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ON THE CONSTRUCTION OF WATER WHEELS.
In the present age, the same importance is not attached to water power as before the introduction of steam, as has been already shown. Nevertheless, since water is still largely employed in some districts and for certain kinds of work, it is of importance that the machinery for rendering it useful should be constructed upon the best principle, so as to secure a maximum effect. In numerous localities in Europe and America, water is the principal motive agent by which manufacturing processes are carried on; and the time has not yet arrived when it can be dispensed with even in our own country. We shall therefore endeavour to pointout the difference between the ordinary and improved forms of water wheels, and to lay down sound principles of construction, accompanied by examples for the guidance of the millwright.
CLASSIFICATION OF WATER MACHINES.
Water may be expended upon water machines, 1st. By gravitation, as in vertical wheels generally; 2nd. By pressure simply, as in the water pressure engine, where the water acts on a reciprocating piston; 3rd. By the impulse of effluent water striking float boards, as in the Poncelet wheel; 4th. By the reaction of effluent water issuing from an orifice, as in the Barker's mill and Whitelaw's turbine; or lastly, by momentum, as in the case of the water ram.
It is not, however, always possible in practice to classify water machines according to the mode in which the water expends its force, and hence it will be more convenient to divide them according to the point at which the water is applied, and the direction in which it passes through the wheel, as in the following summary:—
1st. Vertical Water Wheels, the plane of rotation being Vertical and the water received and afterwards discharged at the same orifice on the external periphery. These may be subdivided into:—
a. Overshot wheels, where the water is applied over the crest, or near the upper extremity of the vertical diameter.
b. Breast wheels, where the water is applied below the crest at the side of the wheel.
c. Undershot wheels, where the water is applied near the bottom of the wheel, and acts, 1. By gravitation, as in the improvedunders hot wheel; or 2. By impulse, as in the ordinary undershot and Poncelet wheels.
2nd. Horizontal Wheels, the plane of rotation being horizontal and the water passing through the wheel from one side to the other. These may be subdivided into:—
a. Horizontal wheels strictly so called, in which the water passes vertically down through the wheel, acting as it passes on curved buckets.
b. Turbines, annular wheels in which the water enters the buckets at the internal periphery, and passing horizontally is discharged at the external periphery.
c. Vortex wheels, in which the water entering at the external periphery flows horizontally and is discharged at the internal periphery.
3rd. Reciprocating Engines, in which the water is applied upon a piston and regulated by valves on the same principle as the steam engine.
The Improvements of the Vertical Wheel.—In the present chapter it will be convenient to enter on the consideration of the construction of vertical wheels. Since the time of Smeaton's experiments in 1759, the principle on which vertical water wheels have been constructed has undergone no important change, although considerable improvements have been effected in the details. The substitution of iron for wood has afforded opportunities for extensive changes in their forms, particularly in the shape and arrangement of the buckets, and has given a lighter and more permanent character to the machine than had previously been attained. A curvilinear form for the buckets has been adopted, the sheet iron of which they are composed affording great facility for being moulded into the required shape. It is not the object of the present treatise to enter into the dates of past improvements, but it will suffice to observe that the breast wheel has taken precedence of the overshot wheel, probably from the increased facilities which a wheel of this description affords for the reception of the water under a varying head. It is in most cases more convenient to apply the water of high falls on the breast at an elevation of about 30° from the vertical diameter, as the support of the pentrough is much less expensive and difficult than when it has to be carried over the top of the wheel. In cases of a variable head, when it is desirable to work down the supply of water, it cannot be accomplished without a sacrifice of power on an overshot wheel; but when applied at the breast, the water in all states of the river is received upon the wheel at the highest level of its head at the time, and no waste is incurred. On most rivers this is important, as it gives the manufacturer the privilege of drawing down the reservoir three or four feet before stopping time in the evening, in order to fill again during the night; or to keep the mill at work in dry seasons until the regular supply reaches it from the mills higher up the river. This becomes an essential arrangement where a number of mills are located upon the same stream, and hence the value of small regulating reservoirs behind the mill as a resource for a temporary supply.
Another advantage of the increased diameter of the breast wheel is the ease with which it overcomes the obstruction of back water. The breast wheel is not only less injured by floods, but the retarding force is overcome with greater ease, and the wheel works in a greater depth of back water.
Component parts of Water Wheels.—Vertical water wheels consist essentially of a main axis resting on masonry foundations, and together with arms and braces forming the means of support for the machine. Chambers for the reception of the water constructed of shrouding, sole-plate, and buckets. A pentrough with sluice for laying on the water, and a tail-race for conveying it away; and an internal or external geared spur wheel and pinion for transmitting the power. These parts we shall treat of successively, before describing the modifications of the vertical wheel.
The main axis is a large and heavy cast-iron shaft carried upon plummer blocks bolted to the masonry foundations of the wheel-house. It sustains the weight of all the moving parts of the wheel, and in some cases the power is taken from it, when it is subjected to a force of torsion. It is usually cast with deep ribs or wings, calcu- Pig, 100.
lated to resist the tensile and compressive strain to which they are alternately subjected as the wheel revolves. A section and elevation of the main axis of a water wheel, 20 feet in diameter and 22 feet wide, are shown in figs. 100 and 101.* A A is one half the main axis with its four deep ribs. The part e is the journal on which the wheel revolves, and d is left square for the convenience of fixing a screw-jack should the wheel require raising. B B B are the recesses for the radical arms of 2^-inch round iron fixed by the keys / /; g g the corresponding recesses for the braces which pass diagonally across the wheel and alternate with the arms; c c are the key beds on the main axis for fixing the main centre. It is difficult to estimate the strain on this shaft when the wheel is on the suspension principle, although the work it has to perform is trifling compared with what it would have to sustain in the event of the power being taken from the axle. In the latter case the wheel has to sustain not only the weight of the wheel and the water in the buckets, but also the force of torsion, as the power is transmitted from the periphery through the arms and axle to the main gearing of the mill.
* The wheel is shown in Plate IV. Fig. 110 is also an enlarged detail drawing of this wheel.