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servo arm length vs torque rating

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Old 12-04-2011, 11:01 AM
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Airscrewer
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Default servo arm length vs torque rating



Hi guys, I was just thinking about my setup andwondering how much servotorque I'm loosing by using longer arms.

Say if servo Xis rated at200oz of torque, how much would the torque rating be reducedif you use an arm that is 1/4" longer than stock?

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Old 12-04-2011, 11:13 AM
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Default RE: servo arm length vs torque rating

From what I remember, they rate servo torque directly off the drive shaft. Every step out along the servo arm reduces the amount considerably.
And even worse increases servo slop, that tiny gear flex/slop at center that exagerated the further out along the arm you go.
I've always set up my pattern models with the push rod connected as close to the servo center as possible for both reasons.
I often look at the IMAC type set ups and think... Hummm may be Im wrong?
I'll still continue with my method... stuck in my ways.
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Old 12-04-2011, 11:35 AM
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Default RE: servo arm length vs torque rating

The torque measured at the servo is the same whether you use a 1 inch arm or a 4 inch arm.

The torque measured at the hinged surface depends upon the ratio of the servo arm as compared to the control horn.

So if you have a one inch servo arm and a one inch control horn (as measured from the control horn to the hinge line), then you will have the same torque on the surface as the servo.
So if you have a two inch servo arm and a two inch control horn (as measured from the control horn to the hinge line), then you will have the same torque on the surface as the servo.

But if you have a two inch arm on the servo and a one inch arm on the surface, you will only have half the torque.

Summary:

set up the servo travel so you are using all the travel you can. On the transmitter, set up the travel for 125% - 150% travel (aka End point adjust).
use a relatively short servo arm
use a relatively long control horn.

You will have maximized the torque at the surface. However, if you don't have enoug movement of the surface, then you'll have to sacriface some torque for movement.
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Old 12-04-2011, 11:59 AM
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Default RE: servo arm length vs torque rating

Ok i get it now, this makes sense. Thanks for the tutorial!

ORIGINAL: DMcQuinn

The torque measured at the servo is the same whether you use a 1 inch arm or a 4 inch arm.

The torque measured at the hinged surface depends upon the ratio of the servo arm as compared to the control horn.

So if you have a one inch servo arm and a one inch control horn (as measured from the control horn to the hinge line), then you will have the same torque on the surface as the servo.
So if you have a two inch servo arm and a two inch control horn (as measured from the control horn to the hinge line), then you will have the same torque on the surface as the servo.

But if you have a two inch arm on the servo and a one inch arm on the surface, you will only have half the torque.

Summary:

set up the servo travel so you are using all the travel you can. On the transmitter, set up the travel for 125% - 150% travel (aka End point adjust).
use a relatively short servo arm
use a relatively long control horn.

You will have maximized the torque at the surface. However, if you don't have enoug movement of the surface, then you'll have to sacriface some torque for movement.
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Old 12-04-2011, 12:00 PM
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Default RE: servo arm length vs torque rating

You are mixing up the definition of torque and force; the torgue doesn't change with the length of the arm. However the force exerted by the arm does. The torque divided by the length of the arm determines the force exerted at the end of the arm assuming no friction in any of the pivot points.
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Old 12-04-2011, 02:28 PM
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Default RE: servo arm length vs torque rating

Rodney is right, you will loose no torque whatever length arm you use, but Torque is a product of a Force times a Distance. (T=FxD).
So for your 200 oz.in servo, that is 200 oz of force at a distance of 1 inch from the axis, will be a force of 100 oz at 2", 50 oz at 4" etc. Or 400 oz at 1/2 inch. etc. This is the force delivered then to the controlled surface horn. As to how you arrange the arm/horn length to get the surface movement you want that is entirely up to the operator, but using 100% of the servo movement is always preferable, modern computer radios have not removed the need to set up the mechanical part of the system with the usual care.
Evan, WB #12.
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Old 12-04-2011, 07:42 PM
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Default RE: servo arm length vs torque rating

Hey guys I am looking to build an attachment to go on the rear side of the front shock tower of my losi truggy to flip it back over when i land on the lid. I am considering a moded electric retract but am concernd about the voltage limiter preventing it from exerting enough force at the end of a seven inch rod. the truggy weighs about seven pounds clean but it only needs to be stong enough to push the rear of the truggy over the top. If i used a seven inch servo arm and a high torque servo. how strong would it need to be to exert 64 oz at the end of the arm? Thanksin advance for any good tips or ideas.
P.S. Although weight is an issue it will only be mounted when practicing and will be removed for racing.
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Old 12-04-2011, 11:17 PM
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Default RE: servo arm length vs torque rating


ORIGINAL: BlueStreakOne

Hey guys I am looking to build an attachment to go on the rear side of the front shock tower of my losi truggy to flip it back over when i land on the lid. I am considering a moded electric retract but am concernd about the voltage limiter preventing it from exerting enough force at the end of a seven inch rod. the truggy weighs about seven pounds clean but it only needs to be stong enough to push the rear of the truggy over the top. If i used a seven inch servo arm and a high torque servo. how strong would it need to be to exert 64 oz at the end of the arm? Thanksin advance for any good tips or ideas.
P.S. Although weight is an issue it will only be mounted when practicing and will be removed for racing.
Interesting idea. Will this "attachment" drive a lever arm of sorts?thats what i'm visualizing.If yes then that seems like acontraption (and possibly too fragile)and may notbesturdy enough to withstand the abuse ofa truggy, not that it cant be done that way.

What if you used something like a solenoid that would fire quickly and flip the thing back on its wheels? maybe usea capacitor to fire it so that it doesnt cause a brown out? just thinking out loud here. good luck!
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Old 12-05-2011, 05:14 AM
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Default RE: servo arm length vs torque rating


ORIGINAL: DMcQuinn

The torque measured at the servo is the same whether you use a 1 inch arm or a 4 inch arm.

The torque measured at the hinged surface depends upon the ratio of the servo arm as compared to the control horn.

So if you have a one inch servo arm and a one inch control horn (as measured from the control horn to the hinge line), then you will have the same torque on the surface as the servo.
So if you have a two inch servo arm and a two inch control horn (as measured from the control horn to the hinge line), then you will have the same torque on the surface as the servo.

But if you have a two inch arm on the servo and a one inch arm on the surface, you will only have half the torque.

Summary:

set up the servo travel so you are using all the travel you can. On the transmitter, set up the travel for 125% - 150% travel (aka End point adjust).
use a relatively short servo arm
use a relatively long control horn.

You will have maximized the torque at the surface. However, if you don't have enoug movement of the surface, then you'll have to sacriface some torque for movement.
Great post!
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