power, as the main shafts of mills; and, third, the counters or shafts transmitting the power to the machines. The strain upon a shaft may be transverse, torsional, or both. In all breast, overshot, or undershot waterwheels, the jack-geer may be so placed that there will be no torsional strain on the shaft of the wheel; in many other shafts, no strain will be transmitted through the journal. In these cases, the size of the journal may be estimated from the transverse strain or weight to which it is subjected. The following table is taken from the Practical Draughtsman, calculated on this formula, D = 'Vw x .1938, D being the diameter in inches, and w the weight to be sustained in lbs.

Table of the diameters of the journals of water-wheel and other shafts for heavy work.

The length of the journal should be from once to twice the diameter. The size of the shaft at the point at which the load is applied may be determined from previous rules; but for all shafts less than three feet between bearings, the size as calculated for the journal need only be enlarged enough to cut the key-seat.

Plate IX.-Figs. 1, 2, 3, represent different views of a wooden waterwheel shaft. Fig. 1 shows at one end the side external elevation of the shaft, furnished with its iron ferules or collars, and its gudgeon at the other end. The shaft is shown in sections, giving the ferules in section, but showing the central spindle with its feathers in an external elevation. Generally, in longitudinal sections of objects enclosing one or more pieces,

the innermost or central piece is not sectioned unless it has some internal peculiarity, the object of a section being to show and explain peculiarities, and being therefore unnecessary when the object is solid; on this account, bolts, nuts, and solid cylindrical shafts are seldom drawn in section. Fig. 2 is a cross or transverse section through the centre of the shaft, to show the outward octagonal form. Fig. 3 is an end view of the shaft, showing the fitting of the spindle B and its feathers into the end of the shaft, and the binding of the whole by ferules or hoops a a. From these views we understand, that this shaft is a long octagonal shaft of wood, oak or pine, of which the ends are rounded and slightly conical. The spindles B, which are let into the ends, are cast with four feathers or wings c. The tail-piece. b is by many millwrights omitted. The ends of the beam are bored for the spindle, and grooved to receive the feathers; the casting is then driven into its place, hooped with hot ferules, and after this hard-wood wedges are driven in on each side of the feathers, and iron spikes are sometimes driven into the end of the wood.

Figs. 4, 5, 6, represent different views of a cast iron shaft of a water-wheel. Fig. 4 is an elevation of the shaft, with one half in section to show the form of the core; fig. 5, an end elevation; fig. 6, a section on the line cc across the centre. The body is cylindrical and hollow, and is cast with four feathers c c, disposed at right angles to each other, and of an external parabolic outline. Near the extremities of these feathers four projections are cast, for the attachment of the bosses of the water-wheel. These projections are made with facets, so as to form the corners of a circumscribing square, as shown in fig. 5, and they are planed to receive the keys by which they are fixed to the naves which are grooved to receive them. The shaft is cast in one entire piece, and the journals are turned.

Fig. 227 represents the section of a portion of a water-wheel, with a cast iron shaft, in use in this country, in which stiffness is given to the wheel by wooden trusses, and a tensional strain is given to the centre of the shaft. These shafts are cast circular in two lengths connected at the centre, with circular bosses on which

the naves of the wheel are keyed.

When the load upon a shaft

Fig. 227.

is not central between the bearings, the size of the journals should be pro

portioned to the weight it will be required to support, which will be inversely as their distance from the centre of pressure.

Fig. 228 represents the fly-wheel shaft of a stationary engine. The parts of least diameter are the journals; their length is 14 times the diam

eter; the centre of the shaft is enlarged to receive the hub of the fly-wheel, and for convenience in driving the keys. Shafts of this form are mostly of wrought iron, the reduction being made by steps, as a convenience in swedging. Fig. 229 is a plan of the crank, from the wheel side.

The torsional strain on a shaft is as the power transmitted through it. It is evident, power being weight multiplied by velocity, that the greater the velocity of the shaft, the less the strain to transmit the same amount of power; and it is the modern practice to drive the shafts at high velocities, and reduce the weight of the geering. In first movers, the strain is often compound; and when the journals bear but little transverse strain, the determination of their size must depend entirely on their capacity to resist torsion. The formula given in the Practical Draughtsman for determining the proper diameter is:

C being for cast iron, 1st movers, 419; 2d, 206; 3d, 106.

Which formula is simplified and tabellated, so that it is only necessary to divide the number or revolutions of the shaft by the horse power, and find the diameter corresponding to the quotient in the table.