If counter weights on tall cranes are used for stability, why don’t the crane lean backwards without any load ?
In: Physics
Some cranes can adjust the position of the weights as they move the load around. Weight farther away from the pivot point have a greater effect: the crane will move weights towards or away from the center as needed to maintain balance.
But mostly, the cranes are just built with enough strength to hold up against the counterweights. That also means they’re strong enough to hold up against an unbalanced weight of the load…just not as unbalanced as it would be without the counterweights.
Like, imagine the crane can withstand 1000 pounds of unbalanced weight in any direction. If the crane has 500 pounds of counterweight, then without a load it’s going to “lean” 500 pounds backwards, but that’s within its tolerance. With that counterweight, it can hold a 1000 pound load easily without worrying about becoming unbalanced – the 500 pound weight pulls one way and the 1000 pound load pulls the other, so they cancel each other to 500 pounds in the direction of the load, which is *far* below the point when it would unbalance. Without the counterbalance, the load would be pulling with the full 1000 pounds and it would be really close to unbalancing.
They do, a little, but because the counterweights are much closer to center of the crane body, it doesn’t destabilize the whole crane.
The main issue is that the crane arm is loooong which creates a lot of torque when it’s carrying a weight, and also moves the center of gravity a lot. The counterweight isn’t just to balance the load like a seesaw. Simply making the base of the crane heavier makes it more stable. Think of how easy it is to knock over a traffic cone vs a traffic cone with sandbags on the base. So simply having extra weight down there helps stabilize it when there’s a lot of weight all the way up there and leaning out.
Putting it a little back helps extra when they know the imbalance will primarily be in one direction. But simply making the bottom heavier does tons for stability.
For many cranes, the counterweight is just a bunch of concrete blocks on the ground ([example](https://www.weissbrothers.us/industry-updates/take-a-look-at-those-counterweights/)). Without a load, the concrete blocks rest on their support on the ground.
If it’s something up in the air then it’s either adjustable or it’s just helping a bit, shifting the crane from “balanced — everything on the right” to “a bit on the left — a bit on the right”.
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