WORKING SAFELY

Slinging Techniques – Centre of Gravity Management Through Load Stability

Article contributed by JOHN HUMPHRIES – CICA

When we talk about Centre of Gravity (COG), it goes hand in hand with Load Stability. Let’s discuss working safely with slinging techniques and managing load stability.

Loads are stable when the centre of gravity is below the lifting points. This is not always practical when lifting loads on pallets, particularly multiple stacked and unrestrained items. You can lift safely from

Diagram 1

underneath; however, you must lengthen your slings, chains or wire rope to create a large enough triangle to keep the load COG inside the triangle formed by the slings in Diagram 1.

We must understand that the stability diagrams above act in 2 axes (although being a 2D picture, it can only show one at a time.)

Diagram 2 below demonstrates this by showing stability in Axis 1 but instability in Axis 2.

Diagram 2

The main issue is that in Axis 1, the triangle of stability extends from the pallet to the crane hook where as in Axis 2 the triangle stops 1/3 of the way up the load where the slings join the vertical sling.

We will look at the two axes separately now in Diagram 3. You can see the slinging in Axis 1 surrounds the COG as the slings join the chain or rope at the outer edge of the load. This is further improved by the wrap effect of the slings passing over the top corners of the load creating a compressive effect.

In Axis 2, the load is literally balancing and will tumble off to the left or right with the slightest force from either the wind or acceleration as the load is moved sideways. The only way the load in the photo has not fallen is most likely due to the frictional forced from the wire rope passing over the top corners of the load. This can never be relied upon. A better way to lift this load is to use 4 ropes/chain/slings directly from the crane hook to the base/pallet. If this is not an option, the next best thing would be to lengthen and extend the triangle of Axis 2 to fully surround the COG.

Diagram 3

Diagram 4

In Diagram 4 the slings have been lengthened, and only just surround the COG hence this is still borderline unstable. The rules of basic rigging and load stability need to be followed always.

The two photos in Diagram 5 show how the rigging in the right photo was improved when the slings were lengthened to surround the COG shown in the left photo. As pointed out in above, this is still a critical situation as the load does not have to shift far for the COG to move outside

Diagram 5

the triangle.

The stability of the load is determined by the distance from the COG to the sling. We see in the far right image in Diagram 6, that a lower COG in the load also improves stability. As it increases the distance to the stability lines.

Let’s now look at a different scenario incorporating a lifting beam with 2 slings either side. This is deceptively unstable. (Hint: COG above Lifting points)

Diagram 6

Diagram 7

The stability triangle runs between the top lifting lug and the hook pivot pins on the underside of the beam. And once we transpose the triangle down to the level of the lifting points of the load, we see the true scenario. The COG is well above and outside the shallow triangle. The thickness of the beam and offset from top lug to bottom hooks helps us slightly. Here is the relative triangle with the COG above it and unstable.

Diagram 8

Diagram 9

Diagram 10

Diagram 9 shows the improved scenario with a modified beam and 2 extra slings forming a triangle. Note the relative stability triangle now surrounds the COG of the load.

The photo in Diagram 10 is an example showing 1 stable axis with optimised slings and large stability triangles and 1 unstable axis with the lifting beam and virtually NO stability triangle. Notice the load tilts until it rests on the slings?

Take care out there and always remember to consider both axes when slinging a load.