Prolyte Campus blog: Slinging matters; An Insert is not a basket!
Although slinging is a hot topic (have a look at the Yes we can do better slinging blog series), using inserts in your bridle chain is not a very common issue. However, lately I have seen a couple of examples of the below type of slinging and it bothers me enough to make me want to write about it. So here we go!
The above pictures show 3 types of a two-legged bridle, all more or less the same. But it’s proven again here, the devil is in the details.
Around the beams you can find basket hitches. A Basket Hitch can be defined as: Slinging Method, where the flexible body of the sling goes around an object, with each of the ends of the sling meeting in one and the same component, from where the lifting force is transferred into the lifting structure, or direct into the lifting machinery.
As you might be aware, in slinging methodology we distinguish 4 basic methods:
- Direct (single vertical)
Apart from these, there are also some less common methods, of which the “endless insert” I see sometimes used in our industry. Let me explain this a bit more…
Figure 1 shows a classic bridle. The shackle in the apex connects the two bridle legs and allows the chain hook to connect to the two bridle legs.
Figure 2 shows a classic bridle in combination with an endless round sling (spanset or soft steel). This round sling is positioned through the eyes of the bridle steels, in this way the round sling is set up as a basket. A shackle taking each end of the sling and connecting these to the lifting machinery.
This type of “insert” is used so it’s possible to make small adjustments with regard to the bridle position, without having to change the complete bridle. In essence the reliability of these corrections now depends upon the friction of the endless sling within the eyes of the steels.
Friction can cause heat, so please do not use this method with regular (fibre) spansets. Apart from this, the D/d ratio will seriously limit the capacity of the spanset or soft steel used. All-in-all may be a handy way of adjusting your bridle point, but surely not a safe or controlled one.
Figure 3 shows the endless round sling applied in the ‘endless insert’ slinging method. The round sling is used completely open, with single stands on all parts of the connection. Not conventional at all, and a lot weaker in its application.
Applied like this, the sling is not used as a ‘doubled’ package, but the sling sides are opened up from the normal parallel sides in a straight pull version. Adding a direction of pull, introduces an angle between both sides of the sling, thus reducing the capacity.
The flatter the angle gets, the higher the internal forces will be, even if the load itself does not increase.
Figure 4 Another example of an “insert” bridle on a cantilever
So, using the “insert” method may look like a good and quick solution to position your points, however, there are enough reasons to stay clear of this method.
Please make sure to familiarize yourself with the data given by the round sling manufacturer.
You can find some useful stuff here:
Read more about slinging in the Prolyte BlackBook
Have a look at the Prolyte Technical blogs