The thread can be defined as an inclined surface surrounding a cylinder and is equivalent to the inclined surface. On the contrary, the unfolding of the thread is an inclined plane. Self-locking phenomenon means that if the line of action of the resultant force Q of the main force acting on the object is within the friction angle, no matter how large the force is, there will always be a total reaction force R to balance it, and the object remains stationary.

Conversely, if the line of action of the resultant force Q of the main power is outside the friction angle, no matter how small this force is, the object cannot maintain balance. This equilibrium condition, which has nothing to do with the magnitude of the force but with the angle of friction, is called the self-locking condition. The equilibrium phenomenon of an object under this condition is called self-locking phenomenon. The resultant force R of the friction angle and the normal reaction force N of the self-locking phenomenon and the friction force F is called the total reaction force of the support facing the object. That is, when the frictional force F reaches the maximum value Fmax, the included angle a at this time also reaches the maximum value b, and b is called the friction angle.