Model QTZ250 (7030) tower crane with lifting torque of 250tm.Tower crane: the boom form of tower crane is divided into horizontal type and pressure bar type. When the boom is horizontal, the truck runs along the horizontal boom and the luffing motion is balanced. The boom is long, but the boom weight is large. When the boom is of compression bar type, the luffing mechanism leads the boom to pitch luffing. The luffing motion is not as stable as the horizontal type, but its self weight is small. The lifting capacity of the tower crane varies with the amplitude. The product of lifting capacity and amplitude is called load moment, which is the main technical parameter of this crane. Through the slewing mechanism and slewing ring, the tower crane has large lifting height and large inertia mass of slewing and walking, so it needs to have good speed regulation performance. In particular, the lifting mechanism is required to be fast under light load, slow under heavy load and micro motion in place. Generally, in addition to resistance speed regulation, eddy current brake, frequency modulation, pole changing, thyristor and electromechanical combination are often used for speed regulation.