Mounting Conundrum

So, I'm working on a gizmo to go with a series of control systems training seminars that I'm putting on.
The gizmo is basically a fan on a pivoted stick, with a control system
that works to maintain the angle of the stick relative to its mount at a commanded value. It does a good job of giving people a visceral understanding of how a feedback control system works, and I don't think it's going to cost me much to produce.
But I went and took it on an airline flight for the first time this week, and it didn't survive well. The position feedback from the pivot is provided by a nice inexpensive potentiometer with a D-shaped hole, into which one inserts a shaft of the correct dimensions. Here's a close-up of the pot mounted on the board, with a shaft (and, if you've sharp eyes, a little paper shim that keeps things snug).
https://docs.google.com/open?id 5lSHlBBxGvjcTBCc3VzWDYxZ0E
On the flight (two flights each way, Portland to Ottawa and back), the pot broke off the board. It was obviously punched out of the board by the force of the shaft acting on the back of the pot. Fortunately the training is for engineers, and it was at a corporate site, so my customer was able to repair the thing and I was able to use it for demonstrations.
Unfortunately -- even though I thought I had identified the problem and fixed it -- it broke on the way back, too.
Now, one solution to this may just be that I need to find a different way of putting the whole thing together so that it's easy to disassemble for shipping, and then don't ship it assembled.
But I also want to put it out to the group for suggestions: the shaft needs to be shimmed to a snug fit in the hole of the pot, or the slight play between pot and shaft messes up the control (the arm will hunt within the slop of the connection). But shimming things seems to set up a problem with the shaft transmitting too much force to the pot, and -- ping!!!
This thing has experienced a moderate amount of knocking around in my shop, and use both on the property and around the local area without breaking. But as soon as I go and ship the damn thing it breaks. So not only am I very concerned about shipping, I'm a bit concerned about this being a point of fragility in an otherwise reasonably stout mechanism.
Comments appreciated.
--
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Tim Wescott wrote:

Vertical clear acrylic tube with a fan at the bottom a ping-pong ball and an ultrasonic ranger at the top. I seem to recall this being covered in a Circuit Cellar issue actually.
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wrote:

Affix it to a gear resting on a bearing on the board, gear the pot next to it, and you're fixed. And split the shaft at the gear so it's can be broken down safely shipped.
Oh, and wire the pot backwards to accomodate the difference in shaft spin. ;)

Speaking of Circuit Cellar, has CC or anyone ever digitized those old copies?
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Larry Jaques wrote:

Last I knew you could get CC compilation CDs. I have the first 6 or 8 CDs worth of them. I presume you can get a DVD with a larger range now, or perhaps a flash drive with everything.
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On 11/12/2012 2:27 PM, Tim Wescott wrote:

Expecting that pot to survive shipment with a big lever attached is optimistic. Switch to a real pot that mounts by the shaft bearing, not the element contacts and a setscrew to remove the lever for shipment.
Or you could leave it the way it is and demonstrate that the electrical solution is worthless if the mechanical implementation fails. That's probably a more valuable lesson.
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On Mon, 12 Nov 2012 16:03:47 -0800, mike wrote:

That pot is less than $2.00, and very low friction.
Your "real" pots are big, clunky, expensive, high friction, and require brackets which must be purchased which further ads to the cost.
So finding a way to isolate that pot from damage while still having it work would be a big plus.
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Perhaps the other useful lesson is that it is best to nail down and publish all the desired specs before having a design review or calling the consultants. I am still not clear about what sort of weight and torque is on that small pot. Just how little friction do you need? How cheap does the solution have to be?
It sounds like you want a stronger bearing to take up the mechanical loads, and then have a connecting arm from the pivoting portion to the pot. The stationary part of the bearing would be mounted with a bracket to board above the pot with its axis in-line with the pot. The connecting arm could extend through the center of a ball bearing, or it could be U-shaped and connected to the outside if you need less than 360 deg. movement and the bearing is not hollow. A cheap, easy-to-make, super-low-friction pivot can be made by tapering both ends of a hard shaft to a point, and then clamp onto the ends with two cup-point setscrews. You adjust the set screws to have minimal friction with no slop. You also need threadlocker or a locking nut to keep the setcrews in place.
Another low friction pivot design for small angles is a cross-flexure. For some reason these are expensive to buy but relatively easy to make if you have some spring steel shim stock. You can find one in the read/write arm pivot in some hardrives. Hell, any bearing they use there would probably be good for your purposes.
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On 11/12/2012 7:12 PM, anorton wrote:

Seriously more that it could deal with!
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On Mon, 12 Nov 2012 17:12:28 -0800, anorton wrote:

I should have posted a 3/4 view of the assembly; much would have been clear.
The shaft is supported by bearings in the framework, and the pot has some lateral play. So in theory, the pot sees no lateral force at all. Where I fell down on this is that when the pot sees no axial force it has too much play in rotation; taking up that play means that the pot, rather than the bearings, is locating the shaft.
Your idea of using a flex coupling has merit. I'm not sure if I can do it cheaply, but I'll think on it.
--
Tim Wescott
Control system and signal processing consulting
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On 11/12/2012 3:27 PM, Tim Wescott wrote:

Take a piece of aluminum channel and mount it to a 1"x6" with the sides sticking up. Drill across the sides and put a 1/4" shaft through to use as the pivot for the fan arm. Add a small right angle bracket to support the pot. Connect the pot shaft to the support shaft with a piece of vinyl tubing. The fan arm could be clamped to the board for shipping.
If you want lower friction, 1/4" ID flanged bearings are cheap at Enco, and could be retained by a simple plate clamping them to the outside of the channel.
BobH
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Tim Wescott wrote:

Is that chrome tube on the back the arm you expect the pot to support?
The easier method with the shaft would be a split tapered shaft that uses friction to compress and retain the shaft.

If that rear tube is what you plan on supporting the easiest way would probably be a bracket on the opposite side with a nylon busing supporting the shaft on the other side. If you plan to keep that pot.

OK one possible solution would be to eliminate that pot. Instead use a printed resister on the board with a wiper mounted to the pivoted shaft. Then use a through shaft with nylon bushings to support the pivot. Take a look at just about every fuel level sensor out there.
Or switch to an optical encoder with a supported shaft.
--
Steve W.

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wrote:

Hell..simply give the pot a bit of hotglue and glue the damed thing down to the board. Wont take much and its easily removable
"
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Yet again, (yawn) your signature quote is a nearly complete fabrication: http://www.truthorfiction.com/rumors/c/cicero-plan.htm
Besides, there was no concept of either bankruptcy or long-term public debt in ancient Rome.
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Can you slit the shaft with a junior hacksaw & spring it apart just a little. That would allow it to slide on & off easily whilst maintaining good coupling.
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On Tue, 13 Nov 2012 11:17:56 +0800, Dennis wrote:

I've been wondering that myself. It would certainly work, if the machining cost didn't overwhelm the advantage of using a $2.00 part.
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Tim Wescott
Control system and signal processing consulting
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Tim Wescott wrote:

First question is who'd you ship it with? If UPS, the answer is clear. A little knocking around your shop is not a valid test, you have to put it in the shipping box and throw it 20 feet. That's how UPS unloads their semis - they throw everything off the truck into a big pile. Try sending it FedEx, they are much more gentle with stuff.
Jon
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On Mon, 12 Nov 2012 23:59:20 -0600, Jon Elson wrote:

Alaska Air Lines and United -- it went as luggage.
I could certainly go a long way toward solving the problem with a bit of disassembly before shipment, I do know that.
--
Tim Wescott
Control system and signal processing consulting
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On 11/12/2012 2:27 PM, Tim Wescott wrote:

I think a little more forensic study is needed. Did the solder break? Did the traces on the board pull off the board? Remember when we first learned to solder we were told never to rely on solder to hold anything.
I would suggest you fabricate a tinned clamping piece with a hole for the shaft and bent over the edges of the pot and then soldered to the circuit board. Solder the clamp before you solder the legs of the pot so there is no upward pressure on the clamp.
Paul, KD7HB
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On Tue, 13 Nov 2012 10:22:57 -0800, Paul Drahn wrote:

The solder broke on some pins, and the pins broke elsewhere. But the pot is clearly not designed to be a bearing, either regular or thrust. Reinforcing the solder joints enough will just make the housing break.
Isolating the pot from everything except for shaft rotation -- that should work.
--
Tim Wescott
Control system and signal processing consulting
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Ahh, It's been a little hard to tell what you wanted. Maybe a piece of rubber tubbing as part of the coupling to the pot? (or heat shrink as previously suggested)
Are you making one of these, tens, hundreds? (At the tens level it's silly to worry about a few bucks.)
George H.

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