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Hi Torque servo has very little torque

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Old 02-22-2008, 05:08 PM
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Default Hi Torque servo has very little torque

Hi gents,

I have an Airtronics 94358 Hi torq servo I am using to lift the bucket on my scatch build Dump Truck. Servo is rated at 160 oz/in at 4.8V, 200 oz/in at 6V.

The problem is it wont lift 32 oz.

My guess is the cheapo receiver, a GWS micro GWR-4p.

Also, is the the torque rating on a servo the HOLD torque or the Force torque?

My linkage setup is the servo laying on its side, arm swings away from hinge pin pushing straight up to the bottom center of the bucket. Pics can be provided if you think it would help.


Any assistance would be greatly appreciated.

TIA

Greg


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Old 02-22-2008, 05:45 PM
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Default RE: Hi Torque servo has very little torque

Greg, I doubt your receiver has anything to do with it, what are you using for a power supply?. I suspect when you do the math, you will find your problem. Those torque rating can be bit optimistic some times, and are based on a stalled torque situation, I believe. Just remember that 200in/oz of torque is only the equiv of about 1 foot pound of torque. Its all levers and vectors which is just basic physics.
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Old 02-22-2008, 07:23 PM
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Default RE: Hi Torque servo has very little torque

PICTURES.....
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Old 02-22-2008, 09:54 PM
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Default RE: Hi Torque servo has very little torque

P40, thx for the reply!

I suspected the rcr due to the fact that torque is a byproduct of current and voltage. Small recr MAY = low current availability. (yes, I know current is a product of load. ONLY if that device suppling the current has that LOAD capability. ie a 2 amp power supply will provide UP TO 2 amps at rated voltage)

The math I used to start out with this servo is quite basic - 1 inch servo arm X servo torque = end effect thrust. 1 cubic foot of sand = ~100 lbs.(googled that) Box dimensions- 3X6X10 = 180cu/in, 1cubic foot = 1728 cubic inches. The bed is 1/10 of a cubic foot, so the weight of the sand in the box is ~10 lbs. 200 in/oz = 12.5 in/lbs.hence my choice in this servo.

Hope the pics help.

Greg



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Old 02-23-2008, 10:47 AM
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Default RE: Hi Torque servo has very little torque

bump
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Old 02-23-2008, 01:32 PM
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Default RE: Hi Torque servo has very little torque

the physics of what your doing would require at least 3 to 4 times the amount of torque, then what you have by the mechanical disadvantage built in to your system. you need to take a course on basic physics or find a physics teacher who could explain it to you. You might want to talk to some robotics guys and look into the servo,s they use instead.
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Old 02-23-2008, 08:21 PM
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Default RE: Hi Torque servo has very little torque

Bypassed BEC in my ESC via a $1.29 Voltage regulator from radio shack.

Lifted 9 lbs.


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Old 02-24-2008, 12:21 AM
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Default RE: Hi Torque servo has very little torque

I am a hydraulics engineer and perhaps I can shed some light on this for you as it is the same problem that I deal with on a daily basis.
As a rule of thumb if you are trying to lift a load 12 inches with a 1 inch stroke of the actuator you are going to require 12 times the torque. Think of a teeter-totter with the fulcrum (or pivot) moved close to one end.
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Old 02-24-2008, 07:26 AM
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Default RE: Hi Torque servo has very little torque

Check out this Power gearbox for servos at Servo-City. It might work in your application.

http://www.servocity.com/html/servo_...gearboxes.html
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Old 02-24-2008, 07:38 AM
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Default RE: Hi Torque servo has very little torque

dont know what space you have but a good strong servo is that of mulitplex (rhino digi 4) slightly bigger than the standard servos: http://www.multiplexusa.com/RC-comp/Servos/rhino.htm about £60-70
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Old 02-24-2008, 01:44 PM
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Default RE: Hi Torque servo has very little torque


ORIGINAL: billdor

I am a hydraulics engineer and perhaps I can shed some light on this for you as it is the same problem that I deal with on a daily basis.
As a rule of thumb if you are trying to lift a load 12 inches with a 1 inch stroke of the actuator you are going to require 12 times the torque. Think of a teeter-totter with the fulcrum (or pivot) moved close to one end.
Thx for the info, but boy I hope yer wrong!! LOL

Some actual numbers.

Front end of bed moves a total of 5 1/2 inches from down to up.

Link connection at horn is 30 mm (1.2 in).

Total travel of link at bed connection is 1 1/2 inches from down to up. (" Stroke of the actuator")

Using your calculation, 5.5/1.5=3.67 times the the torque. ewwwww!

If I set the clevis connector point at the tip of the bed, I would need a 5 1/2 in stroke. or at 180 degree rotion on servo a 2 3/4 in servo arm. which would also decrease torque.

For the record, I was able to lift a 1 gallon container of water after installing a seperate home made BEC. 1 gal is 8.34 lbs. (the bed is about 1/2 lb) Although it strained to do it, its a big improvement. My next step, I think is a bigger better servo.

http://www3.towerhobbies.com/cgi-bin...0&I=LXRRC5&P=K

Dont know how I can get 7.4V at servo as this spec indicates "Torque: 333.29 / 416.61 oz.in (6v/7.4v)" Currently using 5v at 2 amp BEC with a TP 3s 2100 mA LiPo.

rcairflr,

Thx for the link, I did see that power gearbox and I am a bit short on real estate, but thx for the info.

rctruckz,
I am a bit confused at the specs on that servo, "Torque 6.0V: 205 oz/in 186 Power 6.0V: 383 oz/in " anyone know the difference?

Thx all for the replies and let me know yer thoughts on this new information. [sm=drowning.gif]


Greg



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Old 02-24-2008, 02:02 PM
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Default RE: Hi Torque servo has very little torque

http://www.servocity.com/html/12v_linear_actuators.html i dont think servos are going to cut it for something as specialized as this.
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Old 02-24-2008, 02:45 PM
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Default RE: Hi Torque servo has very little torque


ORIGINAL: carlosponti

http://www.servocity.com/html/12v_linear_actuators.html i dont think servos are going to cut it for something as specialized as this.

The smallest one is 7.4 inches long, looking at his truck in the first post, I don't think the linear actuators will work, too big....
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Old 02-24-2008, 02:47 PM
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Default RE: Hi Torque servo has very little torque

Those are tempting, again, real estate my biggest issue there. Their shortest, 7.4 inches long bearly fits, but stroke would be too short at 2 inches. they have a 20 lb thrust "coming soon". Hmm, wonder how big they will be in a 4 or 6 in. stroke?

Took a second look at their Power servos. Need to consider them a bit more where space is concerned. If not build my own...

Greg
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Old 02-25-2008, 07:07 AM
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Default RE: Hi Torque servo has very little torque

Torque = (distance from center of rotation) x (perpendicular component of the force to the radius) This means that if you have a 160 in oz servo it will apply a force of 160 oz at eactly one inch from the center of the servo arm exactly perpendicular to the that arm. From the pictures your load is not perpendicular to the servo arm which means actual force would be the weight of the bed multiplied by either 1/sine or 1/cosine (which is greater than the actual bed weight) plus it appears greater than one inch from the center of the servo arm all of which is why it does not work. Remember that 160 in oz servo will only provide a force of 80 oz if the load is applied two inches from the center of the servo.

You can figure out what is going on by using a first semester physics book (pick one that uses algebra instead of calculus) and using the principal of (1) sum of the torque = 0 and (2) sum of the forces = 0. You must understand torque to resolve this so spend some time reading and re-reading on that subject. Most folks don't fully completely understand torque and usually lump it into the mysterious area of mechanical advantage - which it is, just not mysterious.

If all else fails you can send me a schematic/picutre and the numbers and I can let you know how much torque you will need.

Hope that helps.
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Old 02-25-2008, 06:57 PM
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Default RE: Hi Torque servo has very little torque


ORIGINAL: UTRecon


ORIGINAL: billdor

I am a hydraulics engineer and perhaps I can shed some light on this for you as it is the same problem that I deal with on a daily basis.
As a rule of thumb if you are trying to lift a load 12 inches with a 1 inch stroke of the actuator you are going to require 12 times the torque. Think of a teeter-totter with the fulcrum (or pivot) moved close to one end.
Thx for the info, but boy I hope yer wrong!! LOL

Some actual numbers.

Front end of bed moves a total of 5 1/2 inches from down to up.

Link connection at horn is 30 mm (1.2 in).

Total travel of link at bed connection is 1 1/2 inches from down to up. (" Stroke of the actuator")

Using your calculation, 5.5/1.5=3.67 times the the torque. ewwwww!

If I set the clevis connector point at the tip of the bed, I would need a 5 1/2 in stroke. or at 180 degree rotion on servo a 2 3/4 in servo arm. which would also decrease torque.

For the record, I was able to lift a 1 gallon container of water after installing a seperate home made BEC. 1 gal is 8.34 lbs. (the bed is about 1/2 lb) Although it strained to do it, its a big improvement. My next step, I think is a bigger better servo.



Greg




Howard,

Thx for the reply. Please note the dimensions above. I will try to add a few more i think you are referring to.

Bed is 10" long, connecting clevis is dead center at 5"

Weight to be lifted ~10lbs.

Servo arm is straight down (6 o'clock) when bed is down. At 8 o'clock (from drivers side) Connecting arm from bed to servo arm is straight up, directly inline.

Let me knw yer thoughts.

Thx

Greg
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Old 02-26-2008, 12:48 PM
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Default RE: Hi Torque servo has very little torque

You also need to give us the distance between the pushrod and the center of the servo, like...how far out on the servo arm the pushrod is.

From all the numbers, we could figure it out...but it'd be twice as much work to get an accurate result than most of us would be willing to spend. Also, as Howard said, the circular effect of the servo is even more work. You could go two routes, exact numbers or fairly close numbers. Exact is WAY harder, and you need calculus to get a good number. Anyone with a semester of engineering physics could do it. I'm sure I could if I stared at a pen and paper for long enough.

Someone please correct me, it's been a while since my last physics class and I'm doing all of this off of the top of my head. A simple formula is T=F*d. The torque is equal to the product of the force and the distance from the center. The servo has 160oz/in of torque...which means 10lb/in. A little trigonometry with your numbers gives me a guess of about 1.375". I got that by taking the total travel of the front of the bed(5.5"), using triangles to figure the vertical travel at the same angle 5" from the front of the bed. That's 2.75", divided by 2=1.375" from the center of the servo. That would mean it can lift 7.273lbs ((10lb/in)/(1.375in)) linearly (the inches cancel out, leaving only pounds...which is why I showed it like that). So for all intents and purposes, using simplified numbers and math, we have an actuator with 7.273lbs of force and a 2.75" travel length. Again, as howard said, to be accurate we need to figure out the angular forces and stuff which will make it a little weaker than that. A good guesstimate for the true force is just above 7lbs, so you need to go lower for a minimum set on the range. For that, I'll do it rounded down to 7lbs, and again at 7.273lbs. Now, this is the part I really need checked:

T(7&7.273lbs)=F*distance(5")
T/D=F
(7&7.273)/5=F
F=1.4lbs & F=1.4546lbs
I think that the servo should be able to lift 1.45 lbs with your setup.
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Old 02-26-2008, 05:38 PM
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Default RE: Hi Torque servo has very little torque

Keep in mind another fact: The manufacturers inflate their specs by at least 20% and the servo will heat up in such an application and lose another 20%. To be sure get a ser vo rated at least double the torque you calculate you need.
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Old 02-26-2008, 05:47 PM
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Default RE: Hi Torque servo has very little torque


ORIGINAL: victorzamora

You also need to give us the distance between the pushrod and the center of the servo, like...how far out on the servo arm the pushrod is.

I think that the servo should be able to lift 1.45 lbs with your setup.
"Link connection at horn is 30 mm (1.2 in)."


"For the record, I was able to lift a 1 gallon container of water after installing a seperate home made BEC. 1 gal is 8.34 lbs. "

Bit the big dollar bullet and placed an order last night...
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXRRC5
and

http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXSUM9

So, although my fortay is electronics, I believe I have exhausted the simple Voltage reg's short of breaking out printed circuit board and making something messy, I went with an ESC that could provide 6 Volts to get to the higher torque range of any servo (along with 5Amp capability to drive it) then topped it off with a double-the-torque-mack-daddy robot servo. The airtronics will work nicely for steering, so no waste there.

The ESC also offers a "bit" more options than my $30 test esc where motor options are concerned. (This 3 speed tranny is gonna ROCK!!)

Thanks Victor for your input. Those calculations reminded me why I went to school for electronics and not ME. LOL

Greg
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Old 02-26-2008, 05:49 PM
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Default RE: Hi Torque servo has very little torque


ORIGINAL: dirtybird

Keep in mind another fact: The manufacturers inflate their specs by at least 20% and the servo will heat up in such an application and lose another 20%. To be sure get a ser vo rated at least double the torque you calculate you need.

Agreed!!! and DONE
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Old 03-02-2008, 01:12 PM
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Default RE: Hi Torque servo has very little torque

Just an update...

Installed Hitec servo and Novak crawler ESC, wow. Lifts 10+ lbs no problem (dont want to find its limit to avoid damage.) The ESC really makes it sweet at crawl speeds as well.

The radio I am using is a JR XF622. I want to go digital or maybe Spektrum. I see the DX6i is coming out soon, what are your thoughts on the new radio and what are the major differences of the DX7 and DX6?



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