In the field of transmission, ball screws are an indispensable key mechanical component. They are the most commonly used transmission elements in machine tools and precision machinery, and their main function is to convert rotary motion into linear motion. Due to their very low frictional resistance, ball screws are widely used in various industrial equipment and precision instruments.
ball screw
01
A ball screw consists of a screw, nut, steel balls, preload plates, a reversing device, and a dust cover. Its function is to convert rotary motion into linear motion, transforming the sliding action of a bearing into rolling action.
In 1898, people first tried adding 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, the rotational motion of the steel balls in the nut was transformed into linear motion, and the torque was transformed into axial repetitive force, thereby improving the poor positioning and easy damage of the traditional screw.
The ball screw was invented by Rudolph G. Boehm of Texas and was granted a U.S. patent in 1929.
When the ball screw acts as the driving element, the nut will be converted into linear motion according to the lead of the corresponding specification as the screw rotates. The passive workpiece can be connected to the nut through the nut seat, thereby realizing the corresponding linear motion.
Ball screw assemblies are made of metal, usually steel, and consist of an internally threaded nut and a screw, with the helical grooves of the nut matching the helical grooves of the screw.
Inside the groove, contained within the nut, are numerous small balls made of chrome steel. As the balls circulate within the nut, they provide smooth movement beneath the screw, while a guide plate or return system holds the balls and circulates them through the nut.
When used with electric motors, ball screws are up to 90% efficient. They are incredibly precise, with an accuracy of a few thousandths of an inch per foot. Many industries use ball screws for precision control, including aerospace, computer, electronics, automotive, and medical industries.
Ball screws are also frequently used in manufacturing processes and are commonly found in robots, automated assembly lines, material handling equipment, conveyors, machine tools, wire control, and precision assembly equipment.
02
A ball screw assembly consists of a screw and a nut, each with matching helical grooves between which balls roll, providing unique contact between the nut and the screw.
As the screw or nut rotates, the balls are deflected by a deflector into the ball return system of the nut, and they travel in a continuous path through the return system to the opposite ends of the ball nut. The balls then exit the return system into the raceways of the ball screw and nut threads for recirculation in a closed loop.
The ball nut determines the load and lifespan of the ball screw assembly. The ratio of the number of threads in the ball nut circuit to the number of threads on the ball screw determines how quickly the ball nut will reach fatigue failure (wear) compared to the ball screw.
Ball nuts include two types of ball return methods: external circulation and internal circulation.
External circulation type: The balls return to the opposite end of the loop through the return tube, which protrudes above the outer diameter of the ball nut.
▲External circulation ball nut
Internal circulation type: The balls pass through or return along the nut wall, below the outer diameter of the ball nut.
▲Internal circulation ball nut
The internal circulation type has a rotating loop (as shown in the diagram above), where the balls are forced to climb over the top of the thread on the screw via a return system, which is called a cross-deflector type internal return system. In a cross-deflector type ball nut, the balls only cause the shaft to rotate one revolution, and the loop is closed by a ball deflector (B) in the nut (C), allowing the balls to cross between adjacent grooves (A) and (D) at a point.
▲Internal circulation ball nut
When a long ball screw rotates at high speed, it will begin to vibrate once the slenderness ratio reaches the natural harmonics of that shaft size. This is called the critical speed, which can be very detrimental to the lifespan of the ball screw. (The safe operating speed should not exceed 80% of the screw's critical speed.)
▲Rotating ball nut assembly
Some applications require longer shaft lengths and higher speeds, which is where rotating ball nut assemblies come in.
03
Ball screws were originally only mass-produced by advanced industrial countries such as the UK, the US, Germany, and Japan. Starting in 1982, Ho Fung Precision Machinery Co., Ltd. set up a factory in Tanzi, Taichung County to produce ball screws, officially ushering in the era of Taiwan producing such high-precision parts on its own.
In 1989, Ho Fung was restructured and renamed HIWIN Technologies. HIWIN has become the world's third largest manufacturer of precision machinery such as ball screws and linear guides (the top two being Japanese manufacturers THK and NSK).
The second largest market share holder in the ball screw market is NSK Corporation of Japan, founded in 1916. NSK has always been the number one in the bearing industry in Japan and is also among the top in the world.
NSK ball screws are widely used in semiconductor manufacturing, machine tools, injection molding machines, and other general industries.
