scubyfan

Hey guys,

When specifications rate a servo's max power as "8 inch-ounces," for example, what does this mean in practical terms?

I hope I have not upset y'all with my recent wave of questions.

FHHuber

You can think of it 2 ways. for your 8 in-oz servo (the smallest I've seen is 17 in-oz... A "standard" servo is about 42 to 52 in-oz)

If you have a balanced servo arm (such that it balances like a prop at the servo output shaft) you can take a 8 oz weight and hang it from the one-inch arm (with the servo mounted to lift vertically when it rotates the arm) and the servo can move the weight up and down at the rated speed.

Or...

with the same balanced arm, you can hang a 1 oz weight from an 8 inch arm. and do the same thing.

Its the same force measurement used in torquing bolts in a car... just with a lower value than you'd see in a typical car's specs. but there are torque wrenches that will actually measure the tiny value of the servo's torque.

The stall-torque of the servo (the force it will apply and barely hold the weight from dropping in the above test method) is higher than the torque at rated rotational speed. But the gears can be stripped (teeth chipped off of one or more gears) if you exceed the rated torque.

scubyfan

Thanks, FHHuber! I thought it had something to do with control surface loading, etc. [&:]

FHHuber

In a way it does have to do with control surface loads... The expected load the servo will have to deal with.

In flight the air being deflected by the control surface can exert a large force on the control system. The servo provides the power to keep the surface deflected the correct amount... and that is where the torque value is needed. If you don't have the torque to move the control surface to the desired position, you can't make the airplane do what you want.

So we use the little appx 15 in-oz (HS-55) servos in park flyers. Appx 30 in-oz servos (HS-81) on a typical .15 to .25 powered plane would be more than adequate. for a .40 to .60 size plane you need the appx 45 in-oz "standard" servo (Futaba S-3004/3004). Then the 90 in-oz and larger servos are used in the larger airplanes.

Generally we put in servos that will handle double the expected load in normal flight, so that there is a large margin of power if something comes up requiring extreme deflection at high speed.

scubyfan

Ah, I see. I've gotta thank you for all the help you've provided me with in the last couple of days; I really appreciate it.

tailskid

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That's what this group is for !!!

Jerry

scubyfan

Indeed!