The general outlines and dimensions of the pole pieces for the fields are determined by the electrical design, but they must all be covered completely by drawings so that they can be built in the shop. The main fields have laminated pole pieces, that is, the pole piece is made up of thin sheets of steel punched out to definite form, Fig. 1, enough being assembled together to give the required thickness, Fig. 2. The commutating fields, Fig. 6, have solid pole pieces of forged steel. Plate I covers both of these pole pieces.
The main pole pieces must be riveted together to form a solid piece, and the rivets must be spaced and placed in such a manner as to give the best mechanical construction. The pole tips must be shaded; that is, the amount of iron in the tips must be reduced for electrical reasons. Note how this is accomplished: Each lamination has one tip cut off in a definite manner. The laminations are then assembled with alternate pieces having this cut tip on opposite sides. The actual amount of iron in the tips is then reduced by one-half. The tips are also cut back slightly from a true arc, so that the gap between the pole and the armature is greater at the tip.
The manner of holding the completed pole piece in place is also interesting. Each lamination has a rectangular hole near the top. When they are assembled, this forms a rectangular passage through the whole pole piece, Fig. 2. Part of the laminations also have another hole punched which cuts across the first one and runs out to the top edge so that a square hole is formed from the top into the first hole, shown by dotted line in Fig. 2. Bolts are passed through the frame into these holes and screwed into a threaded key, Fig. 3, placed in the first hole. Thus, when these bolts are tightened up, the pole pieces are drawn up against the frame solidly. In order that there may be some adjustment of the gap between the poles and the armature, shims, Fig. 4, are provided which can be slipped between the frame and the pole pieces, thus bringing the pole piece nearer the armature. These shims are provided with slots instead of holes, so that they can be slipped in after the poles are in place. The air gap can thus be adjusted when the machine is being assembled, without entirely removing the holding bolts.
The pole pieces for the commu-tating fields are simpler than the main pole pieces, consisting of a rectangular block of forged steel. The corners at the armature end are slightly rounded, Fig. 6. Holes are drilled and tapped in the opposite end for the bolts which hold the pieces to the frame. Shims, Fig. 7, are provided for these pieces the same as for the main poles. Pins placed in the armature end are used for holding the field coils and spools.
It should be noted that a number of the dimensions on this drawing are given in tables instead of being placed on the drawing itself. The reason for this is one of economy. Any manufacturer making a number of machines of the same general type but of slightly differing characteristics finds that some parts for the machines of different rating vary only in a few dimensions. By placing these variable dimensions in a table a large number of pieces can be covered by the same picture and many drawings saved. Of course, the drawing will not be to scale for more than one of these pieces, but on simple pieces this is not objectionable. The different parts can be distinguished by assigning a specific group number to those parts wanted for any particular machine. This group will be referred to in a general specification covering the machine wanted. Thus in the present machine the specification would call for pole pieces according to a group in a certain drawing. By reference to this group certain pieces would be called for by number; these numbers in turn being given in the dimension table, would fix the dimensions of the piece wanted. This is a method used in many drafting rooms where many similar pieces are used which can be treated in this way.