed in much the same manner as the cap of the piston rod, only that the sole is flat, as in ordinary plummer blocks, and is fitted between projecting lugs of the framing, to prevent side motion. In the bearings fitted on this plan, however, the upper brass will generally acquire a good deal of play after some amount of wear. The bolts are worked slack in the holes, though accurately fitted at first; and it appears expedient, therefore, either to make the bolts very large, and the sockets through which they pass very deep, or to let one brass fit into the other.

632. Q.--How are the trunnion plummer blocks made?

A.--The trunnion plummer blocks are formed in the same manner as the crank shaft plummer blocks; the nuts are kept from turning back by means of a pinching screw passing through a stationary washer. It is not expedient to cast the trunnion plummer blocks upon the lower frame, as is sometimes done; for the cylinders, being pressed from the steam trunnions by the steam, and drawn in the direction of the condenser by the vacuum, have a continual tendency to approach one another; and as they wear slightly toward midships, there would be no power of readjustment unless the plummer blocks were movable. The flanges of the trunnions should always fit tight against the plummer block sides, but there should be a little play sideways at the necks of the trunnions, so that the cylinder may be enabled to expand when heated, without throwing an undue strain upon the trunnion supports.

633. Q.--What kind of paddle wheel is supplied with these oscillating engines?

A.--The wheels are of the feathering kind, 9 feet 8 inches in diameter, measuring to the edges of the floats; and there are 10 floats upon each wheel, measuring 4 feet 6 inches long each, and 18-1/2 inches broad. There are two sets of arms to the wheel, which converge to a cast iron centre, formed like a short pipe with large flanges, to which the arms are affixed. The diameter of the shaft, where the centre is put on, is 4- 1/2 inches, the external diameter of the pipe is 8 inches, and the diameter of the flanges is 20 inches, and their thickness 1-1/4 inch. The flanges are 12 inches asunder at the outer edge, and they partake of the converging direction of the arms. The arms are 2-1/4 inches broad and half an inch thick; the heads are made conical, and each is secured into a recess upon the side of the flange by means of three bolts. The ring which connects together the arms, runs round at a distance of 3 feet 6 inches from the centre, and the projecting ends of the arms are bent backward the length of the lever which moves the floats, and are made very wide and strong at the point where they cross the ring, to which they are attached by four rivets. The feathering action of the floats is accomplished by means of a pin fixed to the interior of the paddle box, set 3 inches in advance of the centre of the shaft, and in the same horizontal line. This pin is encircled by a cast iron collar, to which rods are attached 1-3/8 inch diameter in the centre, proceeding to the levers, 7 inches long, fixed on the back of the floats in the line of the outer arms. One of these rods, however, is formed of nearly the same dimensions as one of the arms of the wheel, and is called the driving arm, as it causes the cast iron collar to turn round with the revolution of the wheel, and this collar, by means of its attachments to the floats, accomplishes the feathering action. The eccentricity in this wheel is not sufficient to keep the floats in the vertical position, but in the position between the vertical and the radial. The diameter of the pins upon which the floats turn is 1-3/8 inch, and between the pins and paddle ring two stud rods are set between each of the projecting ends of the arms, so as to prevent the two sets of arms from being forced nearer or further apart; and thus prevent the ends of the arms from hindering the action of the floats, by being accidentally jammed upon the sides of the joints. Stays, crossing one another, proceed from the inner flange of the centre to the outer ring of the wheel, and from the outer flange of the centre to the inner ring of the wheel, with the view of obtaining greater stiffness. The floats are formed of plate iron, and the whole of the joints and joint pins are steeled, or formed of steel. For sea-going vessels the most approved practice is to make the joint pins of brass, and also to bush the eyes of the joints with brass; and the surface should be large to diminish wear.

634. Q.--Can you give the dimensions of any other oscillating engines?

A.--In Messrs. Penn's 50 horse power oscillating engine, the diameter of the cylinder is 3 feet 4 inches, and the length of the stroke 3 feet. The thickness of the metal of the cylinder is 1 inch, and the thickness of the cylinder bottom is 1-3/4 inch, crossed with feathers, to give it additional stiffness. The diameter of the trunnion bearings is 1 foot 2 inches, and the breadth of the trunnion bearings 5-1/2 inches. Messrs. Penn, in their larger engines, generally make the area of the steam trunnion less than that of the eduction trunnion, in the proportion of 32 to 37; and the diameter of the eduction trunnion is regulated by the internal diameter of the eduction pipe, which is about 1/5th of the diameter of the cylinder. But a somewhat larger proportion than this appears to be expedient: Messrs. Rennie make the area of their eduction pipes, in oscillating engines, 1/22d of the area of the cylinder. In the oscillating engines of the Oberon, by Messrs. Rennie, the cylinder is 61 inches diameter, and 1-1/2 inch thick above and below the belt, but in the wake of the belt it is 1-1/4 inch thick, which is also the thickness of metal of the belt itself. The internal depth of the belt is 2 feet 6 inches, and its internal breadth is 4 inches. The piston rod is 6-3/4 inches in diameter, and the total depth of the cylinder stuffing box is 2 feet 4 inches, of which 18 inches consists of a brass bush--this depth of bearing being employed to prevent the stuffing box or cylinder from wearing oval.