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506 HANDBOOK OF PROJECTION FOR
ing compound. The obvious remedy is to clean the dirty parts.
To clean the commutator, use a brush stiff enough to remove any foreign matter which may cling to its surface, yet not stiff enough to injure the surface. If the brush will not remove the deposit, then use 00 sand paper (NEVER use emery paper or emery cloth on a commutator) applying the same while the commutator is revolving, but with just barely enough pressure to clean the metal. After having cleaned the surface, put a few drops of light oil on a cloth, or use the pad already described, holding it lightly to the commutator as it revolves. Don't get much oil on the surface of the commutator — just a "suspicion/' as it were.
If it is a carbon brush which is dirty, or which does not fit the curve of the commutator, raise it just enough to slip a piece of No. y2 sandpaper between the brush and commutator, with the sand side against the brush, and pull it back and forth around the curve of the commutator until enough of the brush has been ground away to clean the surface, or to make it fit the commutator. This is illustrated in Fig. 150.
Do this very carefully. If the brush fits loosely in the hole it is best to sand in the direction of rotation only, else the brush will wiggle back and forth with the sandpaper, thus injuring rather than improving the contact. Be sure and always clean the commutator thoroughly after doing this, since if carbon dust is left adhering to its surface it may work into the insulation and cause a local short circuit between two bars.
(d) The brush not making proper contact with the commutator, which may be due to (1) tension spring not strong enough; (2) tension spring having lost its temper; (3) brush stuck in its holder; (4) brush not fitting the curve of the surface of the commutator ; (5) brush holder set at the wrong angle ; (6) high bar or insulation.
The remedies are: (1) Stretch the spring, if it is a spiral spring and in compression, or cut down its length if the spring be in tension. If it is not a spiral spring, do whatever is needful to make the spring stronger, installing a new one, if necessary; (2) put in a new spring and, since the fact that the old spring has lost its temper is evidence that the spring itself is carrying too much current, reinforce it with a current-carrying jumper; (3) the remedy is obvious: do whatever is needed to loosen the brush ; (4) use sandpaper, as before described, until the brush fits the commutator surface; (5) straighten the holder; (6) see section f, further on.