# Selecting a Servo Motor

Hi,
I'm a newbie in robotics. In my current project, I needed to rotate (pitch and yaw) an object which weighs approx. 5kg (11.023lb), about
it's center of mass (approx). How can I select an RC servo motor which are strong enough to do the job? I know the manufacturers specified the servo motors' torque in oz-in or kg-cm. How does this relate?
TIA
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If you are moving an object which is balanced at its exact CG point, you don't need much force at all. A little hobby servo would work. They control those giant lock doors on the Panama canal with 35hp motors. Balance and lack of friction is the key. However, if you need to move the object from further away, you would need much more force. Think about torque this way. If you had a one inch lever attached to a bolt head and hung a 7 oz weight from the end of the lever, you would produce 7 oz-in of torque at the bolt head. If you had a 1 foot lever and hung 200 lbs from the end, you would have 200 ft lbs of torque at the bolt head. It's the reverse with servos. If you had a one inch long servo arm mounted on a 7 oz-in servo, you could lift 7 oz at the end of the servo arm. Hope this helps. -- Shawn

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Thanks for the help. I got 2 more questions about servo motors. One is the speed specs, like "0.20sec @ 60 degree". Does this means that the servo max. turn (degree of travel) is 60 degree?
The other, what is the meaning of "retract servo", "airelon servo", etc in the servo descriptions?
TIA
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It means the servo can turn 60 degrees in 200ms. The servo travel limit should be in the specs. There's usaully an internal pot that sets the limit. I have no idea what your limit is without seeing the specs. It's easy enough to test if you have a compass guage. The servo types you mentioned are special purpose servos. They are designed for model airplane (R/C) applications. Aileron servos are usually mounted out in the wing with a direct connection to the aileron. The retract servo is for use on retractable landing gear. I've been out of R/C for about 10 years so I have never seen those up close. They didn't have those types back then. I fly the real thing now.
-- Shawn

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Uh, forgot to ask. What if a 2 in. long servo arm is mounted on a 7 oz-in servo. Does the weight allowance at the end of the servo doubled (14oz), or halved (3.5lb)?
TIA
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Xeon wrote:

----------- Think!: If you made a 5' or 10' lever arm, you could get rich one of those ways, so that one must be wrong!
Half! -Steve
--
-Steve Walz snipped-for-privacy@armory.com ftp://ftp.armory.com/pub/user/rstevew
Electronics Site!! 1000's of Files and Dirs!! With Schematics Galore!!
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It will lift half the weight. You can increase the weight by using things like gears or pulleys. Or use a more powerful servo.
-- Shawn

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Xeon wrote:

Though it's always possible to counter-weight an arm so that the lifting force of even a heavy weight can be minimal, some well-placed horsepower never hurts! Personally, I'd stay away from standard-size servos for this task. Go with quarter-scale or "mega" servos, which are sold by Hitec, GWS, and others. Torques start at about 200 oz-in at 6 vdc.
For any servo where the motor shaft must support weight it's always a good idea to select a model with metal gears. This adds maybe \$25-35 to the cost of the servo, but the motor will last longer. This type of servo comonly uses ball bearings on the output gear.
The rest is in your design, and how well it keeps lateral forces off the output gear. You might think of a small 3" or 4" lazy susan ball bearing, for example.
-- Gordon Author: Constructing Robot Bases, Robot Builder's Sourcebook, Robot Builder's Bonanza
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