In order to transfer an engine or carriage laterally to another line of rails, at a station or terminus, circular platforms called turntables are established upon each set of rails, which turn as upon a pivot in the centre of each line, each contiguous pair of tables being connected by short branch rails, standing at right angles to the line of rails. The engine or carriage to be transferred is brought to rest wholly upon one table, which is then turned a quarter round, and the carriage is then wheeled on to the next turntable, which being likewise turned a quarter round, the engine or carriage will then be in a position to proceed on the line to which the second table appertains. Turntables are variously constructed; the annexed figure (p. 433) represents a section of one which' is in pretty extensive use on several railways; amongst others (we believe) the Great Western.
It is the invention of Capt. Handcock of Birmingham, who obtained a patent for it in 1840. The base plate is formed of cast iron, and consists of a deep socket a in the centre, from which branch several radiating ribs b b, which are braced together by two broad flat rings c c; d is a column or pedestal firmly keyed into the socket in a perfectly vertical position, and froming the support to the platform of the table; e e are loose collars of gun-nietal, resting upon shoulders formed on the pedestal, which is accurately turned at those parts so as to fit the collars without shake or friction; f is a cylindrical tube or casing surrounding the pedestal, and collars with recesses accurately bored to fit the collars; g is the turning platform, having in the centre on the under side a steel pivot h, which rests in a steel step k; in the top of the pedestal m m are a series of struts or stays, resting upon a stout flanch at the bottom of the casing f, and supporting the platform at the circumference. To provide for the efficient lubrication of the collars, a fiat plate n having metallic rings covered with leather to form a tight joint, is screwed up to the bottom of the flanch of the casing, and the space between the pedestal and the interior of the casing is filled with oil.
The following cut (page 434) represents a turntable of a different description, which was invented by Mr. Robert Mallett, of Dublin, C.E.; from whose able pen an ample description of it appeared in the Mechanics' Magazine; we shall, however, endeavour to explain the construction in a more concise manner.
The platform a of this table is much the same as usual, consisting of four or more interlacing ribs b, of about 12 inches in depth, at the centre, connected by a ring at the circumference, and all cast in one piece. The tops of the ribs carry the crossing rails, and the interspaces are planked, or open cast-iron trellis gratings are dropped into them and rest upon a rebate. The central portion of the ribs is secured by bolts to the projecting flanches of the large vertical pillar or pivot c, upon and with which the whole revolves. The lower portion of this pivot, as well as a broad collar, or neck d, close under the platform, are turned truly cylindrical, while the form of the remainder is generally conical.
This central pillar, or pivot, is cast hollow, of thickness suitable for sufficient strength. At the same level, and concentric with the turned collar d of the pivot, is placed a cast-iron bored ring e, considerably larger in internal diameter than the external diameter of the pivot, and which is sustained in its place, and all lateral or other motion thereof prevented, by a number of diagonal struts h, and by four vertical bolts k, fitted and bolted to the outside of the ring, or "bearing collar" e, and also to a large concentric ring casting l, which is built into the side walls of the turntable pit.
The lower part of the main pivot consists of a turned cylinder, like the plunger or ram of an hydraulic press, and which, also, like the latter, drops into a strong bored cylinder m, of a few inches in length, prepared at its upper lip to receive a double leather collar n, upon Bramah's plan, so as to remain water-tight under considerable pressure; the cylinder has a close bottom, and is provided with a small tube o, opening into one side, and closed by a screw-plug valve of simple construction, by which the interior of the cylinder may be filled with water. The lower part of the turntable pit consists of an inverted arch, or rather dome, of brick or stone, resting upon a bed of concrete when requisite, and this small hydraulic cylinder is bedded down upon a cast-iron ring, forming the centre or crown of the inverted dome, and is secured and adjusted to the latter by means of bolts passing through a projecting flanch cast round it. To counteract the lateral pressure, when an engine is passing on to the table, is the object of the upper collar d, and bearing collar e:
Description Of The Engraving
a surface of table; 6 6, the ribs of frame; c, the hollow conical pivot; d, the collar; e, the outer, or bearing collar; f, the roller frame and rollers; g, the small supporting rollars; A, the struts; A, the vertical holding-down bolts; l, the strut ring; m, the water cylinder, or fluid pivot; n, the leather gland; o, the supply pipe; p p, the moving gear; ss, the masonry.
between these, into the annular space, left by reason of their different diameters, is dropped a circular wrought-iron framef, consisting of two rings of flat bar iron, carrying six turned cast-iron rollers, revolving horizontally between these collars, upon wrought-iron pins, which pass through and connect both wrought-iron rings. The weight of these rollers and rings is sustained by four small rollers g, which are fixed as to position, and revolve vertically in pairs, cast in brackets, projecting inwardly from the turned bearing collar.