When renting cranes in Hefei, it is found that small cranes will be unable to lift during operation, so troubleshooting methods should generally be adopted. Bridge crane is a device for lifting heavy objects with hooks (also called grab or electric panel). Its appearance is bridge-shaped, and both ends of the bridge are supported on the track through rollers. The road roller walks along the track, and the Hefei crane is hired to drive the bridge to move. Structurally, the bridge crane is mainly composed of three parts, namely, metal structure, mechanical transmission and electrical equipment. This small crane can't be lifted. The general reasons are: the oil leakage of hydraulic motor is too large, the volumetric efficiency is obviously reduced, and the lifting is weak; The control element overflow valve (pilot type) leaks oil, so the system pressure can't go up and the lifting is weak. Hefei crane rents the motor oil drain, and operates the lifting mechanism to observe the normal oil drain; After removing the lifting relief valve (plug-in type) of the upper valve group, it was found that the 1 O ring was slightly damaged, resulting in oil leakage and the pressure could not go up. After replacing the O-ring, the adjustment pressure reaches the design value. The small crane was hoisted for trial operation and the work returned to normal. The hoisting mechanism of small crane is noisy. Through observation and analysis, the fault may be caused by the balance valve, which is imported from Italy, and its remote control interface is equipped with a one-way damping combination valve (damping is adjusted by thread). Its principle is to open slowly (delay 2s) and return oil quickly, so that the balance valve of small crane can protect the lifting mechanism more safely. The fault may be that the long-term use of Hefei crane leads to untimely maintenance of the hydraulic system, and the damping part is slightly greasy, which aggravates the damping, resulting in a long opening delay (about 8s), which leads to the disharmony between the opening of the balance valve and the opening of the hoisting brake, and makes a clicking sound. After the balance valve was removed, cleaned, assembled and adjusted, the delay met the factory requirements and the fault disappeared.