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I am trying to build a robotic arm and for the base I need a servo with a lot of torque. After browsing through hobbyking's servo selections I noticed that winch servos offer a higher torque for generally a lower price, however, I do not have any experience using them.

Before I commit to spending a lot of money on potentially incorrect servos for my project does someone know whether there would be a problem using winch servo for the base that would only need to rotate around 90 degrees?

I have read that winch servos are not as precise, but as long as the servo is within 1-3 degrees of what it should be that is fine with me.

Also can you still tell the servo to turn to 0 degrees or 35 or 87 degrees like a normal servo or is it like a continuous rotation servo where it keeps turning until it hits the end of its rotation and writing 100 vs 180 simply controls the speed?

Thanks for your help.

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  • \$\begingroup\$ Sail Winch Servomotors are not the same as continuous rotation ones. Instead, they come in various rotation ratings, i.e. 3.5 turn, or 5 turn servo. That means giving them a pulse duration for say 70% left or right would cause a given number of revolutions of the servo. \$\endgroup\$ – Anindo Ghosh Apr 18 '13 at 22:11
  • \$\begingroup\$ @AnindoGhosh Can you set the exact position of the servo or does writing(100) simply tell it the speed to rotate at that direction? \$\endgroup\$ – ben Apr 19 '13 at 0:55
  • \$\begingroup\$ It's positional, just like conventional servos. Not speed controlled. \$\endgroup\$ – Anindo Ghosh Apr 19 '13 at 3:02
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A Sail Winch Servo is not ideal for driving something that needs to rotate merely 90 degrees: The winch servo is designed to provide a fixed number of rotations from maximum to minimum "angle" setting. More on that later.

For identifying a suitable servo motor:

  • Define torque specifications, to help narrow down the search
  • Search for servos specifying all metal gears: For instance, this one boasts over 15 KG/cm torque.
  • Give preference to Brushless DC servo motors: They are generally more efficient, generate less EMI, and often are less acoustically noisy as well.
  • Servo motors specified with metal bearings, or better yet dual metal bearings, can handle more stress outside of the plane of rotation, and they generally will last longer under load. This may or may not be a concern depending on the robotic arm design.
  • If the torque requirements are not met by hobby servos, look at industrial servo motors, both DC and AC, which can be found for a range of power / torque ratings.
  • An alternative is to use a stepper motor, if necessary with a gear reduction, if the simplicity of servo control can be dispensed with.

An alternative is mechanical torque multiplication using either a stepper or a conventional servo motor, such as by internal planetary gears:

Internal planetary drive gears (Source and additional information)


A sail-winch servo takes the same servo control pulse duration modulation signals as conventional hobby servos, but is designed such that a "full-scale" traversal is several complete rotations.

For instance, comparing a +/- 90 degree (i.e. 180 degree) servo with a 3.5 revolution sail winch servo, a signal of ~ 1250 millisecond pulses, i.e. 50% anticlockwise rotation, would achieve around -45 degrees on a regular servo, and around 315 degrees rotation on the sail winch, just short of full circle.

If the control signal were a 2000 millisecond pulse, typically "clockwise maximum", the regular servo would rotate to +90, while the sail winch would rotate through 1.75 revolutions, nearly two full circles.

With this proportionate increase in angular rotation comes the corresponding decrease in precision: If the hypothetical regular servo above could be precisely controlled in 1 degree increments, the sail winch servo's precision would be merely 7 degree increments, by proportion.


The "writing 100 vs 180 simply controls the speed" situation applies to continuous-rotation servos, not sail winch servos. A CR Servo essentially has had its rotation sensor (usually a potentiometer) disconnected, so the controller inside starts working as a pure direction and speed control.

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