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I need to spin a threaded rod to raise a picture on a frame. The weight of the picture + the parts that it hangs on will be around 6 pounds. Two motors are going to be used to lift it via threaded rods (like in 3d printer platforms) but I need them to lift about 8" in less than 4 seconds.

I am unfamiliar with the differences between a DC and stepper motor and my worry is that when the picture is coming back down, a DC motor could overrun or would it not have enough power to strip the rod or nut (even over time)?

With a larger stepper, say like a 128oz/in and an M8x1.25 rod (.31" with 20 threads/in) is it reasonable for me to use steppers or should I go with DC and rely on timing to stop the lowering motion?

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  • $\begingroup$ Why not use a limit switch to determine the endpoints? $\endgroup$ – wwarriner Nov 19 '15 at 19:31
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    $\begingroup$ Also at rapid speeds threaded rod might seize and is more prone to degradation and wear due to the sharp threading. Have you considered acme, lead (pronounced like leader), or ball screws? They are more expensive but are designed for turning rotational into linear motion with various levels of precision and low wear rates. $\endgroup$ – wwarriner Nov 19 '15 at 19:37
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    $\begingroup$ @starrise - I came across those screw types but only in name, now that you have clued me in I will use those instead. Thanks $\endgroup$ – Magic Lasso Nov 19 '15 at 19:42
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    $\begingroup$ Power required at 100% efficiency = m.g.h/t. < 1.5 Watt. My brain tells me that 10 Watts is a trivial amount of power for an M8 thread in steel. E&OE - check that yourself but I think it's right. $\endgroup$ – Russell McMahon Nov 22 '15 at 13:27
  • $\begingroup$ I was going to shoot for something in the 56 Watt range using a 24" long x 3/8" dia acme screw lifting just under 7 pounds. I found some linear actuators produced with similar mechanism that move at 9"/sec. Much faster than I need and theirs are standard operating at 9amps and 12 volt. I am just kinda guessing that 56 watts will be a workable power for what Im trying to accomplish. $\endgroup$ – Magic Lasso Nov 23 '15 at 21:09
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Direct answers to question:

  • Whether a motor could overrun depends on motor power, momentum of the load, and resistance of the system. This could be a problem even with switches, but can be overcome with careful design.

  • Steppers are overkill if you need to move between two points without stopping between them. See below.

What you need is a linear actuation system, translating rotational motion from a motor into linear motion.

Threaded rod can be used in principle, but it is not designed for linear actuation. It is designed for static structural applications. Threaded rod has sharp threads which can suffer damage and can wear quickly. The products of wear, and damaged (especially folded-over) threads can cause the actuator to seize during operation. Instead I recommend using something like an acme screw with appropriate nuts. The threads are broader and blunted, and the steels used are typically much harder than those of threaded rod with a smoother surface, all contributing to lower wear rates and less risk of damage, reducing the risk of seizing. Threaded rod also has finer pitch, resulting in less distance per rotation, and is less mechanically efficient than acme, lead, or ball screws.

The choice between a DC brushless motor and a stepper motor should be based on repeatability. A stepper is designed for ensuring that X number of electric power pulses translates into exactly Y distance, every time, within some tolerance, without any feedback mechanism. A brushless motor can not achieve the same repeatability without a tight feedback mechanism. If you don't need repeatability then a brushless motor is cheaper and easier to build working electronic systems for as it doesn't require pulse train generation, and operates with continuous DC power. I am assuming that you intend to raise/lower something between two positions and not stop anywhere in between.

Depending on momentum, braking is also a concern to ensure you don't overrun the limit switches or damage anything when stopping. You have a 6 pound mass, but you don't give a speed. If the speed is significant, then braking could be an issue. Otherwise, the limit switches could swap the DC motor from the power source to a bank of resistors tied back into the motor: a braking loop. The short-circuit from the motor to itself provides braking, and the resistors dump the energy as heat outside the motor.

If I have your scenario correct, then a brushless motor directly connected to an acme screw (or similar) with limit switches at each end, with a braking loop as required, is a simple design.

Proper sizing and quantitative design of each component of the overall system could each be its own question, if you need more information.

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