Waste Container Handling Crane -- Why are we getting this lift offset?

[AlanThomas]

TL;DR Summary

When in operation the a hoist path of a waste container handling crane is not vertical - it suffers drift [non verticality] of up to 100mm.

I am dealing with a waste container handling crane. Waste container is 12000kg. It is lifted by a grapple by twist locks at each corner. Height of lift is 14m. The contents of the container leads to an offset c of g which cannot be avoided. The containers are to be stacked 6 high.

The crane hoist mechanism has two synchronised rope drums with ropes falling vertically and attached to grapple at each corner [ 4 point lift]. The grapple is 2000mm square in plan ha a mass of 2000kg. When in operation the hoist path is not vertical - it suffers drift [non verticality] of up to 100mm measured at lowest elevation.

The degree of drift cannot be explained by differential rope stretch due to offset c of g and grapple tilt. A pendulum action is suspected with the offset c of g aligning with the virtual centre of lift. Confirmation of pendulum action gratefully received.

 

[berkeman]

Welcome to PF.

Can you post pictures of the operation? Use the "Attach files" link below the Edit window. Thanks.

 

[DaveC426913]

Any chance the ropes (cables?) have a twist in them that's generating a small torque? You wouldn't be able to tell unless you took all tension off them and see what they want to do when not under load.

 

[AlanThomas]

The 6 machines in service [which show 'drift'] have regular proof tests and rope examinations. Torque from ropes is minimise by left hand and right hand lays. Tension meters are also used to ensure equal loading and grapple level . The Techs are very skilled at rope replacement / adjustment and rope twist has not been revealed.

No photos available - sketch attached showing balanced and unbalanced configurations. 4 more machines are in the pipe line and the same problem is anticipated. The crane makers say drift does not occur [apart from that caused by differential rope stretch] so long as the offset is inside the support points. Commissioning tests for those in service are in conflict with this and some other physical cause is present. [pendulum effect ?]

 

[jack action]

Assuming the tension is the same in both ropes and the container doesn't rotate, no matter how I look at it (mostly a Free Body Diagram), I end up with this equation:
$$\tan \theta = \frac{Y_r - Y_l}{2L}$$
Where:

• $\theta$ is the rope angle (+ve as shown in your drawing);
• $Y_r$ and $Y_l$ are the horizontal distances from the CG to the right and left ropes' attaching points respectively;
• $L$ is the vertical distance from the CG to the ropes' attaching points.

This should be constant, no matter the weight or the acceleration.

If $Y_r = Y_l$ then $\theta = 0$.
If $Y_r < Y_l$ - as shown in your drawing - the angle $\theta$ would be negative, thus the offset would be in the opposite direction.

 

[AlanThomas]

Thank you
Witl MathCAd it up and review result.

 

[Lnewqban]

Welcome, Alan!

Confirmation of pendulum action.
The center of mass of the container will always move to a horizontal position as close to half the distance between the tops of the wire-ropes as those allow it to be.

To counter-act that lateral off-set, you either must push or pull the container sideways or to add weight to bring the CM to the midpoint of the container.

 

[AlanThomas]

Thank you for reply

 

[Nik_2213]

IIRC, similar issues with garage / work-shop 'engine' hoists are addressed using a side-shift 'leveller' beam.

 

[Tom.G]

And what happens if you slightly shorten or lengthen the rope on one end if the load is off-center?

 

[tech99]

Could this be a measurement error, as 100mm is quite small.

 

[Nik_2213]

Is this ~100mm offset more than stacking lugs' capture radius ? Given the crane / gantry 'knows' where it is to a few mm,, the offset must be apparent as soon as lift begins...

I take it there's a sudden, obvious side-shift: Must be disconcerting !!

 

[tech99]

Is it a measurement error caused by deflection of the support structure?

 

[tech99]

Welcome, Alan!

Confirmation of pendulum action.
The center of mass of the container will always move to a horizontal position as close to half the distance between the tops of the wire-ropes as those allow it to be.

To counter-act that lateral off-set, you either must push or pull the container sideways or to add weight to bring the CM to the midpoint of the container.

If the centre of mass is off=centre, that does not create a horizontal force. However, if one of the ropes is not vertical, there will be a horizontal force. Then the load will move horizontally until the centre of mass lies beneath the intersection of the projected lines of the lifting ropes.