These very common, older motors do not perform very well, but in many cases, replacement with newer types is not too practical without major surgery and drive train alterations. To test the possibilities of improving these older motors, newer magnetic materials have increased the field strength drastically. Having some spare motor's on hand and after measuring the space, 1/2" cube magnets appeared to fit.
First tests were run with the original. Then the magnets were exchanged, followed by tests for comparison.
Note: Adjust brightness and contrast for optimum viewing.
Original magnet on left. New on right with hole for bearing.
Note small original magnet.
CAUTION: Do all worm pressing before exchanging magnets while the motor is in the original more rigid state. Since the rear screw, which holds the frame in alignment, will not be used; pressing gears on shaft can distort pole pieces and front bell alignment. I learned the hard way and had to line things up by ear. Applying some ACC to the pole piece to bell joint beforehand can prevent bell rotation and movement.
Check the polarity of both magnets to assure the same orientation in the motor and mark the top face for reference. Simply remove rear screw and magnet, which will now lose most of its field strength. This is not the case with the new magnet. Later motors are riveted, so the swaged end must be drilled to remove the flange. By placing the pole piece end on a bench anvil, the rivet can be punched out with a flat end drift pin.
Place the new magnet in the opening. It will probably snap in, pulled by the magnetic force. Line it up with the sides. The rear end will extend past pole pieces; but, except in tight cases, causes no problem except a slight loss in flux path.
If necessary for rear clearance, one side face of the magnet may be ground with a wet diamond drill to clear the shaft end and the 3/16 OD bearing. Note that the shaft is not centered vertically in the magnet opening. It is about .220" down from top. The ceramic material is very abrasive and will dull most standard tools rapidly.
In most cases the magnetic force will hold it in place, but for security a few drops of ACC around the edges and down the screw holes will help. Tests were run without cement with no problems. Just mount and test.
The speed of these motors varies over the years and with the Bowser versions. The one used ran light at just over 21 k RPM after about 15 minutes running, while it dropped to close to 14 k RPM with the new magnet. This may be attributed to the expected increased torque. Current dropped from about .4 amp to about .25 amp. Stall torque and other measurements are compared in STALL TORQUE DATA ,
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