Ball Screws

Introduction

Ball screws are mechanical linear actuators that play a crucial role in transforming rotational motion into linear motion with high precision and efficiency. They consist of a screw shaft and a nut, which contain rolling balls that move between their matching helical grooves. In this comprehensive guide, we will delve into the world of ball screws, exploring their components, accuracy, types, and the advantages they offer over lead screws.

Chapter 1: What is a Ball Screw?

Ball screws excel at converting rotational motion into linear motion with remarkable accuracy, precision, and repeatability. The rolling balls in the helical grooves act as a replacement for traditional sliding friction, significantly reducing mechanical contact and increasing power conversion efficiency. The efficiency of ball screws is measured by their ability to convert rotational force into linear distance covered.

While ball screws may have a more complex structure than other types of screws, their benefits far outweigh their cost. They are widely used in various industries due to their exceptional performance and reliability.

Chapter 2: Components of the Ball Screw

Understanding the main components of a ball screw is crucial for selecting the right type for your specific application. The key components include the screw shaft, the nut, and the ball bearings.

Screw Shaft

The screw shaft is responsible for receiving the rotational force and converting it into linear motion. It features a continuous groove, known as the ball groove, that runs helically along its length, forming the thread of the screw. The ball groove serves as a pathway for the rolling ball bearings. The profile of the ball groove can be either a semi-circular arc or a gothic arc, depending on the contact points they provide on the ball. Ball screws can be classified as right-hand or left-hand based on the direction of rotation and slant of the groove.

Specifications of the Screw Shaft:

• Nominal Diameter: The maximum diameter of the screw shaft excluding the ball bearings.
• Ball Circle Diameter: The center-to-center distance of two opposite balls when in contact with the grooves.
• Root Diameter: The minimum diameter of the screw shaft, measured from the bottommost section of one groove to the bottommost section of the opposite groove.

The pitch and lead are two important specifications of a ball screw. The pitch refers to the axial distance between two adjacent threads, while the lead represents the linear distance covered by one complete rotation of the screw. As the lead increases, the linear distance covered and the speed of the ball screw increase, but the load capacity decreases.

Nut

The nut of a ball screw houses the ball bearings and their recirculation system. It contains internal ball grooves that match the grooves on the screw shaft. The nut plays a crucial role in transmitting forces to a stationary or dynamic load with high accuracy and precision.

Specifications of the Ball Nut:

• Circuit: The closed path in the recirculating system of the ball nut. Multiple circuit ball nuts can carry heavier loads than single circuit ball nuts.
• Turn: The number of trips the ball makes before being recirculated in the circuit. The turn is dependent on the recirculation method of the ball screw.

Ball Bearings

The ball bearings, or balls, are the essential components of ball screws that reduce friction between the nut and the screw shaft. They move within the clearance of the nut and the shaft, ensuring smooth and efficient motion. The ball bearings are in contact with both the screw shaft and the nut, and their contact points are determined by the groove profile of the two components.

The two common groove profiles are the gothic arc and the semi-circular arc. The gothic arc profile has two contact points on the screw shaft and two contact points on the nut, while the semi-circular arc profile has one contact point on each component.

To protect the entire ball screw assembly, a seal is used to prevent contaminants and foreign materials from entering the clearance between the ball and the nut. The seal also helps retain lubrication, ensuring the efficient operation of the ball screw.

Chapter 3: Accuracy of Ball Screws

The accuracy of ball screws is a critical factor for their successful application in various industries. Highly accurate ball screws have minimal lead error, which refers to the difference between the theoretical and actual distance traveled by the nut when the screw shaft rotates. Lead error depends on the manufacturing accuracy of the ball grooves, compactness, and setup precision of the assembly. Preloading, lubrication, and increasing the mounting accuracy are techniques used to improve the lead accuracy of ball screws.

Preloading

Preloading involves applying axial force to the balls and the grooves of the screw shaft and the nut to compact them. The primary purposes of preloading are to increase rigidity and eliminate backlash in the ball screw assembly. Backlash refers to the lost motion caused by the clearance between the ball and the nut and screw tracks, which can negatively impact accuracy and repeatability in precise positioning applications.

Different preloading mechanisms are used in ball screw assemblies, including spacer preloading, spring preloading, offset lead preloading, and oversized ball preloading.

• Spacer Preloading: A spacer is inserted between two ball nuts to achieve the desired preload. The spacer applies force on the adjacent sides of the nuts, which is then transmitted to the nut and the grooves.
• Spring Preloading: A spring is placed between two ball nuts to transmit the preload. Tensional forces are applied to two adjacent sides of the nuts due to the spring force.
• Offset Lead Preloading: During machining, an offset is created in the middle of a recirculation circuit, resulting in a more compact structure without the need for additional spacers or springs. However, offset lead preloading leads to longer leads, which reduces the load capacity of the ball screw.
• Oversized Ball Preloading: Preloading is achieved by using larger ball bearings, which increase the contact area of the balls with the grooves. Oversized ball preloading generates the smallest preload and is suitable for applications where precision is less critical.

Proper optimization and maintenance of the preload value are essential to ensure the optimal performance and longevity of ball screws.

Lubrication

Lubrication is of utmost importance in ball screws to reduce friction, minimize heat buildup, and prevent premature breakdown. Proper lubrication lowers the coefficient of friction and has a cooling effect, which helps maintain the efficiency and accuracy of the ball screw.

Continuous rubbing of metal components without adequate lubrication can lead to galling, a form of abrasive wear that can impair the functionality of the ball screw. Therefore, ensuring proper lubrication is essential to optimize the performance and lifespan of ball screws.

Increasing the Mounting Accuracy

Accurate mounting of the ball screw is crucial to benefit from its lead accuracy. Inaccurate mounting can result in noise, vibration, positioning errors, material failure, and accelerated wear during continuous operation. Checking and correcting any faults in the installed auxiliary components, such as bearings, couplings, and nut brackets, ensures the precise mounting of the ball screw.

Chapter 4: Types of Ball Screws

Ball screws can be categorized based on their ball recirculation mechanism, the manufacturing method of the threads, and the preload method used.

Internal Ball Recirculation

In an internal ball recirculation system, the balls remain within the nut housing during recirculation. This system offers compactness, less noise, and vibration due to the recirculation occurring inside the nut. The two types of ball screws in this category are:

Deflector-Type Ball Screws

Deflectors are used to lift the balls over the diameter of the screw shaft, guiding them back to their starting point. This design is suitable for applications requiring fine leads and features a compact size, making it ideal for small clearance assemblies.

End Cap-Type Ball Screws

End caps are mounted to guide the balls through a tunnel within the ball nut’s internals. This design is suitable for high lead ball screw applications, as it can withstand the force generated by fast-moving balls.

External Ball Recirculation

In an external ball recirculation system, the balls traverse outside the nut housing during recirculation. This system supports fine lead ball screw assemblies and a wide range of screw shaft diameters. It is more economical than internal recirculation systems; however, the recirculation mechanism influences the overall size of the ball screw.

The return pipe-type ball screw is the only type that uses an external ball recirculation system.

Another classification of ball screws is based on the fabrication method of the threads:

Ground Screw Threads

Ground screw threads are produced through an abrasion process, where an extremely hard abrasive cutter carves out the metal to form the grooves. This method results in highly precise ball screws but is more expensive and time-consuming.

Rolled Screw

Rolled screw threads are created through cold work deformation, with the uncut blank shaft passed through rotating tool dies. This method is more cost-effective and easier to fabricate than ground screw threads, but the surface produced is rougher, leading to higher friction and decreased efficiency.

Preload Method

The preload method used in the ball screw assembly also varies. The discussed preloading mechanisms, such as spacer preloading, spring preloading, offset lead preloading, and oversized ball preloading, dictate the level of preload applied to the ball screw.

Chapter 5: Comparison of Ball Screws and Lead Screws

While both ball screws and lead screws serve the purpose of converting rotational motion into linear motion, there are significant differences between the two.

Mode of Operation

Ball screws use rolling and recirculating ball bearings in the grooves of the screw and the nut, while lead screws have deeper helical threads that slide with the matching nut.

Efficiency

Ball screws offer higher torque conversion efficiencies (around 90%) due to the rolling friction, as opposed to the sliding friction encountered in lead screws.

Material

Lead screws are available in polymeric materials with self-lubricating properties, such as PTFE, silicone, and graphite, to reduce friction. Ball screws are typically made from materials like stainless steel for enhanced rigidity and stiffness.

Capacity

Ball screws have a higher load capacity compared to lead screws. Lead screws are more suitable and cost-effective for low to medium loads.

Power Requirement

Ball screws typically require smaller motors due to their lower power consumption compared to lead screws.

Design

Lead screws have simpler and more compact designs than ball screws, making them easier to customize. They are self-locking and do not require additional braking systems, unlike ball screws. The complex design of ball screws contributes to their higher cost.

Application

Ball screws are extensively used in heavy-duty applications that require high speed, accuracy, and long service life. They find applications in precise positioning systems, automotive steering systems, electric vehicles, energy-generating machinery (e.g., wind turbines, solar panels), aircraft machinery, and the photolithography process.

Lead screws are suitable for transfer applications where speed, accuracy, and precision are not of high priority. They find applications in medical equipment, food processing equipment, and laboratory machinery.

Maintenance

Ball screws require more frequent lubrication and maintenance compared to lead screws to ensure optimal performance and longevity.

Noise

Ball screws generate more noise than lead screws.

In summary, ball screws offer superior power efficiency, accuracy, precision, and load capacity, making them the preferred choice for various industrial applications. However, lead screws are a more cost-effective option for low to medium loads and applications where accuracy and load capacity are not critical.

Conclusion

Ball screws are sophisticated mechanical linear actuators used in a wide range of industries. Understanding the components, accuracy, types, and advantages of ball screws is crucial for selecting the right type for your specific application. Whether you require precise positioning, high speed, or heavy load capacity, ball screws excel in providing efficient and reliable linear motion solutions.