Desperately with physics project (building a motorized drawbridge)

  • #1


First, a little background. My daughter (16 years old) is in honors physics. Each month, they are given a project that they must complete that demonstrates whatever they are currently studying. Parents are encouraged to help the children as some of the projects can be quite complex.

Anyway, this month they were given the task of building a motorized miniature golf obstacle. The obstacle must either intermittently block a putt and/or transport the ball. Level of build difficulty, creativity and functionality are what they are graded on.

My daughter has decided to build a replica of the Tower Bridge of London which is a dual drawbridge. We think we have a good idea of how to lift the drawbridges using levers, pulleys, etc, but we can't figure out how to get the required linear power or even where to get a motor that can do it.

Some details of what we think we need:

A high-torque AC electric motor (120v AC). We need a motor and/or gearbox that will produce approximately only 10rpm at final output or a way to reduce the output and still have enough torque to lift 5-10 pounds.

A way to convert the rotation of the motor into a repeating linear motion of 3-8 inches where the bridge is lifted, lowered and then remains in the down position for 3-5 seconds to allow for travel of the golf ball before lifting again. This must be all automatic and require no human control other than turning the device on.

I have looked everywhere I can think of for the motor, but can only find either small, high-rpm hobby motors or large AC motors that still produce too many rpm's.

Any help is appreciated. We are stumped and don't know where to go from here.


A befuddled dad and his daughter.
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  • #2
You may find something useful if you try a google search for "stepper motor". They won't be AC, but adapters are easy enough to find. The weight (5-10 lbs) may be a problem, though. You may want to find a way to reduce that.
  • #3
You're probably going to want some kind of timer and/or string activated switch.

I'm not sure what your planned scale is, but, in general, you'll want to use some sort of gear arrangement for your winch.

For a ready-made solution, you can buy a set of Lego Mindstorms which will have a pre-fab control unit and a pair of motors but are quite pricy. I expect that control units from other programmable toys would also be useable.

My local Fry's has cheap worm gear box sets, and something like Radio Shack is likely to have timer and switch devices if you're inclined to go with a roll-your-own approach.

It's unclear from your description how much power you need. If you had specified force and linear speed, or torque and rpm instead of force and rpm, I would have had more confidence in telling you that a 120V AC system is probably overkill.

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