First, the earth axis.
The earth axis is the oblique axis of the earth, also called the earth rotation axis. Refers to the axis of the earth's rotation. The intersection of the northern end and the surface is the North Pole, and the intersection of the southern end and the surface is the South Pole.
No matter where the earth rotates in its orbit, its north pole always faces the Polaris, and so does its south pole. According to Newton's law of universal gravitation, there may be a star above or below it that attracts the South or North Pole of the Earth.
The earth axis is the imaginary axis of the earth's rotation. The earth has been rotating around this imaginary axis. Therefore, it is also called the earth's rotation axis. This axis connects the north and south poles through the center of the earth, and the included angle with the earth's orbital plane is 66 34'. The earth's axis is facing the North Star. The plane passing through the center of the earth and perpendicular to the earth axis is called the equatorial plane.
The position of the earth axis in the earth is not fixed, but slightly moves, resulting in a "pole shift". Its extension line is "celestial axis". The two intersections between the earth axis and the ground are "poles". The two intersections between the celestial axis and the celestial sphere are the "celestial poles".
Second, the axle.
A machine that consists of wheels and shafts and can rotate around a coaxial line is called an axle.
A lever that can rotate continuously, the fulcrum is on the shaft, and the speed of the wheel and the shaft is the same when rotating.
R is the wheel radius, R is the axle radius, F 1 is the force acting on the wheel, and F2 is the force acting on the axle. According to the balance condition of the lever, F 1R=F2r (power× wheel radius = resistance× shaft radius).
The outer ring is called a wheel and the inner ring is called a shaft. The two ring of that shaft are concentric circles.
As can be seen from the above formula, when the power acts on the wheel, the axle is a labor-saving lever; When power acts on the axle, the axle is a laborious lever.
Therefore, the greater the radius difference between the wheel and the shaft, the more labor will be saved, but the more distance will be traveled.
Axles such as carriages, door handles, steering wheels and carts are the simplest. Without power transmission, the axle of a power car is much more complicated. Of course, so is the wrench.
Take the car as an example, the power is not simply transmitted to the axle. In that case, the car can't turn. There is a "differential" in the middle of the car axle, which drives the left and right wheels through two half shafts, so that when the car turns, the distance traveled by the two wheels can be different. Human tricycles's rear axle, in order to turn, one rear wheel and the axle fixedly transmit power, and the other rear wheel can make a differential turn with the rotation of the axle.
When important official drives the wheel, such as the movement of pulley, it can not only transmit power, but also change the speed.
Definition: A simple machine consisting of wheels and shafts that can be wound on the same shaft is called an axle. A wheel with a larger radius is a wheel, and a shaft with a smaller radius is a shaft.
The essence of the shaft is that the lever can rotate continuously. When using an axle, the lines of action of the forces acting on the wheel and the shaft are tangent to the wheel and the shaft, so their arm of force is the corresponding wheel radius and shaft radius.
Because the radius of the wheel is always greater than the radius of the shaft, when the power acts on the wheel, the shaft is a lever that saves labor and distance (the first picture below). Practical example: Bicycle pedals and roulette wheels (big gears) are labor-saving axles. When power acts on the axle, the axle is a lever that saves labor and distance (as shown in Figure 2 below). Practical examples are: flywheel (pinion) on the rear wheel and wheel of bicycle, blade and fan blade of ceiling fan.
I hope it can help you solve the problem.