Fig. 40 shows the detail of a spur gear with a T-shape arm. The gear drives through fitted bolts in the flange about the hub. Two or three teeth are dotted in, to show their dimensions, which are according to the standard involute system.
Fig. 41 shows the detail of a pinion designed to mesh with the gear in Fig. 40, one view being sufficient.
The gear-cutting machines, on which the teeth of these gears are cut, require the angles as given for the setting of the cutters. It should also be remembered that the casting must be finished by the machinist before the teeth are cut; hence the dimensions for hubs, diameters, etc., must be so put on that they can be conveniently used by the machinist in turning up the gear blank in an ordinary lathe.
Fig. 43 shows the detail of a worm and worm gear. The teeth on the worm gear are twisted; and if it were attempted to show their true projection, it would be a complicated and difficult piece of work. The worm gear is shown, therefore, by drawing the pitch line of the teeth and other circles at the top and bottom of the teeth. At the point where the gear is in mesh with the worm, a portion of the rim is broken away, and the middle section shown. The worm threads, which are lines in the form of helices, are shown in the drawing merely by straight lines, this being the conventional way for representations of this character. Another way of showing the worm would be in cross section, in which case the helical lines would be wholly avoided; as drawn, however, it has the advantage of at once conveying to the eye that it is a right-handed thread.
Fig. 41. Detail of Pinion to Meeh with Gear of Fig.40.
Fig. 43. Detail of Warm and Worm Gear.
Fig. 44. Detail of Cylinder Head of Steam Engine.