Why is a sling angle greater than 45° important in rigging?

[rollingstein]

Was reading a book on rigging and it had this note that intrigued me:

Ensure that the sling angle is always greater than 45° and preferably greater than 60°. When the
horizontal distance between the attachment points on the load is less than the length of the
shortest sling leg, then the angle is greater than 60° and generally safe

What happens if not? Why this admonition? What's the danger in a "flatter" sling?

 

[pbuk]

First of all, note that there are two ways of measuring sling angle. The method commonly used in the US is to measure the angle from the horizontal; I guess you are in the US. In the UK the usual system is to measure the angle included at the apex of the sling. So the equivalent advice in the UK would be "ensure that the sling angle is always less than 90° and preferably less than 60°".

With a "flatter" sling (larger angle in the UK, smaller in the US) the tension in the slings is much greater which can lead to failure of the sling or attachment points. The first diagram on http://www.haroldpotter.co.uk/chain-sling-information-z-14.html#.UlrxHxA8HF8 gives a useful illustration.