My daughter is trying to build a contraption to cause a rocking chair to rock using an electric motor. Imagine like a rocking chair on the porch at Halloween that is slowly rocking as if occupied, but it is empty. She is going for the creepy effect.
What I have envisioned is a motor with a wheel that has a “piston arm” attached to the rocking chair and as the piston rises and falls as the motor spins, it causes the rocking chair to rock.
I am thinking of a fan motor and of course that has a speed control switch, but I imagine I need it to run even slower than the slowest speed on the switch. I am guessing I can change out that switch for something that offers even more resistance, but could I add a resistor to that path that would slow it down (and keep the original switch, or could I wire in another similar switch that when both are set at the lowest, it slows it down dramatically?
A variable speed drill would have everything about ready to go. Including a chuck to easily attach your actuator mechanics. A bent rod grasped in the chuck, could have a length of fishing line attached to the horse. Easy to make. A hose clamp adjusted to pull the trigger to the right speed. Just use the plug to turn off / on.
If the motor is a shaded pole, permanent split capacitor (PSC) type fan motor, there are variable speed fan controls that will give you the full range of speeds from zero to 100%.
If the amp draw of the motor is 5 amps or less, you can get a KBWC-15K fan control for about $15.
You might have problems trying to run a small motor that slowly. It will take a lot of current to get everything moving, but then very little to keep it moving. It will tend to want to run too fast or stall.
I think you would have better luck just gearing it down. Maybe press the motor output shaft against a rubber wheel and attach the piston arm to that. This will allow the motor to run at a more “comfortable” speed, and the wheel will act as a flywheel, smoothing out the whole system.
If you can find a slow motor a cam is a simple way to make the chair rock. It’s just an eccentric wheel on the motor. If you can find an old rotisserie motor for an outdoor grill it has a gearing mechanism to make it run slow, but it’s usually pretty noisy too.
I’d be inclined not to use a cam or level under the rockers: That’s the sort of thing that folks would be looking for, and finding it would make the effect less creepy. Instead, I’d use something mounted to the bottom of the seat, which would shift a weight around.
I’ve used a drill for something very similar and it worked great. We had a zip tie on the trigger and I was worried it wouldn’t be slow enough, but it wasn’t a problem.
It can be battery operated, which is nice, and lasts a surprisingly long amount of time.
The windshield wiper motor does seem perfect, but the place that I found to order them would not have it here in enough time to do the dry run. And locally the motors are more expensive and do not have the hook up for household electricity.
I think the quickest and easiest that I have access to already is the power drill, I think that will do exactly what I need and would not require me to tear anything up to do it.
If the demo goes well, then I think I might consider the wiper motor (if sound of the drill or other things cause any issues).
I like the idea of using a drill to shift a weight around under the seat somehow (credit to the suggestions above). It could be enclosed in a low profile box and there would be no immediately obvious mechanism causing the rocking. Creepy!
Note that the back and forth motion of windshield wipers is effected by the linkage, not the motor. The lever arm that attaches to the motor’s output shaft is not supplied with the motor. It could be quite difficult to fabricate one, and it’s probably only available from the dealership or a salvage yard.
A switch to activate a solenoid would work pretty well. Solenoids activate quickly, it would have to be positioned to just give the rocker a nudge at the end of the stroke.
A weight on an arm that is rotated about the vertical axis mounted under the chair seat would be my design. The side to side moment won’t do anything, but you get the back to front moment. No linkages, minimum moving parts.
A battery powered motor is possible with say simple worm reduction drive. That should be close to silent if you get reasonable quality parts. Plastic gears may be best.
Tripolar’s solenoid idea is very good. A little tuning, maybe use an optical sensor to trigger a simple a capacitor to dump current into the solenoid and charge capacitor from battery. Potentially very very simple. Would need to do some work looking at the resonant frequency and losses.