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Monday, May 4, 2009

Motor Repairs

With a second motor (from the other side of a chair, so the gearbox is mirrored) I was able to fix the motor brushplate problem.

This is the damaged motor taken off the bike; the wiring harness I left attached to the bike and is just unscrewed from the terminals on the motor.

There's just two philips screws holding that black plastic endcap on (right side of pic), so it's easy to remove.

The nuts holding the endcap/brushplate holder/bearing assembly on are a little more difficult. They're slotted, so a flatblade would work, except that they're threaded down far enough on the bolt that the protruding bolt end prevents access to the slot.

Dremel tool and an old wide flatblade screwdriver to the rescue. Comparison with similar screwdriver unmodified (bottom tool in pic).

The damaged motor's endcap interior, showing one side of the black disc of soft plastic used as a brushplate. The screw terminal for the brush's wire is visible, with the brush itself just below that.

The square metal bit on the end of the shaft is part of the electromechanical brake I'd removed; it would engage a steel plate that rides on springs whenever no power was applied to the brake assembly, preventing the motor from turning. With power applied, the brake plate would be pulled away from the shaft area, allowing it to spin. A type of cotter pin is used to hold the square bit on the shaft, and required removal with a thin metal tool and gentle tapping from a brass hammer before I could take the endcap unit off.

Once that is removed, the brushplate itself is held on only with a pair of small nuts and screws (one each to the semi-circular slots you see in the black ring), used to adjust it's angle around the commutator segments.

It's difficult to see the problem with the plastic in this pic, but you *can* see the irregular wear on the brush facing you, where it has been worn at an angle on the bottom (forward) end, due to the brush not contacting the commutator straight-on anymore.

In this pic, you can just see the slight melting of the plastic standoff area just between the black ring and the metal brush holder.

You'll have to click on the image to see it's full-size version to really see the problem.

In this shot it's a bit more readily visible in the small pic, but again clicking on the pic for the full size version makes it much easier to see.

This pic makes it obvious how the whole brush holder is at an angle due to the melting plastic.

Since the brush holder can't stay straight, the brush no longer fully contacts the commutator segments, and arcs, not only causing extra heat but wasting power and damaging the brushes *and* the commutator.

Below is the endcap assembly (including the coils from the electromechanical brake) off the undamaged motor.

Note that the ring is a different color. This is because it is made of a different plastic, one much more like I was describing before that it needed to be made from. It is much harder, and does not appear to be affected by heat (to test it, I poked a non-critical area with a hot soldering iron, which is MUCH hotter than the motor will get, and it did not damage the plastic).

It also has thicker standoffs, although the brush holders are identical.

Instead of using soft brass rivets, it appears to be using some other alloy for the fasteners (which appear to be a different type of pin, rather than a rivet). They may be cast into the plate, or they may be threaded in. I can't tell without taking it apart, and I don't want to risk damaging the only working brushplate I have. :-)

A comparison pic.

The leftmost unit is the better one, with the brownish plastic that is much more like thick circuitboard material, while the rightmost unit is the damaged one with the black soft plastic that melts easily. I have no way to know (since the s/n label on the second motor was unreadable) whether the company that made them found out the black soft plastic was a problem and *improved* their process, or if they created the problem by replacing a working (but more expensive to make) solution with the cheaper soft plastic. I'm really curious as to which it is, but I doubt I will ever know.

This is the core of the undamaged motor, with nice shiny commutator segments. it also has other better-designed features than the one with the soft plastic plate, such as tied-down armature wiring, preventing it from coming unglued and flying out into the magnets or whatever, which would destroy the motor.

The damaged motor does not have any tie-down of the wiring at all. It also has a cheaper way of connecting the wires to the commutator segments, by simply pinching them into contact.

A comarison of the two commutator ends, with the better design on the left, and the damaged motor on the right.

Both do have an anti-cogging feature of spiraled core segments, and both have the same shaft and bearings.

One thing not visible without taking the shaft out of the gearbox end (which requires removing all the grease to see what holds it in place, as the four screws holding that end's plate on are not all that secures it) is the gear teeth on the shaft itself.

This is the open gearbox on the damaged motor, which shows the gear that makes contact with the shaft's teeth on the right center end of the gearbox.

In this motor, there is grease left in the teeth as it turns. In the other motor, the fit is much more precise between teh two, and only shiny metal is visible after one pass, even if I first force grease into all the teeth as they go down into the gearbox--it's all forced out of the teeth by the very good fit between gears, meaning less slop and less wasted power, and also less wear on the teeth when I pedal harder than the motor is outputting power, which forces the output shaft to turn in a way that essentially acts to the teeth meshing as if the motor had reversed directions. Any slop is then turned into wear as the teeth bash against each other at that point, even if lightly.

This is the bolt whose head broke off when I was tightening the motor down, tensioning it's chain.

Fortunately it was easy to remove, by gripping with pliers and turning it, and it had not damaged the threads of the hole.

Because I could not change the entire motor between gearboxes (without further digging into the gearbox to find and remove whatever shaft retaining rings/etc there are), I simply ended up moving the brush rings between the two motors, so the good one is now on the motor I was already using. Since the motor itself has damaged commutator segments (scratched up and burnt on one end of all of them), I kept the original brushes in there to match them.

I did polish the commutators and brushes a bit with my Dremel's jewelry buffing wheels and rouge, and it made some difference, but still needs much better polishing than what I could do with that and my limited skills and shaky hands.

It now works much better than it did before, though, so it was worth the work, and now should not fail again the same way.

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