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I know this question may not belong here but I assume this is most likely where the most well informed people can help me.

I am looking at buying a servo motor for a an application that must be able to lift 4-5 lb at a rotational speed of approximately 1rpm. The servo motor listed here http://www.robotshop.com/ca/en/hitec-hs755mg-servo.html states a stalling torque of 200 oz-in. Is this torque rating at the horn of the servo motor or the torque rating of the actual motor before any gear reduction is done?

Is this motor sufficiently strong for my application?

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In general, for a motor with integrated gearbox, specifications in the data sheet are "at the shaft" which is the shaft that comes out of the gearbox.

In the case of hobby RC servos, like the one you're linking to, it is always the case that the torque takes into account the gear reduction (because it's a bigger number, thus better marketing.)

When you say "lift 4-5 lbs" you do not specify the distance of the lever arm. For a winch that winds a cable, that arm is the distance from center-of-horn to outermost-layer-of-cable. If you attach an arm, it's distance from center of servo horn to center of weight, projected to the ground plane.

You also don't specify how far you need to lift the load.

And you should know that most motors (and certainly all cheaper, hobby-style servos) are specified in maximum "stall" torque, which is not a level of performance they can provide on a continuous cycle. If you stall the motor for more than a few seconds, you are likely to burn it out. The "holding torque" or "continuous torque" is typically 0.3 to 0.15 times the stall torque, depending on motor construction, cooling, etc. This important data isn't even generally available for hobby servos, but is often specified for industrial automation components.

Also note that that servo likely has, at most, a 180 degree rotation angle.

A better Stack Exchange to ask this question on might be the Robotics Stack Exchange. I don't know how to go about moving questions/answers, though.

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  • \$\begingroup\$ Thanks Jon, let me explain what I intend to do. I would like to attach a rack and pinion gear to this motor to essentially push/pull out a container (from an enclosure) that may experience a resistance of approximately 4-5lbs ~ 22N, so it is not a lift per say. With regards to your point about the arm I assume that further gear reduction will be done depending on the size of the pinion gear. The rack will move back and forth approximately 6 inches. Do you think this is feasible with such a servo motor, given that the motor would only need to perform this for a short period of time? \$\endgroup\$ – user1084113 Mar 6 '14 at 1:24
  • \$\begingroup\$ The servo you linked to is not a "continuous rotation servo" and thus can only move a rack a fraction of an inch with a small pinion. As I said in the description, it only moves 180 degrees. The cheapest easy-to-control servo with position sense covering 360 degrees and full-rotation capability is the Robotis MX-12W, which is not yet available in the US. Its bigger cousin, the MX_28T, is about $230. You're probably better off with a stepper motor or DC gearmotor and a limit switch and an apropriate controller. \$\endgroup\$ – Jon Watte Mar 6 '14 at 1:35

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