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Ratchet Strap problem

Ratchet Strap problem

(OP)
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

RE: Ratchet Strap problem

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

RE: Ratchet Strap problem

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

RE: Ratchet Strap problem

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?

RE: Ratchet Strap problem

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

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

RE: Ratchet Strap problem

(OP)
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.

RE: Ratchet Strap problem

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.

RE: Ratchet Strap problem

(OP)
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.

RE: Ratchet Strap problem

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.

Quote (Nescius)

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.

RE: Ratchet Strap problem

(OP)
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_uploa...

RE: Ratchet Strap problem

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.

Quote (jgKRI)

...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.





RE: Ratchet Strap problem

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.

RE: Ratchet Strap problem

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.

RE: Ratchet Strap problem

(OP)
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.

RE: Ratchet Strap problem

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.

RE: Ratchet Strap problem

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" evil 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.

RE: Ratchet Strap problem

(OP)
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.

RE: Ratchet Strap problem

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.

RE: Ratchet Strap problem

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

RE: Ratchet Strap problem

(OP)
Dave the best i can do for you is these pics:

ECU. Ignore the ducts on the end. And the straps weren't crossed in the test. The Load cell was placed between the ratchet and the floor hook.


Case. As with the ECU the load cell was placed between the ratchet and foor. and was also tried on top and on the other side.

RE: Ratchet Strap problem

(OP)
Rob the load cell is measuring the tension in the strap. Not the vertical force on the case. Angle shouldnt matter.

RE: Ratchet Strap problem

The case seems to have two straps going over the top??. Can you sketch out the exact strapping set up as this looks different to the ecu set up.

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

RE: Ratchet Strap problem

(OP)


this is the complete case setup. Sorry thought i had already stated we tried with one strap by itself, as well as with the other straps on too. made no difference.

The ECU is exactly as per the picture in the OP but with one strap on each corner so 4 in total. As you can see in my previous post the strap attaches to a ring at the top of the ECU.

RE: Ratchet Strap problem

Other difference in the set up is you're pushing down on the ecu ratchet, but up on the case. You can get much more force pushing down than you can up.

Was this the case?

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

RE: Ratchet Strap problem

(OP)
I thought the opposite to be honest. was easier with the angle of it to get under and push up.

RE: Ratchet Strap problem

Try it.

Unless you can lift your own body weight you can always get more force down than up.

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

RE: Ratchet Strap problem

(OP)
no i mean i did try it and found up easier.

RE: Ratchet Strap problem

But does "easier" mean less force?

What I meant was did you reverse the operation with a load cell to see if it made the difference?

It's the force on the lever when it's horizontal that makes the difference. Force down at that point is higher than force up unless you're built like Arnie. ...

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

RE: Ratchet Strap problem

(OP)
well easier meant more clicks of the ratchet.
We didnt reverse it. didnt work with the length of straps we had. As i said earlier we tried with the load cell on each side of the case. in one of those the ratchet would have been reversed i guess.

The angle and geometry when ratcheting down meant it was all arms. when going up could get under it and use your legs to help. I dont know too many guys who couldnt squat roughly their own body weight so no arnie like build required.

I am 100% sure that the orientation of the ratchet wasnt responsible for us getting half the tension.

RE: Ratchet Strap problem

Ok,

I still think it's worth a lab experiment to find out, but that is the only key thing I can see that is any different from one system to the other. The other is line length and hence you will get more clicks of the ratchet on the longer straps as they stretch, but the force is what you're after.

The scientific evidence supports my view... see extract from a NASA document attached. Clear difference between force down vs force up. https://msis.jsc.nasa.gov/sections/section04.htm#_...

Anyway, that's me done for now unless some other difference between the two becomes available.

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

RE: Ratchet Strap problem

When ratcheting the case straps, was the ratchet allow to bear on the case wall, helping to keep the ratchet in-line with the strap? It looks like the ratchets on the ECU are farther away from any available bracing, and would tend to twist when tightening; when the handle is released the ratchet body straightens back out, slightly slackening the tension in the strap.

RE: Ratchet Strap problem

Silly question here: Why are you bothered about how much tension you can develop in the ECU scenario?

In the Case scenario, you are relying on the preload in the ratchet straps to apply Normal force to the case/floor interface, generating friction that will stop the load sliding around. I can see how preload tension matters to you here.

In the ECU scenario, the straps are restraining the lateral loads directly and you need do little more than eliminate the slack. Any additional preload just eats into the available strength of the restraint.

A.

RE: Ratchet Strap problem

(OP)
LittleInch i understand what you and NASAs evidence are saying but its not that. I can categorically say that it has nothing to do with it. I weigh 70kg and can easily lift more than that. As i have already stated the angles of the strap made it difficult to put my whole weight onto it and for downwards it was more on the strength of my arm, whereas it was easier to push upwards and i could put my whole body into it. If you are still hellbent on relying on NASAs data for different situation rather than my own experience then you will notice the difference between up and down is a long way from half. lets please let this theory go.

trueblood no it wasn't resting on the case. Yes the ECU ones were slightly further away and prone to a 90deg twist but that would mean the ECU ones should be less rather than the case. Dont think this was a factor. Its more from the incorrect alignment of the tiedown ring in that pic (that wasnt the one in our experiment).

Zues we were more just measuring to help with our calculations to verify they didnt move. Also the floor rings have a lower WLL (working load limit) than the straps so we wanted to make sure we werent overloading them. You are right that the case situation is much more important which is why we were concerned when we were only getting half the tension.

RE: Ratchet Strap problem

It could be because the coefficient of friction of the strap on the top surface of the case is about half the coefficient of friction of the straps on the top of the ECU which has sharp 90 degree edge. The tension then would be the force perpendicular to the strap times the corresponding coefficient of friction.

RE: Ratchet Strap problem

(OP)
Hi Lilliput1,
We thought that that would happen in the situation where the loadcell is on the opposite side of the case, but in the situation where the load cell and ratchet are on the same side friction shouldnt come into it?

RE: Ratchet Strap problem

Friction will be a factor if the load cell is only at one end of the strap. Try using a turnbuckle & load cell at each end. Also I notice that the strap for the ECU is green while that for the case is gray. Is one more elastic than the other?

RE: Ratchet Strap problem

(OP)
Lilliput why would that help? As in the pic in the OP we tried the load cell in 3 different positions. The ratchet stayed in the same position. When the loadcell is in position 1 there should be no friction loss in that section of the strap. The tension should only be affected in section 2 and 3 once it has gone over the case edge.

Also the loadcell was around $4k - we cant justify getting another just for this. Also due to the strap length it gets hard to put multiple sections in without tying knots in the straps.

We used the same straps on each during the test. That is a pic from when they were actually packed ready for transport. and they are from the same manufacturer and have the same specs etc anyway, just different colour.

RE: Ratchet Strap problem

It is like when you push a load on the floor. The weight x the coefficient of friction is the force you need to push. The strap tightening creates a downward force on the top. this force x the coefficient of friction is the tightening force on the strap.

RE: Ratchet Strap problem

Try spraying WD40 on the strap at the top and see if the load cell reading reduces.

RE: Ratchet Strap problem

(OP)
the equipment has left our facility so i cant retest that.
But it is two very smooth slippery surfaces.

try running a strap or a rope over something rough and tightening with a tension loop or truckers knot etc. You will get a much higher tension in the rope on the side you are tightening than on the otherside. Therefore friction has no effect on the section with the tightening element.

RE: Ratchet Strap problem

On the contrary, what you observed proved there is friction!

RE: Ratchet Strap problem

(OP)
Of course it proves there's friction. what i am saying is it proves that friction only effects the tension in the section of the rope after the case edge.
Look at the picture below. If there is friction T1 > T2. but the tension in T1 should be the same as if instead of a pulley it was a hook (and the T2 section didnt exist). Think about if instead of a ratchet there was a weight on the end of T1 and someone holding T2. The higher the friction in the pulley the lower strength required to hold T2 and the lower the tension in T2. the tension in T1 doesn't magically get less because of the friction. It still has to support the weight.



Also by trying the load cell in all 3 positions and getting the same result we have proved that the friction is minimal anyway.

RE: Ratchet Strap problem

The portion of the strap with the turnbuckle will have the higher tension because of friction. You must have mounted the load cell always where the turnbuckle was.

RE: Ratchet Strap problem

(OP)
Read the OP and look at the picture on the right hand side. Where i have written 1,2,3 and circled them are the 3 locations we moved the load cell to and measured. The ratchet stayed at position 1 for all of the tests. we got pretty much the same tension in each location.

btw it is not a turnbuckle. it is a ratchet strap like this:
https://net-at-hand.s3.amazonaws.com/sites/19508/i...

RE: Ratchet Strap problem

There are an astonishing number of misconceptions in this thread...sad

jcmrlec, look what you've started! bigsmile I don't think you're pranking us, but holy smokes, if you started this thread as a joke, it was a masterful display.

RE: Ratchet Strap problem

(OP)
i wish nescius!

RE: Ratchet Strap problem

The load cells for the ECU and the case should have read about the same values( discounting friction at the corners, angle of the straps to the floor, number of wraps around the ratchets,etc...) when tightening the straps as hard as you can regardless on the attachment configuration over the case or ECU. You did not tell us about the load cells so I suspect that one load cell may have had a different magnification factor on their meters. The discounted effects in the parentheses above would affect the reading but not twice as much of a variation.

RE: Ratchet Strap problem

(OP)
Thanks chicopee that was my thoughts hence the confusion. The loadcell was the same unit on both tests.

RE: Ratchet Strap problem

About the reading meter?

RE: Ratchet Strap problem

How about interchanging the load cell/meter locations from case to ECU

RE: Ratchet Strap problem

(OP)
No chicopee, the cell and technique was the same for all tests.

lilliput not sure what you mean?

RE: Ratchet Strap problem

Relocate the load cell on the case strap to the ECU strap. Relocate the load cell on the ECU strap to the case strap. Compare readings.

RE: Ratchet Strap problem

(OP)
it is just one load cell. we did the test on one and took the reading. then disconnected that same load cell and used it for the other case.

RE: Ratchet Strap problem

So, I see a couple of differences between the ECU and the case. The ECU appears to be an aluminum chassis, which the case is some sort of polymer (I guess). The other difference is the placement of the anchors around the two things. I don't necessarily think there's anything important relative to the anchor placements, but I would think that an aluminum chassis is essentially infinitely stiff relative to your straps, but the case probably isn't.

TTFN (ta ta for now)
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RE: Ratchet Strap problem

(OP)
IR i agree but as the case deforms slowly i kept trying ot crank it further and further - thought that would negate this?

RE: Ratchet Strap problem

(OP)
sorry explained that badly. i could crank it up to the point where i couldnt operate the ratchet any more. then the case would slowly deform and release tension (about 5-10kg of tension lost per minute). after a little while i could click it again and add more tension. but i could never get the tension as high as with the ECU (always around half) even when i couldnt physically operate the ratchet any further

RE: Ratchet Strap problem

The only way to get real repeatable data would be to use a fixed bar and a force meter / spring / weight on the end to activate the mechanism.

This would remove the variable human element from the whole thing.

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

RE: Ratchet Strap problem

(OP)
Littleinch think what you are saying. are you seriously trying to tell me that through human variation after roughly 10+ iterations on each scenario where we got 200kg +-15kg everytime on the ECU and 100kg +-15kg everytime on the case that human error or variation is the reason.

RE: Ratchet Strap problem

In essence yes.

If you look at this logically you appear to have the same ratchet, the same straps, your load is measured at different places all the same, but the only difference I can see is that one you press down and one you pull up.

Now the ratchet is essentially a lever arm rotating around the hinge. It looks like you have a lever advantage of about 5:1. Therefore a 100kg force needs a 20 kg force and a 200kg a 40 kg force.

Now I am aware you are totally convinced that the effort/force you're putting in to both ratchets is the same, but humans are not very good at measuring such loads. Therefore my idea is simply to measure that force on the end of the ratchet to make this more scientific. Perhaps something like a luggage weight device or other simple spring force measurement or weight.

e.g. if there was a rope hanging vertically I'm pretty sure I could hold onto it and lift my feet off the ground, hence max force is my weight ( about 95kg ). Anchor that rope to the floor or to some sort of weight and that's a different story. Depending on what stance you can get and avoiding injury I doubt I could get to 40 or 50kg as a single point lift. So this isn't like bench pressing or lifting a bar, but I would tell you that the "effort" I put into the vertical lift up would be higher to the vertical lift down.

All I'm saying is take the unknown out of the equation and actually measure force being applied to the item and stop using your judgment as to how much force you're using.

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

RE: Ratchet Strap problem

(OP)
everytime i post on a forum i remember why i dont do it very often. you are not reading my responses. it is not like pulling up on a rope. it IS closer to squatting or benching a bar.
the ratchet ends up at about chest height. the slight angle of the strap and the nearness to the case or ecu means that it is not possible to hang your whole weight off the ratchet arm. then when lifting up you can get your whole body under it and lift with your legs, shoulders and whole body. it also really hurts your hand going down, once again due to the geometry of the strap, whereas going up you can use the flat of your hand and it is more comfortable.
Also thinking more about it we took it all on and off multiple times so i think we would have had the ratchet orientated in both axis throughout it anyway.

As i have stated numerous times we dont have the equipment anymore so we cant redo the test.

RE: Ratchet Strap problem

I do read them,

So what are your thoughts now?

The orientation was the only difference I could see.

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

RE: Ratchet Strap problem

Been keeping an eye on this thread for a bit and the more I read the comments, the more I am coming back to a single question for jcmrlec.

Why are you looking at the tension in the straps?

Once your setup leaves the lab and you have workers strapping the equipment down, all your calculations and measurements go out the window because you cannot guarantee that the straps will be tightened to the same degree every time because of the human factor. And unless you are there to oversee the process and verify they use load cells, those workers will not use a load cell to verify strap tension; they will work it till the straps are taught and move one. From what I've seen and read, the tension in the straps is a function of the number of clicks put into the ratchets and no two people are going to setup and clinch it down to the same degree.

In the OP, it's mentioned that this is for air transport. How are you accounting for lateral and negative g-forces? What happens when the plan banks or worst case, inverted?

I fail to see what difference the tension in the straps is while the load is sitting still on the ground. As the plane moves you are going to introduce forces on the ECU and crates besides gravity and tension from the straps, and they won't be constant. I think a more important question is do you have straps located so as to prevent the load from tipping over.

RE: Ratchet Strap problem

(OP)
dbill, we design mobile communications cabins for defence forces. the cases and ecus are stored within the cabins during transport and then taken out when they deploy. we have to provide instructions on how to pack them up ready for transport. this includes detailed instructions on how many straps and even how many "clicks". we also have to provide design documentation to ensure it can be certified for air transport. obviously the number of straps we require to hold something down is dependent on the tension we can achieve in the straps. so we take the achievable strap tension, add a safety factor to allow for the human factor, and then use that to make sure we have enough restraint for each item. we took the measurements to work out this achievable tension.

for lateral loads, we know the floor friction coefficient, we know the strap geometry and strap tension, therefore we can work out the clamping force and hence the amount of lateral force required to make the equipment slide. in terms of inverted forces we can just use the load limits on the straps and other equipment, tension doesn't really matter then.
in terms of the stuff tipping over i have done the tip vs slip calcs etc to make sure that wont happen. for the ecu (which is quite tall) as the straps attach to the corners, and there is a bolt through the floor there is no chance of it tipping. also with the case this is very low and wide so again it has no chance of tipping.

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