What is the difference between primary and secondary maintenance?

The following is the difference between the first level of maintenance and the second level of maintenance: 1, maintenance purpose: the purpose of the first level of maintenance is to tighten and lubrication; the purpose of the second level of maintenance is to check and adjust the main. 2, maintenance content: the first level of maintenance in addition to routine maintenance operations to clean, lubricate, tighten the center of the operation and check the brake, manipulation and other safety components; the second level of maintenance in addition to the first level of maintenance operations to check, adjust the steering knuckle, steering rocker arm, brake shoes, suspension and other safety components after a certain period of use are prone to wear or deformation, and dismantle the tires to tire shift inspection. In addition to the first level of maintenance operations to check, adjust the steering knuckle, steering rocker arm, brake shoes, suspension, etc. after a certain period of time is prone to wear and tear or deformation of the safety components and dismantle the tires for tire shift inspection and adjustment of the engine working conditions and exhaust pollution control devices.