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​HELP! Looking for mechanical engineering advice in building a multiplier/compounding gearbox for a hand crank siren project Answered

HELP! I have a project I'm working on that involves making a hand cranked mini air-raid siren utilizing a 12v generator and a 12v siren. I've done a proof of concept rig where I direct wired the siren to the generator and then used a drill to generate enough RPM on the shaft to get the noise I want. Problem is, I'd like to make this entirely manual and will be adding a crank handle once I get the housing built...which means, I need a way to seriously up my RPM.

I've done research into building multiplier gear boxes and compound gear trains and pinions and everything in between, but am going to confess that while I'm obsessed with DIY projects, this one is exceeding my engineering knowledge. I am at a loss for how best to approach this build. What ratio would you suggest? 40:1? 50:1? Does anyone have experience building gear boxes that would be ok emailing me with their advice? The input for my generator is a 20 tooth gear (please don't laugh if I'm totally messing up all the nomenclature...most of my life experience comes from writing and baking and a very short (failed) stint as an EE...this is a totally new world for me.) and I got this information from another sub: it’s a 3600 rpm motor so 60/sec. I think the fastest I would want to crank is 2/sec, so, 30:1 gear box sounds about right. With a nice long crank handle a 50:1 should also work nicely. 10:1 is too slow.

Ideally (and this might be where it gets really tricky) I'd like it to all fit into a space that's about 5"X3"X3, not including the siren (I can run that further out, using the leads).

I'm attaching a photo of the original proof of concept wire up that I did...and don't worry, I know there are exposed wires, I fully plan on isolating the wires, insulating the whole thing and installing the proper resistors.

https://imgur.com/a/MaAR2PQ

Here is a link to the generator: https://ebay.us/zgAoDQ

Here is a link to the siren: https://ebay.us/fEgahD

Thank you in advance!

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Downunder35m

5 weeks ago

Looks like it is a Mabuchi 555 type motor, you can get ready to use gearboxes for them quite cheap....
But then again, I would just crank the siren directly with a gearbox instead of cranking a motor to run another motor ;)

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Jack A LopezDownunder35m

Reply 5 weeks ago

I also was wondering at this notion of converting mechanical power to electrical and then back to mechanical again.

Perhaps there is fear of taking apart the siren, and successfully connecting to it mechanically.

Or perhaps the crank box and the siren are intended to be widely separated, with long distance wires delivering power from one to the other.

Actually this might be be a good idea for the person cranking, if the siren is so loud as to be uncomfortable on the ears of a person standing right next to it.

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Downunder35mJack A Lopez

Reply 5 weeks ago

I have a 3D printed and quite loud siren on Thingyverse, would be easy to add a planetary gearbox and crank...
As for distance: What about the loss in the connecting cable?
Either way I think there is better options to make it spin fast enough....

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Jack A LopezDownunder35m

Reply 4 weeks ago

Can you give us a link, into the Thingiverse, for this, your siren thing?

Regarding power losses, I don't even know much power a motorized siren like this uses, in its typical install-it-in-your-car implementation. I mean, as electrical power: 12 volts multiplied by... some number of amperes.

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Jack A Lopez

5 weeks ago

Did I mention gearboxes from cordless drills are easy to find? The reason why is because people throw away cordless drills as junk when the battery breaks, and a replacement battery is hard to find.

Did I mention gearboxes from cordless drills can be combined; i.e. one can drive another for additional mechanical advantage? That is if you can get the chuck from one drill, to grip onto the motor shaft of a second drill.

I remember seeing this done before, and it seemed so novel to me that I took a picture of it. That picture is around here somewhere... ah, here it is!

Looking more closely at this picture, I think the drill on the left is a modified cordless drill, and the drill on the right is, or used to be, a corded drill, i.e. the kind with a universal motor made to run from AC mains power.

drill-gearbox-driving-drill-gearbox.jpg
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Jack A Lopez

5 weeks ago

A typical cordless electric drill has a gearbox, and a brushed, permanent magnet, DC motor attached to that gearbox.

Also a typical cordless electric drill has a convenient place to put a crank. It can be inserted into the chuck, and the chuck can be tightened, securing it to the crank.

So that turning the crank... turns the chuck... which turns the gearbox... which turns the DC motor.


Some pictures of such a setup, old cordless drill turned into DC generator, can be seen in this 'ible:

https://www.instructables.com/id/Generator-Demonst...

The voltage output from the motor-as-generator, when turned by hand this way, is usually some fraction (e.g. 10% to 25%) of the nominal battery voltage, from when it used to be a cordless drill.

As an example, in that 'ible I just linked to, the drill formerly had a 9.6 volt battery, and in its new life as a generator toy, it is driving a 2.4 volt flashlight bulb, and the ratio of those two numbers is 1:4, or 25%

By the way, I believe the reason why the voltage was much lower as a generator, is simply because I could not crank the chuck, by hand, as fast as it used to turn when driven the usual way, by the drill's motor, powered by its battery.