As the title states, is there a difference? I plan on using to rotate an arm that ways at most 20 pounds probably less so can I just buy the SD?
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The Andymark website has a handy chart that compares the differences. From a quick glance, the HD versions appear to have roughly 20%-30% higher torque ratings than their SD counterparts.
One of these gearboxes could be used to rotate an arm, but it depends on how you’re rotating it. For example, if you’re trying to rotate the arm directly from the axis of rotation (which I assume is what you’re probably trying to do), then a 20lb arm will likely apply far more than 20lbs of force on the gearbox because the further any weight is from the axis of rotation the more force it applies (you basically have a lever, and your gearbox is at a mechanical disadvantage.
You might want to use this calculator to play with the numbers a bit. The “L” value for a direct-drive is effectively 0 (though you have to put a number greater than zero in this calculator so pick something low), the “X” value is the distance between the center of mass of your arm and your drive shaft, and the “W” value is total weight.
I tested out a 20lb load on a 4in center of mass with a 0.25 Fulcrum and got 320lbs, so you’ll definitely want to think about how you’re actuating your arm.
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