The obstruction which a cylinder meets with in rolling along a smooth plane is quite distinct in its character, and far in ferior in its amount, to that which is produced by the friction of the same cy linder drawn lengthwise along a plane. For example, in the case of wood rolling on wood, the resistance is to the pressure, if the cylinder be small, as 16 or 18 to 1000; and if the cylinder be large, this may be reduced to 6 to 1000. The fric tion from sliding, in the same cases, would be to the pressure as 2 to 10, or 3 to 10, according to the nature of the wood. Hence, by causing one body to roll on another, the resistance is dimin ished from 12 to 20 times. It is there fore a principle in the composition of machines, that attrition should be avoid ed as much as possible, and rolling mo tions substituted whenever circumstances admit.
On this principle depends the advan tages resulting from the application of friction wheels and friction rollers. The extremity of an axle, instead of resting in a cylindrical socket, is made to rest on the circumference of two wheels, to the axles of which the friction is transferred, and consequently diminished in the ra tio of the radius of the wheel to the radi us of the axle. This ingenious contriv ance appears to have first been applied by Henry Sully, in the year 1716.
Soapstone has been used for diminish ing friction with great profit and success. It is first thoroughly pulverized, and than mixed with oil, tallow, lard, or tar. It is used in all kinds of machinery where it is necessary to apply any unctuous sub stance to diminish friction, and it is an excellent substitute for the usual compo sition applied to carriage vehicles.
Perkins has avoided the necessity of employing oil, grease, or any other lubri cator to the piston of the steam engine by forming his piston of bell-metal, com posed of the following materials :—cop per, 20 parts ; tin, 5 parts ; zinc, 1 part. This, as well as his cast-iron cylinder, is cast under the pressure of a considerable head of metal; by which means the den sity and closeness of grain of both of them is very greatly increased, and in deed, the cast-iron has as close a grain as wrought-iron itself. These two metals he finds to act so as to polish each other in, me. lie also uses the same dense cast iron to form his steam-engine crank axes, and the spindles of axes of his grindstones, &c., with ; and he runs the cylindrical necks of them upon bearings formed of his bell-metal, placed under neath them, and made with hollow cylin drical cavities, across their upper faces, not exceeding the sixth part of a circle in extent; and yet, upon these very small bearings, his necks run, with a very tri fling portion indeed of grease, as a lubri cator. In this manner, the cylindrical necks of the axis of a large grindstone, employed in grinding large articles, run ; and yet, on throwing off the band from the rigger, or band-wheel, the stone will make fifty revolutions at least before it stops.