Ratchet Strap problem 1

[jcmrlec]

Hi guys,
Really struggling with something at the moment. We are designing some transport arrangements for an air transportable cabin. I have tried to attach a pic twice - hopefully it shows up.
We have two strapping scenarios that we are measuring strap tension using a load cell, one for a case and one for an ECU. Basically we have the case where one strap goes over the whole thing and the ECU where the straps hook to the corners ie the ECU has two straps. The problem we have is for some reason we can only measure roughly half the tension in the strap for the case, compared to the ECU straps. I have cranked the ratchet as hard as I can on both scenarios. For the case, i have tried putting the load cell in position 1 2 and 3 and all get pretty much the same result. To provide some numbers if i do the straps to a reasonable amount I get 200kg on the ECU and 100kg on the case. If I really try hard i can get 260kg on the ECU and 130kg on the case.

For the case, it should be pretty much just a pulley. So you should get the normal force on the case will be twice the measured tension in the strap (have allowed for angle but its pretty much vertical so can ignore it). But I dont see why the tension in the strap should change. There will be friction losses between the strap and case. But I think in the situation where the load cell and the ratchet are on the same side of the case that friction wouldn't affect it. I would expect to get less when the load cell is on the opposite side but this only changed it by about 10kg.

I think its too convenient that its halved. Also the case kind of deforms a bit. but i have tried cranking it more and i cant. Load cell is reasonably new and calibrated but again that shouldn't matter.
I have showed a few other Mech engineers at my firm and we are all confused. The only thing we can think is a combination of the strap friction, the case deforming and the extra strap wound on the ratchet (bigger lever to actually crank the ratchet) all contribute and its just a fluke that its half. It is making us feel stupid!

Hope someone can help.

Cheers

 

[3DDave]

Put rollers under the boxes and see if the strap load equalizes. You can do this with lengths of small diameter pipe.

 

[hydtools]

With the case, more strap is wrapped on the ratchet drum reducing the tension that you can apply with the same length ratchet handle. If the wrapped radius doubled the tension would be half for the same handle force.

Ted

 

[3DDave]

I think Ted is on to something.

The reason is that the amount of wrap should be proportional to the amount of slack and stretch in the strap; a longer strap will start with more slack and be more elastic. This would require more wrap on the handle pulley to get the tension and therefore increase the radius/decrease the max tension for a given load on the the handle.

What is the length of each strap?

 

[IRstuff]

Seems like a good test of that theory would be to take up the slack in the strap some other way, reduce the amount of strap on the ratchet takeup reel, and see if that changes things.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[rb1957]

I'd rather see you use a cargo net ...

another day in paradise, or is paradise one day closer ?

 

[jcmrlec]

Thanks for the replies guys. Sorry forgot to mention that we have already considered this.
We tried a combination of different amounts of strap wound on the ratchet for both cases - starting with heaps of slack and starting with it as tight as possible before ratcheting. Didnt have much affect. All results were still in the ranges weve been getting. There is a bit of variation anyway (+- 15kg) as each "click" of the ratchet adds a reasonable amount so just depends on where it starts etc.

The strap lengths are roughly 1.5m for the ECU and 3m (maybe 3.5) for the case.

 

[Nescius]

If the load cell is in-line with the strap, nothing else matters. All this talk about strap friction, or strap geometry, crushing the object, or anything of the sort, is irrelevant.

If the load cell is reading properly, it's telling you what the tension in the strap is. Your hand on the ratchet is what's tensioning the strap. Nothing else matters.

If the exact same interaction with the ratchet mechanism can generate two different forces, we could build a free energy machine.

 

[jcmrlec]

Nescius I agree. That is why this result is so confusing. The only reason i talked about all those other factors is that is what we have considered and decided they shouldn't affect it. We have tried multiple times with different ratchets, the load cell in different spots etc and got the same result.
The load cell is quite new and calibrated - we also lifted something of known weight to make sure it was ok.

There was a deleted post in there too, not sure why it was deleted but i would argue in the example of pulling up as hard as you can this is limited by my strength. In the case i am talking about it is all in line with the strap so the ratchet should be able to impart the same amount of tension in the strap in either situation. rather than seeing half the tension in the strap, should see double the normal force on the case.

 

[SwinnyGG]

I deleted that post because I didn't want to get into a giant argument.

But, to your second point- you can't apply double the normal force using the same amount of input energy.

The reflex for most people is to compare the two-leg strap arrangement to a pulley- but this is NOT a pulley. Erase that analogy from your mind.

When you tighten the ratchet strap, nothing moves. The strap acts as a spring, and the work you apply to the strap is absorbed as strain energy as the strap stretches.

The amount of strain energy you can put into the strap is always the same- it's based on the amount of work you can apply to the ratchet.

A spring with a fixed rate, which has absorbed a fixed amount of strain energy, can only apply a fixed force in any given direction.

If the exact same interaction with the ratchet mechanism can generate two different forces, we could build a free energy machine

This point is absolutely correct- but in the opposite way that Nescius meant it. The behavior you're seeing is such that the strap applies the same force to the object, regardless of geometry, because the level of work applied to the spring is the same. Hooke's law tells us that you can't double the force applied with the same level of strain energy stored in the spring. It just isn't possible.

 

[jcmrlec]

jgKRI if you believe everything you read on an internet forum without questioning it it wont end well! But seriously not trying to be argumentative, just trying to get my head around it.

I completely get your point about the using 2 straps vs 1 and pulling as hard as you can in your first post - just thought its a different situation as the force you are applying with the ratchet is inline whereas in the lifting example it is kind of external? Also that is more working on the pulley concept which you are saying shouldn't be applied. effectively the same as this:

So can you explain a little more why its not a pulley system? I get that the straps aren't infinitely rigid etc but it still just seems like the pic above but upside down. Additionally we setup the following situation. If thats not clear we setup a loop with the ratchet in it on the lower end of the load cell. When we cranked this up we got around 350kg (compared to 200kg without the loop). We put the 50kg loss down to friction and the straps interfering with each other etc but that is working like a pulley?? If you take W=Fd then you are doing the same Force but double the distance to get twice the Work done?

My final argument is on the link below (and other similar government transport guide pages) to calculate the straps required to restrain stuff during transport you use that equation. At the end you divide by 2 because one strap is providing twice its tension to clamping force. Or thats how I understood anyway.

https://www.allsafe-group.com/fileadmin/user_upload/bilder_sonstige/force_locking.pdf

 

[Nescius]

jgKRI,

Stiffness of the strap doesn't matter. Hooke's law doesn't matter. This is a statics problem. No consideration for the work done, or the potential energy of the strap, is needed. Somebody is pulling on a handle until a certain force on the handle is achieved.

...you can't apply double the normal force using the same amount of input energy.

I assure you, I certainly can. That is the fundamental function of many devices.

 

[LittleInch]

I think there is a variety of things going on here, but you need to look closely at your ratchet device.

If it's what I'm thinking of, the strap circumference on the ratchet increases as the length of strap through the device increases. This means that for a similar force from the user, as the diameter of the reel increases from what is a very low diameter, the maximum effective force on the strap goes down. Right at the start the diameter is really quite small. It doesn't take much to increase the outer diameter to double or more than it does at the start. This is why it probably doesn't make a lot of difference in the amount of slack, but you can't get too much before the strap starts to interfere with the roller. so the difference between twice the diameter and three times the diameter isn't as much.

The effective force on the strap is variable by length of the strap for the same amount of strain. As you stretch the strap, the force increases, but not the same for different length straps.

so the difference between your 1.5m long strap and your 3.5m long strap is that to get the same force you need more than double the amount of strap to go through the ratchet device assuming you start at hand tight. This limits more and more the maximum force you can apply for the same effort on the ratchet strap.

If you put two ratchets on your ECU, one on either side then I think you would get your 200 kg again if you did them up at the same time.

Also try with the ECU to have lots of spare strap before you hit tension - I doubt you'll get to your 200 kg then.

The fact your case also bends a bit also increases the amount of strap length through the ratchet device to get the same level of tension.

The experiment?

Put two ratchets on your case, one either side and gradually tension it up equally.
On your ECU feed in a lot of slack first to simulate the increased strain of the longer strap.
Measure the diameter of the ratchet reel between your 200 kg tension and your 100 kg tension.

Let us know if this makes sense!



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Nescius]

jcmrlec,

Confirm for us that, in the 2-strap ECU instance, you're ratcheting the side with the load cell last and reading the tension at that time, not doing the load cell side first, not touching the other strap after you've snugged up the load cell side.

Edited to add: Please also confirm that the load cell is reading real-time, not some sort of max. hold mode.

 

[jcmrlec]

LittleInch yes we tried that already. We tried on the ECU with heaps of slack so that the diameter of the strap on the ratchet was as big as possible before interfering with the mechanism. made little to no difference.

Nescius i told a little lie, there were 4 straps on the ECU - one on each corner. We tightened them all up together, took measurements then slackened the load cell one off and tried that a few more times all with the same result. We couldn't do it with just the one strap connected as it pulled the ECU over. When it was tight by cranking one of the other straps it also increased the tension in the load cell strap due to the slight angle on the straps.

Also can confirm the load cell was reading real time.

 

[zeusfaber]

I know this isn't the question you asked, but that diagram in the first post, coupled with your description of the "case" scenario has been bothering me ever since I first saw it.

The way you've drawn it, the load isn't central between the two tiedown points. The trouble if you do that in real life is that as soon as the load gets jolted towards the centre of the bed, you will lose all the tension in the strap (probably closely followed by the load itself).

Expect you knew that already and it was just a rough sketch - but there: My conscience is salved.

A.

 

[Nescius]

Well, it certainly is a mystery. It would be driving me nuts, as I'm sure it is with you, jcmrlec.

So, it's not:

1. The strap winding up, changing the effective drum diameter.
2. The instrumented strap being tightened first, and the following strap(s) gaining a mechanical advantage/lever effect from the case leaning over.
3. The load cell reading a non-representative peak load or some such.

Here are some increasingly crazy ideas that you may or may not have eliminated:

1. The load cell has a halfing/doubling mode to "help" when you're dealing with various sling load arrangements.
2. Human error; you really aren't pulling the handle with the same amount of force.
3. Variability between difference ratchet mechanisms; some of them are "rough" or bind under load, maybe only in certain points of rotation.
5. Expanding on #2: The ratchets end up in ergonomically different spots, affecting how you pull on them.

As a sanity check, put the strap across something else, like end-to-end on a heavy duty workbench. Even better, pull the ratchet handle with another load cell or hanging scale, etc.

Edited to add: Be careful, don't get a hook or something slingshotted into your eye.

 

[jcmrlec]

Zuesfaber that is just my bad drawing. It was central in real life.

Nescuis yes thats about right. We havent changed anything on the cell and tried enough times so think that should eliminate human error and differences in ratchets.
unfortunately we have had to move on for work purposes and have just accepted the 100kg at this stage. I would like to try some more sanity checks in the future but i have wasted enough time at work on this already. I will see if i can borrow the load cell and do some extra curricular tests for my sanity though.

Im just happy to know that its confusing others and im not just overlooking something.

 

[3DDave]

It is mostly confusing to others because none here have seen the actual setup. Since there is no explanation of it as conventional or expected behavior it is obvious that something is being overlooked.

 

[oldschool84]

OK, I have a question about your ratchet straps.

What is the angle of the straps relative to vertical?

Are they vertical, 45 degrees, 60 degrees? If the strap are angled then you can easily end up with 1/2 your member axial tension as a normal force.

Think of each strap as being a tension member/cable/etc. If you have placed these straps at any angle outside of vertical then there shall be an Fx and an Fy force based on the angle. (Fy vertical and up, Fx Horizontal and right)

If the straps were completely horizontal you would provide no downward (-Fx) force or normal force no matter how hard you cranked the strap. If the straps were completely vertical then you would have a normal force equal to you 2X the tension (if using 2X straps and neglecting friction)

So have a look at your strap angle if it is 60 degrees measured from vertical then your Fx force would be 0.5 the tension. (Cos60*T)=Fy Using vertical as reference. if you are at 45 then you are .707.

-Rob