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Question Ball screw linear actuator dimensioning and platform stopping

Discussion in 'DIY Motion Simulator Building Q&A / FAQ' started by Thomas Luzat, Mar 26, 2019.

  1. Thomas Luzat

    Thomas Luzat New Member

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    I am thinking of building ball screw linear actuators for a 6DoF platform, quite similar to FlyPT's design and others, but I do have some open questions:
    1. What happens when the power goes down? Does the platform crash (move down)? Fast or slow? Does it move at all or do the (6) actuators offer enough resistance to stop, say, 150 kg? The motors and screws certainly do offer some resistance, but I have no idea how much.
    2. Are 16mm ball screws well dimensioned for a platform of 150-200kg (fully loaded)? I do suspect that they may flex quite easily at 300-500mm extension under acceleration, given that I do consider somewhat more load and stronger motors than some other builds. Is my fear justified and should I choose a somewhat larger diameter (such as a 2005 or 2010 screw) or is a commonly used ball screw such as 1605/1610 easily sufficient for a stable setup?
    3. For a direct drive solution it might make sense to use another screw pitch, such as with a 1610, to get the desired speeds. I have mostly seen 1605's used, though; would 1610 have disadvantages? I am considering motors in the 400W-500W range at 3000rpm which, at that speed, would theoretically give up to 500mm/s, but a somewhat low limit of 250mm/s with a 1605/2005.
    Answers? Thoughts? I'd be happy to hear. :)
  2. BM114

    BM114 New Member

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    1. The actuators should slow any downward movement. Driving a ballscrew backwards is pretty difficult, but if it is a big concern you can look for motors which include brakes. Brakes come into effect only when the power to them dies. It is as a common safety feature in industrial servos.
    There are a few issues of guessing at if it will stop by itself or not depending on the type of ballscrew(precision rolled, ground, tolerencing) and ballnut(lash, preload, bearing material).

    2. I've always heard 1605 and 1610 are the ballscres to go with. I just did a simple static force analysis in inventor on a 550mm 1605 ballscrew model. With a 200kg load(using mild steel as the material, no hardening) it flexed about .028mm, but be aware the higher your impact forces (the force caused by the speed at which you change direction namely) the farther away from static model behavior you will go. So it will depend on more than just length alone, though it is unlikely you'll run it hard enough to matter unless you're trying to injure yourself. Using 6 of these I don't expect you'd run into any issues.

    3. "Direct Drive" in all meanings I understand means a 1:1 gear ratio where the motor is directly connected to the output. A ballscrew is actually a gearing mechanism so I wouldn't call it that. 500mm/s is very fast. I believe @Thanos has a video showcasing an actuator like that. I would recommended sticking with a 5mm pitch with a 400-500w motor range and the 3000rpm motor speed.
  3. Thanos

    Thanos Building the Future one AC Servo at a time... or 6

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    My Motion Simulator:
    AC motor, Joyrider, Motion platform, 4DOF, 6DOF
    • Agree Agree x 1