What is the indispensable key component in the transmission world?

In the transmission world, the ball screw is the indispensable key mechanical component. It is the most commonly used transmission element in tool machinery and precision machinery, and its main function is to convert rotating motion into linear motion. Due to its small friction resistance, ball screws are widely used in various industrial equipment and precision instruments.

The ball screw is composed of a screw, a nut, steel balls, a preload sheet, a reverser, and a dustproof device. Its function is to convert rotary motion into linear motion, that is, to change the bearing from sliding action to rolling action.
In 1898, people made the first attempt to add steel balls between the nut and the screw, transforming the sliding contact of the traditional screw into rolling contact; replacing sliding friction with rolling friction, converting the rotary motion of the steel balls in the nut into linear motion, and converting torque into axial repeated force, so as to improve the poor positioning and easy damage of traditional screws.

The ball screw was invented by Rudolph G. Boehm from Texas and was granted a US patent in 1929.

When the ball screw is used as the active body, the nut will convert into linear motion according to the lead of the corresponding specification with the rotation angle of the screw rod. The passive workpiece can be connected with the nut through the nut seat, thereby realizing the corresponding linear motion.

The ball screw assembly is metallic, usually made of steel, and consists of an internally threaded nut and a screw. The spiral groove of the nut matches the spiral groove of the screw.

Inside the grooves, contained within the nut are many small balls made of chrome steel. When the balls are circulating in the nut, the balls provide smooth movement under the screw. The deflector plate or return system holds the balls and makes them circulate through the nut.

When used with a motor, ball screws are up to 90% efficient. They are quite precise, with an accuracy of a few thousandths of an inch per foot. Many industries use ball screws for precise control, including aerospace, computer, electronics, automotive, and medical industries.

Ball screws are also frequently used in manufacturing processes and are common in robots, automated assembly lines, material handling equipment, conveyors, machine tools, wire controls, and precision assembly equipment.

The ball screw assembly is composed of a screw and a nut, each with matching spiral grooves. The balls roll between these grooves and provide the only contact between the nut and the screw.

When the screw or nut rotates, the balls are deflected by the deflector into the ball return system of the nut, and they pass through the return system in a continuous path to the opposite end of the ball nut. Then the balls exit from the return system into the thread raceways of the ball screw and nut for recirculation in a closed loop.

The ball nut determines the load and service life of the ball screw assembly. The ratio of the number of threads in the ball nut loop to the number of threads on the ball screw determines the extent to which the ball nut will reach fatigue failure (wear) faster than the ball screw.

The ball nut contains two types of ball return methods: external circulation type and internal circulation type.

External circulation type: The balls return to the opposite end of the loop through the return tube, and the ball return tube protrudes above the outer diameter of the ball nut.

Internal circulation type: The balls return through or along the nut wall, below the outer diameter of the ball nut.

The internal circulation type has a rotating circuit (as shown in the figure above). The balls are forced to climb over the top of the thread on the screw through the return system. This is called a cross-deflector type internal return system. In a cross-deflector type ball nut, the ball only makes the shaft rotate one turn, and the return path is closed by the ball deflector (B) in the nut (C), allowing the balls to cross between adjacent grooves at points (A) and (D).

When a long ball screw rotates at high speed, it will start to vibrate once the slenderness ratio reaches the natural harmonic of the shaft's dimensions. This is called the critical speed and can be very detrimental to the service life of the ball screw. (The safe operating speed should not exceed 80% of the screw's critical speed.)

Some applications require longer shaft lengths and high speeds, and these are all places where the rotating ball nut assembly is required.