Slow spinning ventilation fan effect - how to build

[utilityman]

I need to build some set pieces contain several very slowly rotating fans, maybe 1/4 rpm or lower for a fan with a diameter of approx 2 or 3 feet. I think the best real world example is an idle duct ventilation fan, like when you see a spy or burglar crawling through an air duct on TV or movie. the fan would just be rotating slowly due to natural air movement, not under power.

We're looking to build several of these to put upstage behind a band, and shine some strong beams of light through through them to get a cool effect with some haze, etc

Budget is an issue, so I can't go buy a bunch of real ventilation fans. they don't need to push any air, so I'm thinking about fabricating three fan blades out of some sheet material, laun, signboard, etc, then attach to a shaft (large dowel?) through some low tech bearings (speedbor through plywood?). No idea how that would work, but even if that is done, what would you power it with? I've heard people use electric clock motors before but not sure if enough torque. or maybe erector set motors (or Meccano for Europeans), but those sound expensive. I checked a few surplus websites, but didn't find anything that looked ok.

Any other ideas? I have a few week before I need to build but probably need to order parts and do a pilot in two weeks.

thanks

-Brian

 

[n1ist]

Other sources for slow motors are rotisseries and mechanical Christmas deer.
/mike

 

[kicknargel]

McMaster has some relatively affordable gear motors. Be advised that some of them have plastic gearing, and if you stall them once, they strip and are done forever.

McMaster-Carr

 

[Robert]

Might I suggest using ceiling fans with a ceiling fan speed controller. I have seen some mounted sideways as an in-store display. Leave off the light kit and if wanted attach larger cut-out blades to the fan, but balance of the blades is critical, especially at higher speeds. Check with your local big box hardware store or a specific ceiling fan supplier.

 

[Van]

IF you choose to make bearings out of holes in plywood with dowels sticking through you should be aware that you will need sigificant amounts of Lubrication. Also I know it sounds rather easy to build the fan bodies and it is , to a point, You must gothrough and balnce all the blades to each other. Even a small variance between the mass of two blades will be enough to cause surges in the motion of the fan, especially if they are large diameter. Also, if you are limited to low power motors by budget then this issue will impact your motors ability to kepp the fans rotating and a constant speed and will, most likely, lead to premature failure of the motor. Like Mike said above, old Christmast displays and things like that are great resources for motors and a lot of times you can cross reference their part number on the web to find out what their load rating is.

 

[kicknargel]

A hole in a chunk of UHMW plastic from McMaster makes a much nicer bearing than plywood.

Sideways ceiling fans were discussed at length here:

http://www.controlbooth.com/forums/scenery-props-rigging/16068-ceiling-fan-windmill.html

 

[utilityman]

What would we do without McMaster-Carr?

thanks for the rotisserie and other suggestions. I'll see what I can figure out within the budget.

 

[millamber]

You could do the whole thing as a lighting effect. I'm thinking one of these gobos with a twinspin and some haze.

 

[FatherMurphy]

Mirror ball motors can be had pretty cheaply, and most run about 2 rpm. You could layer some thin plywood together to make simple v-belt type pulleys to get a speed reduction, or find sprocket chain gears to fit the shafts.

I once built some rotator effects to go in front of ellipsoidals by simply cutting wavy lines on luan discs, drilled the center for a MB motor shaft, stuck a nail through the s-hook hole, and goobered hot glue all over the connection. Mounted the motor to a bracket that slipped into the gel clips, and it was off and running.