Pushing a small car and goods onto a truck along a slope is much easier than lifting them onto a truck. Slopes are scientifically called inclined planes because moving objects up a slope requires a greater distance than lifting them up directly. Therefore, according to the "golden rule" of mechanics, we apply the experience of inclined planes in terms of force to screw jacks.

Spiral is just a variation of the inclined surface, a curved form of the inclined surface. We can think of it as follows: spiral is a rolled up inclined surface, which restricts the inclined surface to a very small space. When using a jack to lift a heavy object, the handle rotates once and needs to move a long distance, but the screw only moves one pitch. The distance between adjacent threads can be lifted up with a small force using this spiral.

So the spiral in the screw jack is a lifting device that controls the inclined plane in a small space to achieve labor-saving goals, helping us overcome the problem of lifting heavy objects. It is similar to the working principle of inclined plane conveying and spiral elevators.