PARTS OF A DYNAMO-ELECTRIC MACHINE 



55 



on the commutator; under such conditions these insulators 

 would become charred and thus spoiled in so far as their 

 insulating qualitites are concerned. 



Where the spider clamps the assembled copper bars, 

 insulation must be used to insulate the bars from the 

 spider. Ring-shaped pieces, with V-shaped cross-section, 

 are required for this purpose. Mica sheets are very brittle 

 and could not be bent into the proper shape for such use, 

 so micanite is generally used. This is made by cementing 

 very thin, small sheets of mica together by some such 

 flexible binder as shellac. The thin flakes of mica are laid 



FIG. 29. Cross-section of a Small Commutator, Showing the More 

 Essential Parts of the Assembly. General Electric Co. 



in the bottom of a tray to an even depth (perhaps one- 

 sixteenth of an inch) the thin shellac poured in and allowed 

 to drain off. By a proper drying and compressing process 

 a flexible sheet of micanite is obtained which, when warmed, 

 may be formed into any desired shape. 



Spider. The commutator spider is made of cast iron 

 in two or three pieces. The assembly view of a small 

 commutator is shown in Fig. 29. The spider, shown in 

 cross-section, is seen to consist of a sleeve, a, that fits on 

 the armature shaft, a loose end ring, 6, and the lock nut, c. 

 The sleeve, a, is pinned to the shaft when the commutator 



