FLY-WHEEL. A heavy wheel attached to the revolving shaft of a to serve as an accumulator and equalizer of power. Its action depends upon the inecha»i•al law that a body once set in motion retains :t certain amount of moving force or momentum which tuts to be overcome before ceases. Thus, a heavy wheel once set to rotating by some external force, as the idston-rod of a steam-engine, continues to rotate by virtue of its stored energy or mo mentum after this external force ceases to act. In a steam-engine the function of a lly-wheel is: (1) To store up excess of energy received from the piston during the first part of its stroke tin der full steam pressure, and to give it out when. during the latter part of the stroke. the effort has grown less because of decreased pressure due to the expansion of the steam; (2) to equalize the variation in the leverage with which the vary ing steam effort acts upon the crank to revolve the shaft; (3) to give out or absorb energy when variation in the external load or resistance oc 1.1.1re suddenly. The tly-wheel is, therefore, as de scribed, an accumulator and an equalizer, and the reserve which it stores will be greater as its mass is greater, and the leverage greater with which that mass acts. Large mass means great weight and consequently great friction on the shaft-bearings. Large leverage or large radius means more space required for the wheel and in creased centrifugal force tending to disrupt its rim. The designer of a fly-wheel has to integrate these different factors to meet the requirements of space, speed of rotation. and other governing conditions. Modern practice shows a tendency to adopt smaller diameter wheels than was once customary; in early engines 30-foot fly-wheels were often to he met, but now IS to 20 feet are large diameters, and in centre-crank high speed engines six feet has become a large size. The strains which are set up in a tly-wheel because of the work which it has to do are, first, a tor sional strain tending to twist the spokes off the hub, and second. a strain due to centrifugal force which tends to burst the rim. A fly-wheel van easily be designed to resist torsional strains, but there is no possible way to overcome the centri fugal force, hence for a given material there is a definite speed at which disruption will occur regardless of the amount of material used. A re cent authority gives the following simple formula for figuring the disrupting speed of tly-wheels having solid, single-piece rims: v=1.6.,
Tr' In this formula r represents rim-speed in feet per second at which disruption occurs, s repre cents the ultimate tensile strength of the terial per square inch, and w represents the weight of the material per cubic inch. If, in,lcad of ultimate strength, we let a represent the safe strength, then by solving the equation for v we get the safe rim-speed in feet per second. If the wheel is made of sections bolted together. the ulti mate and safe values of s in the above formula must be reduced to from one-half to one-fourth of the figure assumed for solid wheels. The safe rim-speed of a solid east-iron wheel figured ac cording to this formula is about 100 feet per sec ond. of a cast-steel wheel about 233 feet per sec ond, and of a maple-wood wheel about 155 feet per second.
The disruption or bursting of a fly-wheel re volving at high speed occurs with great force. the flying fragments often wrecking the engine and building in which it is boused. and sometimes being thrown several hundred feet away. Fly wheel accidents are less common than formerly, but are still frequent enough to make the design ing of strong and safe wheels a problem for seri ious study by engineers.
En early engines turning with a low number of revolutions the fly-wheel required to be of large diameter, and was for this reason nearly always distinct from the wheel from which the power was taken off. In more modern engines the con venience of having the fly-wheel serve also as an element of the transmission machinery has brought about the use of fly-band wheels, where belts or ropes are used to take off the power from the engine-shaft. It is so much Less the practice in recent years to use gearing in transmitting the power from the engine-shaft that the fly-wheel is seldom a toothed Wheel. Small fly-wheels are usually made in one piece of cast iron or east steel. Larger sizes are east in halves, which are connected by bolted joints to form the complete wheel. and the largest wheels are cast in several segments which are bolted together. Fly-wheels are sometimes made with metal hub and arms and a rim of some tough, hard wood. Other con structions are steel plates riveted together, and cast wheels with their rims wound with steel wire of great strength. (See STEAM-ENGINE.) Very complete data for use in the calculation and design of fly-wheels will he found in Kent's /1/ echu a/cal Engineer's Pocket-Book (New York, 1902).
FO. The Chinese equivalent for Sanskrit Bud dha. See BUDDHISM.