The ball screw is the most commonly used transmission element in tool machinery and precision machinery. Its main function is to convert rotational motion into linear motion, or convert torque into axial repeated force. It also has high precision, reversibility and high efficiency characteristics.

Ball screws generally consist of screws, nuts, balls, and reversers.

Screw nut unit

The screw nut unit is an efficient actuator that converts rotational motion into linear motion. There are usually two types of structures: split nut (left) and integral nut (right).Compared with split nuts, integral nuts have the following structural features and advantages:

① Integrated design of four-point angular contact ball bearing and ball nut → more compact structure;

② The inner ring of the ball bearing is omitted, so that the diameter of the steel ball can be increased → the load-bearing capacity is increased and large torque can be transmitted;

③Save the number of parts and simplify the system assembly process → facilitate assembly and improve quality;

④The ball bearing and ball nut are integrated, with better coaxiality → smooth operation and better noise and NVH

Ball screws are widely used in the automotive field With the trend of electrification in the automotive industry, screw nut units are widely used in new energy vehicle steering systems, braking systems, parking systems and suspension systems.

Electric Power Steering (EPS)

In an electric power steering (EPS) system, a ball screw connects the electric power steering motor and the steering device (such as the steering rack).

When the driver operates the steering wheel, the EPS system will control the electric power steering motor to output the corresponding torque based on the calculated optimal steering angle and force. At the same time, the steering torque will be transmitted to the steering device through the ball screw to ensure stability and control. sex.

The automobile steering system has gone through the following four stages of development:

▶Mechanical power steering system MS

▶Hydraulic power steering system HPS

▶Electric power steering system EPS

▶Steer-by-wire system SBW

Because EPS is small in size, consumes less power, is lightweight and flexible, it is the first choice for new energy vehicles. With the promotion of new energy vehicles and autonomous driving, steering systems tend to choose integrated screw nut-bearing assemblies that can withstand large torques.

Electronic parking system (EPB)

As automotive electronic control systems develop toward integration and intelligence, the caliper-integrated electronic parking system MOCEPB, which has faster response and more precise control, is gradually taking a dominant position in passenger cars and is increasingly used in light trucks. coming more and more widely.

In CNC machine tools and some special machinery, screw rods are widely used. In order to improve the service life and accuracy of the screw rod, the screw rod is hardened. This kind of screw rod is mainly ground in the end, and the accuracy is ensured by grinding with a thread grinder.

Screw heat treatment

Pay attention to the bending deformation of the ball screw during the heat treatment process. Try not to use the straightening process. If necessary, you can only use heat straightening. Because the screw rod that is straightened at room temperature may appear to be straightened in a short period of time, but it will bend and deform again after a period of time or once it is ground.

The processing standard of the screw rod is the center hole at both ends. The center hole should generally use a bell-shaped center hole, which can prevent end surface damage and affect the accuracy of the center hole. At the same time, the hardness of the center hole should reach 6O~65HRC. The accuracy of the center hole It is the key to ensure the accuracy of the screw. Before the rough grinding and fine grinding processes, the center hole grinding process is carried out after quenching and aging.

Screw grinding method and grinding amount selection

Correctly selecting the grinding amount can improve the accuracy of the screw and improve the surface quality, and reduce surface defects such as ripples and burns.

The selection of grinding amount should fully consider the material and heat treatment hardness of the workpiece, surface roughness, the particle size and hardness of the grinding wheel used, rough and fine grinding and other factors.

The key to manufacturing high-precision hardened screw rods depends on the reasonable selection of materials, the reasonable arrangement of hot and cold processes during the manufacturing process to maintain its dimensional stability, and the selection and control of relevant factors during the grinding process. Through production practice, it has been proved that products produced according to the above grinding process meet the design requirements and the quality is stable and reliable.

---EDITOR: Doris Hu

---POST: Doris Hu