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Everything Else » Physicist, mathematicians and machine shop challenge changing linear motion to compression

August 16, 2010
by Ralphxyz
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In conjunction with my strain gage question I need to convert a linear stretching (cable) motion to compression.

My strain gage is just this flat disk with a raised center requiring about a 1/8th inch travel to compress.

So I have thought about maybe a cam action which I can picture but what would be the math, obviously the force would be multiplied rotating a cam. I also thought about some sort of screw action this would have to apply the force of 0 to 400# in <= 1/2 turn again I do not know the math.

Any ideas on how to turn a linear stretching motion into a compression motion? I am open to any suggestion.

I sure appreciate the much needed help.


August 19, 2010
by mongo
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Something you might be wary of... It may have 1/8" movement but it might not be designed to actually move that much. If done, it could damage the device.

Commercial scales have stops under the load cells to prevent excess movement. The term is "sprung" when damage occurs in that manner. The home scales are made with minimally protected components. They are not typically expected to see loads that could do that kind of damage.

The large floor scales I still work on from time to time have much less movement than that and have load ratings of up to 4000#. They usually have a load cell at each corner similarly but they are full load cells, connected to a balancing circuit before they are connected to the display unit.

Smaller industrial floor scales have only one load cell and it is situated at the end of a mechanical balance to give consistent readings across the platform. Usually, it it a parallelogram fixture with spring steel flexture hinges.

There are also some load cells, typically low level like 5# limits, that the parallelogram is built right into the unit itself. These are generally in package filling lines where weight is the method of filling cartons or bags.

August 20, 2010
by Ralphxyz
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Hi mongo, the cell holders on the scale are hollow below the center weight bearing point, so it appears full compression was possible, but I do not know how much travel there was for a 400# load. 400# was the max the scale was rated for.

I am still working on how to change the linear motion to compression. I have a concept that according to my minds eye would work.

I have no way of visualizing the consequences of course.

The best methods I have visualized so far involve using a cam action to compress the cells lever's. Using a cam might have the possibility of multiplying the force so hopefully I will be able to compensate in my code if needed. But of course I do not have any idea what might happen to the cell or what it's tolerance might be.

I sure am doing a lot of thinking on this project.

Currently I am thinking of having the four strain gage cells stacked in a tube or some sort of vertical holder and then having a cam mounted to the top which would be rotated by a pulling cable applying the compression/pressure.

I really need some mathematician to help me understand what is going on with a rotating cam. Will the pressure being applied be linear or wil there be a curve? I know how to apply pressure with a cam but I do not know the math involved. I distinctly remember being taught this in high school science, math or machine shop class but I wasn't paying to much attention back then.

I need to figure out the required arc required for 1/8" movement in 180˚ of movement. The 1/8" movement of course is to be determined and I am not sure I will get a full 180˚ of movement but if I had the formulas, I should at least be able to establish if it is doable. I sure am open to any suggestions on how to do this.


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