# Impact force from rotating object?

1. Jul 7, 2017

### GuyTom

Hi,

Please could someone help me with the following, I have a rotating cutter and I need to calculate the impact force on an inspection window if one of the cutter blades was to seperate from the main assembly.

The cutter is spinning at 3000rpm
The cutter radius is 115mm (rotating radius)
The distance from the cutter position and the inspection window is 135mm
The weight of the cutter section coming away from the main assembly is 0.240kg

I have calculated that the linear speed of the cutter blade would be approx. 36m/s once it breaks off........

Any help is gratefully recieved.

2. Jul 7, 2017

### person123

You might have made an error when finding the linear velocity. I assume you multiplied the angular velocity by radius; the problem is the angular velocity is in rotations per minute and not radians per minute.

3. Jul 7, 2017

### GuyTom

thats corrct I did multiply by 314 radian/ sec rather than 3000rpm, its just initially when i did the calc (0.115m radius x 3000rpm) the answer of 345m/s seemed high to me.

Also to get a force from this I need an acceleration (F = M x A) and this gives me only the linear speed. How do I acheive the force?

Thanks again for any help.

4. Jul 7, 2017

### person123

In this case, the acceleration would be how rapidly the blade gets slowed down when striking the window. Unfortunately, this is where I can no longer help you, so you're going to have to wait for somebody who knows about the material properties.

5. Jul 7, 2017

### GuyTom

No problem, you have been a great help. thank you.

6. Jul 7, 2017

7. Jul 7, 2017

### tygerdawg

A couple of approaches to consider. Unfortunately not really my area so I can't remember all of the details about momentum & impact. A good mechanical engineering dynamicist could help.

Kinetic Energy KE = (1/2) * (mass) * (velocity)^^2
Material must be able to survive point energy > than KE.
Change in momentum is significant here.
Coefficient of restitution (elastic nature of object-target materials interaction & "bounce effects") is significant here.
Or...conduct a series of simple impact experiments.

F = M * A.
A ~ (change in velocity) / (change in time).
(change in velocity) = your max calculated velocity - 0 (ending velocity).
(change in time) = some reasonable estimate of time for the object to decelerate to 0 velocity.

Once you have force F, then you can estimate stress applied to the material.
Stress = F / (impact area)
(impact area) gets very tricky & subjective:
• object impacts window in which attitude & orientation? Impacts on face of max area? Impacts on corner or point of minimum area?
• at what angle?
• impacts along an edge or line?
With estimated applied stress, can compare against tensile & yield stress of the window material.