Ball screw is the most common transmission method in linear transmission in non-standard fields. Today we will introduce the ball screw.
- Introduction to ball screw
- The composition and classification of ball screws
- Several installation methods of ball screw
- Ball screw selection
- Precautions for using ball screw
Introduction to Ball Screw
Ball screws are ideal products for converting rotary motion into linear motion or converting linear motion into rotary motion.
The ball screw is the most commonly used transmission element in tool machinery and precision machinery. Its main function is to convert rotational motion into linear or torque into axial repeated force. It also has high precision, reversibility, and high-efficiency characteristics. Due to its small frictional resistance, ball screws are widely used in various industrial equipment and precision instruments.
The ball screw comprises a screw, nut, steel ball, preloading piece, reverser, and dust protector. Its function is to convert rotational motion into linear motion, which is a further extension and development of the Acme screw. The important significance of this development is to change the bearing from sliding action to rolling action.
In short, the ball screw converts rotary motion into linear motion and linear motion into rotary motion.
The Composition and Classification of Ball Screws
The ball screw is composed of a screw shaft and a nut, and the nut is composed of steel balls, pre-pressed pieces, reversers, dust guards, etc. See below:
Common Types of Ball Screws
1. Ultra-high DN value ball screw: high-speed machine tools, high-speed comprehensive machining centers;
2. End cover ball screw: rapid handling system, general industrial machinery, automated machinery;
3. High-speed ball screw: CNC machinery, precision machine tools, industrial machinery, electronic machinery, high-speed machinery;
4. Precision grinding grade ball screws: CNC machinery, precision machine tools, industrial machinery, electronic machinery, conveying machinery, aerospace industry, actuators, valve switching devices used in other antennas, etc.;
5. Nut rotating (R1) series ball screws: semiconductor machinery, industrial robots, woodworking machines, laser processing machines, conveying devices, etc.;
6. Rolling-grade ball screw: It has the advantages of low friction, smooth operation, fast supply, and low price;
7. Heavy-duty ball screws: all-electric injection molding machines, stamping machines, semiconductor manufacturing equipment, heavy-duty brakes, industrial machinery, and forging machinery
According to the accuracy level, it is divided into rolling and grinding grade screws. The rolling screw has a relatively low accuracy and is suitable for situations where the accuracy requirements are not very high. The grinding grade screw has a relatively high accuracy and is ideal for applications with high accuracy requirements.
According to the ball circulation method, the nut can be divided into external circulation, inner circulation, and end cover type. Let's talk about the end cap type first. This is a relatively early structure, but its shortcomings are apparent. Now, it has been eliminated and is rarely used.
Let's focus on the inner and outer circulation:
The balls that sometimes break out of contact with the screw during the circulation process are called outer circulation; those that always remain in touch with the screw are called inner circulation. Each closed circuit of the ball is called a row, and the number of leads contained in each closed course of the ball is called the number of turns. Each nut of the internal circulation ball screw pair has 2 rows, 3 rows, 4 rows, 5 rows, etc., and each row has only one turn; each row of the outer circulation ball screw has 1.5 turns, 2.5 turns, and 3.5 turns.
1) External circulation:
External circulation is when the balls return to the space between the screw nut through the spiral groove or intubation on the outer surface of the nut and re-enter the circulation after the circulation process. As shown in Figure 2-3, the return methods of the external circulation ball screw nut pair during ball circulation mainly include end cap type, cannula type, and spiral groove type. Commonly used external circulation methods are end cap, intubation type, spiral groove type. End cover type, a longitudinal hole is machined on the nut as a return channel for the balls. The cover plates at both ends of the nut are provided with return ports for the balls, through which the balls enter the return pipe to form a circulation. The intubation type uses a bent pipe as the return pipe. This structure has good craftsmanship, but because the pipe protrudes outside the nut, the radial size is larger—spiral groove type, which mills a spiral groove on the outer circle of the nut. Through holes are drilled at both ends of the track and are tangent to the threaded raceway to form a return channel. This structure has a smaller radial size than the intubated structure but has a smaller radial dimension than the intubated structure. Manufacturing is more complicated. The external circulation ball screw has a simple outer circulation structure and manufacturing process and is widely used. The disadvantage is that it is difficult to make the raceway joints smooth, which affects the stability of the ball raceway.
2) Internal circulation:
The internal circulation uses a reverser to realize ball circulation. There are two types of reversers. The cylindrical convex key reverser has a cylindrical part embedded in the nut and a reverse groove 2 at the end. The reverse groove is positioned by the round key 1 on the outer surface of the cylinder and its upper end to ensure alignment with the direction of the threaded raceway. Oblate insert reverser: the reverser is a general round-headed flat key insert. The insert is embedded in the nut groove, with a reverse groove 3 at its end, which is positioned by the outer contour of the insert. Comparing the two reversers, the latter is smaller, thereby reducing the radial size of the nut and shortening the axial size. However, the outer contour of this reverser and the dimensional accuracy of the grooves on the nut require relatively high accuracy.
Installation Methods of Ball Screw
There are three main types of typical applications of ball screws:
1. One end is fixed, and the other is supported. This is the most common installation method, suitable for medium-speed to high-speed rotation, and the accuracy level can reach medium precision and high precision;
2. Fixed at both ends, only suitable for medium-speed rotation and with high precision;
3. One end is fixed, and the other is free, suitable for low-speed rotation and medium precision.
In the Yiheda selection manual, you can see the specific models and installation methods, which are very practical.
Ball Screw Selection
1. Screw shaft diameter
Typical specifications are (unit: mm): 8, 10, 12, 14, 15, 20, 25, 32. Generally, the larger the shaft diameter, the greater the load.
The load that screws with different shaft diameters can be viewed in the manufacturer's catalog. The relevant parameters are dynamic-rated load and static-rated load. The static rated load refers to the rated burden that the screw can bear in a dormant state, but we cannot keep it stationary with the screw, so here, we only care about the dynamic rated load. Then, the smaller the ratio between the actual load and the rated load, the longer the theoretical life of the screw. (Note: The rated load does not represent the maximum load, but generally, it should not exceed the rated load. Otherwise it will reduce the service life of the screw)
Lead refers to the distance the nut moves linearly when the screw rotates once (many engineers don't know this, so mark it carefully). Common leads are (unit: mm): 2, 4, 5, 8, 10, 20, 25, 32. It is important to mention here that there is no special relationship between lead and axial clearance. The size of the lead is determined by the linear speed of the transmission.
Ball screws, according to domestic classification, have precision grades of P1, P2, P3, P4, P5, P7, and P10. Japan, South Korea, and Taiwan Province of China adopted JIS grades, namely C0, C1, C2, C3, C5, and C7. , C10; the standards of European countries adopt IT0, IT1, IT2, IT3, IT4, IT5, IT7, IT10.
Generally, our company purchases ball screws from Taiwan, which are more cost-effective, followed by those from Japan.
The accuracy is expressed as follows: No matter how long your ball screw is, if you pick a section of 300mm, the error will be within the accuracy represented by the grade. The accuracy represented by each grade is as follows:
Generally speaking, ordinary machinery adopts C7, C10 level, CNC equipment generally adopts C5, C3 level (C5 is more, most domestic CNC machine tools are C5 level), aviation manufacturing equipment, precision projection and three-coordinate measuring equipment generally use C3, C2 accuracy (as far as I know, ball screws above C3 are not available in the domestic civilian market).
In addition, grades C7 and C10 are generally manufactured by rolling methods, and grades C5 and above are manufactured by grinding processes.
To sum up, the accuracy grade of ball screws commonly used in non-standard designs is C7 (manufactured by rolling method or some people call it transformation), while those with higher requirements on the accuracy grade of ball screws are C5 (manufactured by grinding process). It's enough.
4. Preload level
It is also called preloading. Regarding preloading, we do not need to know the specific preloading force and preloading method. We only need to select the preloading level according to the manufacturer's sample. The higher the preload level, the tighter the nut and screw fit; conversely, the lower the level, the looser the fit (this is very important in the SCARA robot application industry, as the load will affect the preload, thereby affecting accuracy).
The principles to be followed are large diameter, double nuts, high precision, and large driving torque. When the above situations occur in the application of the screw, the preload level can be selected higher. Otherwise, the preload level can be selected lower.
The following is an introduction to how to select a screw rod:
First step, is to determine the type of screw that is suitable for your working conditions based on the application scenarios of the various screws mentioned in the "Classification of Ball Screws" above; you can also determine the accuracy level of the screw (usually C7) and Preload level;
Second step, is to determine the shaft diameter of the ball screw according to the size of the load;
Third step, is to determine the lead according to the moving speed required by the load; after selecting the lead, determine the torque to be provided by the drive motor based on the relationship between thrust and lead.
Details as follows:
The object moves vertically up and down, the weight is 60, and the required moving speed is 1m/s;
1) If you choose a servo motor as the drive and the rated speed is 3000r/min=50r/s, according to the formula v=r*i, determine the lead to be 20;
2) Then calculate the size of the load: Assume that the acceleration and deceleration time of the servo motor is set to 0.3s, then the acceleration is 3.3m/s², and the load F=600+60*3.3=798N (friction is ignored here);
3) According to the formula: F=(2*π*T*n)/i, n is taken as 90%, and T≈2.82N·m is calculated. Check that the rated torque of the 1KW servo motor is 3.18N·m, which meets the requirements.
4) Choose an appropriate itinerary.
The above is a brief introduction to the above steps. The YIHEDA selection manual has more detailed steps. If you want to know more, please get in touch with us.
Precautions for Using Ball Screw
1. When installing, be careful not to damage the tooth surface of the screw; after the screw is purchased, the nut and screw are together, so when installing, pay special attention. First, at both ends. Tie a tie or install a limit block. Do not separate the nut and screw. Once separated, it will be challenging to install them. If it is accidentally separated, do not force yourself to install it. You can contact the supplier and ask them to install it;
2. Lubrication: There is metal friction between the ball and the screw, so lubricating oil or grease must be added between the two. Otherwise, the service life will be reduced; the general adding methods include manual and automatic refueling. Manual refueling is performed regularly by workers. Oil filling and automatic oiling require an automatic oiling system;
3. Dustproof: Ball screws are the same as rolling bearings. When mixed with debris, water, or cutting fluid, the wear will increase, sometimes leading to damage. When designing, pay attention to the dustproof ness of the screw;
4. When the eccentric load phenomenon occurs, it will directly affect the life and noise of the screw and is accompanied by a rough operating feel. If the smoothness of the screw is different when it is unloaded and after assembly, in addition to paying attention to the accuracy of the screw itself, most of them is the phenomenon of eccentric load caused by poor combination accuracy, as shown in the figure below:
5. The shortcomings of the screw are also very obvious. It cannot be self-locking and has reversibility of transmission.