00 CO
CP
MODERN GASWORKS PRACTICE
an additional (outer) lift at some later date, in which case the tank rollers will have a considerable overhang.
Per unit of capacity the spiral holder is considerably cheaper than the guide-
Fig. 411.—Multiple Roller Carkiagb for Spiral Holder.
framed type. It requires, however, great accuracy of fitting ; and, when composed of a number of lifts, an dement of danger from “Jamming” or breakage is introduced by the numer-ous sliding brackets and rollers. Trouble might chiefly be expected in countries in which marked variations of temperature are met with, for the bell is subject to a certain amount of radial contraction and expansion. This difficulty has been overcome by the introduotion of special compensa-ting carriages. The bell of the spiral holder is of much stifter construction than that of the ordinary holder, and some form of trussing is usually em-
ployed in the down. A suitable form of crown curb for a holder of about 125 feet ■diameter working on this principle is seen in Kg. 409.
The guide rollers are either of the dual or multiple type. The dual type illus-trated in Fig. 410 are provided with ■continuous automatic hibrication, being so designed that the axle runs in a bath of oil formed in the bed-
plate of the carriage. The multiple roller carriage, in which four instead of two rollers are employed, is shown in Figs. 411 and 412. It will be ob-served that the bolts securing the carriage are brought as near as possible to their work, thus reducing the strain from leverage upon the tank and the carriage. As regards provision for any radial contraction or expansion of the beil, this is usually arranged for by fitting a sliding axle as in Fig. 413. The ,, T> „
. 6 . ? 6 . Fig. 412.—Multiple Roller Carriage.
oil grooves in the axle, and the special
packing rings at front and back to prevent the lubrication from being lost, will also be noticed. The roller is not free to revolve on the axle, but fixed to it. The space for expansion is shown between the back of the roller and the front of the •carriage. A recent feature is the “ run down stop ” which is affixed at the upper