Ball screw is also called ball screw. Today, starting from the characteristics, composition and classification of ball screw, several installation methods and main parameters, let’s talk about ball screw in detail.
Regarding the introduction of ball screws, the content of Baidu Encyclopedia is as follows:
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 machine tools and precision machines. Its main function is to convert rotational motion into linear motion or convert torque into repeated axial force. It also has characteristics of high precision, reversibility and high efficiency. Due to their low friction resistance, ball screws are widely used in various industrial equipment and precision instruments.
Simply put, ball screw is a mechanism capable of converting rotational motion into linear motion or vice versa. However, its general application is to convert rotational motion into linear motion.
Ball screw characteristics
1. Low friction loss and high transmission efficiency
Since many balls perform rolling movement between the screw shaft and the nut of the ball screw pair, higher movement efficiency can be achieved. Compared with the previous pair of sliding screws, the driving torque is less than 1/3, that is, the power required to achieve the same movement result is 1/3 that of using of the pair of sliding screws. Very useful for saving energy.
2. High precision
Ball screw sets are typically produced continuously using the highest level of mechanical equipment in the world. Especially in the factory environment during the grinding, assembly and inspection of each process, the temperature and humidity are strictly controlled. Thanks to the comprehensive quality management system, the precision is fully controlled. guaranteed.
3. High speed power supply and micro power supply possible
Since the ball screw pair uses ball movement, the starting torque is extremely small and there will be no crawling phenomenon such as sliding movement, ensuring precise micro-feeding.
4. High axial rigidity
The ball screw pair can be preloaded, as the preload can cause the axial clearance to reach a negative value, thus achieving greater rigidity (by adding prepressure to the balls in the ball screw, when effectively used in mechanical devices, etc., due to (the repulsive force of the balls can increase the rigidity of the nut part).
5. It cannot self-lock and has transmission reversibility.
The composition and classification of ball screws
The ball screw consists of a screw, a nut, a steel ball, a preload part, a diverter and a dust protector. Its function is to convert rotational motion into linear motion, which is a further extension and development of Acme screw. The important significance of this development is to change the bearing from a sliding action to a rolling action. Due to their low friction resistance, ball screws are widely used in various industrial equipment and precision instruments.
Regarding ball screws, there are too many types. Here we will list some of the most common ones.
1. Self-lubricating ball screw: A self-lubricating ball screw with a removable oil removal device does not require lubrication piping systems or equipment, reducing the cost of oil changes and maintenance. disposal of used oils.
2. Silent ball screw: Its principle is to install a special groove-type ball spacer ring between the balls, which can suppress the noise caused by the collision between the balls and make the ball screw more stable when silent and smooth movement.
3. High-speed ball screw: It has the characteristics of high acceleration, high rigidity, high-speed feeding, low vibration and low noise. It is used in the fields of rapid feed of machine tools, high-speed grinding tool cutting centers and high-speed longitudinal load cutting centers.
4. Heavy-duty ball screw: It can withstand large axial loads and is suitable for all-electric design machines, air compressors, semiconductor manufacturing equipment and forging manufacturing equipment.
There is also a distinction between rolling screws and grinding screws. The precision of rolling screws is relatively low and is suitable for situations where the precision requirements are not very high, while grinding screws, as the name suggests, are more; accurate. High, suitable for occasions with high precision requirements.
According to the circulation method of the ball in the nut, it can be divided into external circulation, internal circulation and end cover type. First, let’s talk about the tip type. It is a relatively old structure, but its flaws are evident. Now it has been virtually eliminated and is rarely used, so I will mention it here.
We will not talk about the specific structure of internal and external circulation nuts. After all, we don’t need to make a threaded rod. You just need to know the difference between the two and their respective advantages and disadvantages.
Multiple ball screw installation methods
The general installation methods are the above four. You can choose the screw installation method according to your own working conditions. Different installation methods have different ends of the screw.
Main parameters of ball screw
When it comes to selecting a ball screw, we must first talk about its common parameters, and then we can start from these parameters to determine its model.
1. Nominal diameter
That is to say, the outer diameter of the screw, 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, most of the other diameters are term products (made-to-order products, the delivery time is about 30-60 days, Japanese products about 2-2.5 months, European and American products about 3-4 months).
The nominal diameter is essentially proportional to the load. The larger the diameter, the greater the load. For specific values, please refer to the manufacturer’s product sample. Only two concepts are explained here: dynamic load rating and static load rating. The first refers to the rated axial load in the moving state, and the second refers to the rated axial load in the static state. Just refer to the first one when designing. It should be noted that the rated load is not the maximum load. The lower the ratio between the actual load and the nominal load, the higher the theoretical life of the screw. Recommendation: Try to choose diameter 16~63.
2. Lead
Lead refers to the distance the nut travels linearly when the screw turns once. Common pitches are (unit: mm): 2, 4, 5, 6, 8, 10, 16, 20, 25, 32, 40. The parameters related to the pitch are the speed of movement of the nut and the linear thrust available of the ball screw.
The larger the feed, the faster the linear motion speed under the same rotation speed. The specific calculation relationship is: v=ri. Where v – the speed of movement of the nut (unit: mm/s); r – the rotation speed of the screw (unit: r/s); i – the pitch (unit: mm).
The relationship between the pitch and the thrust of the screw: F=(2πTn)/i. Where F – screw thrust (unit: N); T – torque supplied by the motor (unit N·m); transmission efficiency (ball screw transmission efficiency is generally 85%-95%); (the unit here is m).
3.Length
There are two concepts of length, one is total length and the other is wire length. Some manufacturers only calculate the overall length, but some manufacturers must provide the thread length. There are also two parts in the thread length, one is the total thread length and the other is the effective stroke. The first refers to the total length of the threaded part and the second to the theoretical maximum length of linear movement of the nut. Thread length = effective stroke + nut length + design margin (if a protective cover is to be installed, compression is required). the length of the protective cover must also be taken into account. Usually calculated as 1/8 of the maximum length of the protective cover).
During design and drawing, the total length of the screw can be roughly accumulated based on the following parameters: total length of the screw = effective stroke + length of nut + design margin + length of bracket at both ends ( bearing width + locknut width + margin ) + power Enter the connection length (approximately half the coupling length + tolerance if using a coupling). What needs special attention is that if your length is too long (more than 3 meters) or the aspect ratio is large (more than 70), it is best to consult the manufacturer’s sales staff in advance to know if the overall situation is feasible. is that the maximum length of conventional products of domestic manufacturers is 3 meters, 16 meters for special products, 6 meters for regular products of foreign manufacturers, and 22 meters for special products. Of course, this doesn’t mean that domestic manufacturers can’t produce longer ones, but the price of custom products is quite outrageous. Recommendation: try to choose a length less than 6 meters. If it is longer than 6 meters, it is more cost-effective to use a rack.
4. Nut shape
Each manufacturer’s product catalog will have many types of nuts. The first letters of the general model number indicate the type of nut. According to the shape of the flange, there are round flange, single gasket flange, double gasket flange and no flange. According to the length of the nut, there are single nuts and double nuts (note that there is no difference in load and stiffness between single nuts and double nuts. Don’t listen to the talk of The main difference between single nuts and double nuts is that the latter can adjust the preload, the former cannot, and the price and length of the latter are approximately double that. those of the former). When the size and performance of the installation allow, the designer should try to choose a conventional form when selecting to avoid the problem of delivery date of spare parts during maintenance. Recommendation: Choose double-sided nuts for frequent movement and high-precision maintenance, and double-sided and single-sided nuts for other situations. Recommendation: Where possible, the shape of the nut should be a single nut with a double internal circulating packing flange.
5. Accuracy
Ball screws, according to the national classification, have precision grades P1, P2, P3, P4, P5, P7 and P10. Japan, South Korea and Taiwan Province of China adopt JIS grades, namely C0, C1, C2, C3. C5 and C7, C10; European country standards 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.
Accuracy is expressed as follows: Regardless of the length of your ballscrew, take any 300mm segment and the error will be included in the accuracy represented by the grade. The precision represented by each grade is as follows.
Generally speaking, ordinary machines adopt C7 and C10 grades, CNC equipment generally adopts C5 and C3 grades (C5 is more common, most domestic CNC machine tools are C5 grade), manufacturing equipment Aeronautics, precision projections and three-coordinate measuring equipment generally adopt C3. , precision C2.
Additionally, grades C7 and C10 are generally manufactured by rolling methods, and grades C5 and above are manufactured by grinding methods.
To summarize, the precision level of ball screws commonly used in non-standard designs is C7 (manufactured by rolling method or some call it transformation), while those with higher precision level requirements of Ball screws are C5 (made by grinding). method). ) is enough. Of course, I still have to say that specific issues need to be analyzed in detail.
6. Preload level
It is also called preloading. When it comes to preloading, we don’t need to know the specific preloading force or the preloading method. We just need to select the preload level based on the manufacturer’s sample. The higher the preload level, the tighter the fit of the nut and screw; conversely, the lower the level, the looser the adjustment;
The principles to follow are: large diameter, double nuts, high precision and large driving torque. When the above situations occur when applying the screw, the preload level can be selected higher, otherwise the preload level can be selected lower. .
Selection
After understanding the main parameters of the above screw, we can make the selection according to our own requirements.
Step 1: Based on the application scenarios of different screws mentioned in the “Classification of Ball Screws” above, determine the screw type suitable for your own working conditions. You can also determine the precision level of the screw (usually C7); and preload level;
Step 2: Determine the ball screw shaft diameter based on the load size;
Step 3: Determine the feed based on the travel speed required by the load; After determining the feed, determine the torque to be provided by the drive motor based on the relationship between thrust and feed.
The details are as follows: the object moves vertically up and down, the weight is 60 kg, and the required moving speed is 1 m/s.
1) If you choose a servo motor as the drive and the rated speed is 3000 rpm = 50 rpm, according to the formula: v = ri, determine the feed rate to be 20;
2) Then calculate the size of the load: suppose the acceleration and deceleration time of the servo motor is set to 0.3 s, then the acceleration is 3.3 m/s² and the load F=600+60 *3.3=798N (friction is ignored here);
3) According to the formula: F=(2πTn)/i, n is taken equal to 90% and T≈2.82N·m is calculated. Check that the rated torque of the 1 kW servo motor is 3.18N·m, which is compliant. the requirements.
The above basically determines the model of the ball screw. Finally, you can determine the length of the screw based on the stroke you need and the screw installation method mentioned above.
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