Torque when rotating mass stops

[cjholl0]

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!

 

[MintJulep]

when a rotating mass comes to a sudden stop

The velocity is constant so acceleration (alpha) =0

If acceleration is 0 it isn't going to stop.

 

[cjholl0]

I mean the object is rotating at a constant speed and then it suddenly strikes an object. Think maybe a gear or fan where the tooth/blade hits something massive that doesn't move.

 

[desertfox]

Hi cjholl0

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

 

[tygerdawg]

Better ask for a refund on your Dynamics class.

Delta-Torque = (MassMomentInertia) X (Delta-Alpha)

Delta-Alpha = (Delta-RotationalVelocity) / (Delta-Time)

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering

http://www.bluetechnik.com

 

[MintJulep]

If I think about a fan blade hitting something I think there is no torque reaction.

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

 

[BobM3]

You'll have to know something about the springness of the collision. If you assume that all the energy from the rotating mass goes into potential energy of the compressed springs in the system and you assume constant deceleration then you can calculate the force on the spring.

 

[drawoh]

cjholl0,

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.

JHG

 

[TheTick]

Nothing is instantaneous. Force = dI/dt. Your time to stop is small but not zero.

You're not going to get this from a simple calculation.

 

[cjholl0]

The object is a fan rotating at a constant, max speed. I'm concerned about the reaction at the fan support if the blades were to crash into an object. I know in reality nothing is instantaneous but I also don't have a way to measure the time from impact to actual stoppage(it's very short). I was kind of thinking I wouldn't be able to do this via a simple calculation but I wanted to make sure I wasn't missing something.

 

[zekeman]

Get a hold of a fan blade and get its spring constant k empirically,
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

 

[drawoh]

cjholl0,

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.

JHG

 

[FeX32]

"sudden stop" dt=0, T=infinity.


Fe