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.
The unique element of ball screws is the rolling balls in the helical groove which reduces the mechanical contact inside the screw assembly and replaces sliding friction with rolling friction. This mechanism significantly lessens the friction generated, which results in highly efficient power conversion. The efficiency of screws is measured by their capability to transform power utilized in exerting rotational force to the linear distance covered.
Ball screws have more complex structures and components and appear to be bulkier than other types of screws. They are more expensive than other screw types, but their benefits and capability outweigh their cost.
The main components of ball screws are the screw shaft, the nut, and the ball bearings (or balls). We will discuss their important specifications and their impact on the operation.
The screw shaft is the component of the ball screw that receives rotational force for it to rotate about its axis; this is translated to linear motion. The power to rotate the shaft is supplied by a motor, which is situated on its end.
The screw shaft is a long cylindrical shaft that has a continuous groove, called the ball groove, that runs helically around the length of its shaft, which is referred to as the thread of the screw.
The ball groove serves as the pathway for the rolling ball bearings. The ball groove profile of a ball screw may either be a semi-circular arc or a gothic arc. The semi-circular arc profile is formed from a single arc, while the gothic arc profile has an ogival shape formed from two arcs. The two groove profiles differ on the contact points they touch on the ball.
The ball screw is identified as a right-hand ball screw if the groove is traced in a clockwise direction around the screw shaft and slants to the right. It is a left-hand ball screw if the groove runs in a counterclockwise direction and it slants to the left.
Specifications of the Screw Shaft:
The nominal diameter is the maximum diameter of the screw shaft excluding the ball bearings.
Ball circle diameter (or pitch circle diameter) is the center-to-center distance of two opposite balls measured when the ball bearings are in contact with the grooves.
Root diameter is the distance between the bottommost section of the groove to the bottommost section of the opposite groove. It is the minimum diameter of the screw shaft.
The nominal diameter, ball circle diameter, and root diameter are used in calculating the application characteristics and in sizing the screw.
Pitch is the axial distance between two adjacent threads.
Lead is the linear distance along the axis of the screw that is covered by one complete rotation (3600) of the screw. It is an important specification of a ball screw which determines linear travel and speed and load capacity.
As the lead of ball screws increases, the linear distance covered and the speed increase. However, the number of balls accommodated around the screw shaft decreases, resulting also in a decrease in load capacity.
The start is the number of independent helices that run around the screw shaft. Screws typically have one, two, or four starts. The lead of a screw is equivalent to the number of starts multiplied by the pitch. Single start screws are the most common. In this type of screw, the lead is equivalent to the pitch.
Multiple start screws are used when a rapid linear movement is for a lesser number of rotations and high load capacity screws. This solves the consequences of using higher leads. For instance, in double-start screws, the lead is equivalent twice its pitch; this means that the axial distance covered is two-pitch units for one full rotation completed by the screw. The higher number of starts has the higher linear distance covered in one revolution
The nut of a ball screw is a cylinder that houses the ball bearing and its recirculation system. Ball grooves are also present in the internals of the nut that match the grooves of the screw shaft.
Specifications of the Ball Nut:
Circuit refers to the closed path in the recirculating system of the ball nut. Multiple circuit ball nuts have two or more independent closed paths. They are capable of carrying heavier loads than single circuit ball nuts.
The turn of the circuit refers to the number of trips the ball travels before being recirculated in the circuit. The relationship between the turn and the circuit depends on the recirculation method of a ball screw.
The ball recirculation system in the nut allows the balls to be "recycled" in the operation by returning them to their starting point in the circuit. Deflectors, return tubes, and end caps are devices used to feed the ball back to its original position.
The ball bearings, or the balls, are the most prominent component of the ball screws that moves between the clearance of the nut and the shaft. They make up the component dedicated to reducing the friction generated by the nut and the moving screw; this friction would be too great if the balls were not present in the screw assembly. The balls are usually made from steel.
The ball is in contact with the screw shaft and the nut. The contact points of the ball between the screw shaft and the nut are distinguished by the groove profile of the two components where the ball touches. The common groove profiles are the gothic arc and the circular arc:
This is the most common groove profile. It is made by two intersecting arcs. With this profile, the ball has two contact points on the screw shaft and two contact points on the nut.
This groove profile has the shape of a semicircle. With this profile, the ball has one contact point on the screw shaft and one contact point on the nut.
The arc in both profiles has slightly larger radii than the ball used. With this design, clearance between the ball and the ball or nut is inevitable. This clearance is unwanted because it causes backlash. It can be prevented by preloading the ball screw.
The seal is a minor component of a ball screw that protects the entire ball screw assembly. It preserves the efficiency of the ball screw by keeping contaminants and foreign materials from entering the clearance between the ball and the nut; it also retains the lubrication of the ball screw assembly.
A highly accurate and precise ball screw has a minimal lead error. The lead error is the difference between the theoretical and actual distance that the nut has traveled when the screw shaft has rotated. It is dependent on the manufacturing accuracy of the ball grooves, compactness, and set-up precision of the assembly. This value can vary from one lead to another.
Preloading, lubrication, and increasing the mounting accuracy are techniques used to increase the lead accuracy of ball screws.
Preloading is the application of axial force on the balls and the grooves of the screw shaft and the nut to make them compact. The purposes of preloading are to increase the rigidity and to eliminate the backlash of the ball screw assembly. Backlash is the lost motion caused by the clearance between the ball and the nut and screw tracks. It can disrupt the accuracy and repeatability, which are required for precise positioning applications.
The preloading mechanism dictates if the ball screw has a single or double ball nut. The preloading mechanisms commonly utilized in ball screw assemblies are as follows:
In spacer preloading, a spacer is inserted between two ball nuts to achieve the desired preload. The spacer applies force on the adjacent sides of two ball nuts; this force is transmitted to the nut and the grooves. This method is used for setting 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.
During machining, to create the grooves, an offset is created in the lead, which is in the middle of a recirculation circuit. Since there are no spacers or springs required, it is more compact than double nut mechanisms. However, this preload mechanism creates longer leads, which 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.
It is important to optimize the preload value in ball screws and maintain it during the operation. A large preload requires more torque and can cause excessive heat generation.
Lubrication is important in ball screws. It avoids the premature breakdown of the ball screw and increases its performance by lowering the coefficient of friction and by minimizing the heat build-up resulting from the motion of the ball screw components. Oil and grease lubricants have a cooling effect. Heat causes the thermal expansion of the components and negatively affects the accuracy of the ball screw.
Continuous rubbing of the metal components with poor lubrication causes galling, a form of abrasive wear that causes tearing of the metal surface at the microscopic level. Galling is common in threads of nuts and bolts, threaded fasteners, and threaded inserts, including ball screws. It has negative effects on the functionality of the ball screw.
Mounting is how the ball screw is supported during its operation. To benefit from the lead accuracy a ball screw offers, it must have an accurate mounting. Inaccurate mounting causes noise, vibration, positioning errors, and may cause material failure and accelerated wear during continuous operation. The faults of the installed auxiliary components such as bearings, couplings, and nut brackets must be checked and corrected immediately to preserve the mounting accuracy.
Ball screws may be categorized according to the mechanism of their ball recirculation. The ball recirculation mechanisms are grouped into internal recirculation and external recirculation systems.
In an internal ball recirculation system, the balls remain on the nut housing when they are recirculated. Since there are no external protrusions, ball nuts with this return system are more compact. They also generate less noise and vibration than external systems because recirculation only occurs inside the nut housing. The types of ball screws under this classification are:
Deflectors are used to lift the balls over the diameter of the screw shaft to guide the balls back to the adjacent groove, which served as their starting point. For every turn, there must be one deflector to close the pathway of the ball. Therefore, the number of turns is always equal to the number of circuits for internal ball return systems.
This design is used for applications that require fine leads. It has a compact size, which is suitable for small clearance assemblies.
When tubes or caps are used to return the ball, they can be designed to create multiple independent circuits that are placed adjacent to each other.
End caps are mounted to guide the balls through a tunnel within the ball nut‘s internal; this directs them back to their starting point. This design is ideal for high lead ball screw applications because it can endure the force from the fast-moving balls.
In an external ball recirculation system, the ball traverses outside the nut housing when it is recirculated. This system can support fine lead ball screw assemblies and a wide range of screw shaft diameters. It is suitable for mass production and is more economic than internal recirculation systems. However, the recirculation mechanism greatly affects the overall size of the ball screw.
The only type of ball screw that uses an external ball recirculation system is the return pipe-type ball screw.
Another classification of ball screws is according to the fabrication method of producing the ball grooves:
Ground screw threads are fabricated by an abrasion process. The blank shaft is positioned horizontally, and an extremely hard abrasive cutter carves out the metal to form the grooves. The groove surface produced is smoother than the rolled screw. Ball screws produced using this method have high precision but are more expensive and are produced slowly.
The balls pass through an external tube that is protruding from the ball nut‘s wall. The return tube directs the balls back to their starting point. Fingers are fixed on the endings of the return tube to guide the balls in entering and leaving the return tube. The length of the return tube is supported by a tube holding bracket. The return tube assembly is easy to install and dismantle.
Rolled screw threads are fabricated by a cold work deformation process. The uncut blank shaft is passed through rotating tool dies to form the grooves. The large plastic deformation of the blank results in a high strength screw shaft. It is more economical and easier to fabricate than the ground screw. However, the surface produced is rough, which decreases the efficiency and wear resistance of the ball screw due to the higher friction it will encounter.
The type of ball screw may also be distinguished according to its preloading method, which was 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 key difference between ball screws and lead screws lies in their manner of transmitting loads. Ball screws use rolling and recirculating ball bearings in the ball grooves in the screw and the nut. Lead screws have deeper helical threads in the screw shaft and a matching nut that slides with each other.
Lead screws do not typically reach the torque conversion efficiencies of ball screws (around 90%). Power losses due to friction are one of the causes of low efficiency. Ball screws address the sliding friction generated by metal lead screws. The friction acting on the ball screw component is the rolling friction, which is relatively smaller compared to sliding friction.
Lead screws are available in polymeric materials coated with materials with self-lubricating properties such as polytetrafluoroethylene (PTFE), silicone, and graphite; these generate less friction. Ball screws are made from materials with high rigidity and stiffness such as stainless steel.
Ball screws have a higher load capacity than lead screws. Lead screws are suitable and more cost-effective for low to medium loads.
Ball screws typically have smaller motors because they consume less power than lead screws.
Lead screws have simpler and more compact designs than ball screws. They are more customizable than ball screws. They are self-locking and do not require braking systems. Meanwhile, ball screws need braking mechanisms to eliminate back driving. The complex design of ball screws adds up to its costs.
Ball screws are used for heavy-duty applications requiring high speed, high throughput, high accuracy, and long service times. They are used in precise positioning systems, in automotive steering systems, in electric vehicles, in energy-generating machinery such as wind turbines, solar panels, and hydroelectric equipment, in aircraft machinery, and as a viewing system for the photolithography process.
Meanwhile, lead screws are used for transfer applications where speed, accuracy, and precision are not of high priority. Lead screws are also better in transmitting loads vertically. They are used in medical equipment, food processing equipment, and laboratory machinery.
Ball screws are more difficult to maintain than lead screws. They require more frequent lubrication to stay in top condition.
Ball screws generate more noise than lead screws.
To summarize, ball screws are valued for their power efficiency, accuracy, precision, and load capacity. Though they appear to have more superior features than lead screws, lead screws find their worth in applications where they are deemed to be more cost-effective than ball screws.
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