Sunday, December 25, 2011

How to Re-wire a Potted Motor, Part 14: Replacing the Motor Housing Grommet

I almost forgot! Before we take the big step of soldering the new motor leads on, there’s one more thing we ought to check, as this is easier to do while we’ve got the motor leads clipped and out of the way. Take a look at the rubber grommet covering the hole that the motor leads will eventually go through.

Here I can see this one has a crack in it:

And when I pull at it even gently with my fingers, it breaks apart into this:

This is relatively simple to replace and costs about 5 cents. Your local hardware store ought to have a selection of grommets in a bunch of little drawers. This took me some trial-and-error to figure out, but to save you some trouble, here is the exact type and dimensions you’re looking for: What you need is a “Push-in Grommet, 3/8" Inside Diameter, 5/8" Outside Diameter, 3/32" thick, for a 7/16" diameter hole.”

(In the event your hardware store doesn’t carry them, you can order them from McMaster-Carr at this link; the part number is 9600K33, but the problem is they only sell them in bags of 100 for about six bucks. That's a lot of potted motors.)

To replace the grommet, let’s look at the hole where it’s going to go. If any bits of the old grommet are stuck to any part of the hole, you can clean it off with a moistened Q-tip.

Don’t forget to check the inside of the hole. Here, on a Worst-Case Machine, the oil and grease mistakenly injected into the motor has melted the grommet to the inside of the hole, creating a mess. A new grommet will not fit because all of that melted gunk is clogging up the inside of the hole. This requires more aggressive cleaning, which is much easier for us to do before we install the new wiring, which might get in the way.

I clean this crud out by using Q-tips moistened with rubbing alcohol and a dental pick. (Caution: Be sure to blot the Q-tips well--if any alcohol runs out of the hole and onto the exterior finish of the motor, it will eat through the paint.) What I do is use the Q-tip to apply some alcohol to the melted rubber, let it soak in for a bit to break it up, then I start scraping out the old rubber with the dental pick. The alcohol really helps it come off in nice, meaty chunks. After about 10-20 minutes, it looks like this:

Now it’s ready for the new grommet. To insert it, I use a cheapie plastic stick-thingy I bought in the clay section of an art supply store for less than a buck.

But you can use anything thin and cylindrical that has a blunt tip, like if you have a set of those long plastic chopsticks; just make sure they’re not the kind with the pointy ends or you’ll poke through the rubber. (You could probably even use a spool pin, though I’d wrap it in masking tape first to avoid accidentally scratching any paint.)

The grommet has two rims, one that will wind up inside the motor hole, and the other of which stays outside the hole.

 Think of the grommet’s inner rim as a shirt, and the motor hole as a pair of pants. You need to tuck the shirt into the pants. Use the chopstick or what have you to go around the grommet, tucking that inner rim into the hole.

Once you’ve got most of it tucked in, smooth the inside of the grommet out by pressing with your tool, and then it all just kind of “pops” into place.


Go on to Part 15: Re-wiring the New Motor Leads.

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