Interpretation of the reaction forces

ddraht

Hallo,

I have an assembly with a total weight of 110 kg.
The assembly is fixed at the 4 holes. The reaction force should be approx. 6930 N (F=m*a) but is 70554 N
What have I done wrong?





Many thanks for your support

Dieter

Victor

This is the sum of the magnitudes, rather than the magnitude of the sum. I realize that's probably not what anyone would want and it recently tripped up someone else. You have to calculate the magnitude of the sum by hand, unfortunately.

ddraht

Thanks Victor for the clarification.

disla

There is another source of discrepancy when looking at the "reaction forces".
When the applied force is distributed (like gravity), part of the force ends up at the constrained nodes and it is deduced when evaluated.

Reaction force = Sum of all External forces except those acting on the measuring point as it cancels.

In the attached example, a 110Kg Cube is under a Gravity Load of 10m/s2

If one request the Sum of External forces in Y direction one would expect to get:

Action:110Kg*10m/s2= 1100N
Reaction: -1100N

But we get -962.5N.



There are -137.5N missing that corresponds to 1/2 the weight of the elements where one is measuring the Reaction.

27.5 Kg * 10 m/s2 / 2 =137.5 N



ccx manual remarks this effect. The smallest the mesh around the BC area, the smaller error.

cwharpe

(sorry, I am posting late...)

This has come up before, and I have it noted in my Caution file: Summing reaction force magnitudes generally does NOT equal a System's Resultant (unless [rarely] all node resultants point the same direction).

That particular output vector [ SUM reaction force magnitude ] has ambiguous meaning to me, as all directional sense is lost in the nodal magnitude calculations. However, you may confidently rely on the other output vectors [SUM reaction force x,y,z] to inform the appropriate calculation:

"...The tried and true way to find a System Resultant Fr for multiple nodes is first find Sum_Fx, Sum_Fy, Sum_Fz. Then calculate the (scalar) Fr = sqrt(Sum_Fx^2+ Sum_Fy^2+Sum_Fz^2). (Resultant direction is calculated using direction cosines.)..."

Mishal

Random question, is that a pan feeder or something similar?