How strong are servos?

[claesson92]

If i buy the servos in the 50$ robot tutorial, or some servos in that pricerange. How heavy can the robot be? But still moving in a reasonable speed? (With the batteries as in the tutorial)

[Steve Joblin]

Plan on the robot being able to carry its batteries, the electronics, the servos, the robot body, and maybe a few ounces... that is about it.  Let's put it this way... it can carry a barbie doll, but not a brick

[frank26080115]

you can calculate how much weight it can carry with it's torque rating

[Fredrik Andersson]

Much depends on the size of the wheels.

Large wheels -> Faster but weaker
Small wheels -> Slower but stronger

[edit]

But still moving in a reasonable speed?

Guess you figured this out already

[Admin]

Feel free to calculate it
http://www.societyofrobots.com/RMF_calculator.shtml

[nottoooily]

Feel free to calculate it
http://www.societyofrobots.com/RMF_calculator.shtml

That calculator seems to ignore rolling friction, which I think on carpet is pretty significant. In fact that's the only hard part that you'd want a calculator to work out for you. Looks like that's just using F=ma and the odd equation of motion which anyone can do by hand anyway and doesn't tell you much.

[Admin]

This is the equation (no, its not F=ma):
torque*rpm=mass*(acceleration+gravity*sin(angle*PI/180))*velocity*(1/efficiency)/(2*PI)/(#_wheels)

Unfortunately no equation can accurately approximate friction - meaning, it requires measured experiments. Of course, you can approximate it by normal_force*coef_friction, but in reality thats a very rough approximation. And seriously, whats the coefficient of friction between your foam wheels and a persian rug? Or a tile floor? There is no data for this . . . And friction is also dependent on both speed and acceleration.

The only way to know friction is to already have your robot built and tested, which defeats the point of the equation - to design before building.

Sooooo this is why I have efficiency in the equation, to estimate. Friction on the ground only accounts for a percentage of losses, while you also have gear train losses, aerodynamic losses, etc.

 

[nottoooily]

Oh, that's your site? Hope I didn't sound a bit harsh 

A load of data with interpolating functions would be amazing for most robot builders. People actually want to know how much load their robot can carry and how fast it will go. Simple physics doesn't even approximate that. That's what I thought it was when I saw the link and was dissapointed.