Centre of gravity

A freely suspended load swings until its centre of gravity lies vertically below the suspension point. That is not a matter of design but of physics. If the load is slung off-centre, it tilts into this position when lifted – slowly at first, then with a jerk as soon as it is clear.

The first rule of slinging follows: the hook belongs above the centre of gravity, not above the middle of the part. On symmetrical components these are the same. On a geared motor, a housing with attachments or a container filled on one side they are not.

The CAD model gives it most accurately, provided materials and attached parts are fully defined. Without a model it can be weighed: the part is supported at two points a distance L apart and the reaction force is measured at both. The centre of gravity then lies at a distance x = F2 · L / (F1 + F2) from the first support.

In the workshop a trial lift is often enough: raise the part a few centimetres, watch which way it tilts, set it down, move the sling point. For series operation this is not a procedure but the way to a fixed sling point.

A manipulator with a rigid arm does not let the load swing out. If the centre of gravity lies beside the gripper axis, a moment arises from weight times offset, which gripper, rotary unit and arm have to take. An 80 kg part with a 0.15 m offset produces about 118 Nm.

This moment has to be known when sizing. It determines whether a gripper holds, whether a rotary axis can be moved by hand and whether the part swings round when a brake is released.

Some loads change their centre of gravity.

For changing centres of gravity there are adjustable suspension points on the spreader beam that can be moved over the centre of gravity by hand or by motor. Several sling points share the load, but not equally: the leg closer to the centre of gravity carries more. For turning, it helps to run the axis of rotation through the centre of gravity – the part then stays put in any position instead of swinging round.