[HN Gopher] Planetary Roller Screws
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       Planetary Roller Screws
        
       Author : everlier
       Score  : 19 points
       Date   : 2026-02-03 08:59 UTC (3 days ago)
        
 (HTM) web link (www.humanityslastmachine.com)
 (TXT) w3m dump (www.humanityslastmachine.com)
        
       | Animats wrote:
       | Nice. Here's a video by a maker of planetary roller screws
       | showing how they work.[1] The pitch shown is high enough that
       | those don't look back-driveable. So if those drive a leg joint,
       | they have to be able to absorb impacts directly. They can't pass
       | them back to the motor, which can absorb them in a magnetic
       | field. ("You cannot strip the teeth of a magnetic field." - GE
       | electric locomotive salesman, circa 1900)
       | 
       | There's a basic conflict. Small electric motors want to turn
       | fast, so they're usually followed by gear reduction. But that
       | loses feedback precision and back-driveability. A pure direct
       | drive motor works great, but they're large diameter devices. Some
       | SCARA robots use them, but the motor is a foot across. Washing
       | machines have gone direct drive, since there's enough space for a
       | large diameter motor. There's a direct drive electric motorcycle
       | with a hollow rear wheel. There are "pancake" motors, with large
       | diameter but little thickness. None of those devices have a good
       | form factor for humanoid robots.
       | 
       | That leads to tradeoffs such as quasi-direct drive, where there's
       | some gear reduction, but not too much. The article suggests that
       | 20:1 is an upper limit for back driveability. That's pushing it
       | for a leadscrew-type device, but maybe it's possible now.
       | 
       | It's neat seeing all this progress in robotic components.
       | Historically, robotics has been a small niche, and had to use
       | components developed for other purposes. This made for clunky
       | robots. Now we're seeing more purpose-designed components made in
       | volume. Drones made 3-phase synchronous motors and their
       | controllers small, light, and cheap. Now the same thing is
       | happening for other needed components.
       | 
       | Looks like, when the AI guys get their act together on
       | manipulation, the machinery will be ready.
       | 
       | [1] https://www.youtube.com/watch?v=3pMN3BqGk_o
        
       | vintagedave wrote:
       | > Planetary roller screws are the gold standard for high-
       | performance joints such as knees, ankles, and hips.
       | 
       | It's hard to understand how these are used for joints. I think of
       | a screw as something that rotates many times. Are these used for
       | things that rotate only a few degrees, as a knee might?
        
         | convolvatron wrote:
         | these translate rotary motion into linear motion. if you hold
         | the screw fixed (with bearings), and let the nut float, then
         | turning the screw moves the nut back and forth along the screw
        
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       (page generated 2026-02-06 23:00 UTC)