This article presents all the information you need to know about ball screws.
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Ball screws are mechanical linear actuators that consist of a screw shaft and a nut that contain a ball that rolls between their matching helical grooves. The primary function of ball screws is to convert rotational motion to linear motion. Ball nuts are used in transmitting forces to a stationary or dynamic load with high accuracy, precision, and repeatability.
Ball screws are distinguished by their use of rolling balls within a helical groove, which minimizes mechanical contact inside the screw assembly and replaces sliding friction with rolling friction. This mechanism greatly reduces friction, leading to highly efficient power conversion. The efficiency of ball screws is assessed by their ability to convert rotational force into linear movement.
Ball screws feature a more complex structure and are generally bulkier compared to other types of screws. Although they are more expensive, their advantages and performance often justify the higher cost.
The primary components of ball screws are the screw shaft, the nut, and the ball bearings (or balls). We will explore their critical specifications and discuss how each affects the performance of the ball screw.
The screw shaft is the central component of the ball screw that receives rotational force, enabling it to rotate around its axis and translate this motion into linear movement. A motor, located at one end of the shaft, provides the power required for rotation.
The screw shaft is a long, cylindrical piece featuring a continuous groove, known as the ball groove, which spirals helically along its length. This groove is referred to as the thread of the screw.
The ball groove acts as the track for the rolling ball bearings. The profile of the ball groove can be either a semi-circular arc or a gothic arc. A semi-circular arc profile is shaped from a single curve, while a gothic arc profile features an ogival shape created from two curves. These two profiles differ in the contact points they establish with the ball bearings.
A ball screw is designated as a right-hand ball screw if the groove traces a clockwise direction around the screw shaft and slopes to the right. Conversely, it is a left-hand ball screw if the groove follows a counterclockwise direction and tilts to the left.
Details of the Screw Shaft Specifications:
The nominal diameter refers to the maximum diameter of the screw shaft, not including the ball bearings.
The ball circle diameter (or pitch circle diameter) is the distance between the centers of two opposing balls, measured when the ball bearings are in contact with the grooves.
The root diameter is the distance between the lowest point of one groove to the lowest point of the opposite groove. It represents the minimum diameter of the screw shaft.
The nominal diameter, ball circle diameter, and root diameter are essential for calculating application characteristics and determining the appropriate size of the screw.
The pitch is the axial distance between two consecutive threads.
Lead is the linear distance traveled along the axis of the screw with one complete rotation (360 degrees) of the screw. It is a crucial specification of a ball screw, affecting linear travel, speed, and load capacity.
As the lead of ball screws increases, both the linear distance covered and the speed rise. However, this also reduces the number of balls that can be accommodated around the screw shaft, leading to a decrease in load capacity.
The start refers to the number of independent helices that wrap around the screw shaft. Screws typically have one, two, or four starts. The lead of a screw is calculated as the number of starts multiplied by the pitch. Single-start screws are the most common, where the lead is equal to the pitch.
Multiple-start screws are used for applications requiring rapid linear movement with fewer rotations and high load capacity. This approach mitigates the limitations of higher leads. For example, in double-start screws, the lead is twice the pitch, meaning the axial distance covered in one full rotation is two times the pitch. Generally, a higher number of starts results in a greater linear distance covered per revolution.
The nut of a ball screw is a cylindrical component that contains the ball bearings and their recirculation system. It features internal ball grooves that align with the grooves on the screw shaft.
Key Specifications of the Ball Nut:
The circuit refers to the closed path within the recirculation system of the ball nut. Ball nuts with multiple circuits feature two or more independent closed paths, allowing them to support heavier loads compared to single-circuit ball nuts.
The turn of the circuit describes the number of times the ball travels around the circuit before being recirculated. The relationship between the turn and the circuit varies based on the recirculation method used in the ball screw.
The ball recirculation system in the nut enables the balls to be "recycled" by returning them to their starting point within the circuit. This process is facilitated by components such as deflectors, return tubes, and end caps, which guide the balls back to their original position.
The ball bearings, or balls, are a key component of ball screws, moving within the clearance between the nut and the shaft. Their primary role is to reduce friction between the nut and the rotating screw, which would otherwise be excessively high without their presence. Typically, these balls are made of steel.
The ball contacts both the screw shaft and the nut. The points of contact are determined by the groove profiles on each component where the ball interfaces. The common groove profiles are the gothic arc and the circular arc:
This is the most prevalent groove profile, created by two intersecting arcs. In this profile, the ball has two contact points on both the screw shaft and the nut.
This groove profile has a semicircular shape. With this design, the ball has one contact point on both the screw shaft and the nut.
Both groove profiles feature arcs with slightly larger radii than the ball itself. This design inevitably creates some clearance between the ball and the screw shaft or nut, which can lead to backlash. To mitigate this issue, preloading the ball screw is employed to eliminate the unwanted clearance.
The seal, a small yet crucial component of the ball screw, safeguards the entire assembly. It maintains the ball screw's efficiency by preventing contaminants and foreign materials from entering the clearance between the ball and the nut, while also ensuring the retention of lubrication within the assembly.
A high-precision ball screw exhibits minimal lead error. Lead error is the discrepancy between the theoretical and actual distance traveled by the nut as the screw shaft rotates. This error depends on the manufacturing accuracy of the ball grooves, the tightness of the assembly, and the precision of the setup. Lead error can vary depending on the specific lead of the screw.
Techniques to enhance lead accuracy in ball screws include preloading, lubrication, and improving mounting precision.
Preloading involves applying axial force to the balls and grooves of the screw shaft and nut to ensure they are tightly fitted. This process enhances rigidity and eliminates backlash, which is the lost motion resulting from clearance between the ball and the nut or screw tracks. Backlash can affect accuracy and repeatability, making it crucial for applications requiring precise positioning.
The choice of preloading mechanism determines whether the ball screw uses a single or double ball nut. Common preloading mechanisms in ball screw assemblies include the following:
Spacer preloading involves placing a spacer between two ball nuts to achieve the desired preload. The spacer exerts force on the adjacent sides of the two ball nuts, transferring this force to the nut and grooves. This method is typically used to set a large preload.
In spring preloading, a spring is placed between two ball nuts that transmit the preload. Tensional forces are applied on two adjacent sides of the ball nuts due to the spring force.
In machining, grooves are created with an offset in the lead, positioned in the center of a recirculation circuit. This method is more compact than double nut mechanisms because it eliminates the need for spacers or springs. However, this preload approach results in longer leads, which can reduce the load capacity of the ball screw.
Preload is applied by assembling larger ball bearings. The oversized balls produce a more compact structure by increasing the contact area of the balls to the grooves. However, this mechanism produces the smallest preload and is suitable in applications where precision is not that crucial.
Optimizing the preload value in ball screws and maintaining it during operation is crucial. Excessive preload demands more torque and can lead to increased heat generation.
Lubrication is crucial for ball screws as it helps prevent premature wear and enhances performance by reducing friction and minimizing heat buildup from the motion of the components. Both oil and grease lubricants provide a cooling effect. Excessive heat can cause thermal expansion of the components, which negatively impacts the ball screw's accuracy.
Poor lubrication can lead to galling, a type of abrasive wear where metal surfaces tear at a microscopic level due to continuous rubbing. Galling is commonly observed in threads of nuts, bolts, threaded fasteners, and ball screws. This issue can adversely affect the functionality of the ball screw.
Mounting refers to how the ball screw is supported during operation. To fully utilize the lead accuracy of a ball screw, precise mounting is essential. Incorrect mounting can lead to noise, vibration, positioning errors, and may result in material failure and accelerated wear during continuous use. It is crucial to inspect and promptly correct any faults in auxiliary components such as bearings, couplings, and nut brackets to maintain mounting accuracy.
Ball screws can be categorized based on their ball recirculation mechanism. The two primary types of ball recirculation systems are internal and external recirculation systems.
In an internal ball recirculation system, the balls remain within the nut housing during recirculation. This design avoids external protrusions, making ball nuts with internal recirculation more compact. Additionally, these systems generate less noise and vibration since recirculation occurs solely inside the nut housing. The types of ball screws in this category include:
Deflectors guide the balls over the screw shaft's diameter and direct them back to the adjacent groove from which they originated. Each turn of the screw requires a deflector to complete the ball’s path, meaning the number of turns equals the number of circuits in internal ball return systems.
This design is ideal for applications needing fine leads and is compact, making it suitable for assemblies with limited clearance.
When using tubes or caps for ball return, they can be designed to create multiple independent circuits that are placed adjacent to each other.
End caps are installed to direct the balls through a tunnel within the ball nut, guiding them back to their starting point. This design is particularly suited for high lead ball screw applications as it can withstand the forces from rapidly moving balls.
In an external ball recirculation system, the balls travel outside the nut housing during recirculation. This system supports fine lead ball screw assemblies and accommodates a wide range of screw shaft diameters. It is more cost-effective and suitable for mass production compared to internal recirculation systems. However, the recirculation mechanism can significantly increase the overall size of the ball screw.
The only type of ball screw employing an external ball recirculation system is the return pipe-type ball screw.
Another way to classify ball screws is based on the fabrication method used to produce the ball grooves:
Ground screw threads are produced using an abrasive process. In this method, the blank shaft is placed horizontally, and a very hard abrasive cutter carves out the grooves. This process results in a smoother groove surface compared to rolled screws. Ball screws manufactured by this method offer high precision but are more expensive and produced at a slower rate.
In an external ball recirculation system, the balls travel through a protruding external tube from the ball nut's wall. This return tube guides the balls back to their starting point. Fingers attached to the ends of the return tube help direct the balls as they enter and exit the tube. The length of the return tube is supported by a tube holding bracket, making the return tube assembly easy to install and remove.
Rolled screw threads are created through a cold working deformation process. The uncut blank shaft is passed through rotating tool dies to form the grooves. This process induces significant plastic deformation in the blank, resulting in a high-strength screw shaft. While this method is more economical and simpler to fabricate compared to ground screws, it produces a rougher surface. This roughness can reduce the ball screw's efficiency and wear resistance due to increased friction.
Ball screws can also be classified based on their preloading method, as discussed in the previous chapter.
Lead screws, like ball screws, serve a similar purpose to mechanical linear actuators that translate rotational motion to linear motion. However, these screws differ in many aspects. Their differences are enumerated in this chapter, as well as the pros and cons of each type.
The main difference between ball screws and lead screws is their method of load transmission. Ball screws use rolling and recirculating ball bearings within the grooves of the screw and nut. In contrast, lead screws feature deeper helical threads that slide against a matching nut.
Ball screws typically achieve higher torque conversion efficiencies (around 90%) compared to lead screws, which are less efficient due to higher friction losses. Ball screws reduce sliding friction by utilizing rolling friction, which is less than the sliding friction encountered with lead screws.
Lead screws are often made from polymeric materials coated with self-lubricating substances such as PTFE, silicone, or graphite, which reduce friction. Ball screws are generally constructed from high-rigidity materials like stainless steel to handle more demanding applications.
Ball screws offer a higher load capacity than lead screws. While lead screws are suitable and cost-effective for low to medium loads, ball screws are better suited for heavy-duty applications.
Ball screws usually require smaller motors due to their lower power consumption compared to lead screws.
Lead screws feature simpler and more compact designs and are more customizable. They are self-locking and do not need braking systems. In contrast, ball screws often require braking mechanisms to prevent back driving due to their complex design, which also contributes to their higher cost.
Ball screws are ideal for applications demanding high speed, accuracy, precision, and durability. They are used in precise positioning systems, automotive steering mechanisms, electric vehicles, energy-generating machinery (e.g., wind turbines, solar panels, hydroelectric equipment), aircraft machinery, and photolithography systems.
Lead screws are better suited for applications where speed, accuracy, and precision are less critical. They excel in vertical load transmission and are commonly used in medical equipment, food processing machinery, and laboratory equipment.
Ball screws require more frequent maintenance and lubrication compared to lead screws to maintain optimal performance.
Ball screws generally produce more noise than lead screws.
In summary, ball screws are prized for their power efficiency, accuracy, precision, and load capacity. While they offer superior performance features compared to lead screws, the latter are more cost-effective for applications where their benefits align with the requirements.
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