Loadcell mechanical offset through springs...

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Thread Starter

Rick Kelly

Hey Gang...

It has been proposed by a supplier that we try a load cell that has a spring inside of it to offset a tare weight. In other words the first x number of grammes (in this case up to several kilos worth) is taken up by a mechanical spring assembly inside the cell and any weight over this value will impact on the cells bridge and supply an output.

This would allow us to use a cell with a smaller weight range with a weighing tablet that is much heavier. This will increase the resolution of
the cell and therefore the accuracy of the checkweigher.

My first thought is as the spring assembly ages it will require adjusting... anyone have any experience with this. The cell I am being asked to consider is a Tedea 9010.

As always I thank all for responding.

Best Regards... Rick Kelly

Chief Technician N/C
Cheese Operations
Kraft Canada Inc.
Ingleside, Ont.
V (613) 537-8069
F (613) 537-8044
 
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Richard O'Donnell

Rick Kelly wrote:
> It has been proposed by a supplier that we try a load cell that has a
spring inside of it to offset a tare weight. In other words the first x number of grammes (in this case up to several kilos worth) is taken up by a mechanical spring assembly inside the cell and any weight over this value will impact on the cells bridge and supply an output.
>
> This would allow us to use a cell with a smaller weight range with a
weighing tablet that is much heavier. This will increase the resolution of the cell and therefore the accuracy of the checkweigher.
>
> My first thought is as the spring assembly ages it will require
adjusting... anyone have any experience with this. The cell I am being asked to consider is a Tedea 9010.

Hello:

I like to keep things simple. I feel you are better off to use the load cell 'as-is' :

(1) The "spring" will be imposing an initial "mass" on the cell. If the error of the cell is % of mass , then you are increasing your error.

(2) I would take the 'raw' signal and scale (via electronic A to D converter) to the desired engineering units.

(3) Without further discussion, I cannot 'see' an advantage.

(4) Have you considered a cantilever setup (as found on Belt Scales) ?

(5) If you "go with" the Spring, don't forget Hooke's Law: Will you ever exceed the "linear (m*k)range?

(6) What does the Manufacturer (not the supplier) recommend ?

Regards,

Rick
 
You could lose your temperature compensation. The spring my not have the same characteristics at different temps. The load cell is compensated for this.

In my opinion I would NOT consider the spring, it will probably cause more problems then it will solve.

Increase your A/D resolution and/or your sampling rate.


Steve
 
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Robert McDonald

My understanding of a spring and load cell combination is that any force being applied to one side of the spring would be applied to the opposite side of the spring (conservation of energy). Thus, correct me if I am wrong, the effect you are after would not work.

regards,

Robert

Tristar Electrical
www.tristar-elec.com.au
 
Robert

You are correct if the spring and the load cell are in series this will be the case. If the setup is such that the two are in parellel (i.e the spring sits around the load cell and on the same support)then it will take the initial load and the load cell will take the load once the spring has been compresed to its height.

Steve

ABB UK
 
Sorry I was lazy to read another clever advices and even the question carefully.

The spring rate of the spring will stay the same despite on its age because the geometry of the metall stays the same.

The force will decrease with a time but not too soon and monotone(ly).

The accuracy can increase if the preload is just several % from FS: You close gaps for sure and probably the "table" is always in the connection with sensor. It is good.

The spring must have no friction.

Some inspector can blame you in bad intents.

Dmitri

[email protected]
 
I re-read the question.
It is good proposal, about spring.
But to sleep well you will need periodical tests, in summer and winter.
I think it is done alredy and you turn adjusting screw weekly when the scale is empty.
Regards
Dmitri.
 
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