# Perpendicular Gravity Torque on Horizontal Rotating Arm Servo

Hi @cbenson,

Thanks so much for all you’re doing here - great work!

This is following on from the tutorial question I asked yesterday, and yes it seems like we didn’t quite communicate clearly. As you said in Building robotic arm with servos, attaching joints to horn?, it’s not a good idea to hang a robotic arm segment directly off the horn (haha I had a bad feeling about that idea but it actually took a few days of research to figure out what we’re supposed to do instead - thanks! :). Here are the specs I’m looking to build for context:

• Both the upper and lower arm segments will be fully horizontal in the worst case, and mostly horizontal 100% of the time
• Lower arm segment will be ~20cm long and < 600g total with center of mass ~3/4 towards the end effector

I’d add a drawing after posting this text.

So as per Building robotic arm with servos, attaching joints to horn?, we should use a servo that has a bearing at the bottom of the case in line with the output shaft, and connect a C bracket across them, similar to as shown in the picture of this servo: https://www.robotshop.com/en/lynxmotion-smart-servo-lss-motor-standard-st1.html

Hopefully that’s correct so far.

BUT… my question is - if the two arm segments are in the horizontal x-y plane, and the servo rotation sweeps the lower arm segment only within that plane, then the gravitational force on the lower arm segment will produce a torque on the servo that’s perpendicular to the direction of rotation, and which I could imagine if the lower arm were heavy, would tend to wear the servo joint prematurely.

Is there any rule of thumb to determine if a servo should be robust enough to withstand this perpendicular torque without significantly impacting its service life? I’m hoping to get at least 100k movements from these joints (haha and if that’s unrealistic for RC servos, or will require titanium also please let me know as I’m a total servo noob)

Thanks so much!

Matt

Hopefully I’ve drawn that correctly; it’s meant to show the lower arm will rotate into and out of the page.

@Robo_Matt

Is there any rule of thumb to determine if a servo should be robust enough to withstand this perpendicular torque without significantly impacting its service life?

Nothing in particular comes to mind. The support should ideally have bearing support both top and bottom. Looking forward to confirming with your sketch.

Wait, you don’t see the sketch? It should be posted… it’s there when I look at the thread.

@Robo_Matt Odd, now I can see the image. Are you talking about a SCARA robotic arm of sorts? Have not created one, but the technology has been around for quite a while and an online search shows a LOT of equations used. For example “Design and Implementation of a SCARA robot arm”:
https://sagar.se/files/scara-robot-report.pdf
Perhaps that will lead you in the right direction.