Rigging forces

[stewmo]

Am I right to say that the greatest force when rigging with a pulley and friction device will be at the pulley attachment?

 

Thanks

[Pete Mctree]

Yes - it will be (theoretically) twice the load applied.

[jfc]

Yes - it will be (theoretically) twice the load applied.

Two times loading only applies to a static load. When pieces are freefalling (before the slack is taken up) the forces multiply significantly. Up to 12 times the weight of the piece, although up to about 6 times is more the norm. With a good groundie letting stuff run and slowly decelerate that greatly lessens shock loading the system.

If you have a 100kg piece, rigged from below, with 1 to 2m slack, stopped dead you could create a force equivalent to 600kg on each side of the pulley, meaning 1200kg at the rigging point.[emoji15]

Something to think about.

J.

[Pete Mctree]

15 minutes ago, jfc said:

Two times loading only applies to a static load. When pieces are freefalling (before the slack is taken up) the forces multiply significantly. Up to 12 times the weight of the piece, although up to about 6 times is more the norm. With a good groundie letting stuff run and slowly decelerate that greatly lessens shock loading the system.

If you have a 100kg piece, rigged from below, with 1 to 2m slack, stopped dead you could create a force equivalent to 600kg on each side of the pulley, meaning 1200kg at the rigging point.

Something to think about.

J.

I know the math and have destroyed rigging & rigging points. Well explained 

[daltontrees]

The 'twice the load' idea in most rigging situations is a myth. The pulley in rigging is being used for a change in direction, not to create a mechanical advantage.

 

That is the case with static and dynamic situations.

[jfc]

8 hours ago, Pete Mctree said:

I know the math and have destroyed rigging & rigging points. Well explained 

Hi Pete, I was just expanding on your answer for the OP.

[jfc]

The 'twice the load' idea in most rigging situations is a myth. The pulley in rigging is being used for a change in direction, not to create a mechanical advantage.
 
That is the case with static and dynamic situations.

Sorry jules, you are totally wrong on this, very unusual for you to make a wildly inaccurate statement.

I decided to do a we demo.

Photo 1, drill on weighing scales. Approx 2kg.
Photo 2, drill on pulley with rope, approx 4kg.

To hold the drill steady, you have to apply a 2kg downward force on the other side to oppose the 2kg drill, total 4kg.

RIGGING POINTS EXPERIENCE DOUBLE LOADING VIA PULLEY.

J.

[Dan Maynard]

The 'twice the load' idea in most rigging situations is a myth. The pulley in rigging is being used for a change in direction, not to create a mechanical advantage.
 
That is the case with static and dynamic situations.

Got me confused with that one.

I'd say most rigging situations have two ropes leading down from the pulley, close to straight up and down? At least once the piece is getting closer to the floor.

[Mr. Squirrel]

8 hours ago, daltontrees said:

The 'twice the load' idea in most rigging situations is a myth. The pulley in rigging is being used for a change in direction, not to create a mechanical advantage.

 

That is the case with static and dynamic situations.

I mean you're right in a sense, in that peak forces at the anchor point will likely be significantly higher than twice the load. 

[stewmo]

Thanks love an Arbtalk debate