No announcement yet.

Hoop Stress Question

  • Filter
  • Time
  • Show
Clear All
new posts

  • #16
    Presuming this is a repetitive/cyclic stress, I'd be designing for a max strain of < 0.5%. High tensile steels have a tensile strength of up to 60,000 PSI, IIRC.
    Just got my head together
    now my body's falling apart


    • #17
      The most important consideration in this instance will be air drag if a rod or other non aerodynamic shape is used. For a flywheel that size if it isn't aerodynamic the losses due to churning will be the limiting factor. To even approach the tensile limit of ordinary materials the overall shape will have to be an unbroken smooth shape of some sort. Have a look at high speed lab centrifuges for design hints.

      Free software for calculating bolt circles and similar: Click Here


      • #18

        My "gut" feeling is much the same as Evan's.

        It will require very high orders of dynamic balance as well as design for minimising "stress-raisers".

        I was going to suggest using an "on-Campus??" or "in-house" centrifuge with equal loads equally applied and dynamically balanced.

        Perhaps some of the aero-dynamics would be over-come if it were "spun-up" in a vacuum.

        As hinted in a previous post, it would have been preferable if the project/subject was identified as a personal/educational/"for work" project.


        • #19
          My, what a lot of singing and dancing.

          The acceleration of a point rotating at a speed W radians/sec at a radius R meters from some axis is a=R(W**2), a in meters/sec squared, naturally.


          • #20
            "hoop stress" -- if I understand you correctly -- doesn't
            really apply here so looking up hoop stress formulas will
            lead you down the wrong path.

            if you're only interested in rod failure (which really is going
            to be the least of your problems!) it doesn't really matter
            to the rod that it is spinning. all that spinning does is
            set up an acceleration and a force that will try to stretch
            your rod (look up 'centrifugal' force) -- use this number
            in one of the basic stress/strain formulas. be sure to add
            the weight of the electronic package, connectors, wires,

            again, all you are doing here is (from the rods point of view)
            is stretching it.

            remember that physics problem where you had a ball tied
            to a string and you spun it over your head as fast as you
            can and were asked how fast till the string broke? same

            this is assuming that you spin it up to speed slowly.. ie
            you don't need to get from 0 to 2000 rpms in a 1/10th
            of a second. if that were the case you'll need to worry
            about bending in the rods.

            the real problem is going to be that hub (the welds) and
            the bearing assembly.

            that and keeping your wires from getting all twisted up

            the rest of the problem is going to be decided by your
            electronics and the purpose of your experiment -- how
            "balanced" is "balanced"?

            Evan's cross-section is correct from a stress point of
            view -- but not very good for a flywheel (since the point
            is to store energy) -- again, since you're not trying
            to store energy, "flywheel equations" will also lead you
            down the wrong path.

            your best bet, if all you are looking for is tangential speeds,
            is to use something really lightweight -- maybe even an
            aluminum disk -- and have it dynamically balanced before
            you mount it (bolted!) to your hub. drill holes in the plate
            to balance it or add through bolts on the opposite side of
            your package. the disc wouldn't need to be turned to a
            perfect circle -- as long as it is properly balanced.

            the enemy here is vibration & what it'll do to your machine.

            wow, that was long-winded.



            • #21

              18" diameter and 2.000rpm doesn't sound like a problem. We're running turbine-driven centrifugal gas compressors here offshore. The rotors are about 2 feet in diameter and they run at 16,000rpm day in, day out. These are industrial machines, not especially high-tech.

              The rotors look a bit like ventilated brake discs and weigh something like 100Kg each. Never heard of one failing.


              All of the gear, no idea...


              • #22
                Oh, on the vibration issue; the rotors are dynamically balanced by the vendors, but then we get hold of them :-)

                In service, deposits build up in the gas passages (we're compressing wet natural gas, mostly methane). We inject various chemicals to stop this, but buildup still happens. Every so often, a bit of foulant breaks off and we see vibration. If left unattended, this tends to wear the floating pad capsule bearings to the point where we have to fit a rebuilt bearing (at $10k a pop). That sorts most of the vibration out. Every so often, we change the entire compressor stage out for a factory rebuilt one.

                The rotors are made of some kind of stainless steel.

                All of the gear, no idea...


                • #23
                  So Fasttrack,

                  This is what you are doing?

                  I assume that you are trying to demonstrate special relativity by skewing the frequency of an oscillator!

                  The hint for the stress distribution is to remember that the formula for centripetal force as mentioned by Rantbot is mw^2*r. (I cheated and looked it up on the web as my employment of late usually involves rotating images rather than masses). This is only good for a particle of a given mass on a string.

                  So, being the good engineers we are, we integrate. So first we must find mass in terms of r.

                  A*rho*dr should be the mass at r. Where phi is the density and A is the cross sectional area. This should be sufficient as long as the shape is symmetric.

                  Thus F=integral(A*rho*r*w^2*dr) from 0 to r. which should mean F=A*rho*r^2*w^2 /2.0 for one half of the assembly.

                  Total Force should then be A*rho*r^2w^2; Stress however is force divided by area so rho*r^2*w^2 appears to be the stress for a rotating object that is of fixed cross section.

                  I had a few too many last night so I may have made an arithmetic mistake but considering what you appear to be up to, I suspect that you can derive the correct stress. I got N/m^2 as the unit of the above so it's not insane but check the equation before using it. . .

                  At any rate, have fun and good luck with whatever you are actually doing. P.S. What degree are you studying



                  • #24
                    Smaller diameter, higher revs

                    What about going to 18" diameter and 4000 rpm, or 12" diameter and 6000 rpm? At 12" diameter a machined disc becomes more feasible, or maybe use an automotive flywheel and skip some machining. Plus you know a modern flywheel is good for 6000 rpm, no worries at all.

                    Set up the flywheel on the end of a 1 1/2" dia countershaft, select 2 V belt pulleys to provide a countershaft speed of 6000 rpm from the electric motor you had in mind.

                    Switch on from a safe distance and see what happens.


                    • #25
                      You guys are way over my head with all your high level cipherin' & guzintas (as Jethro Bodine would say) but bob ward's solution looks to be the easiest & safest solution. A good used one at the junkyard should be next to nothing. Just don't mention that it's going in a college engineering project....that'd drive the price way up there.

                      Also, isn't it coincidental how the optimum flywheel cross-section Evan posted mirrors the shape of a "NACA duct." Strange things at work here I tell ya!

                      Originally posted by Evan

                      "Accuracy is the sum total of your compensating mistakes."

                      "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton


                      • #26
                        just duct tape whatever your testing to the chuck of a large CNC lathe.
                        I kid. I kid.


                        • #27
                          Originally posted by DICKEYBIRD
                          Also, isn't it coincidental how the optimum flywheel cross-section Evan posted mirrors the shape of a "NACA duct." Strange things at work here I tell ya!

                          pretty observant of you DB, I just got a blast of that kinda music you hear when they show extraterrestrials in movies and crap... (also very similar to the tunes they play when you die and your body's going to heaven)


                          • #28
                            Flywheels have no more to do with the problem than hoop stress does. Flywheels are designed to store energy, test packages held on the ends of sticks are not. Entirely different problems with entirely different solutions.

                            The easiest way to generate high G loads on a test object is to drop it on the floor. The subsequent load is, of course, not continuous. For a continuous load a centrifuge of some sort is needed. In-between would be something like a rail gun.


                            • #29
                              Fasttrack wrote:
                              4) I don't know for sure what speed I need, or rather I have some wiggle room. The faster the better. The minimum speed is 36,000 inches per minute. This an 18" radius wheel spinning at 2000 rpm and the package on the outside.
                              Rantbot wrote:

                              Flywheels have no more to do with the problem than hoop stress does. Flywheels are designed to store energy, test packages held on the ends of sticks are not. Entirely different problems with entirely different solutions.
                              It appears from what Fasttrack has written a flywheel is indeed what is needed. The fact that it isn't intended to store energy is irrelevant. It will just as the medical centrifuge does.

                              I agree that the use of an automotive flywheel is likely the best approach. However, to be safe I would have it spin tested to at least 50% greater rpms than it will operate at in the experiment. I would also provide a scatter shield and would operate it in the vertical alignment to reduce the hazard. Finding bearings able to withstand a continuous 4000 to 6000 rpm will also be a challenge as they will develop a significant amount of heat in the size needed.
                              Free software for calculating bolt circles and similar: Click Here


                              • #30
                                I don't have much to add except most of the above is spot on.

                                I was just thinking I shifted my old 327V8 in my 55 chevy once at 7200 rpm.

                                I looked up the diameter of the flywheel... 14"

                                pi x D=44"

                                7200 x 44=316,672 in per min

                                You only need 36,000 ipm?

                                No worries! (unless I screwed up my figurin)