The ball screw is also called the ball screw. Today, starting from the characteristics, composition and classification of the ball screw, several installation methods and main parameters, let's talk about the things about the ball screw in detail.
Regarding the introduction of the ball screw, the content of Baidu Encyclopedia is the following explanation:
Ball screws are ideal for converting rotary motion to linear motion, or linear motion to 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 repeated force. It also has high precision, reversibility and high precision. Features of efficiency. Due to its small frictional resistance, ball screws are widely used in various industrial equipment and precision instruments.
To put it simply, a ball screw is a mechanism that can be converted from rotary motion to linear motion or from linear motion to rotary motion, but its application is generally converted from rotary motion to linear motion.
Features of ball screw
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1. Small friction loss and high transmission efficiency
Since there are many balls rolling between the screw shaft and the screw nut of the ball screw pair, higher motion efficiency can be obtained. Compared with the previous sliding screw pair, the driving torque is less than 1/3, that is, the power required to achieve the same motion result is 1/3 of that of using the sliding screw pair. Very helpful in saving power.
2. High precision
Ball screw pairs are generally produced continuously with the world's highest level of machinery and equipment, especially in the factory environment of grinding, assembly, and inspection processes. The temperature and humidity are strictly controlled. Due to the perfect quality management system So that the accuracy can be fully guaranteed.
3. High-speed feeding and micro-feeding possible
Because the ball screw pair uses balls to move, the starting torque is extremely small, and there will be no crawling phenomenon like sliding movement, which can ensure accurate micro-feeding.
4. High axial stiffness
The ball screw pair can be preloaded, because the preload can make the axial clearance reach a negative value, and thus obtain higher rigidity (the preload is added to the ball in the ball screw, and when it is actually used in a mechanical device, etc., due to The repulsion of the ball can increase the rigidity of the mother part).
5. Can not be self-locking, with transmission reversibility
Composition and classification of ball screw
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The ball screw is composed of screw, nut, steel ball, preloader, reverser and dust collector. Its function is to convert rotary motion into linear motion, which is the further extension and development of Acme screw. The significance of this development is to change the bearing from sliding motion to rolling motion. Due to its small frictional resistance, ball screws are widely used in various industrial equipment and precision instruments.
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There are too many types of ball screws, here we will list some of the more common ones.
1. Self-lubricating ball screw: The self-lubricating ball screw with removable degreasing device does not require lubrication pipeline system and equipment, which reduces the cost of oil change and waste oil treatment.
2. Silent ball screw: Its principle is to set a special groove-type ball spacer ring between the balls, which can suppress the noise generated by the collision between the balls and the balls, so that the ball screw is more stable when it is moving. Quiet and smooth.
3. High-speed ball screw: It has the characteristics of high acceleration, high rigidity, high-speed feed, low vibration and low noise. It is used in the fields of rapid feed of machine tools, high-speed cutting centers for abrasive tools and high-speed longitudinal load cutting centers.
4. Heavy-duty ball screw: able to withstand large axial loads, suitable for all-electric design machines, air compressors, semiconductor manufacturing equipment and forging manufacturing equipment, etc.
There is also a distinction between rolling and grinding grade screw rods. The precision of rolling screw rods is relatively low, which is suitable for occasions where the precision requirements are not very high; while grinding grade screw rods can also be seen from its name, and the accuracy is relatively high. High, suitable for occasions requiring high precision.
According to the ball circulation mode in the nut, it can be divided into outer circulation, inner circulation and end cap type. Let me talk about the end cap type first. This is a relatively early structure, but its shortcomings are obvious. Now it has basically been eliminated, and it is rarely used, so I will mention it here.
Let's not talk about the specific structure of the inner and outer circulation nuts. After all, we don't need a wire rod, we only need to know the difference between the two and their respective advantages and disadvantages.
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Several installation methods of ball screw
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The general installation methods are the above four. You can choose the installation method of the screw rod according to your own working conditions, and different installation methods have different ends of the screw rod.
The main parameters of the ball screw
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When it comes to the selection of ball screw, we need to talk about its common parameters first, and then we can start with these parameters to determine its model.
1. Nominal diameter
That is, the outer diameter of the screw, the common specifications are 12, 14, 16, 20, 25, 32, 40, 50, 63, 80, 100, 120, but please note that among these specifications, each manufacturer generally only prepares 16~50 That is to say, most of the other diameters are futures (see the order production, the delivery time is about 30~60 days, the Japanese product is about 2~2.5 months, and the European and American products is about 3~4 months).
The nominal diameter is basically proportional to the load. The larger the diameter, the greater the load. For specific values, please refer to the manufacturer's product catalog. Only two concepts are explained here: dynamic rated load and static rated load. The former refers to the rated axial load in motion, and the latter refers to the rated axial load in static state. Design can refer to the former. It should be noted that the rated load is not the maximum load, the smaller the ratio of the actual load to the rated load, the higher the theoretical life of the screw. Recommendation: try to choose a diameter of 16~63.
2. Lead
The lead refers to the distance that the nut moves linearly when the screw rotates one revolution. The common leads are (unit: mm): 2, 4, 5, 6, 8, 10, 16, 20, 25, 32, 40. The parameters related to the lead are the nut movement speed and the ball screw can provide straight line thrust.
The larger the lead, the faster the linear motion speed at the same speed. The specific calculation relationship is: v=ri. Among them, v——moving speed of nut (unit: mm/s); r——rotating speed of screw rod (unit: r/s); i——lead (unit: mm).
The relationship between lead and screw thrust: F=(2πTn)/i. Among them, F—screw thrust (unit: N); T—torque provided by the motor (unit N m); n—transmission efficiency (the transmission efficiency of the ball screw is generally 85%~95%); i— - Lead (here the unit is m).
3. Length
There are two concepts of length, one is the overall length and the other is the thread length. Some manufacturers only calculate the overall length, but some manufacturers need to provide the thread length. There are also two parts in the thread length, one is the full length of the thread, and the other is the effective stroke. The former refers to the total length of the threaded part, and the latter refers to the theoretical maximum length of the nut moving in a straight line. Thread length = effective stroke + nut length + design margin (if a protective cover needs to be installed, the compressed length of the protective cover should also be considered. Generally calculated according to 1/8 of the maximum length of the protective cover).
When designing drawings, the overall length of the screw can be roughly accumulated according to the following parameters: overall length of the screw = effective stroke + nut length + design margin + support length at both ends (bearing width + lock nut width + margin) + power Enter the connection length (approximately half the coupling length + allowance if using a coupling). In particular, if your length is super long (greater than 3 meters) or the aspect ratio is large (greater than 70), it is best to consult the sales staff of the manufacturer in advance whether it can be produced. The general situation is that the maximum length of domestic manufacturers’ conventional products is 3 meters. meters, 16 meters for special products, 6 meters for regular products of foreign manufacturers, and 22 meters for special products. Of course, it does not mean that domestic manufacturers cannot produce longer ones, but the price of customized products is outrageous. Recommendation: Try to choose a length of less than 6 meters. If it exceeds, it is more cost-effective to use a rack and pinion.
4. Nut form
There are many types of nuts in the product samples of various manufacturers, and the first few letters in the general model indicate the nut type. According to the flange form, there are roughly round flange, single trim flange, double trim flange and no flange. According to the length of nuts, there are single nuts and double nuts (note that there is no difference in load and rigidity between single nuts and double nuts. Don’t listen to the speeches of the sales staff of the manufacturer. The main difference between single nuts and double nuts is that the latter can be adjusted. The former cannot, and the price and length of the latter are roughly twice that of the former). When the installation size and performance permit, the designer should try to choose the conventional form when selecting, so as to avoid the delivery time problem of spare parts during maintenance. Recommendation: choose double nuts for frequent action and high precision maintenance, and double and single nuts for other occasions. Recommendation: Try to choose the single nut of the inner circulation double trimmed flange as the nut form.
5. Accuracy
Ball screw, according to the domestic classification, the accuracy grades are P1, P2, P3, P4, P5, P7, P10, Japan, South Korea, and Taiwan Province of China adopt JIS grades, namely C0, C1, C2, C3, C5, 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 relatively cost-effective, followed by Japanese ones.
The way to express the accuracy is: No matter how long your ball screw is, take any section of 300mm, and the error is within the accuracy represented by the grade. The accuracy represented by each grade is as follows.
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Generally speaking, ordinary machinery adopts C7 and C10 grades, CNC equipment generally adopts C5 and C3 grades (there are more C5, and most domestic CNC machine tools are C5 grades), aviation manufacturing equipment, precision projection and three-coordinate measuring equipment generally adopt C3, C2 precision.
In addition, grades C7 and C10 are generally manufactured by rolling, and grades C5 and above are manufactured by grinding.
To sum up, the precision grade of ball screw commonly used in non-standard design is C7 (manufactured by rolling method or some people call it transformation), while the precision grade of ball screw has higher requirements, C5 (manufactured by grinding method ) is also sufficient. Of course, we still have to say that we need to analyze specific issues in detail.
6. Preload grade
It is also called preload. Regarding preload, we don’t need to know the specific preload force and preload method. We only need to choose the preload grade according to the manufacturer’s sample. The higher the preload level, the tighter the fit between the nut and the screw; on the contrary, the lower the level, the looser it is.
The principles to be followed are: large diameter, double nuts, high precision, and large driving torque. When the above conditions occur in the application of the screw rod, the preload level can be selected higher, otherwise, the preload level can be selected lower.
selection
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After understanding the main parameters of the above screw rods, we can select the type according to our own requirements.
Step 1: According to the application scenarios of various screw screws mentioned in the above "Classification of Ball Screws", determine the type of screw screw suitable for your own working conditions; at the same time, you can also determine the accuracy level of the screw screw (generally C7) and Preload grade;
Step 2: Determine the shaft diameter of the ball screw according to the size of the load;
Step 3: Determine the lead according to the moving speed required by the load; after determining the lead, then determine the torque to be provided by the drive motor according to the relationship between thrust and lead.
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 you choose a servo motor as the drive, the rated speed is 3000r/min=50r/s, according to the formula: v=ri, 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 force is ignored here);
3) According to the formula: F=(2πTn)/i, taking 90% of n, T≈2.82N·m is calculated, and the rated torque of the 1kW servo motor is 3.18N·m, which meets the requirements.
Above, the model of the ball screw is basically determined. Finally, according to the stroke you need to use and the installation method of the screw mentioned above, you can determine the length of the screw.
