Ball screws are ideal products for converting rotary motion into linear motion, or converting linear motion into rotary motion.
Ball screw is the most commonly used transmission element in tool machinery and precision machinery. Its main function is to convert rotary motion into linear motion, or convert torque into axial repetitive force, and it has the characteristics of high precision, reversibility and high efficiency. Ball screws are widely used in various industrial equipment and precision instruments due to their low frictional resistance.
In short, a ball screw is a mechanism that can convert from rotary motion to linear motion or from linear motion to rotary motion, but its general application is to convert rotary motion to linear motion. Regarding this, we know that it is working fine.
The ball screw is composed of a screw shaft and a nut. The nut is composed of a steel ball, a preload, a commutator, and a dustproof device. . For more detailed information, see the figure below:
There are too many types of ball screws. Here, we will list some of the more common ones.
1. Self-lubricating ball screw: self-lubricating ball screw with detachable oil removal device, no need to lubricate pipeline system and equipment, reducing oil change and waste oil treatment costs;
2. Silent ball screw: The principle is to set a special groove-shaped ball spacer between the balls, which can suppress the noise generated by the collision between the balls and make the ball screw move more quietly and smoothly;
3. High-speed ball screw: It has the characteristics of high acceleration, high rigidity, high-speed feed, low vibration and low noise. Used for rapid feed of machine tools, abrasive high-speed cutting centers and high-speed longitudinal load cutting centers;
4. Heavy-duty ball screw: It can withstand larger axial loads and is suitable for all-electric design machines, air compressors, semiconductor manufacturing equipment and forging manufacturing equipment.
There are also differences between rolled and ground grade screws. The precision of the ball screw is low, which is suitable for occasions where the precision is not high. And you can see the screw of the grinding grade according to its name. High, suitable for occasions requiring high precision.
According to the ball circulation method in the nut, it can be divided into outer circulation, inner circulation and end cap types. Let me talk about the end cap type first. This is a relatively early structure, but the shortcomings are more obvious. It has basically been eliminated and is rarely used, so I will mention it a little bit here.
We will not talk about the specific structure of the internal and external circulation nuts. After all, we don't need to make screws, we just need to know the difference between the two and their respective advantages and disadvantages.
The advantages and disadvantages of internal and external circulation ball screws:
1. The internal circulation ball screw is small in size, the reverser is fixed firmly, the rigidity is good, and it is not easy to wear;
2. The reverse stroke of the ball screw is short, the number of balls is small, the fluency is good, the friction loss is small, and the transmission efficiency is high;
3. The outer diameter of the inner circulation nut with the same nominal diameter is smaller than that of the outer circulation, which is suitable for occasions where the installation space is relatively compact;
4. One end of the ball screw shaft that matches the internal circulation nut must be smaller than the valley diameter;
5. Compared with the external cycle, the impact resistance of the internal cycle is not as good as that of the external cycle, so most of the large screws and high-load screws adopt the external cycle structure.
6. The internal circulation type multifunctional plastic frequency converter has certain requirements on the working temperature.
Initially, the installation method of the ball screw should be placed on the back, and then I thought about it. When determining the choice of screws, you need to determine the installation method first, and then move it to the front.
I have listed several common installation methods that have not been mentioned, and welcome to add them in the comment area.
The conventional installation methods are the above four. You can choose the screw installation method according to your own working conditions. Different installation methods have different screw ends. Please note.
Speaking of ball screw selection, you need to talk about its common parameters first, and then you can start from these parameters to determine its model.
1. Nominal diameter of screw
It is the shaft diameter of the screw. Common specifications are (unit: mm): 12, 14, 16, 20, 25, 32, 40, 50, 63, 80, 100. Regarding the shaft diameter of the screw and its related bearing capacity. Generally, the larger the shaft diameter, the greater the load.
The load that a non-coaxial diameter screw can bear can be viewed in the manufacturer's catalog. The relevant parameters are the dynamic load rating and the static load rating. The static load rating refers to the rated load that the screw can withstand in a static state, but we cannot keep the screw still at all times, so here, we basically only care about the dynamic load rating.
Then, the smaller the ratio of the actual load to the rated load, the longer the theoretical life of the screw. (Note: The rated load does not represent the maximum load, but generally do not exceed the rated load, otherwise the service life of the screw will be reduced)
2. Lead
Leading refers to the distance the nut moves linearly after one revolution. Common leads are (unit: mm): 2, 4, 5, 6, 8, 10, 16, 20, 25, 32, 40. The parameter related to the lead is the moving speed of the nut, and the ball screw can provide linear thrust .
The larger the lead, the faster the linear motion at the same speed. The specific calculation relationship is:
v=ri
v: the moving speed of the nut (unit: mm/s)
r: screw speed (unit: r/s)
i: Lead (unit: mm)
The relationship between lead and screw thrust:
F = (2 π T n)/ i
F: Thrust (unit: N)
T: Torque provided by the motor (in N•m)
n: transmission efficiency (the transmission efficiency of the ball screw is generally 85 percent -95 percent )
i: lead (the unit here is m)
3. Accuracy
Ball screws are classified according to the country/region, and the accuracy grades are P1, P2, P3, P4, P5, P7, P10. Japan, South Korea and Taiwan use JIS grades, namely C0, C1, C2, C3, C5, C7, C10; European countries/regions adopt IT0, IT1, IT2, IT3, IT4, IT5, IT7, IT10.
Usually, the ball screws purchased by our company come from Taiwan, which is more cost-effective, followed by Japan.
The accuracy representation method is: no matter how long your ball screw is, any 300mm segment, the error is within the accuracy of the grade, the accuracy of each grade is as follows:
Generally, C7 and C10 are used for ordinary machinery, C5 and C3 are usually used for CNC equipment (more C5, most CNC machine tools in China are C5), aviation manufacturing equipment, precision projection and coordinate measuring equipment are usually used for C3, C2 accuracy (As far as I know, there is no ball screw above C3 in the domestic civil market).
In addition, C7 and C10 grades are usually manufactured by rolling methods, while C5 grades and above are manufactured by grinding methods.
In summary, the precision grade of ball screws commonly used in non-standard designs is C7 (manufactured by rolling or some people call them remanufactured), while the precision grade of ball screws has higher requirements, C5 ( Manufactured by grinding method) is sufficient. Of course, we still have to carry out specific analysis on specific issues.
4. Preload level
Also called preload. Regarding the pre-tightening force, we do not need to know the specific pre-tightening force and the pre-tightening force method, but only need to select the pre-tightening force level according to the manufacturer's sample. The higher the preload level, the tighter the fit of the nut and the screw; otherwise, the lower the level, the looser it will be.
The principles to follow are: large diameter, double nuts, high precision and large driving torque. If the above situation occurs when using screws, you can choose a higher pre-tightening force, and vice versa.
After understanding the main parameters of the above screws, we can choose according to our needs.
The first step is to determine the screw type suitable for your working conditions according to the various screw application scenarios mentioned in the "Classification of Ball Screws"; at the same time, you can also determine the screw's accuracy grade (usually C7) and pre- Tight grade
The second step is to determine the shaft diameter of the ball screw according to the load;
The third step is to determine the wire according to the moving speed required by the load. After determining the wire, determine the torque to be provided by the drive motor according to the relationship between the thrust and the wire.
The details are as follows:
The object moves vertically up and down, the weight is 60Kg, and the required moving speed is 1m/s.
1) If the servo motor is selected as the drive, the rated speed is 3000r / min = 50r / s, according to the formula: v = r i, determine the wire to be 20;
2) Then calculate the size of the load: assuming that the acceleration and deceleration time of the servo motor is set to 0.3s, the acceleration is 3.3m/s², and the load F = 600 plus 60 3.3 = 798N (the friction force is ignored here);
3) According to the formula: F = (2 π T n)/i, n takes 90 percent , and the calculated value is T≈2.82N•m. The query 1KW servo motor has a rated torque of 3.18N•m, which meets the requirements.
Above, the type of ball screw is basically determined. Finally, determine the length of the screw according to the required stroke and the installation method of the above screw.
1. When installing, be careful not to damage the tooth surface of the screw; in addition, after purchasing an ordinary screw, the nut and the screw will come together, so pay special attention to the installation. Do not separate the nuts and screws, otherwise you will not be able to install them yourself. If you accidentally separate it, please don't use it forcefully, you can contact the supplier and let them install it.
2. Lubrication: There is metal friction between the ball and the screw, so lubricating oil or grease should be added between the two, otherwise it will reduce the service life;
The general adding method is manual refueling and automatic refueling. Workers regularly refuel manually; automatic refueling requires an automatic refueling system, which will be described in detail later.
