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Countersink vs. Counterbore

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  • #31
    What I had meant was its not smearing or flaring when tightening taper head fasteners as someone suggested on here..

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    • #32
      Originally posted by 754 View Post
      I disagree that taper head cannot hold torque , we used them a lot. And flat head cap screws should be 82 degrees NOT 90 degrees.
      and yes the angles should match.
      I didn’t say they won’t appear to hold torque. I’m saying you are not getting the tension you think you are. The torque will not result in a constant tension relationship do to the variables mentioned and more. In joint design where a particular clamp load Is desired, we unfortunately cannot measure tension. We rely on something we can measure and that is torque. Variability of torque to tension can be as much as 50% of the calculated nominal value for tapered heads. Structural joints I worked with could never tolerate this variability. If the joint is in torque or shear there is no possibility of an equal sharing of load between the fasteners. That is why I’m saying don’t use them if clamp load is critical. Your designs apparently can tolerate this variability.
      The motorcycle wheel example removes some of the position and angle variability by extruding the aluminum wheel to fit the screw head. Is is done, howeve, at the expense of the torque tension relationship.

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      • #33
        Well how does the torque on lug nuts on automobile wheels hold. lug nuts are angled to match the counter sink in the wheels. John b.
        John b. SW Chicago burbs.

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        • #34
          The standard torque settings probably do not work with the countersinks, because the area is different, the accuracy of centering makes a difference, surface roughness, etc make a difference. The torque tables are maybe 30% accurate on a good day, with standard heads, and smooth surfaces, at the force per unit area existing with those.

          Throw in the countersunk head on top of everything else, and the sky s the limit on errors of actual bolt tension.
          CNC machines only go through the motions.

          Ideas expressed may be mine, or from anyone else in the universe.
          Not responsible for clerical errors. Or those made by lay people either.
          Number formats and units may be chosen at random depending on what day it is.
          I reserve the right to use a number system with any integer base without prior notice.
          Generalizations are understood to be "often" true, but not true in every case.

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          • #35
            Originally posted by john b View Post
            Well how does the torque on lug nuts on automobile wheels hold. lug nuts are angled to match the counter sink in the wheels. John b.
            every time i see one i wonder about lug nuts. i once calculated the contact pressure for a 12 mm lug nut torqued to 130 nm (without even considering the wegde effect) and it was so high i cant understand how it can work in low strenght cast aluminum. are they using inserts in the wheel? never heard about it.

            another interesting question concerning countersunk heads: how thick does the material need to be to support the rated load? is the height of the head enough, for what strength of material? are there any standards/rules for this?

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            • #36
              For thin materials, the forces are usually normal to the screw axis, because thin materials do not carry bending very well in the thin direction.

              So I do not think the question you ask comes up much with thin material.
              CNC machines only go through the motions.

              Ideas expressed may be mine, or from anyone else in the universe.
              Not responsible for clerical errors. Or those made by lay people either.
              Number formats and units may be chosen at random depending on what day it is.
              I reserve the right to use a number system with any integer base without prior notice.
              Generalizations are understood to be "often" true, but not true in every case.

              Comment


              • #37
                Wheel nuts / bolts on cars are a whole subject on their own. Long before you worry about friction losses in the taper seat, sort out whether the thread should be lubricated or not. Good luck in getting a consensus on that one.

                Ian
                All of the gear, no idea...

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                • #38
                  Originally posted by J Tiers View Post
                  The standard torque settings probably do not work with the countersinks, because the area is different, the accuracy of centering makes a difference, surface roughness, etc make a difference. The torque tables are maybe 30% accurate on a good day, with standard heads, and smooth surfaces, at the force per unit area existing with those.

                  Throw in the countersunk head on top of everything else, and the sky s the limit on errors of actual bolt tension.
                  When used as a screw, yes countersinks have lots of friction, and limit the torque that creates tension in the fastener.
                  When used as a bolt, no the countersinks do not matter, because you are tightening the nut.

                  -Doozer
                  DZER

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                  • #39
                    Originally posted by J Tiers View Post
                    For thin materials, the forces are usually normal to the screw axis, because thin materials do not carry bending very well in the thin direction.

                    So I do not think the question you ask comes up much with thin material.
                    not sure why normal to axis? i was debating if material as thin a the height of the counter sink is usually able to take up the typical strain of the screw or if it needs to be thicker resisting a force parallel to screw axis.

                    and i was not talking about loss of torque by friction (about half of which is under the head, btw) but about surface pressure in the contact area.

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                    • #40
                      Because thin materials tend to tear out when pulled away from the screw heads. The material is also not strong in bending in that direction because it is thin. The screws are used to attach it, but the material is more often used for strength as a shear panel.
                      CNC machines only go through the motions.

                      Ideas expressed may be mine, or from anyone else in the universe.
                      Not responsible for clerical errors. Or those made by lay people either.
                      Number formats and units may be chosen at random depending on what day it is.
                      I reserve the right to use a number system with any integer base without prior notice.
                      Generalizations are understood to be "often" true, but not true in every case.

                      Comment


                      • #41
                        Originally posted by john b View Post
                        Well how does the torque on lug nuts on automobile wheels hold. lug nuts are angled to match the counter sink in the wheels. John b.
                        Three major differences. Nuts are spherical, you have studs not cap screws and most importantly the wheel pilots on a diameter. It is the combination of these that make it work But the pilot is the controller along with a sequenced torque procedure.

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                        • #42
                          round wheel nuts/bolts are rare and there is plenty of bolt centered wheels out there.
                          Last edited by dian; 07-05-2020, 06:18 AM.

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                          • #43
                            Originally posted by rickyb View Post
                            Three major differences. Nuts are spherical, you have studs not cap screws and most importantly the wheel pilots on a diameter. It is the combination of these that make it work But the pilot is the controller along with a sequenced torque procedure.
                            Most european cars are opposite just like dian mentioned.
                            Location: Helsinki, Finland, Europe

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                            • #44
                              Originally posted by rickyb View Post
                              Three major differences. Nuts are spherical, you have studs not cap screws and most importantly the wheel pilots on a diameter. It is the combination of these that make it work But the pilot is the controller along with a sequenced torque procedure.
                              Yep, as suggested by others, on the Volvo wagon I have it is not as you state. The nuts are the ONLY centering mechanism, the wheel is a loose fit otherwise. IIRC, on the old S10 the system was like the Volvo. On the new Ranger, it may be as you say, but that is a Ford, no telling what was done by their "newly graduated engineers".

                              The concept of having conical or spherical nuts in conical recesses AND centering on a boss of the hub is a flawed idea, it is kinematically wrong. Unless perfect, the two centering means will "fight" and there will be internal stresses set up by the errors in location of the studs/nuts vs the centering boss. A stupid system indeed, from that point of view. It may transmit torque OK, but so does the old system.
                              CNC machines only go through the motions.

                              Ideas expressed may be mine, or from anyone else in the universe.
                              Not responsible for clerical errors. Or those made by lay people either.
                              Number formats and units may be chosen at random depending on what day it is.
                              I reserve the right to use a number system with any integer base without prior notice.
                              Generalizations are understood to be "often" true, but not true in every case.

                              Comment


                              • #45
                                In the automotive world there are hub-centric and lug-centric wheel mountings. Google the terms.

                                lg
                                no neat sig line
                                near Salem OR

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