STEEL SHRINK RING COMMUTATORS
Known as Exciter and Dynamometer units, these commutators are very specialized and require extra attention from design through completion.
On these commutators, the only restraining force on the copper and mica segment pack is exerted by the forged steel rings on the brush surface of the commutator. The strength of the forgings is required to allow for very fast acceleration to exceedingly high RPM. This differs from v-ring commutators in which the copper and mica segment pack is retained by the force exerted by the steel cap on the 30 degree internal dovetail angle.
The commutator copper bars remain stable in their extreme duty applications as a result of the fit of the steel rings to the segment pack. To obtain this fit, close attention must be paid to the thermal cycling of the commutator and the measurement of the growth of the steel rings. The steel OD is measured cold, the rings are then heated, seated on segment pack and measured again. Typically we are trying to achieve a final fit of 0.005" to 0.010" (dependent on size) which results in pressure of 30,000 psi per 0.001" of retained fit. On many units there is an additional consideration.
These commutators can also be designed with a tapered segment pack I.D. Further, they may sometimes be installed directly to the armature shaft. This feature ensures better performance at high RPM, and this taper is best machined on a vertical turret lathe for accurate fit. Experience is key in the refill of these commutators to ensure safe, long term operation under extreme duty conditions.