What If We’re Lifting a Load with More than 2 Chain Motors?

What If We’re Lifting a Load with More than 2 Chain Motors?

CX 95 Staging What If

The issue of whether or not we need to use load measurement with chain hoists has been coming up quite a bit recently, so now seems to be as good a time as any to open up that particular can of worms.

Seeing as the tone of the last few articles regarding chain hoists has been about D8 and D8+ machines, we’ll keep this discussion heading in the same general direction and make a few basic assumptions, namely:

  • The lifting process is in a controlled space and nobody will be under the load when it’s moving.
  • In choosing to use D8/D8+ hoists you have a copy of the VPLT.SR2.0 Code of Practice for Event Technology and you are using this as the basis for developing your own work methods.
  • The lifting process is being carried out by experienced and qualified riggers.

The simplest lift is a single point lift (single chain hoist), and assuming that you know the weight of the load you are about to lift before lifting, then we can reasonably proceed without load measurement in the hoist.

Similarly, if you are lifting a rigid load (e.g. truss segment) with 2 hoists then the distribution of the load between the 2 hoists is unlikely to change during the lift, even if the load is not evenly shared. It would be reasonable in this case to use hoists without load measurement, although you’d be well advised to use a D8+ hoist or a D8 hoist with an (optional under the Code of Practice) overload device fitted if the total load exceeds the individual lifting capacity of either of the 2 hoists.

When we get to lifts involving more than 2 hoists (or with 2 hoists where the load is guided), we enter into that arcane and difficult territory known as “Statically Indeterminate Loads”. To best understand this, we suggest you use your favourite search engine and download a copy of “igvw SQ P2” which is the German standard for electric chain hoists that forms part of the VPLT SR2.0 Code of Practice.

Table 4 on Page 18 of this document describes a number of Statically Indeterminate loading cases and makes for a compelling read if you’re so inclined, or alternatively simply explains a complex concept in a couple of diagrams, if calculus isn’t a recreational or leisure activity in your world.

Lifting a single line of truss with 3 or more motors is the first example used. Assuming that you are using fixed speed, conventional AC induction motor chain hoists, operating directly from the 3 phase supply, then your hoists will run at (very approximately) the same speed when evenly loaded, but will operate at significantly different speeds (anything up to 5% or more difference) when unevenly loaded.

As the lift progresses, a hoist that is running faster will start taking up more of the load, and a hoist running slower will start seeing less of the load, with a risk of a chain going slack or a hoist being overloaded.

If the load is simply a 3 point lift on a straight truss and the operators are experienced and observant, then this is unlikely to cause a serious problem, but in less than ideal circumstances it is not that difficult for this situation to get away from an operator.

The second example shown in this table is a rectangular truss grid with a centre span, picked up by 5 hoists, 1 in each corner and one on the centre of the centre span element. This type of lift using fixed speed, un-synchronised hoists, is going to be a challenge for an experienced rigger – not impossible and done on a regular basis, but certainly not a simple task to be undertaken lightly.

Our second assumption at the start of this piece was that we were using VPLT SR2.0 as the basis for our work methodology, and VPLT SR2.0, “Section 3.1.3 Overload shut-down/overload monitoring” relating to the operation of D8/D8+ hoists states that “For special load types where there is a risk of specific elements... being overloaded, a load measurement is required...”

This doesn’t mean that we need a complex control system with a load measuring system deciding when a hoist should run or stop, but we do need to know what each hoist is lifting.

A good rule of thumb would be that if you can’t reliably predict what load a hoist is carrying during a lifting process then you need to measure the load on each hoist.

Jands Staging
provide articles to the "Staging What If" section in CX Magazine. If you have any questions regarding this article then please comment below or email info@jands.com.au.

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