THREE:A learner should, in fact, consider the application and operative conditions of gearing as one of the main parts of the subject, and the geometry or even the construction of wheels as subsidiary; in this way attention will be directed to that which is most difficult to learn, and a part for which facilities are frequently wanting. Gearing may be classed into five modificationsspur wheels, bevel wheels, tangent wheels, spiral wheels, and chain wheels; the last I include among gearing because the nature of their operation is analogous to tooth wheels, although at first thought chains seem to correspond more to belts than gearing. The motion imparted by chains meshing over the teeth of wheels is positive, and not frictional as with belts; the speed at which such chains may run, with other conditions, correspond to gearing.A planing machine invented by Mr Bodmer in 1841, and since improved by Mr William Sellers of Philadelphia, is free from this elastic action of the platen, which is moved by a tangent wheel or screw pinion. In Bodmer's machine the shaft carrying the pinion was parallel to the platen, but in Sellers' machine is set on a shaft with its axis diagonal to the line of the platen movement, so that the teeth or threads of the pinion act partly by a screw motion, and partly by a progressive forward movement like the teeth of wheels. The rack on the platen of Mr Sellers' [134] machine is arranged with its teeth at a proper angle to balance the friction arising from the rubbing action of the pinion, which angle has been demonstrated as correct at 5°, the ordinary coefficient of friction; as the pinion-shaft is strongly supported at each side of the pinion, and the thrust of the cutting force falls mainly in the line of the pinion shaft, there is but little if any elasticity, so that the motion is positive and smooth.
REVIVE YOUR WARDROBE WITH CHIC KNITS
