A Treatise On The Principles And Practice Of Dock Engineering

ROLLERS AND ROLLER PATHS. 337 the centre of section. Accordingly, we must find the stress in each flange separately. Outer Flange. Inner Flange. /=-+£^ FxÇb-p^ AiI _ 109,000 + 109,000 x 17-45 x 251 _ 109,000 109,000 x 17-45 x 15-1 45 11,957 “45 11,957 =2-86 tons per square inch. = 20 Ibs. per square inch. It will be observed that all the foregoing stresses are weil withinjthe safe limits for mild steel. The thickness of the lowermost plating works out as follows : — *=*\/3/= 15'9 J^15^000 = ’377 inch‘ All the plates are actually made ^ inch = -4 inch. Such is a very condensed outline of the calculations entailed in con- nection with the design of dock gates. Gate Fittings.—We now turn our attention to some of the more prominent details connected with gate construction, leaving aside for the present those matters which relate to the working of the gates. These will be more advantageously dealt with in the chapter on Working Equipment. Hollers and Holler Paths. —Gates may be entirely hung upon a pivot or axis at the heel-post, or they may derive partial support from truck wheels, or rollers, placed under them at one or more points. There is much conflict of opinion among engineers as to the value or otherwise of the latter method. On the one hand, it is urged that rollers add unnecessarily to the weight and expense of the gates, that they are liable to get out of order, that they are diflicult to adjust and repair, and that, generally, they are a source of much anxiety and inconvenience. On the other hand, it is contended that they are a valuable means of support, that they reduce the friction on the heel-post and relieve the stress on the anchor blocks, and that they can be maintained in a state of efficiency with very little trouble. Generally speaking, Continental (more especially Dutch) practice inclines to the former view, English practice to the latter, but there is no absolute uniformity in either case. Rollers have been, and are being, dispensed with at Hull, while on the Mersey, the Manchester Ship Canal, and elsewhere they are still the invariable rule. It may, however, be fairly conceded that for small wooden gates and for iron gates with buoyancy chambers, rollers are not absolutely essential. Heavy wooden gates of large span certainly do gain in steadiness by the attachment of rollers to their outer extremities. Types of rollers in use at various ports åre illustrated in figs. 276 to 278, 279, and 294. 22