Ball Screw Buy
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Precision rolled ball spindels (screws) for CNC machines and other automation applications which require precision linear motion. Ball screws are available in two lead accuracy classes C5 (0,023/300 mm) and C7 (0,050/300 mm). Recirculating ball nuts and screws are available with normal or big lead for high-speed applications. Ball screws can be cut to length and end machining of the screw can be done to fit ball screw end support. End supports with bearings for ball screws are available for vertical or horizontal mounting.
Lead screws are an excellent linear motion product that converts rotary motion to linear motion very economically and until recently offered only moderate accuracy and repeatability. Today's top manufacturers are producing standard lead screws with .003"/inch lead accuracy and even better on request. These companies are also producing wide ranges of sizes and leads to choose from and readily produce custom sizes and shapes of nuts to fit these screws. Here are four things that every engineer should keep in mind when selecting any lead screw.
NSK standard linear ball screw assemblies have been developed using cutting-edge friction control technology. NSK's pioneering manufacturing methods leave no room for error, making NSK ball screws highly precise and reliable linear machine components.
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 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.
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
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:
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 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.
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.
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.
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.
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. 781b155fdc