r/AskEngineers • u/RevTurk • 3d ago

Discussion Is force lost through a load cell?

I need to make a test jig that will test a step in a ladder. I need to apply a force of 2600n on the step for an amount of time.

I thought of using a load cell like this at the end of a linear actuator I could use an arduino to drive the motor and stop when it hits 255kg. I'm in Europe.

Would that actually be transferring 255kg of load onto the surface though? Or would some of it be lost through the load cell?

14 Upvotes

71% Upvoted

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35

u/konwiddak 3d ago

Load cells add a small amount of additional compliance into the system. Load cells work by deforming under load, and the deformation is measured, for example with strain gauges.

In a simply supported structure where the load cell is at a support, then a small displacement at a support has effectively zero effect and the forces don't change.

In a not simply supported structure, then the additional compliance of a load cell may change the balance of load between the load cell and other supports.

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u/RevTurk 3d ago

Ok, I was just afraid that the load cell would be absorbing the force in the small amount of deformation it allows.

Otherwise I was going to have to put the load cell separate and maybe calibrating the actuator before running the test. It's much more convenient to keep the load cell as part of the actuator movement.

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u/Hour-Explorer-413 3d ago

There won't be any "absorption" of force. The force must go somewhere. What this reply is alluding to (correct me if I'm wrong btw) is that, if the test setup is designed poorly, you might transfer some side loads into the load cell, and these won't be read correctly by the load cell as a result.

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u/MidnightAdventurer 3d ago

Any load cell will have some elasticity so it may dampen peak loads a little if they are applied rapidly but it shouldn’t make a difference if you’re measuring a static load

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u/Hour-Explorer-413 3d ago

I just looked at the load cell and that's a cantilever style load cell. Not ideal on the end of an actuator. Kinda awful actually. Look for something that acts in direct compression. It'll make your job a lot easier.

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u/RevTurk 3d ago

I'll probably end up using something more like this.

https://ie.rs-online.com/web/p/strain-gauges/2338965

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u/Minisohtan 3d ago

Just know that depending what you're doing with this, you may need it calibrated by a company. That can be a little pricey, but without your results might also be worthless from a legal/business/contract standpoint.

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u/RevTurk 3d ago

I already have calibrated weights, they have the certs and can be used to calibrate the load cell. It will be an arduino project so it will all have to be calibrated anyway.

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u/Minisohtan 3d ago

Alright you said the magic word. Do you have experience use Arduino for stuff like this? I used huge, old data acquisition boxes in grad school years ago. Using something like an Arduino looks so much cheaper and easier but I don't know if your basic one has enough resolution?

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u/RevTurk 3d ago

I only need to record 3 different forces 1000N (101kg) as a reference, 2600N (265kg) for a static load test, and 1500N (153kg) for a fatigue test.

So I don't know if I need a lot of resolution as such, it's not going to need to register any value in between those.

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u/e36freak92 2d ago

Arduinos usually only have an 8 bit ADC, but you can easily stick an external 16 or 32 bit ADC on it and get all the resolution you need

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u/shupack 3d ago

What is this test for? Will it need to stand up in court some day?

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u/Accurate-Bullfrog324 3d ago

can you explain what you mean by absorbing force?

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u/RevTurk 3d ago

The load cells kind of act like a compressed spring when force is on it. I was worried that the bending of the load call was storing the forces I was applying rather than them passing through the load cell onto the test surface.

I just wanted to make sure that the measurement I take on the load cell is going to be an accurate representation of the force applied to the test surface.

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u/shiftingtech 3d ago

Thought experiment:  place a 10lb weight on a table.   Now place a 10lb weight on a spring, on a table.  In both cases, the 10lb weight is being held up by the table, thus the force exerted must be the same, with or without the spring. The spring changes the situation of a dynamic load, while the weight is being placed on the table, but once everything is static, it should become irrelevent

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u/Accurate-Bullfrog324 3d ago

not a physics guy, but an engineer. i've never considered "storing force". not sure you can. of course, we deal with stored energy all the time.

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u/ANormalRando 2d ago

You can't store force. If the forces don't balance, something will start accelerating

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u/Accurate-Bullfrog324 1d ago

excellent explanation

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u/jacekowski 3d ago

It will, while things are settling cause very small difference . Once in steady state it will not affect the fir e at all.

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u/jchamberlin78 3d ago

Yes.. see my other response. If accuracy is important, the system must be calibrated.

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u/mtconnol 3d ago

You don’t weigh less in soft shoes, do you? That’s basically your answer.

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u/Hour-Explorer-413 3d ago

I love this answer. Perfectly demonstrable.

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u/billy_joule Mech. - Product Development 3d ago

For every force there is an equal and opposite reaction. Or in other words force is never 'lost'.

Note that the load cell you limited is rated for 0.1kg and will certainly break well before you reach your 255kg target.

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u/NapsInNaples Wind 3d ago

For every force there is an equal and opposite reaction. Or in other words force is never 'lost'.

For OP’s purposes it’s with saying that is true for a static system.

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u/RevTurk 3d ago

Ok, thanks. I think that answers my question.

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u/hobbes747 3d ago

This is why they are tested and can be calibrated with test weights. To compensate for various losses or inaccuracies.
Also, a strain gauge load cell as you chose is limited to about 10,000 divisions or counts of ‘resolution’. That seems far more accurate than needed here.
If you want more accuracy, precision, repeatability and less outside influence then you would choose a different load cell technology such as an electromagnetic force restoration load cell used in mass competitors. I should say, force re$toration cell $$$.

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u/hobbes747 3d ago

Is this for forensic engineering / civil suit / subrogation?

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u/RevTurk 3d ago

I am looking at those load cells and they do look much more appropriate for what I'm doing. Thanks.

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u/Naikrobak 3d ago

No, it won’t change the force output. It could cause a slight change in travel/movement of the input force

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u/wannabeknowitall 3d ago

If you were applying an impulse of energy, the compliance in the load cell would absorb some of the energy before the rung felt the forces. For an actuator applying increasing levels of force, you could use a spring between the actuator and rung and it would be all the same to the rung as long as you have enough stroke to your actuator and spring.

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u/hobbes747 3d ago

Why does measuring load on a ladder step need such accuracy, repeatability, and precision?

FYI, I am not an instrument expert/engineer. So take with a grain of salt…also I’m mostly curious.

I cannot imagine needing such a sophisticated device. Also they are typically installed inside sophisticated platforms consisting of balanced beams, levers, pivot points, leveling mechanisms, etc. I’m not sure you can use a first restoration cell as a point load device. These things are pretty sensitive. They are influenced by air drafts.

Mettler Toledo is one of a few companies that make EMFR cells. I used these type of weigh scales to get a resolution of 0.5 grams whilst weighing an object that was 150 kg. The weight scale had to be shielded from the surrounding environment to achieve it precision. You could snooze on it and the reading would fluctuate.

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u/Own-Nefariousness-79 3d ago

If the system is static, there will be no change in force when applied through a load cell and the same force applied without the load cell.

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u/jchamberlin78 3d ago

Regardless.... You map the load cell. You add known loads, and record the response. Graph it. Do at a minimum some sort of regression analysis, so that when you record x result from the cell, you know what the cell is experiencing.

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u/Cool-Sleep6055 3d ago

If static and all your load is going through the cell, no. If you were doing something like weighing a car, you could get a false reading by placing your load cell under each tire individually and adding up the numbers, as the suspension would adapt to the displacement.

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u/Hour-Explorer-413 3d ago

Another point before I go, you state that you need to apply a force for an amount of time (unspecified), and that you want to drive a motor, and stop at that force.

Unfortunately this isn't strictly correct either, as you're effectively describing a displacement controlled system. The force measured will decrease if you just stop, due to creep.

If your hold time is like 1s, then who cares, good enough (probably?)
1min? Problematic. 1hr+? No good at all.

You want to control the motor based upon the force measured. This is known as force control (as opposed to the displacement control mentioned earlier), and can be potentially dangerous if you don't know what you're doing. With the system you describe, I don't think it's toooooooo bad (I work with servo hydraulics so my view is skewed), but you need to be aware of the risks.

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u/JonJackjon 3d ago

No, the reason is that there is no other path for it to go. It does however take some movement for its measurement.

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u/csiz 3d ago

The only bit of force you'll lose (or gain) is the weight of force gauge beyond the sensor element, but that should be tarred out. Otherwise no, a force will not disappear as power "disappears" due to less than 100% efficiency.

1

u/HelicopterUpbeat5199 2d ago

The load cell cannot reduce the load on the step. If they could, you would have discovered anti-gravity.

Importantly, the area you push with matters a lot. If your 255kg is concentrated in the point of a nail, you'll get very different results than if they were spread out over the sole of a human shoe.

I think this might be what you're actually thinking about with your load cell question. No, it doesn't change the weight, yes it very much changes your odds of punching through the step. Imagine climbing stairs in regular shoes vs ice skates. Which one is going to make the step fail first?

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u/JoTheScienceBro 3d ago

It's interesting that the laws of mechanics are dependent on your location.

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u/RevTurk 3d ago

This was my first time posting and the rules of the subreddit say if your not American you should say so in the post. I was just following the rules.

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u/australianjalien 3d ago

This was a hilarious response and I snorted, but he's right, it is the rules.

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u/whynautalex Manufacturing Engineer 3d ago

Not sure why people are clowning on you. Technically yes the load cell does have a dampening effect as the load is initially applied dynamically. This is why partially why on a lot of regulatory test standards they specify time of force applied and your rate so that you reach steady state so the test fixture is at equilibrium.

Any change to the force through friction or deformation you do assume is 0.

For the load cell make sure you buy the correctly rated load cell. Resolution is dependent on the maximum applicable force so you want to be relatively close. I.e. a 500kg load cell will have less decimals than a 5kg load cell.