What is the difference between the main arm and the basic arm of a truck crane?
The secondary arm is for distinguishing from the main arm, and the auxiliary arm includes the first section of the auxiliary arm and the second section of the secondary arm (if there is a second section of the auxiliary arm).
The main arm is for distinguishing between the main arm, the main arm including the basic arm (ie, the first arm), the second arm, the third arm, the fourth arm, the fifth arm (if there is a fifth arm
The basic arm is to distinguish it from the second (three, four, five) arm, which is part of the main arm.
