Thursday, March 13, 2008


Coupling gets too hot

It's a RepRap day. I like RepRap days - courtesy of Catalyst IT I get one a week. If anybody wants to sponsor me for another day a week I'd love to hear from you!

So following on from the saga of the disintegrating Y Motor coupling, I printed out another even denser PLA Y Motor Coupling with yet more plastic in the stressed areas. I can't really continue replication with a straight face without resolving this one way or the other.

At 47C it was showing signs of de-laminating at the 8mm shaft end and the 6.5mm motor shaft started slipping. I tensioned it up once more, but as the temperature hit 60C the PLA softened and was no longer sound enough to hold the motor shaft under tension from the grub. The grub moved, slackened, and the coupling failed to grip the motor shaft once more, this time allowing the shaft to rotate freely. Ugly.

The group photo shows one pristine ABS part in fetching green, one less dense PLA part with a split marked in red, and one showing how the nut & grub go a-wandering when the plastic warms up.

Solutions as I see it:

1. Make the coupling from a higher MP plastic.

2. Cast the coupling from epoxy in a reprap'd mould.

3. Find motors that run cooler.

4. Cool the coupling with a fan or similar.

5. Make the coupling from non-fabricated materials.

6. Try powering down the stepper between steps to lessen the heat

7. A combination of the above.

8. Something else.

Note that with the X motor we're not having melting problems. The CAPA gear sits directly on the shaft at 51C and is not melting. It's just the Y axis with the problem, probably due to the high load that motor experiences.

So, having a small 12V cooling fan handy made the selection much easier. Number 4 please, Bob! A few cable ties later and some impromptu ducting made from - yes, duct tape, we have the following monstrosity. If my head had been functioning properly this ducting would be under the cable ties, not stuck on top of it but hey.

So I've thickened up the coupling design some more, made it taper out, away from the motor end where space is tight, and started printing it while monitoring the motor temperature. So far the Y motor has reached a maximum temperature of 42C compared with the uncooled X casing at 69C, which may be good enough.

As you can see from the graph on the left, the temperature seems to stabilise around 41C under ambient conditions of approx 24C. The X axis snarled, so I stopped and left the trace going for people who like cooling curves. Thanks to Suz for the birthday present of a RS232-enabled QM1538 meter by the way. Very thoughtful of her :)

Vik :v)

What about water cooling?

Otherwise go with a ceramic or epoxy part.
Water, high currents, microelectronics, what could possibly go wrong?

Vik :v)
Here's what I ended up using: Z coupling. It's just a threaded-rod coupling, drilled out and fitted with a couple grub screws. I've got one for the Y motor as well.
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