WATER WHEEL, a machine by which the energy in falling water is utilized to per form mechanical work. through its tend ency to seek the lowest level — the point near est to the centre of the earth as a motive power by its weight. When the water is con fined, as in a vertical pipe, this weight becomes pressure. When the water has acquired a ve locity in flow its motive power is called impulse. Water wheels adapted to these various condi tions may be divided into two general classes— the INertical,* consisting of the overshot' *breast* and 'undershot' wheels; and the •horizontal,' which includes a great 'variety of turbine or reaction wheels. The impact or im pulse wheels are represented in both classes. The term water wheel is correctly applicable to all forms of water motors that rotate, hut in this article it will be restricted to those of the vertical class. For those belonging to the hori zontal class, see TPRRT!V P..
The overshot wheel is so called because the actuating water is fed to it from the top. It is provided with a number of buckets fixed to its periphery in such a way that as the wheel re volves the buckets on the descending side have their tops upward, and being filled with water at or near the top of the wheel, the weight of the water exerts a downward pull and the axle of the wheel being free to turn in its bearings, a rotary motion is imparted to the axle from which it is transmitted by belts or gearing to the mechanism to be operated. The water is admitted to the wheel by various methods, such as the overfall sluice, the penstock and the guide bucket, in all of which the water is guided in a course tangential to the perimeter of the wheel. If the water is in motion, that is, has a certain velocity, when it enters the wheel buck ets, it acts partly by impact, but in usual condi tions the motive power of the wheel is simply the weight of water in its descending buckets, which may be roughly estimated as the cubic content in feet of two-thirds of the number of buckets, multiplied by 6214 pounds. To obtain
the greatest efficiency, the number of buckets, ought to be as large as possible and so arranged as to hold the water as long as practicable. The buckets vary in form, but the greatest effect is obtained from curved buckets, the *radial° giving only about 75 per cent of the power obtainable by the use of (elbow' buckets. Overshot wheels may be constructed of wood or iron in any size. The earlier forms were mas sive structures built of wood; the modern wheels are generally built of iron and are prac tically limited by constructionproblems to falls less than 60 feet in height. The average sizes range from 8 to 40 feet in diameter.
In the breast wheel the water is received by the buckets on a level with or but slightly above the axle of the wheel. It is most effectively employed in connection with falls ranging from S to 20 feet, and where the flow ranges from 5 to 90 cubic feet per second. The buckets are generally held in a curb, and, not being re quired to hold water, are set radially. Their number is made as large as possible, and they are set at such an angle that they leave the water vertically so as not to carry up water from the tailrace. The breast wheel is more effective than the overshot where the fall is low, since it can be made of nearly double the diameter and affords an increased facility for the reception and discharge of the water by the buckets, besides gaining some power from impulse.
The undershot wheel receives the water be low its horizontal centre line. Instead of buck ets it is equipped with a series of paddles or vanes fixed to the periphery in such a way that the flowing water passing under the wheel pushes against the paddles, causing rotation. It L: usually set in a curb with a minimum amount of clearance for the escape of the water. The most effective curbs arc concentric to the wheel, those set straight or tangential allowing too much waste. The paddles are set radially and inclined upward in order to be readily emptied of water upon the return side.