Technical Director of Britlift, Anthony Culshaw, shares simple explanations of commonly seen but not always fully understood lifting and rigging scenarios
What is the Centre of Gravity of a Load?
The centre of gravity (CoG) is a theoretical point of an object, which engineers use for convenience in calculations, as the single point where all of that objects weight is concentrated. For a symmetrical object, made of the same material, the centre of gravity is central – but how often in the world of rigging are we lifting a perfect cube of the same material?
Let’s consider some 2-dimensional examples to better explain:
In figure 1 we can see that the centre of gravity for load A is central as shown by the symbol (seen in the centre of figure 1 diagram A) which is commonly used to mark the centre of gravity of an object. In load B the shape of the load is no longer uniform and so the centre of gravity has moved accordingly.
But what does the centre of gravity mean?
In the simplest terms the centre of gravity can be explained as the tipping point. Imagine both these objects were to be pushed off a cliff. The centre of gravity is the point at which the object would tip and fall off the edge.
A final point on the definition of the centre of gravity is that it is not just dependant upon the shape of an object but also its density – what it’s made out of. Consider an empty box, provided the box is of uniform shape and construction, the CoG will be central. If the box were to be pushed off a shelf, then the box would tip and fall at the point that more than half of the box was hanging over the edge. But if you filled one half of the box with something. The box would tip and fall much sooner, with less than half of the box hanging over the edge. This would be because the CoG had moved towards the heavier end of the box. See the below example:
So, the CoG is not just dependant upon the shape of an object but also its composition – what it’s made out of.
But how does this translate to a lifting operation?
How does the Centre of Gravity Affect a Lift?
In the previous example you could say that the edge of the shelf (or cliff) was acting as the pivot point, and the relationship between it and the location of the CoG was obviously critical to the stability of the object.
In the same way, in most lifting operations either the crane hook or the load connection point is acting as a pivot, and the relationship between it and the CoG of the load being lifted is also critical.
When a load is lifted by a crane, the load is free to tilt and move, in most simple lifts (not using any special equipment) the load will tilt until the CoG is directly under the crane hook.
See the simplest example below, a load being lifted from a single top point with an offset CoG:
The box being lifted is heavier on one side and as a result has an offset CoG but the lifting point is located in the middle of the box. When the box is raised into the air it will tilt until the CoG is underneath the crane hook.
This means that if the CoG of a load is known, or can be calculated – the exact angle of tilt can be predicted before the lift. This is critical when planning a lift and determining whether or not the lift will be stable. It is also incredibly important in the design of lifting aids such as counterweighted lifting beams.
Even at this stage, using the simplest examples, the height of the CoG affects the angle of tilt. A higher CoG means that the load has to tilt further to bring the CoG under the hook.
In figure 4 the horizontal location of the CoG has not been changed but it has been moved higher up the load. As you can see from view D, this has resulted in a higher tilt.
How does the Location of the Lifting Points Affect a Lift?
You may have heard an AP or rigging engineer state before that lifting points located on the top of a load are “safer” but why is this? Well one of the most important reasons is related to the stability of the lift and the CoG location.
In lifting, if the CoG is located above the pivot point (the load connection point) then potential for the load to “topple” immediately becomes a possibility.
Consider a very simple example: a bucket full of water. Most buckets come with a pre-installed lifting beam for you to use – in the form of the handle. The handle of a bucket is connected to the top of the bucket, for a very good reason – it makes it impossible to topple.
Bucket A in figure 5 shows a traditional bucket. Consider the handle a piece of lifting equipment and the bucket of water itself the load. The lifting aid is connected to the load at the top and there shouldn’t be any stability concerns lifting this bucket.
Bucket B however uses a similar lifting aid (handle) but this time it is connected to the bottom of the bucket. You can imagine that this bucket would be almost impossible to lift without it flipping over. You would stand a better chance if the bucket was empty and you might be able to make it balance for a while. That would be because the CoG would be lower (a standard bucket doesn’t have a lid and is usually made of thicker material at the bottom so the CoG is naturally lower). When the bucket is full the higher CoG would make it even more difficult.
This is one of the reasons that a full glass of water is easier to knock over than an empty one!
The examples given above were all 2 dimensional which helps to simplify the problem, but the same rules apply to a 3D scenario. Complications arise when loads with offset CoGs require lifting/spreader beams or frames to lift them.
Generally when lifting a simple load from the top of a load you only need to worry about the angle of tilt rather than the stability of the lift as a whole, but if you are connecting to a load at the bottom (underneath its CoG) then you must take extreme care that the lift is planned correctly.
Above all I would recommend the use of experienced and knowledgeable Riggers, Operators and Appointed Persons who should be familiar with such concepts. Furthermore, I would encourage the less experienced APs and lift planners to seek the aid of a peer if in doubt – guessing the right way to carry out a lift doesn’t help anyone, but in engaging with a peer you may both learn something. In this age of social media, one of the (few!) benefits is at least that is easier to do than ever.
I hope this article has been of interest, any questions or anything you feel you can add to this article, then please leave a comment below, email firstname.lastname@example.org or message me on Linkedin.
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Anthony is currently the Technical Director at Britlift, designers and manufacturers of lifting beams, spreader beams and custom below the hook frames. He is a former member of the LEEA Technical Committee and has spoken at lifting conferences around the world on the subject of below the hook lifting equipment.