Expanding and shrinking mechanism

The invention relates to an expanding and shrinking mechanism which is suitable for walking in a pipeline and is tightly clung to the pipe wall with the change of the inner cavity of the pipeline. The expanding and shrinking mechanism is applied to the field such as non-destructive inspection of oil and gas transmission pipelines. The expanding and shrinking mechanism comprises a mandrel, a front fixing plate, a connecting rod, a pipe wall, a spring, a detecting block, a slide arm and a back fixing plate. The detecting block is tightly clung to the pipe wall all the time and can be freely expanded and shrunk under the action of the spring; after pipe detecting equipment which takes the expanding and shrinking mechanism as a carrier enters the pipeline, during deformation of the pipeline, the pipe wall extrudes the detecting block and then compresses the spring, so that the peripheral size of the expanding and shrinking mechanism is reduced, the expanding and shrinking mechanism can smoothly walk in the pipeline. The expanding and shrinking mechanism has the characteristics of compact and flexible structure, capability of freely expanding and shrinking, and the like, setting for functions of the equipment is easily realized in the limited space of the pipeline.