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Ball Screw Components, Working, Types, Applications, Advantages

Ball screw is also known as ball bearing screw or recirculating ball screw. A ball screw is a mechanical linear actuator and it is used to convert rotary motion into linear motion or torque to thrust, with the features of high accuracy, reversibility, efficiency with little friction. In ball screw Sliding friction is replace by rolling friction due to this very less and negligible friction produces. There is almost no heat generation in ball screw due to negligible friction. Ball screw gives accurate motion after elimination of backlash and backlash can be eliminate by preloading (tension or compression) as well as while manufacturing, it made with close tolerances to achieve high precise motion. It is also able to apply or withstand high thrust loads.

Ball screw Components

Figure 1. Ball screw Components

Ball Screw consists of a screw spindle, nut, balls and integrated ball return mechanism. Ball screw component is shown in figure 1; Information of these ball screw components is as per following. 1.1. Screw spindle Screw spindle having external helical groove, which forms threads in screw spindle. Thread form used in this screw spindle can either be gothic arc type (figure 2) or circular arc type (figure 3). Screw spindle threads may be right handed or left handed but mostly right handed screw spindles are used.

Figure 2. Gothic arc thread form of screw spinlde

Figure 3. Circular thread form of screw spindle

1.2. Nut Nut have internal helical groove, which forms threads and thread form used in this nut can either be gothic arc type or circular arc type. Sometime ball recirculation hole or channel is provided in the nut. In figure 1, nut of ball screw with circular arc thread form is shown. 1.3. Balls Ball is a spherical shape component which is used in ball screw to convert Sliding friction into rolling friction. In ball screw, number of balls is used. In figure 1, balls of ball screw is shown.

1.4. Ball returns mechanism or system There are two types of Ball return mechanism or system, which are External Ball Return System and Internal Ball Return System. External Ball Return System In this type of return system, the balls are returned to the initial position through a ball return tube. Ball return tube is assembling at outside the nut. Internal Ball Return System In this type of return system, the balls are returned to the initial position through ball return channel or hole and this channel or hole is into the nut as shown in figure 1.

As shown in figure 1, screw spindle assemble into the nut and balls are inserted into the groove of screw spindle and nut. In this ball screw, ball return channel or hole is in the nut. In some ball screw, return tube is used instead of this hole or channel. This return tube creates path to the balls for recirculation. As per operations and purpose, nut is assembling with machine parts which help to move it. A movement of nut is guided with the help of guide ways.

2. Working of ball screw As screw spindle rotates, the nut translates the moving part linearly (which is assembled with nut) along the guide ways. In this way rotary motion is converted into linear motion. The balls of ball screw rolls along the helical groove and then they are guided at a certain spot to change their path after they have reached a certain spot with the help of ball return mechanism or system. Due to this, balls one after another reach to their starting point. In this way recirculation of balls is done as well as the sliding friction is converted into rolling friction. The movements of ball is shown in following video.

3. Preloading in ball screw Preloading is a method used to eliminate backlash in ball screw by creating elastic deformations in balls and ball grooves. Backlash of the nut and the screw spindle is eliminating in advance by providing an axial load. There are two methods of preloading and these are as follow. 3.1. Double Nut Preloading In double nut preloading, two nuts are used and preloading is achieved by applying axial tension or compression load on the nut. And this is done by following method.

By Tension Method Tension preloading is obtained by inserting an oversize spacer between the two nuts and effectively pushing the nuts apart. Which creates axial tension into two nuts and tension preloading is achieved in ball screw. As shown in figure 4

Figure 4. Axial tension preloading in ball screw

By Compression Method Compression preloading is created by inserting an undersized spacer and correspondingly pulling nuts together. Due to pulling of these two nuts axial compression load is develop and compression preloading is achieved in ball screw.

3.2. Single Nut Preloading There are two methods of preloading in single nut ball screw. One is called oversized ball preloading method and other is offset pitch preloading method.

In oversized ball preloading, slightly larger balls are used than groove of screw spindle and nut. These oversized balls are inserted into groove. Due to oversize ball it makes contact at four points to groove profile of screw spindle and nut. In this way, backlash is eliminating in the single nut preloading using oversized balls.

In offset pitch preloading method, groove pitch in the middle of the nut is changed to achieve preloading. It is more compact than the double-nut method.

3.3. Effects of mounting errors while preloading In case of mounting errors between the screw spindle and the nut while preloading may causes to 1. It will Reduced positioning accuracy. 2. It will affect to smooth operation. 3. It will cause to breakage of screw shaft. 4. It will reduce service life. 5. It will produce noise and vibration.

4. Ball screw types There are different types of ball screw which are classified on the basis of screw spindle, ball circulation and preloading method and these are as per following

4.1. According to screw spindle

4.1.1. According to direction of turns of ball screw thread Right hand screw These types of ball screw uses right hand screw spindle and hence these are known as right hand screw type ball screw. Left hand screw These types of ball screw uses left hand screw spindle and hence these are known as left hand screw type ball screw.

4.1.2. According to thread form of screw spindle Gothic arc type Thread form used in this screw spindle is gothic arc type. Circular arc type Thread form used in this screw spindle is circular arc type.

4.2. According to ball circulation method In this category, ball screws types depend upon circulation method of balls and these are further divided into internal ball return system and external ball return system. Types of this category are as following. 4.2.1. Return tube or return pipe type ball screw This is external ball return type ball screw in which return tube is used for circulation of balls located at the outside of nut. The return tube allows balls to be picked up, pass through the tube, and return to their original positions to complete recirculation of balls. Balls complete their recirculation in two or more turns. 4.2.2. Deflector type ball screw This is internal ball return type ball screw in which deflector is used to change travelling direction of balls. Balls complete their recirculation in only one turn and return to their original positions. 4.2.3. End cap type ball screw This is internal ball return type ball screw, in which end caps are used at the both side of nut and used to picked up balls from groove space and pass through the through hole of the nut, and return to their original positions to complete recirculation of ball screw.

4.3. According to preloading method 4.3.1. Single nut preloading type ball screw In this type of ball screw, single nut is used for preloading and hence it is known as single nut preloading type ball screw. 4.3.2. Double nut preloading type ball screw In this type of ball screw, double nut is used for preloading and hence it is known as double nut preloading type ball screw.

5. Characteristics of ball screw

1. High mechanical efficiency

2. Low in wear and tear

3. Estimation of life is possible

4. Specifications are standardized

6. Selection criteria of ball screw There are some factors which consider while selecting a ball screw and these are as following.

1. Accuracy of motion

2. Stiffness of ball screw

3. Maximum rotational speed

4. Applied load or thrust force on nut

5. Required torque

7. Dust Prevention of Ball Screw When dust, debris or foreign matter enters inside of the nut, it could affect smoothness in operation, causes to premature wearing, breakage of components of ball screw. To prevent such things from occurring, seals are provided to keep contaminants out. There are various types of seals like plastic seal or brush type of seal, bellows, screw cover, wiper ring used in ball-screw drives.

8. Precautions on Using the Ball Screw

1. While mounting of ball screw components, necessary care and correct procedure must be followed to avoid mounting errors effect.

2. Disassembling component or removing the ball screw nut from the ball screw shaft causes to enter dust inside to the system and balls or the nut to fall off.

3. Prevent or avoid to dust, debris or foreign matters from entering inside to ball screw system for continuous better performance or function.

4. Avoid speed exceeding than permissible rotation speed may cause breakage of a component or accident.

5. Overrunning of ball screw nut will cause balls to fall off or the ball circulating component to be damaged. So avoid to overrunning of ball screw.

9. Ball screw standards To design a ball screw National and international standards are used to standardized ball screw. There are several different standards used to govern various aspects of ball screw design, like DIN, ISO, and JIS

10. Applications of ball screws

1. Ball screws are employed in cutting machines, where accurate positioning of the table is desired.

2. Used in inspection apparatus where precise positioning is vital

3. Used in robotics application where precision positioning is needed.

4. Used in medical examination equipments since they are highly accurate and provide smooth motion.

5. CNC Machinery- CNC machine centre, CNC lathe, CNC machine, CNC EDM, CNC grinder, boring machine, etc.

6. Precision machine tools-Milling machine, Grinder, manufacturing, Planer, etc.

11. Advantages of ball screws

1. Highly efficient and reliable.

2. Less starting torque.

3. Lower coefficient of friction compared to sliding type screws.

4. Power efficiency is very high near about 95 %.

5. Could be easily preloaded to eliminate backlash.

6. Has longer thread life.

7. Smooth movement over full range of travel.

8. No heat generation during operation.

12. Disadvantages of ball screws

1. Tends to vibrate.

2. Require periodic overhauling to maintain their efficiency.

3. Ball screws can have problems with vertical applications

4. Ball screws require braking systems.

5. Inclusion of dirt or foreign particles reduces the life of the screws.

6. They are not self-locking screws hence cannot be used in holding devices such as vices.

7. Proper lubrication is required.

8. Ball screws are more expensive than lead screws.