The brush holder yoke, which is in two pieces, Figs. 11 and 12, so that it can be slipped into the slot on the bearing when the machine is being assembled, is made in the form of a thin wheel with projecting slotted lugs for carrying the studs. These pieces are made of cast iron, and sufficient details must be shown to enable the pattern maker to provide a pattern of proper section at all points. The enlarged sections at the right are for this purpose entirely. Notice on this drawing another example of a bent section line used to save drawing other views. The line B B B is used since it will then take in one of the holes in the hub and permit clearer delineation.
It will be noted that there are two holes in the hub and two in the rim which are for the brush shifting device; the details of the latter must be worked out before the proper location of the holes can be determined.
It will be noted that the arrangement for shifting the brushes around the commutator, in order to get the proper location for good commutation, consists of a shaft, Fig. 8, having mounted at one end a worm, Fig. 2, which engages the gear or the rack, Fig. 9, mounted on the bearing housing. When the handwheel on the shaft is revolved, the worm shifts the brush holder in one direction or another, until the proper location of brushes is secured. It is necessary, of course, to place the shaft in such position that the worm engages the rack properly, that is, so that the pitch line of the rack and worm are tangent. The holes in the brush holder yoke can be located as soon as the worm and rack have been laid out; it will be noted that four holes are provided, although only two are necessary, so that the shaft and handwheel can be assembled on either side of the commutator, depending upon which is more convenient for operation.
The worm, detailed in Fig. 2, is a good example of the double rectangular thread. In order that the motion may not be too slow a one-inch pitch has been determined upon, but if a single thread were used with this pitch the thread would be entirely too deep; therefore, a double thread is used, which reduces the depth to a reasonable amount. If the drawing is checked carefully, it will be noted that the worm is not drawn to scale, that is, the draftsman has made the picture in the most convenient way and has used proper dimensions. The threads are not detailed, but are covered by a note, giving the number of threads and the pitch. The teeth on the rack are, of course, determined by the pitch of the worm, so that no further information is necessary. It will be noted that the dimensions of the rack are given in degrees, since the amount of the shift required for the brushes would be expressed in this way.
The shaft, Fig. 8, for operating the worm is, of course, simply a cylindrical bar of sufficient length to bring the handwheel, Fig. 3, to a convenient point. The only detailed dimensions necessary are those referring to the holes for pins at various points.
For bearings for this shaft a steel casting, Fig. 6, is used, having a stud which passes through the brush holder yoke and is secured by a nut. The shaft is held in the proper position by means of the worm which is pinned to the shaft, Fig. 2, and a collar, Fig. 5, which is also pinned to the shaft and located below the upper bearing. The construction here does not require fine work since the brushes are only shifted at long intervals and easy operation or freedom from friction is not required. The cast-iron handwheel, Fig. 3, is of simple construction, with a hub which fits over the shaft and is pinned to the shaft. The whole construction is simple enough to be easily understood, and great elaboration is not necessary for the workmen in the shop.