[ARC5] BC-348 DYNAMOTOR

[WA5CAB at cs.com]

In a message dated 1/23/2008 10:29:11 PM Central Standard Time, jfor at quik.com 
writes: 
> >If the former, another possibility to add to Mike's holder list is a
> >scissors type tube puller with the plastic covering over the curved tips.  
> Hold
> >the tube puller in one hand (left for most people) and grab the cap with 
> it.
> >Grab the outside of the puller tips with pliers and loosen the cap.
> 
> In a later email, it became clear that the brush caps are screw plug type, 
> rather than
> pipe cap type. That makes grabbing them by the OD impossible.  8=((


Well, in that case use a close fitting screwdriver blade.  Grind the edges if 
necessary in order to ensure that the blade gets to the bottom of the slot.  
Or clamp the dynamotor to a drillpress or mill table, put a properly fitting 
screwdriver blade in the chuck, and use the downfeed to ensure it doesn't pop 
out of the slot while turning the quill by whatever method is convenient and 
safe for the quill.   If the cap slot is in plastic and it breaks, it breaks.  
You're no worse off 
> 
> >Assuming you can rotate the armature (brushes aren't stuck to commutators),
> >remove the leads from the brush holder terminals.  Remove the two screws or
> >studs holding the cast aluminum bearing and brush holder carriers to the 
> field
> >coil assembly (steel cylinder).  Pull one carrier away from the field 
> housing
> >far enough that you can move the carrier from side to side.  Then grasp the
> >bearing end and wobble or rock the carrier back and forth slightly in the 
> plane of
> >the brushes while pulling it away from the rest of the dynamotor.  Once one
> >is clear of the dynamotor, pull the armature out the open end.  You can 
> then
> >get at the commutator end of the brushes with the wood stick and push them 
> into
> >the brush holder slightly.  At that point, you should be able to remove the
> >brush by pulling gently on the spring.  If it is still stuck, cut a piece 
> of the
> >stick short enough to fit between the two brushes.  Grab it with long nose
> >pliers and use it to push the brush out.
> 
> I'd be really reluctant to pull the machine apart as you suggest. The 
> bearings often
> have preload springs and getting them back correctly is distinctly 
> non-trivial. Also,
> differing torques on the bolts holding the parts together can impact the 
> shims. If the
> preload on the bearings is too much, it will damage them.

Disagree.  I did forget to mention removing the bearing caps and any shims or 
Bellville Washers.  But all that you have to do to get them back together as 
they were is remember how they came out.  And as far as bearing preload is 
concerned, if you are concerned that it might be different once you reassemble 
the thing (not my general experience), take a spring scale and length of nylon 
line.  Tie a loop in one end of the line for the hook on the spring scale.  
wind up the line around the armature in the area between the commutator and the 
windings.  Hook the spring scale to the loop in the line and pull smoothly on 
the spring scale noting the ounce-force reading at steady pull.  When you put 
it back together, adjust the shim pack if necessary for the same approximate 
reading.

Or as another field expedient, after reassembly, loosen the four or eight 
bearing cap screws about half a turn.  Start the dynamotor and run it for a few 
minutes to reseat the brushes and to let everything settle in.  Note the 
no-load current.  Tighten all screws in 1/8 turn increments (45 degrees).  When 
fully tightened, the no-load current should be just slightly higher (say 1-3 %).  
If it is more (or if it goes more before getting all the screws fully 
tightened), secure the power and remove one of the thinnest shims (usually either 
0.002" or 0.003").  Repeat.  If the current doesn't increase just slightly from 
loose to tight, you need to add a shim.  

Some dynamotors have screw-in bearing caps (DM-21-B comes to mind).  On 
those, no shims are used.  Just be sure whichever type bearing cap is present that 
the brushes remain in the wear pattern area as you alternately tighten one 
then the other cap.

> 
> On some machines the brush holders are held in with set screws. If those 
> screws are
> loosened, you may be able to pull the holders out radially and get access 
> that way.
> You need to watch out for the connecting wires though.
> 
Generally, those set screws are glued in.  I've removed some successfully and 
been able to reuse the screws but not all.  Plus you really need a jig 
mounted in the bearing housing bore to reset the brush holder properly, both in 
depth and angle.  Lacking the jig, I really wouldn't remove the bearing holder 
except as a last resort.

Robert Downs - Houston
<http://www.wa5cab.com> (Web Store)
MVPA 9480
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