Pop Rivet vs Screw..Which is stronger?

Discussion in 'Materials' started by rebar, May 9, 2012.

  1. rebar
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    rebar Junior Member

    Sorry if this has been discussed before, but I need to get the most pulling, not shear strength, from 1/8" 6061 alum without a nut behind.

    What would be stronger? A self taping screw, tapping a machine screw or a monel pop rivet of the same diameter? The piece I need to secure has been counter sunk already.

    Thanks!
     
  2. jehardiman
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    jehardiman Senior Member

    What diameter hole?
     
  3. rebar
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    rebar Junior Member

  4. jehardiman
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    jehardiman Senior Member

    You need a backing plate if you want to support anything like the rated load. Where is this?
     
  5. alan white
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    alan white Senior Member

    Screws (wood and sheet metal) are stronger than rivets of the same diameter because they have more cross section, but they have little backing area.
    Machine screws with washers and nuts are not only extremely strong, they also have a large backing area. Rivets can also be used with washers. They are far cheaper, so if tensile strength isn't a major issue, they can save big money. Screws can be removed. Rivets must be destroyed to remove them.
     
  6. rebar
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    rebar Junior Member

    Sorry, no can do.

    Its on a interior aluminum wall with 2.5 X 3/4 X 1/8" 6061 aluminum studs.
     
  7. SamSam
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    SamSam Senior Member

    I had to reattach the pivot part on the bottom of my mast and was told in the thin wall of the mast rivets would hold much better than screws because there was so little direct contact with screw threads.
     
  8. PAR
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    PAR Yacht Designer/Builder

    The best thing you can employ is a threaded insert, which is a rivet with female threads, that will accept a machine screw or bolt when fastened.

    [​IMG]

    The image shows an open back, threaded insert (a type of rivet, sometimes called a rivet nut)) installed. They are available in closed or open ended, so it can be a sealed environment if necessary. A counter sunk or oval headed machine screw would be the obvious choice for your application, naturally with the threaded insert.
     
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  9. rebar
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    rebar Junior Member

    Thanks PAR. Any suggestions on a entry level rivnut kit?
     
  10. jehardiman
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    jehardiman Senior Member

    Yep, the best thing he can do if he can't get a backing plate in to spread the load. Just be warned that if the fitting base is really stiff and the load is perpendicular to the surface as stated, it will still fail by ductile rupture of the sheet metal around the insert and pull out (i.e. the sectional area of the screw is still larger that the shear area around the insert, these things work much better in peel loads on thin sections).

    Here is a quick google on a typical blind nut rivet. There are several types and different installation methods. While the "pull' style setter is popular for line processes, there are some that are made to be hand set, usually with an allen wrench and socket headed cap screw.

    http://www.boltproducts.com/blind-rivet-nuts-c-110-l-en.html
     
  11. rebar
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    rebar Junior Member

    You kind of lost me there..

    Fastenal wants almost $200 for a kit not including the 100 rivnuts.

    Would this one be ok?
     
  12. jehardiman
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    jehardiman Senior Member

    Ok lets say that those are #10 screws (0.190 in od). Max tensile load of a #10 is ~ 1094 lbs (0.0199 in^2 * ~55 ksi for mild steel which is consistant with the fittings rated load of 1300 lbs if they used high strength screws)

    The shear area of the panel is the circumfrence of the od * the thickness, ~0.0746 in^2 (i.e. pi*.190* 0.125) Force needed to shear the plate in way of the threads is then ~984 (0.6*0.0746*22 ksi for as formed, temper 0) to ~1879 lbs (for t6 temper). which looks pretty good. However, yeild is half or less (8-15 ksi) of ultimate for aluminium so you get pullout rather than shear. The aluminiun yeilds and stretches rather than shear. This is why you get the classic "volcano" raised area around a pulled out rivet or screw in a mast. http://ars.sciencedirect.com/content/image/1-s2.0-S0261306909004543-gr7.jpg

    You need something on the back side of the plate to spread the load. So looking at the od of a #10 washer.... (pi*.5625*0.125*.6*8ksi) = 1060 lbs....and imagine that, even in the weakest aluminium you will effectively break the screw before you yeild the plate....;)... and that is why the fitting came with a backing plate.
     
  13. rebar
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    rebar Junior Member

    Thanks.. I cannot install a backing plate.

    My ancra tie downs are designed to strap down motorcycles.. Im straping a 32" x 38" rolling tool box which could weigh as much as 400 lbs, to a wall to prevent it from rolling. Worst case scenario.. Each fastener (four) could see at the most 200 lbs of pull if the weather got really rough and the vessel was capsizing.

    Your saying under those circumstances, a rivnut would pull out?:confused:

    I was hoping someone would say.. " Mike, in your situation where you cant install a washer or nut. I would suggest...
     
  14. PAR
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    PAR Yacht Designer/Builder

    Mike, it's helpful if you tell us what you're trying to do rather then ask for physical properties data on fasteners.

    Again, making some assumptions about this, a threaded insert can work for you, naturally sized to the fastener hole available. There are several types of insert, some include a threaded stud, which may be handy for you, but then I'm guessing as to what you actually need or want. Take a look at Hansonrivet.com
     

  15. tunnels

    tunnels Previous Member

    A big headed pop rivit with a flat washer on the back side will never come loose !!.
    I remember once using coins as washers , the big ss washer was a dollar each so for 50cents i used a 50cent coin !!! Half the price and just as good NZ 50c works well !! :D
     
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