296. Advantages of Steam Road-rolling.—The advantages of roadrolling may be summed up as follows: : (1.) The roads being made for the traffic, a saving of metalling is effected, as the stones are interlocked by the process of consolidation and present only one surface subject to wear, compared with the abrasion inseparable from the system of patching and consolidating by wheel traffic. (2.) A harder and more regular surface is obtained, the road generally has a better appearance, ease of traction is promoted, injury and suffering to animals are avoided, and the damage to the wheels of vehicles is reduced, owing to the absence of newly-laid or loose metalling lying on the surface of the roads. (3.) The crust of the road contains only a small amount of binding or soluble matter, wherefore there is a material reduction of the quantity of mud in wet, and dust in dry weather; and the surface of the roads do not require the constant attention which is necessary with metalling during the long process of consolidation by vehicular traffic. (4.) Scraping and cleaning are reduced to a minimum by reason of the road being practically impervious to the effects of weather and traffic; the surface wears longer, and the absence of loose stones in dry weather secures greater efficiency and diminished cost of maintenance. Steam road-rollers are now almost universally adopted on account of the superiority and economy of the work done, the shorter time necessary for repairs or construction, and also the indirect saving effected owing to the reduced wear and tear of horses and vehicles. 297. Different Types of Rollers made; Weight and Construction.— Steam road-rollers are made of different weights, chiefly 10, 12, and 15 tons, for use in this country, while a few rollers of 20 and 25 tons have been constructed. The former weight of roller is used in Paris, while the latter is that adopted in Madrid. The 12-ton engine rolls a width of 6 feet 5 inches, and the 15-ton roller 7 feet 3 inches wide, and these are the class generally used in Great Britain for county work, while 10-ton rollers for the most part are employed in the larger towns, owing to the difficulty experienced in connection with gas, water, and other service pipes in the use of a heavier class of roller. A short description of Aveling & Porter's 15-ton roller may be here given. The boiler is of the ordinary locomotive type, the cylinder being placed on the top and at the forward end of the boiler, and surmounted with a steam jacket. The steam is taken direct from the boiler, and admitted to the cylinder without the intervention of steam pipes, thus avoiding loss in the effective pressure of the steam through being condensed. They are made either with a single cylinder or on the compound principle, and, when necessary, can be constructed so as to convert the roller into a road engine for hauling purposes, by substituting traction wheels for the front and rear rolls. The rear driving-wheels are 6 feet in diameter and 20 inches wide, those in front 4 feet in diameter, set close together, and having a combined width of 4 feet 6 inches. The front rolls are surmounted by a swivelling fork having a vertical spindle working in the fore-carriage bracket, which supports the front end of boiler. Provision is made for the spindle having sufficient play to admit of the front rolls accommodating themselves to the contour of the road. By means of this arrangement on the fore-carriage in conjunction with the worm-wheel and side chains, the steering of the engine is provided for, and operated from the footplate, the working of the engine being under the control of one man. The distance between the centre of the driving-wheels and the front rolls in the 15-ton convertible engine is 12 feet 6 inches, and the weight of the ordinary roller in working order, with scarifier attached, is 161 tons, distributed as follows, namely two-fifths total weight on front rolls, and three-fifths of total weight on the driving or hind wheels, or fully 5 tons on each wheel. This gives the driving-wheels, when the roller is working on a level stretch of road, a compressive force of 4.88 cwts. per inch of width, being twice that exerted by the front rolls. When rolling on a gradient these latter figures will be slightly increased, owing to the effects of gravity and the weight of the water in the boiler being then greater at the rear end of the engine. The brackets for carrying the gearing are formed by the side plates of the fire-box being extended upwards and backwards in one piece, thus obviating the usual unsatisfactory method, common to many road-engines and rollers, of bolting them on to the boiler, which causes a considerable strain and injury, and consequently increases the cost of working and repairing. The wear and tear of the spur-wheels and pinions are reduced to a minimum by these being brought close together, and the combination produces the maximum of strength in the working parts. They are fitted with compensatory motion or differential gear, which enables them to be steered round sharp turns easily without slipping. Two speeds are provided, which can be thrown into and out of gear by a single operation by the driver from the foot-plate. As previously mentioned, the steam road-roller is a special adaptation of the ordinary road locomotive either with single or with compound cylinders. Those constructed on the latter principle, although the first cost is considerably greater and repairs more numerous, are notable for their silent working, the steam first doing duty in the high, and afterwards in the low pressure cylinder. Thus the steam, in passing from the engine, is discharged into the atmosphere at a relatively low pressure, reducing to a minimum the noise from the exhaust when at work, an important matter when rolling in towns or on busy suburban roads. An engine constructed on this principle can be started readily in any position of the crank, and there is an entire absence of sudden jerks and shocks inseparable from an engine with a single cylinder only. It is also claimed that a considerable saving in the consumption of coal and water required is effected by a road-roller having a compound cylinder over that of one with a single cylinder only, which may result in greater economy of the working expenses, especially when the fuel has to be brought from a considerable distance, and in districts where water is scarce. Another matter of considerable importance is the reduced temperature of the fire-box, owing to the steam being expanded down before passing through the exhaust into the chimney. The fierce draught caused in high pressure or single cylinder engines creates considerable wear and tear to the fire-box and tubes, which does not occur to any great extent in engines constructed on the compound principle. In selecting a steam road-roller for the consolidation of the metalling in making new or repairing existing roads, it is necessary to consider, in the first instance, the nature or quality of the material which is to be employed for the purpose of construction and repairs. The weight of the roller most suitable in any circumstances will, therefore, have to be in a great measure determined by the quality of the macadam made use of in the particular locality concerned. The great object to be attained is a resulting coat of metalling compressed to such a degree, without crushing the material, that the interstices are just apparent and contain the least possible amount of binding consistent with the proper cohesion of the stones forming the coating. To accomplish this effectually with hard and tough road metal the greatest weight of road-roller permissible should be employed which will consolidate the coating without crushing the stones composing it. 298. Experience shows that for consolidating such material as granite, syenite, basalt, and whinstone of good quality, the greatest amount and the best work are obtained by the employment of a 15-ton roller, which exerts a compressive force on the material under consolidation of from 4.88 to 5.42 cwts. per inch of width of wheel. For convenience and comparison, the weight per inch of the bearing surface of the different kinds of rollers generally employed is given in Table XXXIII. TABLE XXXIII. Class of Road-roller in Steam. 15-ton road-roller, weighing in working order 161 tons, driving-wheels 18 inches wide, 15-ton road-roller, weighing in working order 16 tons, driving-wheels 20 inches wide, 12-ton road-roller, weighing in working order 13 tons, driving-wheels 17 inches wide, Weight per inch of width in driving-wheels. 5 42 cwt. per inch 4.88 4.59 10-ton steam-roller, weighing 10 tons in working order, driving-wheels 16 inches wide, 3.94 |