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

 

[jcmrlec]

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.

 

[lilliput1]

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

 

[jcmrlec]

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.

 

[lilliput1]

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.

 

[jcmrlec]

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/images/22297_full.jpg

 

[Nescius]

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

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

 

[jcmrlec]

i wish nescius!

 

[chicopee]

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.

 

[jcmrlec]

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

 

[chicopee]

About the reading meter?

 

[lilliput1]

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

 

[jcmrlec]

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

lilliput not sure what you mean?

 

[lilliput1]

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.

 

[jcmrlec]

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.

 

[IRstuff]

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)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

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[jcmrlec]

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

 

[IRstuff]

I would think the exact opposite; the case continues to deform, which continues to release the tension you are attempting to apply.

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

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

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[jcmrlec]

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

 

[LittleInch]

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.

 

[jcmrlec]

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.