# The Physics Behind Applying Force with a Load Cell

• Osculum
In summary: Then the other blocks would only support 140 lb each.Not unless something is keeping the other blocks from loading up.
Osculum
We are applying a force with a machine that has a dial that measures lbs.
P=F/A
We have 5 blocks with a combined area of 20 in^2 and we want to apply a pressure of 35 PSI to them. Therefore F(total) for the entire Area is ~700 lbs. This is what the machine reads.
If we put the load cell on top of the blocks it should read 700 lbs (the total force applied) correct?
However, then each block should relieve the force of 140 lb (700/5), correct? Or does each block receive 700 lb?
If we put the load cell next to our blocks and therefore adding support to the entire load. will it read 700 lb or will it read 700 lb/(5+load cell)= ~140lb? do all the blocks receive 700 lbs or is it divided evenly among each block?

My boss with a pHD says the load cell should read 700 lb (the force) but it's divided among the blocks right?

Osculum said:
We are applying a force with a machine that has a dial that measures lbs.
P=F/A
We have 5 blocks with a combined area of 20 in^2 and we want to apply a pressure of 35 PSI to them. Therefore F(total) for the entire Area is ~700 lbs. This is what the machine reads.
If we put the load cell on top of the blocks it should read 700 lbs (the total force applied) correct?
However, then each block should relieve the force of 140 lb (700/5), correct? Or does each block receive 700 lb?
If we put the load cell next to our blocks and therefore adding support to the entire load. will it read 700 lb or will it read 700 lb/(5+load cell)= ~140lb? do all the blocks receive 700 lbs or is it divided evenly among each block?

My boss with a pHD says the load cell should read 700 lb (the force) but it's divided among the blocks right?
It's not clear how these blocks are arranged. Are the 5 blocks stacked one on top of another, laid out flat in some array, what?

Pictures will tell a better story.

they are not stacked. they are spread out evenly, sandwiched between 2 heavy metal plates and then pressed. i probably shouldn't post pics of our work but here is a little example.
thanks

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The structure that you have is statically indeterminate. It means that you cannot predict the force on each block without considering its position and its stiffness. On average each will have about 117 lbs, but the stiffer ones will have more and the placement matters too.

That's what I thought. The blocks are nearly identical and (nearly) evenly separated and the force (nearly) evenly applied. Thus the force (nearly) divided amongst each block.
The load cell (next to/ in line with the blocks) measures pounds, so if 700 lb is applied it should read something close to 117-140 lb (give or take depending on the size of the load itself) but certainly not 700 correct?

Osculum said:
That's what I thought. The blocks are nearly identical and (nearly) evenly separated and the force (nearly) evenly applied. Thus the force (nearly) divided amongst each block.
The load cell (next to/ in line with the blocks) measures pounds, so if 700 lb is applied it should read something close to 117-140 lb (give or take depending on the size of the load itself) but certainly not 700 correct?

This arrangement doesn't seem to make a lot of sense, at least, from the standpoint of calculating what load each block supports.

If you want to test what happens if a block is loaded to 35 psi, design the test so that you know for certain that block is carrying a load which produces 35 psi.

The thickness of the plates, the arrangement of the 5 blocks, and probably a couple other factors which are not apparent, all factor into how a force applied to the plate is shared among the blocks.

Osculum said:
That's what I thought. The blocks are nearly identical and (nearly) evenly separated and the force (nearly) evenly applied. Thus the force (nearly) divided amongst each block.
The load cell (next to/ in line with the blocks) measures pounds, so if 700 lb is applied it should read something close to 117-140 lb (give or take depending on the size of the load itself) but certainly not 700 correct?
if the load cell is stiffer or larger than the blocks then it could be supporting most of the load. Consider the extreme cases.

If the load cell is very stiff and slightly larger than the blocks then it would take the full 700 lb load and the blocks would be unloaded.

On the opposite extreme the load cell could be enough shorter than the blocks that it doesn't even touch the plate and so would be unloaded.

Basically, this isn't a good way to measure anything useful. Small variations in the setup could change the reading from 0 to 700 lbs.

Last edited:
CWatters

## 1. How does a load cell measure force?

A load cell measures force by converting mechanical energy into electrical energy. Inside the load cell, there are strain gauges that bend when a force is applied. This causes a change in electrical resistance, which is then converted into an electrical signal that can be measured.

## 2. What is the relationship between force and strain in a load cell?

The relationship between force and strain in a load cell is linear. As the force applied to the load cell increases, the strain on the strain gauges also increases. This results in a proportional change in electrical resistance, allowing for accurate measurement of the applied force.

## 3. How does the sensitivity of a load cell affect its accuracy?

The sensitivity of a load cell refers to its ability to accurately measure small changes in force. A load cell with high sensitivity will be able to detect even small variations in force, resulting in higher accuracy. However, a load cell with low sensitivity may not be able to accurately measure small changes in force and therefore may have lower accuracy.

## 4. How do you calibrate a load cell?

To calibrate a load cell, you need to apply known weights to the load cell and record the corresponding electrical signals. This creates a calibration curve that can be used to convert the electrical signal into a force measurement. It is important to regularly calibrate load cells to ensure accurate measurements.

## 5. Can a load cell measure force in different directions?

Yes, a load cell can measure force in different directions. However, it is important to consider the orientation of the load cell when using it to measure force. The load cell should be aligned with the direction of the force for accurate measurements. Some load cells may also have multiple strain gauges in different orientations to measure force in multiple directions.

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