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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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  • $\begingroup$ If the rating is given for a product containing internal gear reduction, the ratings should be for the definite output, after any gear reductions. However, you can't always trust what manufacturers say... $\endgroup$
    – Kurt E. Clothier
    Apr 5, 2014 at 18:39

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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

For reputable manufactures and vendors, the ratings will include all internal gear reduction, or external in the case of gear-head motors.

Is this motor sufficiently strong for my application?

That depends more on the details of your application. 200 oz-in of torque means the servo should be able to lift up to 200 oz within one inch of its center shaft. That means it can work on up to [ 200 oz / 16 oz/lb ] = 12.5 pounds within that inch.

To get 4 - 5 pounds out of it, your rotational radius must be within:

12.5 lb-in / 4.5 lb = 2.78 inches.

However, operating the motor continuously under a full load will most likely cause it to fail fairly quickly. It will also slow down the rotational speed.

Speaking of speed, the servo is given at 0.23 seconds / 60 degrees. That means it will rotate 60 degrees in 0.23 seconds. a Full circle is 360 degrees, so 4 * 0.23 = 0.92 seconds for a full rotation.

However, (and correct me if I am wrong) no where in the listing does it mention continuous rotation. A traditional RC servo motor cannot rotate all the way around unless it specifically states it can or is manually altered (voiding any warranties). It will typically only rotate a maximum of 140 to 180 degrees.

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the torque rating is AFTER the gear reduction.

you can calculate the maximum load by the following equation

Torque = Force * Radius

if you try to lift 1.8-2.2kg load with this servo, you need to attach the load no farther than 6 cm away from the center of the horn.

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  • $\begingroup$ Be careful, you are mixing standard and metric here... $\endgroup$
    – Kurt E. Clothier
    Apr 5, 2014 at 18:40

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