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Why no direct drive?


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Posted · Why no direct drive?

Hi,

@Ultimaker:

I have read a few posts about the direct drive mod and I wonder why Ultimaker didn't consider it for the UM2. Did you ditch it for design reasons or are there other, more "serious" implications? I remember that the short belts were a bit of a pain on the Ultimaker Original. Any way to get rid of them would be a big plus.

 

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    Posted · Why no direct drive?

    Having the motors stick out the sides of the UM makes for more things to bump into and your motors mean you have to position the UM farther away from any walls and it's uglier. And the short belts work fine for me.

    If you transport your UM often you will be glad the motors are out of the way.

    However I've been thinking of moving mine to the outside (but still using the belts). It requires no extra hardware, no holes drilled, just swap the X,Y direction bits in software (or in hardware jumpers).

     

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    Posted · Why no direct drive?

    Hi Nico,

    Thank you for your post.

    Obviously, the mod also caught our attention and I can see some pro's and cons to it.

    We choose not to do it because once assembled, it is very reliable but it is a rather precise and difficult assembly and would make the assembly and design more complicated. Getting it accurate for 1 machine is one thing, but for 1000+ machines is a different story. The small belts might need some adjusting in a future stage, but it is much easier, faster to build and easy to adjust if necessary.

    @GR5, why would you move your belts to the outside?

     

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    Posted · Why no direct drive?

    @GR5, why would you move your belts to the outside?

    I'm guessing he wants to close the printer up to create a heat chamber and wants the motors to keep cool(ish).

    Yes. That's it. Chamber at 50C. Can steppers handle this? Maybe. But only the steppers go outside - the belts stay where they are. I have not done this but others have and it's quick and easy.

     

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    Posted · Why no direct drive?

    The first part of the stepper to take damage would probably be the magnet. Depending on the type used, the maximum safe temperature can be as low as 80°C. Above that, the magnet loses (at least part of) it's magnetisation.

    It would make a lot of sense to take a magnet which can take more than 80°C for a stepper motor, but I don't know any details about that. Probably also depends on the quality of the motor.

    For 50, 60 or even 70°C motor temperature, there shouldn't be any issues with the motor. Bearings and oil should be able to take that, too as friction induced heat is common.

    Note however that the motor itself gets hot. If it is put into a hot climate (heated chamber) it will get even hotter. So if you have 50°C inside the heated chamber, your motor will be hotter than that.

    There should be a datasheet for the motor stating the maximum recommended operating temperature. Knowing that would simplify everything...

    I will definetly swap my motors to the outside of the printer, and I'd recommend that to anyone who wants to make a heated chamber. Just to be safe and sound.

     

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    Posted · Why no direct drive?

    I think they get pretty hot in their own right - from reprapwiki on Nema 17s

    Most of the motors specs give the current for two coils that will give an 80 °C rise, i.e. they can run at 100 °C! When using them on plastic brackets you need to under-run them to keep the brackets from melting. With PLA's glass transition temperature between 60-65 °C, you have to seriously under-run them! Fortunately temperature rise is proportional to power, which is in turn proportional to the square of current (P=I2R), but torque is directly proportional so you can keep temperature under control without losing too much torque. For example, running a stepper at 70% of the rated current would result 70% of the torque and 49% (0.72=0.49) of the power dissipation and thermal rise.

     

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