114 



ARMATURE CONSTRUCTION 



freely dissipated) is absorbed by the excess cement, and may lead 

 to acid formations, which if retained anywhere as, for instance, in 

 cavities may ultimately cause disintegration of the mica insula- 

 tion, especially if any trace of oil ever gains access to these parts. 



Where, for a special commutator, it is too expensive to provide 

 the rings of gas-pipe, the heating may be accomplished, though 

 less effectively, by individual Bunsen burners ; but this takes 

 longer, and is far from satisfactory. From one to two hours 

 application of the heat is generally sufficient when rings of gas- 

 pipe are employed, and during this period the gas-pipe should be 



FIG. 130. Machined Commutator ready for Bush, allowing Building Rings and parts. 



frequently slightly shifted to avoid concentrating the heat upon 

 any particular places, and to avoid destroying the leatheroid 

 or fibre insulation. It is a good plan to line the inner surface 

 of the segments with sheet iron, to prevent the flame from 

 carbonising the cement in the mica strips. One should continue 

 to uniformly and gradually tighten up the clamping blocks during 

 the cooling process. When cold, the V-grooves at the ends of the 

 segments are turned on a boring mill, and the segments are then 

 tested for short circuits. Fig. 130 shows the commutator after 

 the V-grooves have been turned in the ends, the commutator still 

 being held in the assembling rings, of the type shown in Figs. 

 121 and 122. On the right-hand side of the figure may be 

 seen the mica V-rings, and on the left the commutator bush and 



