Counterweight
Removable steel or concrete masses placed on the back of a crane’s superstructure to balance the moment created by the lifted load and increase the crane’s rated capacity.
Counterweight is the steel or concrete mass mounted at the back of a crane's superstructure (the rotating upper) to counterbalance the load at the boom head. Without counterweight, a crane would tip forward the moment a load was lifted off the ground. With more counterweight, the same crane can lift more — until other limits (boom strength, ground bearing, slewing brake) kick in.
On mobile cranes, counterweight is modular and removable. A typical all-terrain crane carries a base counterweight as part of its road weight and trucks additional counterweight slabs to site for the lift. The load chart specifies the capacity for each counterweight configuration; reading the chart means matching the configuration on the page to the slabs actually mounted on the machine.
On tower cranes, counterweight is mounted on the rear counter-jib opposite the load-bearing main jib. On crawler cranes for the largest lifts, derrick-style trailing counterweights run on their own tracked transporter behind the crane — this allows enormous lift capacities at long radii without the counterweight having to balance on the crane itself.
For buyers: when a seller lists a crane with a given lift capacity, ask what counterweight configuration the rating assumes. A "200-tonne crane" rated at full counterweight may only lift 110 tonnes in the configuration the seller can actually deliver. The counterweight slabs themselves are sometimes sold separately on the used market — check that the listing includes them.