Fly wheels afford the lathe two important advantages. Their momentum, equalizes the results of the varying muscular effort expended in driving them; storing up all in excess for the work or load to be overcome, and parting again with just so much, as is necessary to carry on an equal revolution under occasionally increased strain, and during the recurring periods of diminished effort. Thus, permitting a maximum of power to be conveyed to the work, with a minimum of fatigue to the operator. In the second place they serve as speed pulleys, and communicate the power with its rate varied to different velocities, suitable to work of different densities or magnitudes.

The momentum of the hand fly wheel allows the operator to exert the principal portion of his force at the two most favourable parts of its circuit; in pulling up, at a small angle with the perpendicular, during the ascent of the handle, and in thrusting down in the same manner on its descent; while it affords him a momentary slight relief at the intermediate points, at which, from the more nearly horizontal position of the handle he can exert little power. The weight and velocity being suitable, the momentum regulates or governs these four unequal efforts, melting them into one uniform force spread over the entire revolution. When therefore two handles are applied to one fly wheel, they are usually placed at right angles to each other, which combines a maximum and minimum effect at one time, and equalizes the power. The handles of large fly wheels usually vary from about ten to eighteen inches in radius. A short radius which reduces resistance and gives a quick pace to the wheel, is usually preferred to the slower and heavier pull necessary to the longer, when the assistance derived from momentum is also less. The extremes of velocity attained with the hand fly wheel, may be considered as from about fifty to twenty-five revolutions per minute, respectively, the former gaining the most advantage from momentum; the pace of the revolutions therefore should not be unduly diminished, as it is far less laborious to the operator to continue using the half of his force, with a velocity of say thirty revolutions, than the whole, with that of fifteen.

With the smaller light hand wheels, the question of momentum becomes nearly unimportant; the limited path of the handle is entirely within the range of action of the hand and arm, and the slight force called for, can be readily applied to all parts of the circuit without the aid of momentum; while from the same reason, the axis of the wheel may generally be placed indifferently, vertically for the turner, horizontally for the lapidary, or in any intermediate position otherwise required.

The action of turning a foot wheel by its treadle, already described by Moxon, somewhat resembles that of ascending stairs; the analogy lying in the manner in which the force is applied to the treadle board. On the stair, the weight of the body is gradually thrown on the one foot, and as gently relieved when it is taken up by the other foot on the next stair. The muscular effort of the foot on the lathe treadle is equally gradual. The knee is bent, and the downward push of the foot which commences quietly, is greatest at the middle of the stroke, then diminishes and entirely ceases at the end, in order to offer no opposition to the rising of the treadle by the revolution of the crank. The jerk to be given at the first moment of the tread, as described by Moxon, was required to overcome the elasticity of the crank thong; with the rigid hook it is no longer necessary nor desirable.

It has been said that the hand wheel receives two impulses to every revolution, the foot wheel can receive but one; and therefore to obtain sufficient momentum to carry this lighter wheel through the two-thirds of its circuit, during which the foot is inert, it is requisite that it should move at least twice as fast as the hand wheel. The motion of the ordinary foot wheel can barely be maintained when it falls below thirty revolutions to the minute, two, three or four times that velocity are usual, while, for many purposes of turning and polishing, the velocity is often very much higher.

The weight of the fly wheel, upon which combined with its velocity, the momentum depends, is determined by circumstances. In the spinning wheel, with which the velocity is very great while the resistance is inconsiderable; the wheel can scarcely be too light, that it may be instantly checked to stop the machine, constantly necessary to repair the thread. With large fly wheels, applied to continuous work in pumping, or in driving heavy machinery such as that for rolling iron, where the stoppages are rare, but, in which the resistance is frequently and considerably varied; the weight becomes very great, the principal limit arising from the friction of the axle, which increases directly with the weight of the wheel.

For the foot lathe, an intermediate course has to be followed; the fly wheel should be sufficiently heavy, for its momentum to ensure uniformity of motion, to overcome an occasional increase in the resistance from a hard place in the material, or from an extra depth of cut; but yet not injudiciously so, as it is continually necessary to stop the lathe to examine the progress of the work. The equal efforts employed in starting the wheel from a state of rest and carrying it to any given velocity, and afterwards destroying this motion in stopping it, are so much taken away from the employed and productive power of the operator; and needless weight in the wheel correspondingly increases this loss. The fly wheels suitable to foot lathes therefore, generally vary from about twenty to thirty inches in diameter, and are from about forty to a hundred and twenty pounds in weight.

As a speed pulley, the fly wheel converts the velocity most easily obtained by the operator, into that called for, by the size and density of the particular material under reduction in the lathe. The speed employed in turning the soft woods can scarcely be too high, but with ivory, the hard woods, the soft, and then the harder metals, a gradually decreasing rate of motion is required; ending indeed for some large works in the last, in power lathes, with the slow rate of ten to fifteen feet per minute, measured at the tool.