Is there a way to calculate how much weight a servo can lift?

I'm working on a 6 legged, 3DOF, spider-like robot, and I was wondering if its possible to find out how much weight the servos I have can lift...

It says they are rated 3.9kg - cm
so for 1 cm pole, its 3.9kg, and how can I convert that to see how much weight it can hold? the servo's that move the legs I'm not worried about but the ones that lift the chassis, make me have second thoughts.. The chassis is less than 1 pound, so at the point of the servo is about an inch and a half away, will it be able to lift.. Also, 6 servos will be lifting, so does this add all the torque together for a total product?



frollard4 years ago
Torque in 'distance-force' scales linearly. foot-pounds or centimeter-newtons can convert directly to any given distance.
Note, kilograms is a mass, not a force - but we'll assume 'the force of 3.9 kilograms accelerated by gravity 9.81m/s^2 '...moot point, but important if you try to do math with the units.

It can lift a mass of
3.9kg at 1cm armature
.39kg at 10cm armature
.039kg at 100cm armature (the armature would probably weigh more than 39 grams if its 1 meter long...probably best to avoid :)

Having 6 lifting motors share the load means they each take 1/6th the force if the platform is level and the center of gravity is shared evenly. Assume if your bot is going to walk, it will either have 1, 2, or 3 legs off the ground at a time (stutter step or one leg at a time move) to maintain balance of at least 3 points.
So...
If you have say, 10cm arms on the servos, they can EACH lift about 390 grams (full stall, not good for them) - call it 300 to be safe, also assume worst case only 3 are holding up the bot at a time...so your bot can weigh approximately 900-1200 grams and still maneuver.

However far you make the lever divide out the force.
+1, nailed it, the part about assuming only 3 points of contact is especially important.
If the servos have 3.9Kgcm of torque, you will be able to lift a maximum of 1Kg, so it will be able to lift about 1Kg at 4cm. Given the chassis only weighs about half a Kg, you should be fine, providing your power supply can provide enough current to the servos.

If you have multiple servos lifting, the weight will be shared out between them, so you will be able to lift more.