Ans. They serve to keep the armature in the proper position with respect to the bearings.
Ques. How is the shaft proportioned?
Ans. If it be proportioned to secure the proper stiffness, it will be found of ample size to resist the twisting strain.
Fig. 349.—Typical shaft for an armature. The illustration shows the keyways for pulley armature and commutator. In the smaller sizes, there is usually a flange at A, and threads at B and C for retaining nuts.
The shaft is subject also to bending by the weight of the armature, by the magnetic drag on its core, and in belt driven machines, by the lateral drag of the pulley. When running, it is also subjected to bending stresses if the armature be not properly balanced. If the bearings do not give, it is evident that all such actions tend to bend the shaft at definite points.
Core.—In the small and medium size dynamos, the core is attached direct to the shaft. There are two kinds of core:
1. Smooth;
2. Slotted.
Ques. What may be said of the smooth type of core?
Ans. It has become obsolete, except in special cases, as for machines used for electrolytic work where a large current at low voltage is required.