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Lug strength may be mostly determined by length? (among other factors...)

Discussion in 'Rifle Country' started by Evil Monkey, Jun 3, 2009.

  1. Evil Monkey

    Evil Monkey member

    Ever take a look at an ar15/ar18/aug/scar/g36/etc bolt? 6 to 7 lugs....

    Ever take a look at an ak/xcr/sig/etc bolt? Usually has 2 or 3 lugs....

    Notice anything else? I notice that the 6/7 lug bolts have lugs that are alot shorter than the 2/3 lug bolts, by 1.5x to 2x just by eye balling.

    I'd bet that if those 2/3 lug bolts had shorter lugs, we would see some lugs shearing off over time. I'd also bet that if 6/7 lug bolts had longer lugs, we would see far less lug shearing issues like those found in the M4.

    What do you guys think?
  2. gvnwst

    gvnwst Well-Known Member

    Not sure about that, seems like the longer thing would shear better. The thing, the 8 lug bolts are so much thinner, and so can shear seperatly more easily.
  3. rcmodel

    rcmodel Member in memoriam

    I think you are wrong.

    Longer lugs, assuming they fit with full contact, would have a longer leverage arm working on the base of the lug, and be more likely to break.

    Thicker front to back would make them stronger in shear strength.

    Longer but the same thickness would make them weaker.

  4. dakotasin

    dakotasin Well-Known Member

    and all too often that is a huge assumption.
  5. Evil Monkey

    Evil Monkey member

    I was thinking that since they were longer, there would be more metal to take the thrust better.

    So lets say we take a 7 lug bolt from an AR and make the lugs twice as long. What other dimensional improvements would be neccessary for stronger lugs? Would we need to make the lugs wider and/or taller?
  6. gvnwst

    gvnwst Well-Known Member

    Which is easier to break, a long pencil or a short one?

    If they are 2 times as long, they will break easier, if you want to make them stronger, make them thicker, side to side and/or front to back.
  7. Jim Watson

    Jim Watson Well-Known Member

    The best measure of bolt strength is the shear area between the lug and body. As wide as you can get through the raceway, as deep as you care to provide bolt travel for, that is what gives strength. More height just provides added engaging area in the locking recesses and once you have enough to avoid compressive deformation, you have enough and don't need to make the bolt and receiver any larger.
  8. Evil Monkey

    Evil Monkey member

    neither pencil is milled from and still attached to a tree trunk like the lugs on a bolts body. That's the point I was trying to make, that the longer lugs meant there was more metal attached to the bolts body.
  9. MrCleanOK

    MrCleanOK Well-Known Member

    gvnwst and EvilMonkey, I think you are applying different names to the same dimensions.

    A lug that is longer in the axial direction (parallel to the bore) will have a greater surface area of material resisting shear, making it stronger. A lug that is longer in the radial direction (perpendicular to the bore) will make the lug more prone to bending.
  10. JWF III

    JWF III Well-Known Member

    I'm not sure if I'm reading what the OP says like it was meant to be read.

    The way that I'm reading it, he's meaning "long" as in down the length of the bolt. Which I'd agree with that longer being stronger. Like the difference in shear strength between a 1/4" bolt and a 1/2" bolt.

    Length away from the center-line of the bolt would provide no advantage or disadvantage in shear strength.

    Length around the circumferance (although not possible with small bolts, with multiple lugs) would give some strength, but not as much as down the bolt. Think about it this way. It's easier to break a 2x4 the narrow way than the wider way.

    The pencil analogy, although seems relevant, is not. The breaking of a pencil does not use shear forces. It uses leverage.

    Scissors use shear force to cut. To use the pencil analogy again, since it was brought up. Try cutting a short pencil, and a long pencil with the same cutting tool. They use the same force.

    Shear strength has to do with a)material used (IOW difference between Grade 5 and Grade 8 bolts, b)the area the item(s) covers.

    This is just a guess, but measure the area of the lugs attachment to the bolt bodies. My bet is the two large lugs, and the 8 small lugs, will come to close to the same area, if they are designed for the same pressures. It would require the measurements to be taken from different designs but similar chambering. (IE- AR and AK rifles, both chambered for the 7.62x39.)


    ETA- Well it looks like my typing skills leave something to be desired. Everyone from Jim Watson on said basically the same thing I did. They just used less words.
    Last edited: Jun 3, 2009
  11. Maverick223

    Maverick223 Well-Known Member

    Yep, the no of lugs doesn't really matter all that much because the bearing surface is the same either way, wider would be stronger but not in shear, deeper (parallel to the bore) would be stronger in shear and compression. Rifles really don't use the best type of breach, a Smith Asbury interrupting screw, and don't really need to.
    I agree 100%, and so does my Strength of Materials book. :)
  12. Evil Monkey

    Evil Monkey member

    Yes! That's exactly what I am talking about.

    So I was right! :D

    The arguements I've read before for the 7 lug bolt was that you don't need a longer cam track in order to rotate the bolt, but the lugs were weaker. Well now it is possible to lengthen the lug parallel-wise to the bolt body/bore to provide a longer bolt life, especially in different barrel lengths/gas systems, and still have a shorter locking/unlocking time.

    But why aren't there any rifles out there that adhere to this concept? There must be a catch....
  13. Badger Arms

    Badger Arms Well-Known Member

    +1 -- Where do AR-15/M16 lugs usually break? In my experience, it's the two lugs on either side of the extractor. Reason being they are the WEAKEST lugs in terms of shear strength. The bearing surface isn't as much an issue as the base of the lugs where they connect to the bolt body proper.

    The M16 bolt is asymetric. The 7th lug, opposite the extractor has no opposing lug and therefore "levers" the bolt to bear a significantly higher proportion of pressure on the two lugs adjacent to the extractor. You can relieve the back of that lug and you end up with a 6-lug bolt with the '7th lug' acting as a safety lug should the others fail. In fact, Mark Westrom of the modern Armalite company thought of this, I didn't. He patented it. I've linked the patent and am also attaching a PDF of same. A good read of this patent might help explain things.


    Attached Files:

  14. gvnwst

    gvnwst Well-Known Member

    Okay....i thought you were saying from the centerline out... Yes, you are correct then.
  15. LoadedDrum

    LoadedDrum Well-Known Member

    When you say length do you mean the dimension on the lug from the bolt face to the back of the lug? Or do you really mean the width of the lug (how much of the bolt face's circumference that the lug occupies)? I think the latter is more important than the former.
  16. Evil Monkey

    Evil Monkey member

    I meant the first one. And no, the latter isn't more important, we just discussed this.
  17. Maverick223

    Maverick223 Well-Known Member

    Basically the length of the lug parallel to the barrel is more important than the width of the lug perpendicular to the barrel. The number of lugs makes no appreciable difference as long as the bearing surface area is the same.

    I wonder why no one has used a interrupted screw design in a bolt gun or the like, it would be much stronger and could therefore be made a bit lighter, they have been used in artillery pieces for decades for just that reason.

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