Static Vs Dynamic Force Analysis in Mechanism

[tetris]

Homework Statement

When should static or dynamic force analysis be applied when analyzing moving mechanisms ?

Relevant Equations

N/A

My understanding is that :
Static Force Analysis

• carried out on objects in static or dynamic equilibrium, that is rigid bodies that are not moving or moving with zero acceleration
• therefore, does not take into account the inertia of the object

Dynamic Force Analysis

• carried out on objects in motion with an acceleration
• therefore, takes into account the inertia of the object (along with the angular velocity and angular acceleration)

However, I have seen both static and dynamic force analysis applied to a moving four bar linkage. In the case where I would like to find the force applied to the coupler to move the mechanism from position A to B, would dynamic force analysis be used? Since, the velocity would not be constant, hence, it has an acceleration.

Any help is appreciated, thanks in advance.

 

[Lnewqban]

Welcome!
It depends on what force is greater.
We are interested on how robust the joint should be to resist those forces.
We can have a light link accelerating much and rapidly, like the connecting rod of a small engine: dynamic forces are more important in that case.

 

[tetris]

Firstly, thank you for your help

In summary, it depends on the speed of the mechanism. So, for the same mechanism, if one is moving at very high speed, then dynamic analysis would be used. On the other hand, if it is moving at low speed, static analysis would be sufficient since the inertia effects are small.

However, is there a velocity or any other value threshold where dynamic analysis should be applied?

 

[erobz]

Firstly, thank you for your help

In summary, it depends on the speed of the mechanism. So, for the same mechanism, if one is moving at very high speed, then dynamic analysis would be used. On the other hand, if it is moving at low speed, static analysis would be sufficient since the inertia effects are small.

However, is there a velocity or any other value threshold where dynamic analysis should be applied?

It depends on the accelerations of the mechanism components. There could be a mechanism with a low output velocity, but that experiences large accelerations in its components. Accelerations and mass are what generate force. It really should be taken on a case by case basis.