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

 

[LittleInch]

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.



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[jcmrlec]

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.

 

[LittleInch]

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.

 

[dbill74]

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.

 

[jcmrlec]

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.