Why Can the Wheels of a Supermarket Shopping Cart Rotate Freely?

Why can the wheels of a supermarket shopping cart rotate freely?
During the holiday shopping trip, I casually pushed a small car, turned left and right, and the wheels followed me as if I understood the language. Have you ever thought: why is it so obedient?
The story starts with the wheels.

Ancient humans discovered that placing logs under heavy objects saves more effort than dragging them hard. So, logs became wheels, wheels added axles, and the transportation revolution began. Without wheels, there would be no city like today, and no supermarkets to visit.

However, wheels are inherently stubborn and can only walk one path until black. Do you want to turn? We need to lift the whole thing up. Later, someone added a “neck” to the wheels – a bracket that can rotate horizontally, which is called a caster wheel, commonly known as a universal wheel. It makes the wheels shake and the car learns to turn.
The bottom four of the shopping cart are all universal wheels: the bracket and axle are not locked, and can rotate 360 ° around the vertical axis. When pushing the cart forward, the wheels always “slip” behind the rotating shaft; Once it retreats, friction will immediately give the wheel a torque, causing it to “shake its head” 180 degrees and continue to obediently move backwards. Simply put, whoever gives friction force listens, that’s why the car points and hits wherever it goes.
Despite its simple structure, it is quite sophisticated in construction:
1. Steel plate punching and punching, one-time forming of bracket and brake pad;
2. Apply oil and drain steel balls in the bowl shaped disc, circle up and down, then press the bracket in and rivet it to death. The steel balls are permanently sealed in the “track” as bearings;
3. The rubber is melted and molded into a wheel, the molding line is polished, the shaft ring, screw, and nut are installed, and only after passing the test can it be put into operation.
It is interesting that the front wheels of the bicycle are also an upgraded version of this “universal” principle: the intersection point between the extension line of the front fork and the ground always runs ahead of the contact point, so pushing forward is stable like a dog, and pushing backward makes it sway like a snake. This small relative position can determine the stability of the entire vehicle more than wheel inertia.
Next time you charge forward with your shopping cart, don’t forget to give a thumbs up to these four universal wheels on your feet – they are the heroes behind your ‘shopping drift’.