Propeller beanie design problem: attaching the propeller to the motor

In a recent column I described how my friend David Ewing, CTO at Synapse Wireless, is in the process of designing and building 250 wireless mesh networked propeller beanie hats. Known collectively as CapNet, these little scamps are going to make their debut at the forthcoming DESIGN West 2013 Conference and Exhibition (see Atmel MCU to power world's first wireless mesh propeller beanie network and Hands-on speed training at DESIGN West).

[Click here to register for DESIGN West 2013, April 22-25 at the San Jose McEnery Convention Center. Options range from an All-Access Pass -- which includes Black Hat (security) Conference Session to Free Expo Admission].

If you had asked me at the beginning of this process, I would have said that getting the wireless technology to work would have been the tricky part, but talking to David has made me realize that there are lots of things that have to be considered with a project of this nature.

Consider, for example, the following, which is in David's words, and which describes a seemingly trivial issue that grew and grew:

For CapNet, mounting the beanie-cap's signature propeller to our little DC motor was an interesting challenge. The motor shaft is 1mm, while the propeller has a 2mm hole. At first, I optimistically started searching the web for a "shaft adapter" with these specifications. It seemed simple enough, but after a few hours of searching, I realized that the task was going to be more difficult than I'd thought!

I looked at all the "radio control hobby" sites, but it turns out that props of the appropriate size for a beanie-cap are not typically used with motors this small. If I'd used a motor big enough to fly the cap around the room, there'd be no problem! But there seemed to be no off-the-shelf solution for adapting a motor/prop combo sized appropriately for the beanie application. So, no help from the RC hobby folks.

An additional constraint in the choosing was that – since we were building 250 of these CapNet hats – we needed a solution which wasn't too difficult to mount. Also, considering the fact that the motor itself costs just under $1, it would be a shame to pay more than a few pennies for the little adapter.

Considering all this, I decided to bring in a professional – as an EE after all, I was clearly out of my element here. So I asked our senior mechanical engineer, Edward Jory, if he had any advice. He searched briefly, and – not finding an obvious off-the-shelf solution – he quickly sketched out a design for a "Shaft Adapter" with the added benefit of rubber-banding the prop, which would serve as what the RC guys call a "prop-saver":

 
Unfortunately, getting 250 of these made would cost us about $6/each – so this was relegated to being a "last ditch backup plan" and the search continued. As we discussed the problem, the idea of using just a simple piece of plastic tubing – which would press-fit on the motor shaft and into the propeller – emerged as the ideal solution. If we could only find tubing of the right inner and outer diameter, made from an appropriate plastic, it could be cut into 1/2 inch lengths to make simple and cheap propeller adapters.

So I started searching for tubing. There's a lot of tubing out there. But unfortunately, I couldn't find what I was looking for. Just under 1mm inner diameter, with just over 2mm outer diameter – so we'd get a nice, tight "interference fit" to the shaft and the propeller.

Well, being an EE, I turned back to familiar territory – where had I seen little rigid plastic tubes before? Flipping through the online catalogs of some of our electronics distributors, I found something that just might work! A little nylon PCB spacer from Ricoh – the R938-7:


Inner diameter 0.8mm, outer diameter 2.2mm, and already cut to 7/16" in length. The outer diameter is a little bigger than I wanted, but it's a minor operation to "clean" the propeller bore with a 0.082 drill bit before pressing the spacer into place. The result is a nice, secure fit into the propeller – and a nice tight fit onto the motor shaft! Furthermore, at only about 10 cents each in the quantities I needed, the price was right on target.

 
So there you have it – quite a saga for a "component" in the CapNet project which will probably go completely unnoticed, despite being crucial to the success of the endeavor!
 

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