Part 2: Mobile Crane Stability – Gravity, Balance & Leverage

Monday, October 16th, 2017

In Part 1 we took a high-level look at mobile crane stability, now its time to bring it back to the basics.

A mobile crane is built around the design principals of balance and leverage. In short, a crane must be capable of lifting heavy loads, through the use of leverage, while remaining in balance.

Before we dig into these concepts, we need to talk about gravity. We know from Newton and Einstein that the natural phenomenon of gravity is alive and well, all you have to do is drop a small object on your foot to be a believer. In material handling, we have to consider the Center of Gravity (or CG) of a load. The “center of gravity symbol”, illustrated by the black and white circles on the illustrations below is used to show the location where an object’s CG lies. The CG of any object is said to be the point in the object where the weight is evenly distributed. Meaning the leverage on one side of the object must be equal to the leverage derived by the other side of the object.

FULCRUM1
Now let’s look at a balance beam, which is similar to a basic crane model. The beam, balanced on a fulcrum, remains in balance as long as the leverage on one side of the fulcrum is equal to the leverage on the opposite side of the fulcrum. If either of the weights placed on the beam is larger than the other, the beam will tip in the direction of the heavier object. The only way to keep the beam in balance is to move the fulcrum closer to the heavier object, as illustrated below. By moving the fulcrum, the beam remains in balance and each object counterbalances the other.FULCRUM2

Counterbalance is dependent on each object’s leverage being equal. Leverage is the result of an object’s weight, multiplied times the distance the object lies from the fulcrum. In the illustration below, the load of 300 pounds is multiplied times the distance the load lies from the fulcrum (5 feet). The 100-pound weight is multiplied times the distance it lies from the fulcrum (15 feet). If the two sides of the equation are equal, the beam must remain in balance. The math is fairly simple. You multiply the 300 pounds times 5 feet which equals 1,500 foot pounds. Then multiply the 100 pound weight times its distance from the fulcrum. The result of 1,500 foot pounds illustrates that the same leverage is derived on both sides of the fulcrum (foot pounds is the result of multiplying distance in feet times weight in pounds.)

FULCRUM6

Now let’s apply this to a mobile crane. In the illustration below, the right side of the balance beam is raised up, much like the boom of a crane. As long as each object’s leverage remains equal, the beam remains in balance.

FULCRUM3

Now let’s take it a step further and suspend the load below the beam like it would be from a crane. In the illustration below, the beam can remain in balance if the leverage derived from each object remains equal. This example is great, as long as nothing changes, but we know this is not likely to happen. As soon as the weight of the object on the right is increased (like picking up a heavier load) you must compensate by increasing the leverage on the left side of the beam. If the object on the right moves closer to the fulcrum (like retracting a boom), you must compensate by decreasing the leverage of the object on the left side of the beam.

FULCRUM5

While these examples show how balance is derived, we know a crane must be able to remain in balance under a variety of load changes, radius changes and other factors which influence balance. In the field we think of balance as “stability”—and the ability for a crane to stay stable while accomplishing the task at hand is the end goal.

Stay tuned next week as we will take a closer look at how the components of the crane translate into the crane’s stability load ratings, followed by the next segment on how balance and the crane work together to lift safely.

We’d love to hear from you, please don’t hesitate to leave a comment.

 

(Article originally published October 13, 2015, updated 2017)

5 Responses to “Part 2: Mobile Crane Stability – Gravity, Balance & Leverage”

  1. p k mahalik says:

    How the load distribution takes place while lifting with vector diagram

  2. Charles Kemp says:

    I never thought that the mobile cranes were anything different, but now I understand what you are saying. There are quite a few cranes in town that are mobile and you can get because they have they have these different kinds of posts that go into the ground. I thought that was to make sure that the crane didn’t move but now I understand that it has more to do with balance than anything.

  3. Camilo Filho says:

    Very good and very well explained article. In reality, the crane works like a seesaw.
    The heavier kid is the crane and the thin kid is the load. As the thin guy gets far from the fulcrum point it comes to an unstable balance point and if he keeps going away, the fat kid will be in trouble. That’s why cranes overturn, simple like that!

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