Mechanics and Construction > Mechanics and Construction

To balance or not to balance the weight on a Servo?

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enoon:
Hello everyone,
this is my first practical robotic project, and even though i have a decent theoretical background (Robotics course at university), I have very little knowledge on how to put things together :)


What i'm trying to accomplish:
A simple drawing bot, with 3 degrees of freedom (actually it's a 2R planar robot plus one extra servo to put the marker up and down),
something like this:


Materials:
The bigger servos are Parallax Normal Servos, the little one holding the marker i found in the Arduino starter kit. Arms made in plywood and attached to the servos with hot glue. Everything controlled by an Arduino Uno.

My question:
I'm afraid that having all that unbalanced weight on the servos (the first two) might strain the rotation axis leading to poor performance and/or breakage. Is this a real issue?
As a solution i thought of extending the arms the other way (as in the next image) in order to bring the center of balance on top of each servo. Is this a good idea? Are there better way to handle this?

Thanks :)

MrWizard:
Very very light weight can be handled like this....but......the construction will be loose and badly controlled. So it is not practical. You will not get secure results and will get an extreme failure rate.  Use bearings and box or trangle constructions for a more sturdy solution. Check out some other constructions of similar ideas.

jwatte:
 If the servos have ball bearings on the output spine, they probably won't break, however, you will have significant play, flex, and vibration issues, as well as not very good positioning.

Google "egg Bot" for some ideas. Typically you will want to use stepper motors and ideally some gearing and/or belts for xy movement.

enoon:

--- Quote from: MrWizard on February 23, 2013, 12:07:54 PM ---Very very light weight can be handled like this....but......the construction will be loose and badly controlled. So it is not practical. You will not get secure results and will get an extreme failure rate.  Use bearings and box or trangle constructions for a more sturdy solution. Check out some other constructions of similar ideas.

--- End quote ---
Thanks, could you link to some?

 
--- Quote from: jwatte on February 23, 2013, 10:10:49 PM --- If the servos have ball bearings on the output spine, they probably won't break, however, you will have significant play, flex, and vibration issues, as well as not very good positioning.

Google "egg Bot" for some ideas. Typically you will want to use stepper motors and ideally some gearing and/or belts for xy movement.

--- End quote ---
Thanks, any other suggestions on where to look?

MrWizard:
http://www.crustcrawler.com/products/AX-18F%20Smart%20Robotic%20Arm/images/Side_View1.jpg

Flat but still a box type construction.

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