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  • How to make U-bolt from threaded rod?

    Discussion in 'The main mechanical design forum' started by thorq, Mar 2, 2016.

    1. thorq

      thorq Active Member

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      Hello, I have an application that I can't easily find U-bolts of specific dimensions for and it isn't worth it ordering custom made ones. The u-bolts are intended to fasten square tubes -alu or steel profiles- to form a frame that should be sturdy enough to take some weight, vibrations, etc. Nothing extraordinary. I am thinking a cardboard/vinyl cutter frame.

      I don't have ALU profiles to use so I have to rely on either ALU square tube or Steel square tube. This is for a side project so I don't want to buy brackets/etc. Tubes, (U-)Bolts, Nuts. Here below you can see what I am talking about:

      Can you please tell me if using threaded rod would be acceptable to bend (hot bend not cold) in a u-bolt shape (around another round object so I can achieve the radius I need)?

      Do I need to treat the newly shaped piece of threaded rod to have it hardened somehow?

      [​IMG] [​IMG]

      The design above prevents to any member all degrees of freedom, be it rotational or translation.

      Thanks.
       
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    3. robertjeffery

      robertjeffery Active Member

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      Looking at the images attached, you would have to heat the threaded rod up to cherry red - golden to bend as tight bend radius as that. And I'm not entirely sure you could achieve a bend radius as tight as that. The issue with trying to bend threaded rod, is that the threads in the rod create loads of little notches, perfect stress risers.

      What dimensions are you looking to achieve, if you post the thread size and inside bend radius, I will give it an attempt tomorrow and post the results.

      Just to put a spanner in the works, I'm assuming you have no welding equipment. could you not use an angle iron, or angle aluminium, and flat profile. So that the bolts clamping the frame together actually have solid material between the head of the bolt and the nut. As I fear, you will find when you clamp the U-bolts to make the frame more rigid, it will have a tendency to crush the box section and therefore nether making the system that ridged.
       
    4. thorq

      thorq Active Member

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      The square tube is 20x20mm so at 2mm thick walls I am left with 18mm of U-span. If I use 4mm threaded rod, that would leave 10mm for the radius. But I'd even try 5mm rod/ 8mm bolt for the radius.

      Are you saying to build the entire frame with angle profile? I find steel square tube to be strong enough and because the u-bolt's legs are at the edge of the square, they are basically clamping on solid stuff. Only the M5/M8 stopper bolt sits in the middle of the tube, but it's size should be enough to offer sufficient clamping surface.

      I am also researching a very cheap way (something I can source from the stuff I have around) to built a jig for bending the threaded rod. It might also come in handy in future projects.

      Thanks.
       
    5. thorq

      thorq Active Member

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      The length of the U-bolt is the real issue here, as I can't find such small U-bolts that have legs longer than 20mm or so and I must span through the connected tube so the bolts should have 32-35mm legs for this application.
       
    6. robertjeffery

      robertjeffery Active Member

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      all I was suggesting is box section frames, with out welding techniques, are quite a tricky profile to join together.
      say if you look at most forms of truss bridges, they are generally angle and flat profiles bolted and riveted together.
      [​IMG] [​IMG]

      you can also add diagonal bracing with ease.




      Sorry, this isn't the point of this thread!!!!!

      Can we bend threaded rod??? YES
      Can we bend it cold??? YES

      Now you asked about 4-5mm threadded rod, now the smallest i had on the shelf was 8mm, so i just scaled the bend radius up.
      8mm stainless bend radius 20mm
      10mm BZP bend radius 25mm

      all i did was place them in the vice with a bolt of the bend diameter desired, and smacked them around with a hammer. I was expecting them to tear across from a notch in the thread, but they kept in very good nick.

      [​IMG]

      If you make some tooling up to create these bends, i don't think you will have any issue bending them cold what so ever,

      especially as you are looking to bend much smaller bars. as with a press brake bending sheet metal, when you get to the larger material eg 8-12mm and above your bend radius has to be larger then the thickness of the material in order for it not to tear on the outer edge, where when bending 3-5mm the bend radius can be the same as the material thickness and maybe even slightly smaller the material thickness.
       
    7. thorq

      thorq Active Member

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      Thanks for doing that, I wasn't expecting anyone making experiments on my behalf.

      I am aware of the angle trusses stuff but I find it a bit flimsy - I am thinking of those angle shelves they sell in general hardware stores -unless welded that is. With square tubes it seems on paper a lot sturdier, chunkier, more professional - they build car chassis out of them (welded of course).

      I was worried about the small diameter being more prone to tearing up because there is less material to spread the tension through. I am also a bit worried if by doing that I would affect in any way the integrity of the bent threaded rod to withstand the pulling forces necessary for making the frame rigid.
       
    8. robertjeffery

      robertjeffery Active Member

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      My gut feeling is that the thread would strip on the inside of the nut before the actual bar fails from snapping. even around the deformed radius created in the bending process.

      I have a feeling (really don't quote me on this) that threaded rod is produced to a 4. something grade (most every day bolts are 8.8 grade which is something like 1000mpa UTS and 750 Yield.) I would guess that threaded rod is produced from a steel like EN8 kinda grade - which is something like 700UTS and 450 Yield??

      What I am trying to suggest is that the thread form on the bar is going to only be half the strength of what you expect threads to be able to withstand.

      I know it is floating off topic slightly, do you have any machining capability's, ie, drill press and tap and die set?

      http://www.radmir.pl/en/images/stor...eczkoweBP/Nakrętki wałeczkowe 2-otw BH_3D.JPG

      because if you have any concerns that the threaded bar will not be strong enough, you could always use a system like the link above with 8.8 bolts, this would allow the maths to be considerably easier.

       
    9. thorq

      thorq Active Member

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      Yes indeed that would be sweet and I thought of that even with off-the-shelf (sort of) parts. Ikea has a bin of unused/returned metal miscellaneous stuff and I found one such thing with one hole only, and believe me I was even thinking of two such one holed things in a small diameter pipe that I cut to length and insert in the square but that would be way too complicated (for now).

      Another idea was to use the 2-holed thing of a U-bolt cabble stoper, like this one:

      [​IMG]

      And use normal M4 bolts through it (I am in Europe :) ). That would be my go to solution if U-bolts fail to deliver. I don't know though if this metal cast thing is weaker than the threaded rod. I would insert bolts from the back, on the flat side, and catch them with locknuts on the other side of the square tubing.
       
    10. robertjeffery

      robertjeffery Active Member

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      Rudimentary test, using nuts to try and stretch the cold formed U bolt to see whether the bar snaps before the threads rip out. as you can see from the image, the threads ripped out of the threaded bar but not the nuts.



      [​IMG]

      [​IMG]


      [​IMG]
       
    11. robertjeffery

      robertjeffery Active Member

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      I must admit, that I was a little sceptical of the joint design initially, but doing some simple calculations does suggest the joints to be more robust then they look. This calculation is making the assumption that the threaded rod will yield at 450mpa, where maybe if a conservative guess would be that the threaded rod would fail at 225mpa, and therefore the max side load you could apply would be in the region of 25Kg, I haven't done the beam bending calc, but I would expect a 20mm @2mm wall thickness to be at its yield stress 25kg at 900mm.


      [​IMG]
       

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