In the transmission industry, the ball screw is an indispensable mechanical component.
It is the most commonly used transmission unit in tool machinery and precision machinery,
Its main function is to convert rotary motion into linear motion.
Due to its low friction resistance, ball screws are widely used in various industrial equipment and precision instruments.
In this post, we will explain the concept, structure, principle and manufacturer of the ball screw.
Let’s dive into it.
1. Concept and structure of ball screw
The ball screw is composed of screw, nut, steel ball, pre-compression piece, reverser, and dustproof device.
Its function is to convert rotary motion into linear motion, which is to change the bearing from sliding motion to rolling motion.
In 1898, people first tried to add steel balls between the nut and the screw to convert the sliding contact of the traditional screw into rolling contact;
Rolling friction is used instead of sliding friction, the rotary motion of the steel ball in the nut is converted into linear motion, and the torque is converted into axial repeated force, so as to improve the poor positioning and easy damage of the traditional screw.
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 an active body, the nut is converted into linear motion with the angle of rotation of the screw in accordance with the guide of the corresponding specification, and the passive workpiece can be connected to the nut through the nut holder.
The ball screw assembly is metallic, usually steel, and consists of a female nut and a screw whose helical groove matches that of the screw.
Inside the groove, contained in the nut are many small balls made of chrome steel.
As the balls circulate in the nut, the balls provide smooth motion under the screw and the deflector or return system holds the balls and allows them to circulate through the nut.
When used with a motor, the efficiency of the ball screw is up to 90%.
They are quite accurate, with an accuracy of a few thousandths of an inch per foot.
Many industries use ball screws for precise control, including the aerospace, computer, electronics, automotive and medical industries.
Ball screws are also often used in manufacturing processes like robots, automated assembly lines, material handling equipment, conveyors, machine tools, wire control and precision assembly equipment.
2. How does a ball screw work?
The ball screw assembly is composed of screw and nut, each has a matching spiral groove.
The balls roll between these grooves, providing the only contact between the nut and the screw.
When the screw or nut rotates, the balls are deflected by the deflector to the ball return system of the nut, and they travel through the return system in a continuous path to the opposite end of the ball nut.
The ball then exits from the return system to the ball screw and nut threaded raceway for recirculation in a closed loop.
The ball nut determines the load and life 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 that the ball nut will reach fatigue failure (wear) faster than the ball screw.
Ball nuts include two types of ball return methods: outer circulation and internal circulation.
Outer circulation: the ball returns to the opposite end of the loop through the return pipe, and the ball return pipe protrudes above the external diameter of the ball nut.
ball nut with external circulation
Inner circulation: the ball passes through or returns along the nut wall, which is lower than the outer diameter of the ball nut.
ball nut with internal circulation
The internal circulation has a rotating circuit (as shown in the figure above), the ball is 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 the ball nut of the cross deflector type, the ball only rotates the shaft one turn, and the circuit is closed by the ball deflector (B) in the nut (C), allowing the balls to cross between adjacent grooves at a point (A ) And (D).
ball nut with internal circulation
When a long ball screw rotates at high speed, once the slenderness ratio reaches the natural harmonic of the shaft size, it will begin to vibrate.
This is called the critical speed and may be very detrimental to the life of the ball screw. (The safe operating speed should not exceed 80% of the critical speed of the screw)
Rotation ball nut component
Some applications require longer shaft lengths and higher speeds, all of which are areas where a rotating ball nut assembly is required.
3. Ball screw manufacturers
- South Korea: SBC.
- Japan: NSK, THK, IKO.
- Sweden: SKF, Schneeberger Schneeberg.
- Germany: IF, Rexroth.