Safe Working Loads

[davidatefi]

I'm currently doing a Systems & Control GCSE at school, and for my coursework I am making a working model theatre with a fly gallery.

I know about SWLs in theatre, but I'm just wondering how many times the actual capacity of a bar / rope / whatever is compared with the SWL. (as in, 'the SWL is always a fifth of what the item can actually support')

Can anyone help me?

Thanks a lot

David

[Andrew C]

Depends on what it is. Others may have chapter & verse, but 8 and 10 times is used sometimes even 5.

 

WLL is different again.

[Jivemaster]

There are many indicative loads. SWL ia as it says the Safe Working Load there is also the Min load at Failure. the factor between mlf and SWL is a design criterion that can be chosen by a competent designer there used to be various safety factors used now the nost used figure is 8 so an item may fail at 8 times its rated load.

[Nick Evans]

Either the designer or the legislator will specify the safety factor for the item. SWL is (crudely) ultimate strength divided by safety factor.

[Chris Higgs]

Brilliant to hear of your project for S&C. I have a lad of similar age and I'm always interested to help if I can.

 

The legislation in the UK requires a factor of safety for lifting equipment roughly thus:

 

Forged steel components like shackles and things 4:1

Wire ropes and their terminations 5:1

Chains in slings 4:1

Fibre ropes and slings 7:1

Chain hoists and wire rope winches and counterweight bars, for example work rather differently, and are generally designed to lift a designated load, to sustain periodic test loads and simple overloads.

The 'test' load values range from 1.5 x design load for manually operated machines and as little as 1.1 for the load tests on power operated lifting machines.

The machines are obviously subject to an inspection and maintenance regime as well. (The whole idea is a bit like designing and maintaining a car to allow it to be driven in safety, really)

 

However, the best practice represented in standards for theatres and places of entertainment is roughly thus:

 

Anything used overhead 8:1 except:

Barrels and trusses where something between 3 and 4:1 is likely. Because there are a number of design and engineering considerations a crude ratio is not really appropriate for a structure (same as the car).

So for trusses, barrels and the like, use the information supplied by the supplier/installer or better yet manufacturer (like the car, again).

Hoists are another issue again, and typically, you'd find a factor of between 6 and 10:1 on most.

I hope that helps, but you can see, it isn't as easy as a 'blanket 'figure.

[davidatefi]

thanks very much guys, most appreciated