What If I Need To Choose A Chain Hoist Control System? - Part 3

What If I Need To Choose A Chain Hoist Control System? - Part 3

Jands Staging What If Chain Hoist

Part 3

In the last issue, we looked at multi-point lifting of a truss using 4 chain hoists, and the basic safety requirements for controls when using high safety specification, low performance fixed speed hoists, covering service limits, emergency limits, overloads, underloads and the concept of a “group-stop”.

In this issue, we’ll look at synchronous operation, with position and speed management.

In the last few articles relating to this one, we’ve looked primarily at the standards or codes of practice that apply in Australia, the UK and Germany and we’ll continue here along the same lines.

Neither the Australian nor the UK standards specifically mandate the use of any particular control methods for our application (multi point lifting, by chain hoist, of a truss grid structure) but do require the use of the appropriate “Safety of Machinery” standards (AS4024/EN954 and/or AS/EN62061) to determine the level of risk associated with the application and to implement controls that can be validated as having adequately addressed the hazards and risks involved.

The principal risk involved in a move of this nature is that the motors will operate at different speeds, and if not managed correctly, the truss could be deformed or damaged, or worse still, someone could be injured.

It can be argued, not unreasonably, that if the move is under the supervision of well trained and experienced riggers, then the move can be carried out safely, especially in light of the fact that you have, of course, got at least a controller and hoists that will initiate a “group stop” in the case of things getting away from you.

If there is a reasonably foreseeable risk that the truss will be moved by operators with relatively poor training and experience, then your hazard and risk assessment is going to dictate the use of synchronous controls, and it is going to be difficult to mount a reasonable case against the use of synchronous controls no matter what the level of training or experience of the operator if the truss is to be moved over persons.

The German codes of practice dictate that when a load is moved over persons, then the hoists must comply with BGV-C1, AND the control system must be synchronous in operation. Furthermore, if the risk category requires a C1 hoist, then the control must be synchronous, whether or not the move is over persons.

So if you are using Australian or UK codes in a high risk category application or if you are using German codes with BGV-C1 hoists and controls, then you need to synchronise the operation of the hoists.

Synchronisation requires, at the very least, that each hoist operates with a variable speed drive and is fitted with calibrated position encoders. The control system must then process the data from the hoist position encoders and modify each hoist’s speed so that they maintain a constant position relative to each other.

The effectiveness of the synchronisation is largely determined by the resolution and stability of the position encoders, the degree of precision to which the variable speed drives can control the motors and the speed and accuracy with which the controller can process the position data and make adjustments to speeds.

In addition to maintaining synchronisation between motors, the control system must also monitor safety circuits and load measuring devices and keep an eye on other safety functions such as limit switches, electrical circuit monitors and thermal protection devices.

With higher specification machines there is also a requirement to manage over-speed and with certain hoists, this is achieved inherently with the use of dynamically irreversible gearboxes, which, by design, cannot “run-away”. These gearboxes have the drawback of overheating fairly quickly, so high endurance machines and their controllers will also need to incorporate a speed management circuit, using either a dedicated “tacho” encoder, or the position encoder, or a combination of both.

The technique of using synchronisation of hoists to mitigate the risk of damage to the load or injuries to persons is only as good as the reliability of the system, so next issue, to wrap this topic up we’ll take a look at levels of certification for safety related control systems and try to de-mystify the “Category/SIL/Performance Level” acronyms.

Read 'What If I Need To Choose A Chain Hoist Control System' Part 1 here.
Read 'What If I Need To Choose A Chain Hoist Control System' Part 2 here.
Read 'What If I Need To Choose A Chain Hoist Control System' Part 4 here.

 

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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