Monday, January 21, 2019
A load’s centre of gravity should be one of the first considerations, says Steve Hutin, the managing director of Rope and Sling Specialists Ltd.
It might not be the first question one asks. That might be to enquire as to the weight of the load. Dimensions are important too, as are a host of other factors—environment, location, restrictions, etc. But in the early stages of dialogue about a lift, before planning even begins, the relevant parties should verify the centre of gravity.
As Lifting Operations and Lifting Equipment Regulations 1998 (LOLER) asks, where is a load’s centre of gravity? And how will you attach it to the lifting machinery?
Fact is if a crane’s hook is not directly above the centre of gravity, the load will be lifted at an angle. Consider that the centre of gravity will always try to end up as far away from the hook as possible. It might be planned for a load to be raised titled—that’s ok, as long as it’s stable and has been rigged accordingly. Lifting or spreader beams, and other rigging gear come into play here. Plan A should always be for a balanced, level load, however.
LOLER makes few direct references to centre of gravity, yet it can be applied to great swathes of the document.
It says: “Lifting equipment should be positioned or installed in such a way as to reduce the risk, as far as reasonably practicable, of the equipment or load striking a person, or of the load drifting, falling freely or being unintentionally released.”
And: “Measures should be taken to reduce the risk of load drift (e.g. spinning, swinging, etc.)…”
One can’t adhere to this guidance if the centre of gravity is unknown or calculated via guesswork. On critical or complex lifts, it’s rarely if ever overlooked, but sometimes when the lift is deemed to be less significant, or appears to be more routine, certain principles can, wrongly, be abandoned. It’s always prudent to find out as much as possible about a load prior to the rigging team arriving at a jobsite. This isn’t an industry where surprises are a good thing. Improvisation can be even worse.
My company is often contracted to deal with the necessary equipment between the load and the crane hook. Depending on the load, we might have to go to various sources to gain information about it. The customer might have the necessary detail and / or we find out who manufactured the engine, part, container, etc., and ask them everything we need to know. Sometimes it’s been designed with lifting in mind, other times not. Loads can vary from a solid item with one central pick point, to a hollow piece that needs to be lifted at multiple points.
Get to the point
Even if a load appears to be straightforward, the competency of everyone involved in the lift is still important. Ok, an engine part might have a specially machined lifting point, supposed to be at its centre of gravity. But it should still be tested and inspected by a competent person before it is rigged to the crane or hoist. We can learn something about a load’s stability as it is raised off the ground, but it might take until the load is many feet in the air before a faulty lifting point or failure is detected. By then it could be too late.
It might also be possible to attach rigging gear to other parts of the load not necessarily manufactured for that purpose, but the appointed person must verify that the load bearing part is sufficient to take the weight. Welding or bolting on lifting points is an option but, again, they must be properly inspected and tested by… you’ve got it… a competent person. In all cases, knowing the centre of gravity is vital to ensuring the rig is attached at the right points.
A load can be lifted without any pick points at all, say, when a pair of slings is wrapped around the hull of a vessel. We recently supplied two MOD 6 spreader beams at 2.5m and two 10t capacity, 12m-long duplex webbing boat slings in just that case. Centre of gravity remains key, however, because imagine what would happen if a load were heavily weighted towards bow or stern—it’d place too much strain on the rigging at one end and become unstable. Also think about the crushing forces in such instances; the spreader mitigates those stresses.
(Did you know that a lifting beam is loaded in bending but a spreader is loaded in compression?)
Think about how centre of gravity can change, say, from when a load was manufactured to the time of lift. A simple example could be a container that has been altered by a user (they installed an air-conditioning unit) or it has been loaded with equipment, the majority of which is at one end. This needs to be factored into the lift plan and assurances given that the state of the load or its stability won’t change during the lift.
A four-point frame is often utilised when lifting containers. We’ve also prescribed a one-over-two spreader beam configuration when a unit had a slight offset centre of gravity. This blog isn’t about rigging configurations; the first principle is knowing the centre of gravity so one can rig a load safely.
Sling lengths are key to safe lifting, especially when an offset centre of gravity is present. If the load were heavier at one end a rigger would use shorter slings at that point so it will lift evenly. Imagine using slings of the same length in this scenario—the lighter end will lift first and the load will be uneven.
In some cases, a trial lift can provide valuable intelligence when there are no drawings and the centre of gravity is hard to calculate.
Computer-aided design (CAD) makes things easier these days but the same principles must be adhered to.
Rope and Sling Specialists Ltd