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Thread: Ratchet Strap Problem

  1. #1
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    Ratchet Strap Problem

    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 - 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
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  2. #2
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    What do you mean by an "ECU" and can you be more specific about the ECU's location. as opposed to your load cell.

  3. #3
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    ECU is environmental control unit (air con). What do you mean by its position? it has 4 straps, one on each corner - all equidistant from the ECU. It is in a mobile cabin, we hook tie down rings in the floor and connect a ratchet strap between the floor rings and the corner of the ECU.

  4. #4
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    OK, my error, because in your illustration it appeared you had two columns with the left column titled ECU and the right column titled Case, I got the impression you were comparing your load cell values to a "ECU' value of some kind.

    The reason I asked about "location" was that I was wondering about a possible elastic deflection in one connection point that was limiting the amount of load that could be applied vs the other connection point.

    The only thing that seems possible, as you mentioned, is that you have exactly twice the diameter of wrapped strap in one case than the other which would reduce a given wrapping torque by 1/2 for your same applied lever load.

  5. #5
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    Hi Jalberts,
    I have actually posted this on another forum http://www.eng-tips.com/threadminder.cfm?pid=404
    Have had more replies on that so we have covered off a few things

  6. #6
    Administrator Kelly Bramble's Avatar
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    Use vector statics/math and the problem is revealed.

    Assuming the strap angle is 60 deg.

    200 kg * cos 60 = 100 kg

    and

    260kg * cos 60 = 130 kg

    see:

    Statics 2D Force Components

    Solutions right angle calculator


    Free body diagram
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  7. #7
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    Kelly the load cell is just measuring the tension in the strap, not the force down on the case. the tension in the strap has nothing to do with the strap angle.

  8. #8
    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by jcmrlec View Post
    Kelly the load cell is just measuring the tension in the strap, not the force down on the case. the tension in the strap has nothing to do with the strap angle.
    Then what are you looking for? I think your question needs clarification.. My best guess is that you are simply not applying the same tension with the strap ratchet mechanism.

    The math I provided is a resultant vector force applied to the ECU by vector math with the straps at a known angle.

    You sure on the load cell configuration?
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  9. #9
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    Kelly the issue is not the force applied to the ECU/case. The issue is a difference in strap tension between the two.

  10. #10
    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by jcmrlec View Post
    Kelly the issue is not the force applied to the ECU/case. The issue is a difference in strap tension between the two.
    That statement eliminates my confusion with your question as there should not be a difference in tension if the ECU/Case are rigid under loading (compression).

    If the case is collapsing under strap loading then the tension in the strap would appear limited. Eventually the case will collapse or buckle under load and will reveal itself by observation.'

    I would double/triple check that the load cell and associated electronics are functioning properly in both loading senarios..

    ~ The math seemed to work assuming your questions was flawed.
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  11. #11
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    jcmrlec

    Now that I have reviewed the other thread I have found why your description on this thread is not clearer to those of us on this thread. It is because on this thread, only the single drawing shown on your post opens when clicking on the "attached Thumbnails" figure, none of the other 15 photographs shown on the other site are
    accessible.

    That aside, after looking at all of the posts, to me it still appears that the most likely issue is the increase of the effective diameter on the ratchet spool due to the increased wraps on the ratchet because of increased stretch in the longer length of the strap over the case, as opposed to the individual corner strap on the ECU.

    It basically come down to the amount of tension the ratchet can apply to the strap for an equal amount of your force on the handle, nothing else, type of attachment, deflection of the attachment points or the case, etc makes any difference at all other than to possibly add a small bit of more wrap on the ratchet spindle.

  12. #12
    Administrator Kelly Bramble's Avatar
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    So, this brings me to one of several possibilities for the limited tension load.

    1. The case/box is compressing at a certain normal load applied by the strap.
    2. The strap itself is stretching under the tension load and will eventually either break or act as a spring and gradually increase load.
    3. The anchor points are distorting under load and limiting the tension on the strap.
    4. Some combination of all of the above...

    The bottom line is that if the load measurements are being done properly something is not perfectly rigid and changing length or compressing as the strap is cranked and it's length is getting shorter or maybe it;s stretching.

    The following is a typical stress strain curve and what is happening is that the applied load is reaching the elastic region on one or more of the components.




    Quote Originally Posted by JAlberts View Post
    jcmrlec

    Now that I have reviewed the other thread I have found why your description on this thread is not clearer to those of us on this thread. It is because on this thread, only the single drawing shown on your post opens when clicking on the "attached Thumbnails" figure, none of the other 15 photographs shown on the other site are
    accessible.

    That aside, after looking at all of the posts, to me it still appears that the most likely issue is the increase of the effective diameter on the ratchet spool due to the increased wraps on the ratchet because of increased stretch in the longer length of the strap over the case, as opposed to the individual corner strap on the ECU.

    It basically come down to the amount of tension the ratchet can apply to the strap for an equal amount of your force on the handle, nothing else, type of attachment, deflection of the attachment points or the case, etc makes any difference at all other than to possibly add a small bit of more wrap on the ratchet spindle.
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  13. #13
    Principle Engineer Cragyon's Avatar
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    jcmrlec Your welcome!

  14. #14
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    I agree that yielding could be an issue except that at some point, if the ratchet force multiplication is capable of pulling with 200 lbs with his normal force; then, if it isn't a spindle torque issue, one of the following should ultimately occur:
    1) The yielding will continue until the number of wraps on the ratchet spindle is so great that the strap wrap O.D. binds/jams on the surrounding ratchet frame
    2) The yielding of an element(s) continues until there is a failure of that element.
    3) The increase in elastic force continues until the load cell force reaches the 200 lbs that he is capable of achieving.

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