THK INSULATING OF ARMATURE CONDUCTORS 247 



There is less danger of cracking the material at the bends, and if 

 faults exist in the component sheets, the danger of a superposition 

 of these faults is less the greater the number of sheets. Each sheet 

 may also be more thoroughly dried out the thinner it is. One also 

 avoids the necessity of keeping in stock a large number of different 

 thicknesses 



Horn fibre ranks first, both in mechanical and in disruptive 

 strength, but does not run very uniform in thickness. It is also 

 relatively expensive. 



Presspahn ranks first in uniformity of thickness and in surface 

 smoothness; it is tough mechanically, and has high disruptive 

 strength. 



Red rope paper and manilla paper are cheap, and are widely 

 used for slot linings, but care must be taken to test the quality. 

 Manilla paper is preferable. 



When wood is used in armature construction, it must be dried 

 with the greatest care and made waterproof by suitable treatment. 

 It is often used for the retaining wedges for the slot conductors, 

 but has the disadvantage that, when so employed, it will, in the 

 interests of mechanical strength, require more space than is 

 necessary by other arrangements and materials. It obstructs the 

 emission of heat from the conductors. It is also difficult to avoid 

 warping and cracking. It involves the use of more expensive 

 punches and dies. In general it may be said that the use of wood 

 in armature slots is to be avoided. Formerly it was employed to 

 a great extent. It is preferable to let the insulated armature coils 

 come directly to the surface and be held in place by binding bands. 



Great care must be exercised with all slot-lining insulations to 

 subject them to thorough drying and waterproofing processes. 

 A very effective process consists in prolonged immersion 'in hot 

 linseed oil. When recourse must be had to the use of wood for 

 slot wedges, great care should be employed in the selection and 

 treatment of the wood. It should be of hard, fine grain, as, for 

 example, maple or teak. It must be cut with the grain, and must 

 be free from knots and irregularities of all kinds. It must be 

 thoroughly seasoned and dried. The ultimate drying should be in 

 a vacuum oven, and after being taken from the oven it should, 

 while still hot, be immersed in double-boiled linseed oil, and left 

 there for from twelve to twenty-four hours, the temperature being 

 maintained near the boiling-point. The wood thus becomes 

 thoroughly impregnated, making it moisture-proof and improving 

 its insulating qualities. 



