servo trouble

my and my robotics team are trying to make a lift to life racket ball into pvc pipes for a competeion this saturday and we took a servo and switch the gears to those of a motor and now our servo is running uncontrolably as soon as you turn the controler on the servo starts spinning in a counter clock wise motion and will not stop… if someone could tell me if we have a electrica problem or that i cant turn a servo into a motor it would be greatly appericated…

also does anyone know how to get more torque out of a motor module im trying to lift a boom arm we made it lifts the boom life until you put a racketball on it im already using counter weights in the back of the boom if anyone knows how i can get more torque please help


well for more torque, you can buy really strong hi-tec servos that can hold around 60lb of weight for a little over a hundred dollars. or you can gear down the servo ( have a 12 tooth gear on the servo and have the gear turn a 60 or 84 tooth gear). as for the servo thing, mine ussually does that when there is to much weight on it, or the batteries are dieing.

unfortuanlly for the compettion were in we can only use the parts in the starter kit but when i was testing it we didnt even have it hooked up to anything and our controller said our battery level was 10.00volts

we’ve got the servo and motor plugged directly into the boom to lift it now would we have more lifting power if we used gears?

sorry for all the ?'s this is only my 2nd comp

The output shaft of a servo connects to a poteniometer that sends position information back to the servo electronics. The “motor” inside the servo is spun to get the potentiometer position to “match” pwm value coming from the controller. When you change to motor gears, the poteniometer never moves and the electronics spins the motor in a futile attempt to position the potentiometer. This is 100% expected behavior - from an engineering analysis. The quick answer is - don’t put motor gears in servos if you want something useable.

That is what the gears and/or sproket and chain are for. You sacrifice speed for torque (or lifting power). The gears are simple: 12, 36, 60, and 84 teeth. All gears are designed to mesh with each other when placed using the same “row” of holes on a metal piece. 12 teeth on the motor shaft and 36 teeth on the other shaft will reduce speed 36/12 or by 3 (output spins at 1/3 the motor speed.) Torque is increased by the same ratio (roughly). If you put a 12 tooth on the motor and an 84 tooth on the arm, the 84 tooth gear will rotate 1/7th the motor speed and you will get 7 times the torque.

Note: The square shaft hole in the gears is only good to about 30 pound-inches. If your arm is 10 inches long and you are trying to lift 5 pounds, that will place 50 pound-inches of torque on the gear’s square hole and strip it out. This is where you use the other holes in the 84 tooth gear to “bolt” your arm to. The teeth of the 84 tooth gear will handle more than 100 pound-inches of torque. (With torques over 10 pound-inches be sure to support your shafts at both ends with bearing blocks to prevent bending and gear slippage. Too much shaft flexing and the gears will spread apart and slip.)

The motors are only good up to around 5 pound-inches of torque. To lift 5 pounds on a 10 inch arm, you will need at least a 10 to 1 reduction in speed in order for the motor to lift the arm. A 12 tooth on the motor driving a 36 tooth “idler” shaft gives you 3 to 1. If you then have a 12 tooth on the “idler” shaft driving a 60 tooth bolted to your arm, you get another 5 to 1. The product of those to reductions is 15 to 1. Thus, a motor good for 5 pound inches will produce 75 pound inches on the 60 tooth gear and lift 7.5 pounds at the end of a 10 inch arm. The arm will move 15 times slower than the motor.


Forgot to mention that the plastic locking plates with metal inserts are only good to about 30 pound-inches and are pretty much destroyed if they are overloaded. For higher torque loads, drive big gears with holes to mount your arm to.


thanks man u just saved my team from having to stay at school super late:)

One more thing I just remembered. The voltage reading on the RC transmitter is the battery voltage for the transmitter - not the robot.

Using rechargeable batteries (or the VEX power pack) for the bot, when they start getting low, the motor and servo performance seems to slow down. Eventually, the power/batt light on the micro-controller will turn red when the batteries get weak enough. With weak batteries, as you try to drive the bot, the motors will load the battery down and the micro-controller power/batt light will turn red. If no motors are going, the battery load is minimal and the power/batt may turn green again.


so thats why it was making an annoying noise today and had a red light on

ElectroJohn is probably right about the battery level.

also, you could use the chain kit to gear things down. it isnt as strong or effective though.