Crane Load Problem: Understanding Tension Torque and Beam Bending

[MotoPayton]

Can someone explain to me why the torque due to tension is taking from the roller instead of at the end of the beam.

If you were to take it from the end of the beam you would also need to take the torque created by the bending of the cable. This is obviously more complicated.

It just doesn't seem like the way they did it should work.

 

[NascentOxygen]

I can't figure out what is at B. Is the cable attached at C and slides over (above) a peg at B? Or is the cable attached at B, and a separate line connects B to C?

I think the cable might be fixed to the wall at A, and connected by a tensioned cable of fixed length to B. There is a separate lifting cable running from B to C that can be reeled in by a motor at B. That seems how it is analyzed, anyway.

 

[MotoPayton]

It is the same cable and that is a roller at B.

 

[NascentOxygen]

Then it looks like the extra force acting on the beam is a not-quite-vertical force at B, its line of action being midway through angle ABC. But I'm no expert.

Are there online resources where you can analyse beams and forces, as a check?

 

[DeeNos]

I can explain the reasoning behind taking the torque due to tension from the roller instead of the end of the beam.

Firstly, it is important to understand that in a crane, the roller is the point of rotation or pivot for the beam. This means that the beam is free to rotate around the roller, and the tension in the cable creates a torque that causes this rotation.

Now, let's consider the torque at the end of the beam. While it is true that the tension in the cable also creates a torque at the end of the beam, this torque is in the opposite direction of the torque at the roller. This means that the two torques cancel each other out and do not contribute to the overall torque on the beam.

On the other hand, taking the torque from the roller accounts for the full force and direction of the tension in the cable. This is because the roller is the point where the cable is actually pulling on the beam, and thus where the torque is being applied.

Additionally, as you mentioned, taking the torque from the end of the beam would also require considering the torque created by the bending of the cable. This adds an extra level of complexity and may not be necessary for the overall analysis of the crane load problem.

In conclusion, taking the torque due to tension from the roller instead of the end of the beam is a simplified and accurate approach for understanding the forces and torque acting on the crane.