Steve's Junk » strain

Complete 180, well, perhaps 90.

smackaay — Thu, 11 Dec 2008 19:48:13 +0000

So, after doing a bunch of work on the rotary strain gage it’s been declared incorrect. Oh well. Here’s the new concept, although not my own idea, were going to rebuild the entire concept around the notion of a dynomometer. First we’re going to place the motor inside of a housing that allows the motor to move freely with the help of large ring bearings, then we’re going to bind the motor in place with load cells.

While this concept may work, I feel as though it’s lower end repeatability and accuracy will be compromised and will be greatly subject to temperature differences as the bearings heat and cool from ambient and mechanical sources. But perhaps I’ll be proven incorrect.

Here are some images of the idea.

I’m going to keep working on my own sensor design in the meantime, it may come in handy someday.

Rotary Strain Gage update

smackaay — Wed, 10 Dec 2008 21:48:18 +0000

Well, it’s been a little while since the last update, I suppose I should have stated something sooner. So far, the device seems to work but now I have mechanical problems. Friction on the top is preventing the load from being sensed at the paddles. Plus, the strain gages, I believe, are placed improperly. While it looks as though one side gets compressed and the other side gets stretched, I am thinking that this is not the case.
Unfortunately what that means is that the strain gages are counteracting each other, thus the voltage difference is too small to detect effectively.

Gonna design a new one with only two strain gages.

Creating a strain gage coupling

smackaay — Thu, 20 Nov 2008 17:55:13 +0000

So now I have to find some way of measuring the torque applied between the two shafts of the viscometer. Here’s my idea.

Strain Coupling Model View

What you can’t see is that I plan to manufacture it out of plastic, that way the strain gage experiences the most out of the deflection of the part. I’ll use a flexible polyurethane bonding agent for the strain gages, allowing the forces of the strain to work without breaking the bond.

Here’s a through model view:

Strain Coupling Transparent View

Of course, it only took me a few minute to make this model but it helped me see if what I was doing would work, I also used FEA to see if the forces would be transmitted where I wanted them. The FEA results were promising, nice even strain along the gage surfaces. (Those gauge surfaces are seen in light blue). You can see though that the hole through the side relieves some strain on the strain area, though, this shouldn’t be an issue.

Strain Coupling Strain Test view

Here’s an image of the stress as calculated by Cosmos

Strain Coupling Stress Test view

Well, hopefully this will work. First I need to get the strain gages to provide reliable results. By using two of them, temperature concerns should no longer be an issue and should help mitigate innacuracies. I’ll post pictures of the finished part, though I don’t think I’ll do it on the NC, I’ll just do it manually, it may look like ass