Pulley forces on truss crane problem.

[hollidca]

Hi for my design project at uni I am to design a truss crane to a certain spec.
I can easily calculate the forces in a truss without the pulley systems.
However when I have the pulleys I'm not sure how this affects the forces in the truss.
I have the load attached via a 4 cable pulley at the end of the beam of the crane, i understand the force here is still the load * gravity straight downwards. the lifting cable from this pulley goes to a pulley at the joint of the beam and central column, the cable then goes straight to the ground and attached to the cable drum. Is there a 45 degree force from the tension in the cable on the pulley attached at the joint of the beam and column?

Thank you for any help.

 

[Unrest]

Is there a 45 degree force from the tension in the cable on the pulley attached at the joint of the beam and column?

Consider the reaction forces that must be required to oppose the tensions in the cables. Essentially, do a free body diagram of the pulley with a bit of cable on it. Then it becomes obvious.

 

[hollidca]

Okay I think i get what your saying. Consider a 10N Load pulling to the right around the pulley, and the opposite 10N pulling down. Leaving a resultant at 45 degrees from the horizontal and vertical of 10 root 2. This would need an equal and opposite force? Meaning that the beam that the pulley is attached to would take this load?
In which case do I take it into account on the force analysis of the side truss, or do I take it into account on the transverse beam it is held by and the joints connecting it to the truss?

Sorry if this makes no sense.

 

[Unrest]

the right around the pulley, and the opposite 10N pulling down. Leaving a resultant at 45 degrees from the horizontal and vertical of 10 root 2. This would need an equal and opposite force? Meaning that the beam that the pulley is attached to would take this load?

Yep

In which case do I take it into account on the force analysis of the side truss, or do I take it into account on the transverse beam it is held by and the joints connecting it to the truss?

Whichever parts have the load transmitted through them, so it sounds like both tho I'm not quite sure what part is what. You can consider this 45deg down-right force to be an external force applied to the pulley's shaft.

 

[hollidca]

Okay, thank you. I think i have sorted it out in my head. Now to do it on paper... The joys of force analysis.

 

[Studiot]

Sounds like a tower crane from your description.

Pulley bearings have horizontal and vertical reactions, but no moments, like any other hinge.

The angle will only be 45 if you assume zero friction. Of course, friction is how the windlass works and accounts for the differentce in tensions on either side of any pulley.

You should also note that any real world design should account for 'snatch' forces which are impulsive forces approximately double the static load values.
Many a crane has been broken due to the operator snatching a load.

go well

 

[hollidca]

Yes, It's a tower crane.

We are assuming zero friction and also that there are no impact loads such as wind, snatching, or just impact of another object.

Typical undergraduate course lol, always assuming things are insignificant when they are the most significant.