Stability of a crate lifted with a lifting beam and two slings when the center of gravity is above the attachment points of the slings I have a hard time understanding the stability of a crate lift when using a lifting beam. Consider the system in the first figure, where a crate is suspended in two lines using a lifting beam. Presume that the crate does not slide. There is a distance a between the attachment point of the beam and the two slings.
The center of gravity is located a distance b away from the attachment points of the slings. I found a claim that states that:
If the distance b in the figure is greater than the distance a the load may become unstable.

An unstable system means that the center of gravity is outside of a stability triangle. In this case it seems that the triangle is defined from a triangle with the height a and a base corresponding to the length of the beam as shown in the second figure.

This claim neglects the slings length. I cannot figure out how this works, can someone give an explanation?
 A: I think I understand what is going on here.
Instead of two bars connected with vertical lines, imagine a single bar that supports a payload, and the bar is suspended using two ropes that come together to a single point above the center of mass.
Stable

This point is a pivot point and the payload suspended below acts like a pendulum. If you swing the pendulum one way, it would want to correct itself as the center of mass will try to be directly under the pivot at all times.
Unstable

But in the situation where the center of mass is above the pivot point  then the situation is unstable since the center of mass would want to swing around and be below the pivot point. This is like an inverted pendulum where the weight is above the pivot and the slightest movement will topple it over.
Now to consider the effect of having the two bars connected with parallel ropes. This links the upper and lower bars together, and any swing the bottom bar does, the top bar follows.
Stable

The two situations are entirely equivalent in terms of the stability, so when the pivot is above the center of mass in the equivalent case, this corresponds to $a > b$ in the two bar case.
Unstable
And when the pivot is below the center of mass in the equivalent case, this corresponds to $a < b$ in the two bar case.

