This has long been one of my favorite topics. Strong side, weak side, tensile, compressive. I've asked the same question about this each and every time it has arisen here; in a spinefinder like the one Colin Dick designed, is the hard spine on top or on the bottom of the shaft? There have been many reasoned responses to the question, mostly equally divided. Some say top, some say bottom. I say, it doesn't matter. That then starts another round of discussion. Things are kind of slow over the holidays so what the hell, start again.
Wishing all a prosperous and happy new year, I remain, your humble curmudgeon, TFlan ----- Original Message ----- From: "Alan Brooks" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Saturday, December 28, 2002 2:18 PM Subject: Re: ShopTalk: shaft flex v.s. frequency > In an attempt to understand where the 'weak and strong' sides of a shaft > concept came from it occurs to me that one of the problems with > understanding this concept is that it is easy to visualize a shaft that is > stronger on one side than the other; a thicker wall on one side in a steel > shaft, or more fibers on one side in a composite shaft. This will, indeed, > result in a shaft that is 'stronger' on that side - to a tensile (or > compressive) load applied parallel to the axis of the shaft! Assuming the > shaft remains straight the strain and stresses in the shaft material will > be the same through a cross section but, because of the greater cross > sectional area on the 'thick' side of the shaft more of the reaction force > to the axial shaft load will be carried on that side of the shaft, so, in a > sense, it is 'stronger'. In a bending situation, however, because of the > redistribution of stresses that occurs in the shaft to balance the forces > on either side of the neutral axis, this does not result in a shaft being > stiffer in one direction than in the opposite direction. In a given > bending plane the shaft has the same stiffness in both directions. The > 'neutral axis' is defined, by the way, as the line of zero stress through > the cross section of a shaft under bending load and is not always at the > geometric center of the shaft. The stresses in the material on one side of > the neutral axis are compressive and tensile on the other for bending in > one direction and then reverse for bending in the other, but the neutral > axis remains in the same location, hence the resistance to bending > (stiffness) is the same. I hope this helps. > > Regards, > > Alan > > > > At 04:58 PM 12/26/02 -0500, you wrote: > >At 04:32 PM 12/26/02 -0500, Al Taylor wrote: > >>I'm impressed. I have no clue if you answered my question, but I was > >>impressed. John, you still there? ;-) > >>Al > > > >OK, John and Alan and I all said, counterintuitive as it may seem, yes it > >bends exactly the same TOWARD and AWAY FROM the spine, as long as it's in > >the same plane. > > > >You could weld a small steel rod to the shaft, to give it as stiff a spine > >as you want. It will still have exactly the same stiffness in BOTH > >DIRECTIONS in the same plane. > > > >Twirling it in a spine finder might or might not say that. But measuring > >the REAL stiffness will. I have posted here how to measure true stiffness, > >several times over the past week. > > > >Hope this answers it. > >DaveT > >
