DIRECT CURRENT DYNAMOS AND MOTORS. 71 
As a result of the above considerations, the length of the 
air gap with toothed armatures is made from ;; inch in 
small bipolar machines to } or 4 inch in large multi- 
polar ones. 
For smooth-core armatures the air gap is necessarily greater 
than for a corresponding toothed armature, as it includes 
the height of the winding as well as a proper mechanical 
clearance, which ranges from 3°; inch in small machines 
to § inch in large ones. 
TABLE 25. 
RADIAL CLEARANCE BETWEEN ARMATURE AND 
POLEPIECES., 
Diameter Smooth-Core Armature. Toothed 
of Armature, or Perforated 
in inches. Drum. Ring. Armature, 
2 3°3 * 7. 1 
4 % 32 $I 
8 BE $ ne 
12 75 ve $ 
18 3 15 ve 
24 t 33 Te 
30 Py + 32 
40 Nt 35 + 
50 ae sz 
76 $ 1 
100 qs 4} 
125 $ & 
150 tr ie 
20) 2 4 
Table 25 gives the clearances usually allowed for various 
types and sizes of armatures. For toothed and perforated 
armatures the figures given are at the same time the 
lengths of the air gaps, but for smooth armatures the 
clearance must be added to the height of the winding 
space given in Table 14, in order to obtain the fength of 
the air gap. 
