Announcement

Collapse
No announcement yet.

"Weak link"

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • #16
    And when the steel pin shears, there's a swiss file to clean the "smeared" joint faces of the drive.

    Regards Ian.
    You might not like what I say,but that doesn't mean I'm wrong.

    Comment


    • #17
      Originally posted by CalM View Post
      For every aluminum shear pin, there is a smaller , lighter. cheaper steel pin with the same shear value ;;-)))
      If two pins are the same dimension, (in my case, 3mm X 50mm) would not the steel one weigh more than the aluminum one regardless of price or shear value?

      MATERIAL..WEIGHT PER CUBIC FOOT..WEIGHT PER CUBIC INCH

      ALUMINUM ..........169 LB / CU FT.........0.O97986 LB / CU IN

      STEEL.................490 LB / CU FT.........0.28356 LB / CU. IN

      Comment


      • #18
        Originally posted by GNM109 View Post
        If two pins are the same dimension, (in my case, 3mm X 50mm) would not the steel one weigh more than the aluminum one regardless of price or shear value?

        MATERIAL..WEIGHT PER CUBIC FOOT..WEIGHT PER CUBIC INCH

        ALUMINUM ..........169 LB / CU FT.........0.O97986 LB / CU IN

        STEEL.................490 LB / CU FT.........0.28356 LB / CU. IN
        Well, we'd want both to be the same strength, so the steel one would probably be hollow, but overall I would expect the steel one to weigh more, but have less volume of material.

        Comment


        • #19
          When working with an existing arrangement, if one is not satisfied that the current pin will shear at a low enough value, then consider adding two grooves to reduce the pin diameter.

          The grooves are positioned along the length of the pin to match up with where the pin passes from the shaft to the collar with enough groove width to provide some tolerance for pin placement and enough groove depth to reduce the shear load to be more satisfactory.

          .

          Comment


          • #20
            Originally posted by EddyCurr View Post
            When working with an existing arrangement, if one is not satisfied that the current pin will shear at a low enough value, then consider adding two grooves to reduce the pin diameter.

            The grooves are positioned along the length of the pin to match up with where the pin passes from the shaft to the collar with enough groove width to provide some tolerance for pin placement and enough groove depth to reduce the shear load to be more satisfactory.

            .
            I guess that you could groove the pin whether it is made from aluminum or steel but why bother? In my case, the aluminum pins work perfectly well. Whenever I need one, I just take another piece of my lifetime stash of 1/8" aluminum welding rod and whittle out another one. I very seldom need them anymore since I have worked on my lathe technique but if I do, I just make another one.

            Comment


            • #21
              The groove would work to weaken a too-strong pin. But, it's other function is to define WHERE the pin breaks. Helps make sure the pin breaks where it doesn't drag a protrusion into the small clearance space and get jammed up. And gives some place for the metal to go if it DOES break less than cleanly.
              CNC machines only go through the motions

              Comment


              • #22
                This shows the 1/8" brass shearpin in both the lead screw for threading and the drive screw for table traverse on my 12 x 27 lathe. (and no, I didn't realize how dirty that acme thread was when I took the picture.)
                Brian Rupnow
                Design engineer
                Barrie, Ontario, Canada

                Comment


                • #23
                  A quick calculation gets a 6061 T6 pin of 1/8" providing about 11 ft-lb torque at it's breaking point, based on a half inch diameter rod, with pin in double shear. Seems like quite a bit, but there are a number of assumptions that make it maybe not useful for actual figuring, such as alloy type, diameter of rod, exact details of the interface between the rod and the outer sleeve (clearance, shape of edges, etc).

                  "Generic brass" is in the same area.

                  It would seem that a material which is ductile would be poorer, one would probably want a material with the yield and ultimate strengths to be pretty close, so it either is unaffected, or breaks. I can see problems escalating in case of a leadscrew shear pin made of a ductile material that might allow significant misalignment before it broke.
                  CNC machines only go through the motions

                  Comment


                  • #24
                    Originally posted by GNM109 View Post
                    ... but why bother? ...
                    It gives the OP an option for tuning his 'Weak Link' if he finds after this is built that the pin-breaking torque is higher than he is satisfied with.

                    .

                    Comment


                    • #25
                      There is no like button here, but I like this post.



                      Originally posted by Circlip View Post
                      And when the steel pin shears, there's a swiss file to clean the "smeared" joint faces of the drive.

                      Regards Ian.
                      Paul A.
                      SE Texas

                      And if you look REAL close at an analog signal,
                      You will find that it has discrete steps.

                      Comment


                      • #26
                        I would think that calculating the size of a shear pin would involve quite a lot of math. First, you need to know the weakest link, but apparently you do: the thrust bearing. At least, that WAS the weakest link. Did you beef it up? It may no longer be the weakest link.

                        But how do you calculate the force or torque needed to break that weakest link? You would need to do a structural analysis of that part. Not necessarily easy. Of course, you could always slowly apply increasing amounts of torque, measuring as you go, until something gives. It is called destructive testing: not my favorite method. Another way would be to measure the worst case operational stress or torque and then apply a factor for some level above that and use that for your shear pin calculations. At least that would not involve breaking anything. But measuring that operational force/torque could be a challenge.

                        One factor in your favor here is that most accidents involve the sudden application of force. You say you "crashed" your lathe. I assume that means that you ran something into a rotating part that absolutely and instantly stopped that rotation. So momentum was involved and that means that the force/torque applied at that instant was much higher than the normal operating force/torque. So you probably have a wide range between the operating point and the point where something broke. So a fairly good, "ballpark" estimate would probably serve your need. Estimate the operational force/torque and estimate the amount needed to break the weak link and shoot for half way between them.



                        Originally posted by Jim Hubbell View Post
                        Crashed my Atlas and broke the lead screw thrust bearing. Made a replacement and it is up and going. My thought is to build a new bearing assy using a shear pin. Don't know how to size it to be in safe shear/thrust load area.
                        Any ideas/info welcome.
                        Paul A.
                        SE Texas

                        And if you look REAL close at an analog signal,
                        You will find that it has discrete steps.

                        Comment


                        • #27
                          There are, out and about, as well as perhaps right here on the forum, a number of designs for a leadscrew bearing that will pop out non-destructively, and can be popped back easily with no parts to be replaced.

                          You might try searching for them, since that would appear to be better, and in the long term, safer and easier than trying to do a detailed analysis of the situation and deriving a breaking force at the shear pin. There is force multiplication, leadscrew friction, etc, etc to be taken into account, but Atlas did that, and designed a breakaway bearing.

                          As far as that force, with measurements of your broken original bearing, and data on Zamak, a decent estimate can be made of what the folks at Atlas thought was a good "not-to-exceed" force. Failing any other better information, I suspect that is good enough to determine the pop-out force needed.

                          In case you did not have a "breakaway" version, the same info gives you an estimate of the force THAT ONE took to break, and it presumably did not break anything else,, so the result is still good for your purposes.
                          Last edited by J Tiers; 09-28-2016, 04:54 PM.
                          CNC machines only go through the motions

                          Comment


                          • #28
                            Most "crashes"are running a poweredmachine member into something not movable.

                            Originally posted by Paul Alciatore View Post

                            One factor in your favor here is that most accidents involve the sudden application of force. You say you "crashed" your lathe. I assume that means that you ran something into a rotating part that absolutely and instantly stopped that rotation. So momentum was involved and that means that the force/torque applied at that instant was much higher than the normal operating force/torque. So you probably have a wide range between the operating point and the point where something broke. So a fairly good, "ballpark" estimate would probably serve your need. Estimate the operational force/torque and estimate the amount needed to break the weak link and shoot for half way between them.
                            Like a carriage into a head or tail stock. or even into the tightly clamped carriage stop. Once the moan of straining metal fills the air, That's a crash!
                            Heck, I've "crashed" into high end dial indicators and test indicators. They had nothing do do with a rotating spindle.

                            Crashing a power feed into something can certainly pull out or break the bearings.

                            Calculations that would encompass all possible conditions across the full range of speeds and gear ratios would be quite an undertaking, And I doubt one sacrificial element could provide both useful service and comprehensive protection for most machine tools.

                            Comment


                            • #29
                              CalM--I'm with you. There are just too many variables here to determine any kind of "calculated breaking point". ---Brian
                              Brian Rupnow
                              Design engineer
                              Barrie, Ontario, Canada

                              Comment


                              • #30
                                Disagree completely.

                                No reason to sit helplessly in this matter, hands folded, unable to define everything, and so unable to start any action.

                                **What is the threat?

                                Running the carriage into something under power.

                                **What does it do?

                                Potentially break gears, and halfnuts, etc, parts of the lathe in the feed that are expensive to replace.

                                **What about my extra delicate indicator? It might get broken and nothing else is damaged. I wanna protect it.

                                If you want to protect an indicator from crashes, pay attention. Don't expect something else to protect you against being careful.

                                The goal is to do a reasonable job of protecting major structure and mechanical parts from breakage or damage from standard sorts of accidents, the major one being power feed goofups. A pop-out bearing CAN be made to do a decent job of that. It's totally unrealistic to expect protection from every hazard, without some sort of double-check robotic vision system watching over you like a shop class instructor.

                                You have a choice, if you are concerned about this in the first place.

                                1) sit with hands folded and do nothing because you don't know how to protect against every possible problem.

                                2) Take action to provide protection against the typical power feed related crash breaking feed mechanism parts. Accept that there may be other bad things that occur as the chuck jaws beat your tooling, etc. That can't be handlled the same way, and you are free to develop some other idea for that, if you wish. The power feed safety is a decent idea if you are worried about it (I have never bothered about it, but others may)

                                For that, however, a carriage stop comes to mind, set to trip the power feed safety if the carriage is going to power into the chuck. It's not that hard to make the carriage stop easily movable and lockable. And it can be set to allow all the legitimate work you need to do close to the chuck, but keep the carriage out of the chuck's way, for any particular setup.

                                See? That wasn't too hard, was it?
                                CNC machines only go through the motions

                                Comment

                                Working...
                                X