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Old Mar 07, 2013, 07:30 PM
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Servo power

Man ,I'm just full of questions today!

Let's see if we can settle this, or at least clarify it.

I have servos rated at 133ozs. with arms on them that are 3/4" long,center screw to outermost hole in the arm. They are not digital.
New servos.4.8 volt rated....2000ma rated battery,fully charged at 5.4 volts.
Any servo I choose, stalls with 27-29 ozs of weight attached to the arm.

I thought servos are rated at 1" from the center screw..Enlighten me,please.
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Old Mar 07, 2013, 07:47 PM
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This article has some good information in regards to servo's.
http://www.rchelicopterfun.com/rc-servos.html
And if you want a little deeper analysis here ya go.
http://www.rcgroups.com/forums/showthread.php?t=1194285

Hope they were helpfull.
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Old Mar 08, 2013, 03:03 AM
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Just to be picky, no servo is ever rated in ounces. They all specify a TORQUE rating which will always contain a force and a distance figure. E.g. kg-cm, oz-in, ton-mile (o.k. perhaps not that last one).

What is normally quoted is the stall torque not the dynamic or moving torque which is lower. But if what you have is a 133 oz-in servo with an arm less than an inch long and it can't move less than 30oz on the end of that arm then I'd say you've been swindled. It should stall at around 133oz on a 1" arm, 66.5oz on a 2" arm or approx 177oz on a 3/4" arm so any weight noticeably less than that should show movement at nominal voltage.

Steve
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Old Mar 08, 2013, 05:23 AM
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So I shouldn't think of a 133 oz.in.servo as having the ability to move 8lbs 5ozs off the floor under ideal conditions?

Here's what I'm trying to do.....I'm building a Ziroli DC-3, with two aileron servos attached to EACH aileron. I want to build stronger ailerons,using a single servo to motivate it.
I believed that 133oz in. converted to 8 lbs 5ozs of force to move the surface.,more ,with a shorter arm and higher voltage battery..
I don't want a "yes that will work"- I'm trying to understand a testing method.Should I be able to lift 133 ozs off the floor ?
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Old Mar 08, 2013, 06:41 AM
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I think somewhere in those calculations there is a linear equation. In other words the total oz.in is spread across the distance of the servo's movement. For example lets say the servo arm hole is 1" and the weight is divided by every quater inch of movement, the oz.in of a 133oz servo would be 33.25" . And also maybe the arch of the arms movement plays into the rating as well.

I just tried your test on a Futaba 3010 which is 90oz.in @ 6v. It would not even begin to lift the 6.25lbs. I then put a rod on the servo and grabbed hold by hand. And it seems weak to start with (easy to stall) and gets stronger as it rotates. Again this was just by feel. But it does make one wonder how exactly they measure and what they measure.
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Old Mar 08, 2013, 06:57 AM
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Quote:
Originally Posted by epoxyearl View Post
I don't want a "yes that will work"- I'm trying to understand a testing method.Should I be able to lift 133 ozs off the floor ?
Probably not. You might have to ask the specific manufacturer EXACTLY what they're specifying but in general a 133 oz-in servo will stall, i.e. not move, when asked to lift 133 oz with a 1" arm. Exactly how much less than that it will actually lift at any reasonable speed is rarely specified but is typically around 75-80% of the stall torque.

Steve
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Old Mar 08, 2013, 08:04 AM
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Ahh- better answers !

I don't understand "force-feet" etc, but I can tell if a servo does what I want or not..

I'm guessing that with guys using two 54 oz in. servos on the aileron,I'd be safe with one, 133,replacing them.

I'll contact Futaba's gurus and see how they measure the torque.

My question,lurking in my mind,is that wouldn't 133 oz in. of power literally tear a horn off a stalled surface ?.or rip hinges out ? Y'see my concern?

I can only guess that it's an optimistic number from the engineers, like horsepower.
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Old Mar 08, 2013, 08:28 AM
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You got me to wondering as well. So did a little research. It appears there are a couple things needs to be added to the test, one is ability to start and speed. The servo has to be under no load with the load added while under movement. And for the servo to pull the rated oz-in it has to be at the rated speed. So I hooked up a fish scale to my current build. With no load and the frame sitting on the foam and pad it will pull the rated load easily. I used a 3/4" arm(what was already there) and it was pulling over 8lbs. The pics are first at idle then full followed by half. So it would appear the rated oz-in is not a constant dead load but on a curve based on speed and load gain during that speed. I think it is like a monster truck sled, it can pull a large load given a chance to start and the load applied gradually.
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Old Mar 08, 2013, 09:06 AM
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Okay-thanks-my testing procedure was faulty...I positioned the arm straight down, so the servo moved a bit before the load was applied. The load actually exceeded the height-it got moving enough that inertia took it up enough to lessen the load at the top of the travel....Kewl..

Thanks,Ace

Okay-so some diagonal bracing in the aileron against torsional twist,and we're flying with single aileron servos.

-with knowledge ,comes understanding-
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Old Mar 08, 2013, 09:58 AM
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Quote:
Originally Posted by epoxyearl View Post
Okay-thanks-my testing procedure was faulty...I positioned the arm straight down, so the servo moved a bit before the load was applied. The load actually exceeded the height-it got moving enough that inertia took it up enough to lessen the load at the top of the travel....Kewl..

Thanks,Ace

Okay-so some diagonal bracing in the aileron against torsional twist,and we're flying with single aileron servos.

-with knowledge ,comes understanding-
Can I also say _ _ _ with understanding, comes knowledge ?

Zor
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Old Mar 08, 2013, 11:10 AM
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I also learned in that search that useing a size larger than required also uses less battery. The larger servo does not require the energy to do the job that the smaller servo would. They also stated that only 80% of the rated capacity was feasible, under load over 80% was entering the stall parameters of the electronics. I also seen evidence of this in my generic test, as it passed about 6-1/2lbs it had a couple glitches. In my opinion if we use appropriate servo's and our linkage is set up to use 90-100% of the servo's travel there is nothing we can do in the air that will cause an overload to the servo inducing a stall. There is another component to the equation not accounted for when we think "How much torque do we need" and that is simply from the moment we begin to operate the control surface the aircraft reacts accordingly. Which also reduces the load on the surface. Plus most of our flying surfaces have split control's, aileron's two servos, elevator two servos. etc. etc. So there is a lot to account for other than just the strength of the servo. I would think a more important issue would be the power supply, Is there enough?
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Old Mar 08, 2013, 11:30 AM
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Servo torque

You guys got me curious.
I have taken for granted up to now that the torque figures given by servo manufacturers was reasonably accurate.

I happen to have a JR821 that I decided to make tests with.

It is rated at 4.8 volts, 72 oz-in and at 6 volts 88 oz-in.

I wrapped a string around a 2 Kg bag of sugar which is just about _ _
2 Kg. times 2.2 lbs per Kg gives 4.4 lbs or times 16 equals 70.4 ounces of weight.
The servo arm has a 1/2 inch radius so the servo according to my interpretation should be able to lift 144 ounces (9 lbs).

The result of my test is that with the 70.4 ounces of weight the servo was operating normally with full travel in both directions with a neutral position from its rotation axis to the attahed weight being horizontal.

I am satisfied with my understanding of the servo specifications.

Zor
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Old Mar 08, 2013, 01:44 PM
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Practical experimentation....Visible proof !
With these demonstrations, we could do this test with any servo.

These ailerons are 34" long, x 3-1/2 " wide, tapered to 1" at the tip..One 133 oz in. servo should have no problem deflecting it, under normal flight speeds.

Nope- I don't want 4000 ma lipos,at over 11 volts regulated down to 5.2 volts for more power ! Good Lord ! that's like driving 50 miles to go ten!

If my 2000 ma ,4.8 battery tests at full capacity,each time I fly, I'm all set. Oh stop !

The right side of the DC-3 will have a complete airborne radio system, as will the left. Maybe I'll split the rudder and work each half off a dedicated servo....naaahhh

You guys have been a great help....thanks.
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Old Mar 08, 2013, 08:35 PM
Zor
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Quote:
Originally Posted by epoxyearl View Post

Practical experimentation....
>
>
>
The right side of the DC-3 will have a complete airborne radio system, as will the left. Maybe I'll split the rudder and work each half off a dedicated servo....naaahhh

You guys have been a great help....thanks.

epoxyearl,

If I understand you well, you will have two receivers on board both bound to the same transmitter.

? ? ?

Zor
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Old Mar 09, 2013, 05:51 AM
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Zor - yes...The safety resulting from two radio systems is offset (in my mind) by the cost of an extra receiver and battery.

Radio failures don't happen too often, but if I can prevent the loss of an airplane,or injury to someone,I should do the second system,as a safety measure.
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