Fig. 209.
The mangle-wheel[10] in its simplest form is a revolving disc of metal with a centre of motion c ([Fig. 209]). Upon the face of the disc is fixed a projecting annulus a m, the outer and inner edges of which are cut into teeth. This annulus is interrupted at f, and the teeth are continued round the edges of the interrupted portion so as to form a continued series passing from the outer to the inner edge and back again.
[10] From Willis’s “Principles of Mechanism.”
A pinion b, whose teeth are of the same pitch as those of the wheel, is fixed to the end of an axis, and this axis is mounted so as to allow of a short travelling motion in the direction b c. This may be effected by supporting this end of it either in a swing-frame moving upon a centre as at d, or in a sliding piece, according to the nature of the train with which it is connected. A short pivot projects from the centre of the pinion, and this rests in and is guided by a groove b s f t b h k, which is cut in the surface of the disc, and made concentric to the pitch circles of the inner and outer rays of teeth, and at a normal distance from them equal to the pitch radius of the pinion.
Now when the pinion revolves it will, if it be on the outside, as in [Fig. 209], act upon the spur teeth and turn the wheel in the opposite direction to its own, but when the interrupted portion f of the teeth is thus brought to the pinion the groove will guide the pinion while it passes from the outside to the inside, and thus bring its teeth into action with the annular or internal teeth. The wheel will then receive motion in the same direction as that of the pinion, and this will continue until the gap f is again brought to the pinion, when the latter will be carried outwards and the motion again be reversed. The velocity ratio in either direction will remain constant, but the ratio when the pinion is inside will differ slightly from the ratio when it is outside, because the pitch radius of the annular or internal teeth is necessarily somewhat less than that of the spur teeth. However, the change of direction is not instantaneous, for the form of the groove s f t, which connects the inner and outer grooves, is a semicircle, and when the axis of the pinion reaches s the velocity of the mangle-wheel begins to diminish gradually until it is brought to rest at f, and is again gradually set in motion from f to t, when the constant ratio begins; and this retardation will be increased by increasing the difference between the radius of the inner and outer pitch circles.
Fig. 210.
The teeth of a mangle-wheel are, however, most commonly formed by pins projecting from the face of the disc as in [Fig. 210]. In this manner the pitch circles for the inner and outer wheels coincide, and therefore the velocity ratio is the same within and without, also the space through which the pinion moves in shifting is reduced.