Torque when rotating mass stops
Torque when rotating mass stops
(OP)
I'm trying to calculate the torque on a structure when a rotating mass comes to a sudden stop. I know the moment of inertia and angular speed so I can get the energy involved. I've been going through old books trying to get a relationship between that energy and the torque back on the structure, or energy and the force on impact. The velocity is constant so acceleration (alpha) =0. I don't know what the distance after impact is. Is there some equation that can be used to estimate this force? Thanks in advance!





RE: Torque when rotating mass stops
If acceleration is 0 it isn't going to stop.
RE: Torque when rotating mass stops
RE: Torque when rotating mass stops
We need more information, you can use conservation of angular momentum but without knowing masses,dimensions,or whether the impact is elastic or inelastic your questions not to bear very good answers.
desertfox
RE: Torque when rotating mass stops
Delta-Torque = (MassMomentInertia) X (Delta-Alpha)
Delta-Alpha = (Delta-RotationalVelocity) / (Delta-Time)
TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering
www.bluetechnik.com
RE: Torque when rotating mass stops
I think there is a linear force tangential to the point of contact.
The equivalent translating mass is I/R2
Stopping takes some about of time, so there is an acceleration.
F=Ma
RE: Torque when rotating mass stops
RE: Torque when rotating mass stops
What is the mechanism that makes your mass stop?
If you are applying a brake, you should be able to estimate the decelleration and get your forces from that.
If some piece of your rotating mass comes into contact with your fixed structure, then you can use strain energy methods to work out the forces. The forces will of course be reduced to zero if some critical part of your structure is destroyed instantly.
RE: Torque when rotating mass stops
You're not going to get this from a simple calculation.
RE: Torque when rotating mass stops
RE: Torque when rotating mass stops
Next get the moment of inertia of the fan,J
Now if the fan speed is w in radians/sec
and worst case the fan strikes an immovable object, then all of the rotational kinetic energy goes into deflecting a single fan blade assembly (and possibly breaking it).
The energy absorbed post collision is:
1/2k*r^2*@mmax^2
The k.e is
1/2*J*w^2
Equating the two
1/2*J*w^2=1/2k*r^2*@mmax^2
solving for the max force, k*r*@max,
Fmax=k*r*@max=w*sqrt(J*k)
3@max
Fmax=k*@max=J*w^2/r
RE: Torque when rotating mass stops
If you are shoving an object into your fan blades, you are going to have to make assumptions about how stuff will react to the impact. Any plastic deformation or shearing of the blades or object, will affect your deceleration. Is the fan mounted on bearings on a spider at or near the middle of the hub, or is it on a shaft extended from a base? What sort of objects are you going to shove into the fan?
Once you make your assumptions, you should be doing some sort of strain energy.
Mythbusters did an episode on ceiling fans beheading people. They did not seem to be concerned about the reaction forces at the base.
RE: Torque when rotating mass stops
Fe