ROLLERS FOR BROKEN--STONE The roller is indispensable for the economic construction of broken-stone roads. Roads can be built without the use of a roller, but always at large expense to the traffic and with great waste of the road metal; and such roads never have as smooth a surface and are .not as durable as if a roller had. been employed in their construction. With traffic-consolidated roads, much of the metal is worn round and smooth before the fragments become firmly fixed in place; and the dirt brought upon the road by the traffic mixes with the stone and prevents it from ever packing as solidly as the clean stone would, and, besides, the dirt when wet has a lubricating effect upon the stone which under the action of traffic causes the surface to break up readily. Further, during the time traffic is consolidating the stone, the surface is not even approximately water tight; and therefore the subgrade is soft ened by rains, and the stone is mixed with the earth below and vir tually lost. Ordinarily, it is true economy to compact the road by the use of a roller.
Classified according to the power employed, there are two forms of rollers: the horse roller, and the steam roller. The horse roller was first introduced in France about 1834, and the steam roller in 1865. Neither MacAdam nor Telford used a roller in constructing roads, as it was invented after their time.
lighter rollers are drawn by two horses and the heavier by four. The weight per linear inch of face varies from 200 to 300 pounds.
The catalogue price of horse rollers is usually about $100 per ton.
The steam stone-road roller, Fig. 64, is made by a number of manufacturers, but all are practically the same. The total weight .
varies from 10 to 20 tons, and the pressure under the drivers varies from 450 to 650 pounds per linear inch. The cost of these rollers is usually about $200 to $225 per ton.
There has recently been introduced a type of traction engine, the wheels of which may be replaced by heavy rollers, thus con verting the traction engine into a road roller of moderate weight. The rolls cost $200 to $300 in addition to the price of the engine. A machine of this kind would be valuable as a substitute for a road roller where the amount of work will not justify the purchase of a steam roller and where the traction engine could be employed part of the time for other purposes.
The asphalt roller. Fig. 65, page 225, differs from the preceding form chiefly in being lighter and in being so arranged that the front and rear rolls cover the same space. The rear roller may be filled with water or sand. The main purpose of this roller is to smooth rather than to compress, but it can be used for stone-road con struction. The weight varies from 3 to 15 tons. 5 tons being the usual weight. The pressure under the front roll is usually about 200 pounds per linear inch of face, and that under the rear roll can be varied between 200 to 260 pounds per linear inch of face.