Experience with Overshot, Undershot and Pitchback Water Wheels.
Editor of The Miller's Review:
the question relating to certain water wheels has recently been pretty fully discussed though your valuable columns by a number of able writers. In reviewing their conclusions, we have as yet no information from any one of them as to the real superior merit of the overshot wheel over the pitchback wheel, or reversing the order, the pitchback over the overshot wheel under precisely the same conditions of water, and the head and fall being the same. Now, my own personal experience in milling commenced with overshot wheels, under a head and fall of fifteen feet, our wheels extreme diameter being thirteen feet, leaving two feet of head water over the top of the wheel in the forebay. The width of the wheel was four feet between the rims. We admitted the water about one foot on the down stream side of the top centre of the wheel, by a carefully built chute, in such a manner that the wheel receiving the full force of the hydraulic pressure transmitted from the standing head of water as well also of the full weight and gravity of the same until it left the wheel on its under side. This outfit gave us very satisfactory power, under an evenly balanced load, carefully adjusted to the power in hand or applied. But the moment we attempted to add additional load, without adding additional vent to the water gauged, then our mill would radically check up its speed, as for instance, the hoisting of a three-bushel sack of wheat would disarrange the running of the buhrs very materially, which was or is detrimental to good work. The amount of water flowed on either of these wheels was sixty square inches ordinarily, sometimes more or less.
Pencil Note on Photocopy: "Origins" of Wheel Heishman's Mill (?) William Foshag.
My second experience was with a seven foot head and fall, applied to undershot wheels. In this case we had wheels of twelve feet diameter and as the name of the wheel implies we struck the paddles of the wheel on line of a horizontal chute attached to the bottom of the flume or forebay. These wheels were naked save only the paddles grooved into the outer rims of the wheel, there being no sheeting used of any kind to hold or confine the water to the wheel. Pressure alone in this application, is propelling force; weight and gravity could scarcely claim very much credit. We vented 425 square inches of water on each wheel, and the power derived from this application was excellent. Adding additional load did not visibly affect its motion, nor in any manner disturb the grinding. The only, and paramount, question was to be sure of a sufficient supply of water in the mill dam.
My third experience with wooden wheels was with the pitchback under practically the same conditions of water as the overshot spoken above, except the height and fall of the water, which in this case was thirteen and one-half feet, or one and one-half feet less than in the above overshot application. In this mill I found two wheels of thirteen feet diameter each. The flume was brought into the mill directly in the center of the mill, and at a right angle with the wheels. This flume was penstocked directly in the center between the two wheels in the shape very much of a letter V. From that penstock the water was admitted to the wheels about four feet back from the center of the wheel, and as near as my memory recalls, about fifteen inches or perhaps a little more from the top of the wheel down.
Thus, the reader will notice that the wheels revolved toward each other and discharged their tailwater toward each other and discharged tailwater at a right angle to the wheel's right under their centers, giving the water a continuous flow though the bottom of the mill building. This arrangement gave us two pairs of buhrs running with the sun and two against the same. The motion in this manner applied counteracted very effectually the vibrations and tremor of the mill building so often visible with the number of buhrs are running in the same direction at one time. The gearing attached to these wheels differed very materially in comparisons with that in the overshot power described above, viz, that the gyrations of the water wheels were at least one-third faster that the above, and the power transmitted by them in all respect was superior. The construction of the wheels, both overshot and pitchback, was of the very finest workmanship, and they were all alike carefully sheeted, so that no water could be wasted traveling at random on the inner surface of the wheels. These pitchback wheels would take on additional weight of work with less perceptible chocking up, thus making them very desirable power to operate, and I, for one, am firmly of the opinion that even now they could be employed very profitably in places where overshot wheels are hardly available on account of low head and fall.
In making my final comparison between these wheels. I would state that I worked three different mills with overshot power and one pitchback as stated above, and I found the same easy disturbances in all these overshot wheel powers, although one of them, the last one I operated, had powerful wheel sunder a fourteen-feet head, and fall, with width of wheel sixteen feet, and the same susceptibility of checking up under additional load as the one first described above. If I had the time to make diagrams of wheels and forebay and penstock showing precisely the different kinds of wheels and the difference of applying the water I think I could very fully convince students and users of water power of the easy superiority of the pitchback wheel over the overshot, used under similar conditions. Much more could be said in connection with this important problem that would be instructive and valuable.
Gleason, Pennsylvania. Thomas R. Burgner.
July 15, 1907. Miller's Review.
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