Crane safety devices: To ensure safe and reliable operation of lifting a large construction crane, crane equipped with better safety devices, so that in the case of second-hand construction crane accidents, protect the parts or alert the operator attention, thus play a security effect. Each hydraulic system relief valve: Can loop abnormally high pressure in order to prevent damage to the hydraulic pumps and motors, construction cranes, and to prevent the overload condition. Crane Development origin: history. Slightly, difficult to operate. To the 15th century, Italian invented the jib crane, to solve this problem. This crane has roots inclined cantilever arm equipped with a pulley top, can lift can be rotated. It was not until the 18th century, a variety of lifting machinery are also used by humans to human, animal powered on starting weight, range and efficiency is very limited.