Forces on Rotating Member
Forces on Rotating Member
(OP)
As part of a valve analysis I am looking at a swinging disc arm. I have not done an analysis like this for a while so I am using a previous calc (not done by me) as a guide. The portion I am looking at right now considers the bending moment acting on the arm as a function of distance from the pivot caused by the inertia of the arm (it's a pretty big valve). I have a number of reactions while looking at this. First, the original calculator (the person, not the program) defines the bending moment at X as "the moment caused by that portion of the arm to the right of x" (where the pivot is to the left of x). At first this makes sense because if you are considering inertia the more of the bulk of the arm involved the higher it should be. But then, this also means that at X=0 the moment is highest which seems counter-intuitive. This is complicated further (at least in my head) by the fact that the pivot is at the lower corner of the arm as opposed to being located along the central axis. So my questions is this:
How do I consider inertial moment as a function of distance from the pivot when the pivot is offset from the members central axis?
Please explain what this means physically as it is somehow eluding me.
How do I consider inertial moment as a function of distance from the pivot when the pivot is offset from the members central axis?
Please explain what this means physically as it is somehow eluding me.





RE: Forces on Rotating Member
Might I suggest you post a picture, you know clearly in your mind what your describing but unfortunately we don't.
desertfox
RE: Forces on Rotating Member
For example, a torque is delivered to the arm in order to close it in t seconds and you need to know the value of that torque and perhaps the stresses induced in the member.
Absent the conditions and requirements, we can't comment on the solution as presented.
RE: Forces on Rotating Member
RE: Forces on Rotating Member
Now, the original calculation you mention is essentially correct in that you sum the induced moments to the right of the x point The offset center will effectively cause the inertial loading to be oblique to the beam centerline so the shear diagram would have a sine@ factor at each segment along the beam and the orthogonal cos@ factor would cause a compressive stress in the beam.
So, if you do this classically, develop a loading diagram normal to the beam which is the inertial forces on each segment multiplied by the sine term from which you get the flexural stress
MC/I.
To this add the compressive component.
You also have the centrifugal component which will add tension in the beam, which should also be small.
RE: Forces on Rotating Member
If its a valve part your analsysing isn't there more to it than an arm falling under gravity?
I can see that stresses could be induced in the arm but only if it was closed under some resistance, I can't imagine a huge valve being allowed to slam shut under gravity can you please expand on the problem some more.
desertfox
RE: Forces on Rotating Member
RE: Forces on Rotating Member
I am glad you have resolved your problem through thought. Indeed, it is said Nikola Tesla never began work on a project until he had every aspect fully considered and balanced. Wise words if true.