Often seen beside a picturesque rural mill, an overshot water wheel possesses two excellent characteristics: considerable mechanical efficiency and easy maintenance. Many have remained in service for decades, and now lend a nostalgic charm to their surroundings.
Operated by gravity, the overshot wheel derives its name from the manner in which water enters the buckets set around its periphery. Pouring from a flume above the wheel, the water shoots into buckets on the down-moving side, overbalancing the empty ones opposite and keeping the wheel in slow rotation.
Since such a wheel may be located near but not actually in the stream, it offers endless landscaping possibilities for a country home where a stream with sufficient flow is available. If a site on dry ground is chosen, the foundation may be constructed dry and the water led to the wheel and a tailrace excavated. With very little effort, the scene may be turned into an attractive garden spot, the wheel becoming both a landscaping feature and a source of power.
It should be noted, however, that an overshot wheel is practical only for a small-capacity output. How much power it will produce depends upon the weight of water the buckets hold and its radius, or lever arm. Expressed in another way, the output depends upon the weight of water transported and the height, or head, through which it falls while in the buckets. For maximum efficiency, the wheel must use the weight of the water through as much of the head as possible. Therefore, the buckets should not spill or sling water until very near tail water.
Power Increases with Width
Although of simple construction, an overshot wheel is cumbersome in size. For this reason, before attempting to build one be certain you have the facilities to move and lift it into place when completed. Also allow yourself plenty of working floor space. It must be understood, too, that such a wheel is a sizable project and requires a lot of material and time. Extreme care in cutting and assembling the parts is not essential, however, because the wheel, operating at slow speed, need not be accurately balanced.
Our construction plans are suitable for a small wheel suitable for a water head of 6' 3". The wheel itself has a diameter of 5', leaving a flume head of 15" to propel the water into the buckets. You may build the wheel to give a power output ranging from 1/2 hp. to 1 hp. at 10 r.p.m. All dimensions remain the same except the width, the horsepower increasing as this is increased. For 1/2 hp., the wheel should be 15 31/32" wide. For 1 hp., it should be 31 29/32". Before deciding on the wheel size, you'll want to make a survey of the power available in the stream.
Virtually all large wheels are built with wood or steel arms and have a shroud plate only around the outer edge, but you may find it simpler and more satisfactory to build the drum-type wheel described here. In this case, each shroud plate is a disk of 1/8" sheet steel. Each disk is braced by a 1/8" sheet steel sole plate to which it is continuously welded, by the buckets, by one of the two large diameter 1/4" steel hub flanges to which it also is continuously welded, and by the long hub itself.
Large Sheet Required
If preferred, the shroud plates may be made of wood. If so, care should be taken to bolt them securely to the hub flanges. Bushings pressed into the wood for the bolts will give the wheel a longer life expectancy.
Sheet steel for the disks may be ordered direct from several large steel companies in case your local supply house is unable to furnish it. Ordinarily, such steel comes in standard 48" widths, so you may have to weld together two or more sheets to get the required 5' diameter, using either a butt weld or a backing plate. This will produce some distortion or ripple, as will the welding on of the numerous clips required. So long as distortion is local, however, and the main lines of wheel and shaft remain true, this will do no harm.
After the sheet has been prepared, scribe a 5' circle on it and cut it with the cutting flame of a gas welding torch. With ordinary care, this method should give sufficient accuracy. Vent and drainage holes should be drilled around each disk to lessen corrosion with the drum.
Good Buckets Important
The buckets are the most important element of the wheel. To give maximum efficiency, they must be formed so that the water enters smoothly at the top of their travel and remains in them until just before they reach the bottom. For this reason, the bucket form we've indicated should be followed faithfully. Either sheet metal or wood is an acceptable material, but metal is better suited to cold climates, since wood is damaged when absorbed water freezes. Because the buckets are subject to wear from the water and sediment that it carries along, you may want to install them so they can be easily replaced.
In laying out and making wooden buckets, follow these steps:
Using a common center, strike off two arcs, one with a 21 1/2" radius and the other with a 2' 6" radius. Then draw a radius line intersecting these arcs.
From the point where the radius crosses the inner arc, measure 2 3/4" farther along the line and mark the point E .
From the point where the radius crosses the outer arc, draw a chord 10 1/2" long and from the new point where this intersects the outer arc draw a line to point E . You now have the inner trace of the bucket.
Take a piece of the bucket stock and lay it along the upper edge of this inner trace, and you have a cross section through the bucked, Cut your stock accordingly.
Steel Buckets Require Jig
Steel buckets are only slightly more difficult if you follow these steps:
Using a common center, strike off two arcs on a piece of plywood, one with a 21 1/2" radius and the other with a 2' 6" radius.
Draw a radius line and then a tangent to the inner arc, making it vertical to the radius. From the point of tangency, measure 5" along the tangent. Mark this point.
Using this mark as a center, strike off an arc with a 5" radius. This is part of the inner trace of the bucket.
At the point where the original radius line crosses the outer arc, draw a chord 10 1/2" long, and at point F where this chord intersects the outer arc draw a new radius line. Also at point F measure off 15 deg. below the new radius and draw a line FG that is 11 1/2" long.
Then, using G as a center, strike an arc with a 11 1/2" radius. This forms the rest of the inner trace of the bucket.
Cut the plywood along this line and along the lines that form a quarter ellipse. Using this as a pattern, cut several more quarter ellipses from scrap. Nail these to stretchers to make a bending jig around which the buckets may be formed.
Weld Wheel Parts
Welding of the various parts of the wheel produces an exceptionally strong construction. After getting together or making all the required parts, begin the assembly by welding four clips to each end of the hub sleeve. Then weld the required number of clips to the shroud plates for the sole plate, and weld the shroud plates to the clips on the hub sleeve. After welding both hub flanges to the shroud plates and the sleeve with a continuous weld, attach the sole plate to the clips on the shroud plates with No. 8 self-tapping screws. Also weld the sole plate to the shroud plates with a continuous weld, and the bucket-support angles to the sole plate.
Attach wooden buckets to the supports with 3/4" No. 10 roundhead wood screws, and then drill holes 2" from center to center through the shroud plates for 1 3/4" No. 10 roundhead wood screws. If you use steel buckets, rivet or screw 10 clips to each side of each bucket and attach the buckets to the angles with No. 8 self-tapping screws. Then drill holes through the shroud plates in the way of the clips for the same type of screws.
Lubricate Bearings Well
Using locknuts and washers, fasten the hub sleeve to the shaft with two 3/8" by 4 1/2" bolts, placed at right angles to each other. Two bearing mountings having 2 3/8" renewable liners with shoulders should be bolted to the foundation. Place shims about 1/4"thick under the bearings.
Standard bearing mountings, variously called pedestals or blocks, may be bought complete with wick oiler or cup oil reservoir and with built-in self-aligning features. Standard bronze bearing metal liners or inserts likewise may be bought from any machine component supplier. Babbitt liners are equally satisfactory.
Although the wheel turns slowly, it is heavy and will be running almost constantly, so good lubrication of the bearings is essential. To this end, care should be taken to insure that the bearing liners are finished to the correct fit. Porous inserts or inserts containing graphite are excellent for this application, but may cost more than regular bearing inserts.
It is important that the foundation be carried deep enough so that water falling from the buckets will not undermine it. Avoid a long flume if possible, in order to keep the construction as simple as possible. Strengthen it along its entire length with an exterior frame and support it well from dam to wheel with pipe uprights.
Sluice Governs Wheel
The sluice gate may be located at any convenient place along the flume. Since it is the governing mechanism of the wheel, its installation should be anything but slipshod. If it is installed at an angle, water pressure will keep it at any desired position. If installed vertically, some mechanism, such as a rack and pinion, should be provided to keep it in place.
Adjust the sluice so that the buckets will run one-quarter full. This will give a wheel speed of 10 r.p.m. If the buckets are allowed to run more than one-quarter full, the efficiency of the wheel will drop for two reasons. Because of the increased speed, centrifugal force will throw water from the buckets. They also will begin to spill before approaching tail water. Although this practice does waste water, it may be profitably employed during a freshet to increase the power output, for at such times the excess water would be wasted anyway.
Reprinted courtesy of Popular Science Monthly, Popular Science Publishing Co. Inc. 1947