Class 25/33 – Step 12: Magnet & Pole Piece Fitting

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  • Before fitting the magnet and pole pieces, take a look at the commutator and bearings as in picture 1. You are checking to see that the bearings and commutator are correctly aligned. This picture shows the correct alignment. You want the brushes in the middle of the commutator – too far one way and they’ll be half off the end, too far the other way and the commutator will be impeded from rotating. Roll your thumb across the armature to check that the shaft rotates unimpeded.

Note: If you do find that the commutator isn’t aligned, you will have to remove the brushes, springs and base plate and then re-align the shaft by tweaking the shaft length (put the screwdriver up against the spring end and then rotate the armature or worm to adjust the shaft length) as described in Step 9: Driveshaft Fitting (Spring Coupling). You will most likely only have this problem if working with spring couplings, since you have set the length of the shaft by adjusting the springs. If you have the drive dog fittings then the shaft length is fixed. The only way you might have the issue of commutator mis-alignment with drive dogs is if you insert the bearings the incorrect way around.


  • Once you are happy with the brush alignment, place the magnet in its seating, as shown in picture 2. The dimple on the magnet is there for you to keep track of the magnet orientation, since which way around you place the magnet will affect the polarity of the armature. The proper way is to note down where the dimple is and replace it that way. Personally I don’t bother, but rather run the loco first and then see if forwards really is forwards, and backwards really is backwards. If it isn’t, I simply rotate the magnet 180 degrees. The only thing I do ensure, however, is that I have the dimple on top (not for any running direction issues but because dimpled magnets are generally good magnets, and by having the dimple visible, I can quickly identify which type of magnet it is).
  • Lastly, slot the pole pieces into place on either side of the magnet, as per picture 3. If you find this tricky (because the magnet is too strong) then fit the pole pieces into place before seating the magnet.

 

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