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]

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.

 

[jcmrlec]

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

 

[LittleInch]

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.

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[jcmrlec]

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.

 

[LittleInch]

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?

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[jcmrlec]

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

 

[LittleInch]

Try it.

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


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[jcmrlec]

no i mean i did try it and found up easier.

 

[LittleInch]

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.

 

[jcmrlec]

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.

 

[LittleInch]

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#_4.9_STRENGTH

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.

 

[btrueblood]

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.

 

[zeusfaber]

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.

 

[jcmrlec]

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.

 

[lilliput1]

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.

 

[jcmrlec]

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?

 

[lilliput1]

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?

 

[jcmrlec]

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.

 

[lilliput1]

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.

 

[lilliput1]

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