COMMUTA TOR CONSTR UGT10N 117 



commutator, 60 tons pressure should be applied, or, say, 1200 

 Ibs. per inch of circumference for a very rough rule. For 

 very large diameters, some firms have employed massive solid steel 

 rings, with slightly conical internal surfaces, and have forced these 

 over the segments by three presses, equidistant about the periphery. 

 But this is now generally abandoned, and the radial pressure is, 

 for large commutators, practically universally applied by means " of 

 an external steel ring provided with numerous radial bolts, 

 bearing on clamping blocks as already described; and where 

 hydraulic pressure is employed, it is for the purpose of tightening 

 the end clamping rings. Where hydraulic pressure is dispensed 

 with for this purpose, it is the more important to have numerous 

 massive bolts for drawing up and permanently holding the end 

 rings. After again testing for shorts and grounds, the external 

 clamping rings are removed. The commutator is now again 

 heated by means of the outside ring of gas-pipe. A still further 

 amount of cement is thus driven out, and the bolts retaining the 

 end clamping rings are gradually tightened up, a very little at a 

 time, during the process ; or, and preferably, the hydraulic pressure 

 is applied before and during each tightening of the bolts. The 

 tightening is continued during cooling. A test for shorts and 

 grounds is again made; the surface is then turned roughly and 

 the insulation again tested. The commutator is now in the state 

 shown in Fig. 133, and is ready to be bolted on to the armature 

 spider, preparatory to connecting it up to the arma- 

 ture windings, after which the commutator surface 

 undergoes final machining. A good plan some- 

 times employed, consists in using segments with 

 the internal edges rounded, as shown in Fig. 134, 

 the internal surface in this case not being turned. 

 This construction gives a long leakage path at the FIG. 134. Sketch 

 bottom, from segment to segment ; and as the inner showing rounded 



, '. . ... ., & . ,, ' L - , edges of Commu- 



surface is so inaccessible, it is thought that the pre- tator Segments, 

 caution is desirable. It is not so obvious that this 

 is the case, since the corners afford lodgment for dust ; and hence 

 the interior must be made inaccessible to dust, and carefully 

 cleaned out before being closed in. The mica end rings of a 

 commutator should be from 1/5 to 3'0 mm. thick, according to the 

 voltage and size of the commutator. Both for the end rings and 

 the segment mica, the secret of success rests largely upon the 

 application of heat, say, 200 Cent., whenever pressure is used. 

 This facilitates the exudation of all but a minimum residue of 



