TURNTABLE. A rotary platform which turns wagons, carriages, and locomotives. The smallest sizes are less than three feet in diameter and are without rails, being used for light temporary work or with portable tracks. In other cases rails are mounted, flush with the surface, for use inside factories or other places subject to foot or wheeled traffic. A circle of small rollers or a ring of balls supports these tables. An automatic catch prevents the table from making more than a quarter of a revolution at a time, unless it is desired to move further.
In the large tables for carriages and wagons most of the load is carried upon a centre pivot, and the two rollers at each end run on a circular rail or race, and function when the load is unbalanced. The rollers are attached to steel joists bolted to the main girders and the rollers can be taken off for cleaning at any time. A hand lever enables the table to be readily pulled around, then locked with a wedge which also blocks up the end so taking the load off the wheels. When there is insuf ficient depth available for the pit of an ordinary turntable, the "inverted girder" or "surface type" is installed, the girders being placed upside down this type of construction. If locomo
tives cannot be well balanced, or rapid handling is important, power-driven turntables are chosen. In the electric tractor system, a wheel resting on the circular race is driven by a motor so as to turn the table in about a minute. Rotating gear on the winch system comprises a barrel pulling on two steel wire ropes so that one pays out while the other winds in. These operate a wheel on the race. Some turntables, for locomotive manufactur ing works, carry three tracks (e.g., 2 f t. 6 in., 4 ft. 81 in., and 5 ft. 6 in.). The centre-pivot support is not applied in this case, but the uneven loads are sustained by a ring of cast-steel rollers running on the fifty-foot diameter race. Some tables are about 85 ft. in diameter and take engines up to 267 tons, 5 ft. 6 in. gauge. The electric tractor drive is fitted to these. Turntables are also used in motor garages and works.