servo with great torque other than mg995

hi everybody, i’m a fan of robotics, i succesfully build a quadruped, write his program (850+ lines of code) but i have troubles with the servo : mg995. after some reserch on the internet, i know the 995 is poor quality. i want to change and i want your feedback for durable high torque servos you use.

my robit specs:

total Weight = less than 2.5kg with all
every time 3 legs are on the ground
const float fm=80.0; //femur length in mm
const float tb=122.0; // tibia length in mm

so the correct calcul may be: 2.5kg/3= 0.9 kg
each leg should be strong to lift 0.9kg
for femur the minimum torque should be = 0.9kg x 8 cm = 7.2 kg/cm
for tibia the minimum torque should be = 0.9kg x 12.2 cm = 10.98 kg/cm

is it correct? if it is, i could you tell me your expérience about your durability servo with this torque
the speed of the servo and his own weight is’nt important for me, only the size (standart), the torque, and the electricity consumption

thank you

You've made the good case to go for minimum torque for worst case in your calcs but I ask will the mass will be evenly distributed across the 3 legs at all times ? I suggest it wont be, therefore 2.5/3 gives an underestimate of the mass an individual servo has to lift.

May as well keep the logic going in the same direction?

Torque is not measured in kg/cm, but Nm or kgf-cm if you must, it is the product of a force and a distance (or equivalently energy per radian). kgf = kilogram-force (on earth, 9.8N)

The max torque of a servo is not usually a continuous rating, you may need a large margin of safety to avoid frying your servos, something like 3 or 4 perhaps. So 4 Nm might be the sort of rating you need.

A 4Nm servo with normal sort of speed (1 rad/s) is probably going to need over an amp continuous, perhaps 3+ A peak. Look for metal gear high torque servos.

Do any of the vendors have lists sortable by torque? Hobbyking's is sortable by weight (well that's what they say, but they mean mass 8) ) which tends to put the torquey ones at the top seeing as they're physically bigger and heavier. A list by torque would be useful.

MarkT: normal sort of speed (1 rad/s)

Not that anyone measures servo speed in rad/s; the 60 degree time is the standard. Most servos are about 0.15 to 0.2 seconds for 60 degrees (close enough to a radian), so the speeds are typically about 1 rad every 1/5 second, ie in the 5 rad/s not 1 area.

Torque is not measured in kg/cm,

Interestingly I think the OP knows that, and that torque is force x its arm as you say, since his calc showed x although mysteriously the units became / in the answer.

For whatever reasons, the servo community seems to have standardised on quoting speed as simply "seconds" (meaning actually time for 60 degrees) and torque of simply "kg" (meaning kg.cm with kg in turn being a sort of mass based analog for force) Not right in the science or engineering world, but clearly a standard sort of shorthand in the argot of servos.

thanks for explanations , in facts, mg995 (13Kgf/cm) are enought powerfull for this robot. it work well, realy. but they are low quality and low durability. i search for durability servo.this is my first need

someone have feebback about mg996 or/and turnigy ?

Metal gears and metal case would be good - metal gears for durability, metal case for heat dissipation.

The dynamixel servos might be worth checking. With increasing quality the price will also be increasing.

http://www.robotshop.com/en/dynamixel-smart-servo-motors.html