Selection and precautions of ball screw

The common specifications are (in millimeters): 8, 10, 12, 14, 15, 20, 25, 32. Generally, the larger the shaft diameter, the greater the load.
The load capacity that screws with different shaft diameters can withstand can be found in the manufacturer's sample book, and the relevant parameters are dynamic rated load and static rated load. Static rated load refers to the rated load that the screw can withstand in a stationary state, but we cannot keep it stationary with the screw, so here, we basically only need to focus on the rated load of the heart. Then, the smaller the ratio of actual load to rated load, the longer the theoretical life of the screw. (Note: The rated load does not represent the maximum load, but generally should not exceed the rated load, otherwise it will reduce the service life of the screw)
2. Lead the way
Lead refers to the distance that the screw rotates once and the nut moves in a straight line (which many engineers do not know, so it is important to note). Common leads are (in mm): 2, 4, 5, 8, 10, 20, 25, 32. It should be noted that there is no special relationship between lead and axial clearance. The size of the lead is determined by the linear velocity of the transmission, which is very important.
3. Accuracy
Ball screw rod, classified domestically, has precision grades of P1, P2, P3, P4, P5, P7, P10. Japan, South Korea and Taiwan, China Province of China use JIS grades, namely C0, C1, C2, C3, C5, C7, C10; The standards adopted by European countries are IT0, IT1, IT2, IT3, IT4, IT5, IT7, and IT10.
Generally speaking, ordinary machinery adopts C7 and C10 grades, while CNC equipment generally adopts C5 and C3 grades (C5 is more common, and most domestic CNC machine tools are C5 grades). Aviation manufacturing equipment, precision projection and coordinate measuring equipment generally adopt C3 and C2 precision (as far as I know, ball screws with C3 or higher are not available in the domestic civilian market).
In addition, C7 and C10 grades are generally manufactured using rolling methods, while C5 grades and above are manufactured using grinding methods.
In summary, the commonly used precision level for non-standard designs of ball screws is C7 (manufactured by rolling method or some people refer to it as conversion), while C5 (manufactured by grinding method) is also sufficient for those with higher requirements for the precision level of ball screws.
4. Pre compression level
Also known as preload, regarding preload, we don't need to know the specific preload force and preload method, we just need to choose the preload grade according to the manufacturer's sample. The higher the preloading level, the tighter the nut and screw fit; On the contrary, the lower the level, the looser it is (this is important in the SCARA robot application industry, as the load will affect preloading, which in turn affects accuracy).
The principles followed are: large diameter, double nuts, high precision, and large driving torque. When the above situations occur in the application of the screw, the preloading level can be selected higher, otherwise it can be selected lower.
Below is an introduction to how to select a lead screw:
The first step is to determine the type of screw that is suitable for your working conditions based on the application scenarios of various screw types mentioned in the "Classification of Ball Screw" section above; At the same time, the accuracy level (usually C7) and preloading level of the screw can also be determined;
Step 2: Determine the shaft diameter of the ball screw based on the size of the load;
Step 3: Determine the lead based on the required movement speed of the load; After determining the lead, determine the torque to be provided by the drive motor based on the relationship between thrust and lead.
as follows:
The object moves vertically up and down, with a weight of 60Kg and a required moving speed of 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: assuming the acceleration and deceleration time of the servo motor is set to 0.3 seconds, the acceleration is 3.3 m/s ², and the load F=600+60 * 3.3=798N (frictional force is ignored here);
3) According to the formula: F=(2 * π * T * n)/i, n is taken as 90%, and T ≈ 2.82N · m. The rated torque of the 1KW servo motor is 3.18N · m, which meets the requirements.
4) Choose a suitable itinerary.
Precautions for using ball screw
1. During installation, be careful not to damage the tooth surface of the screw rod; After the screw rod is purchased, the nut and screw rod are together, so special attention should be paid during installation. First, tie the tie or install limit blocks at both ends. Do not separate the nut and screw rod, as they will be difficult to install once separated. Accidentally separated, do not force yourself to install it. You can contact the supplier and have them install it;
2. Lubrication: Ball and screw are metal friction, so lubricating oil or grease should be added between the two, otherwise it will reduce the service life; The general ways of adding fuel include manual refueling and automatic refueling. Manual refueling refers to regular refueling by workers; And automatic refueling requires an automatic refueling system,;
3. Dust prevention: Ball screws, like rolling bearings, experience increased wear and tear when mixed with debris, water, or cutting fluid, which can sometimes lead to damage. When designing, attention should be paid to the dust prevention of the screw;
4. When the phenomenon of unbalanced load occurs, it will directly affect the service life and noise of the screw, and it is basically accompanied by a rough operation feel. If the smoothness of the screw is different when it is unloaded and assembled, in addition to paying attention to the accuracy of the screw itself, it is mostly due to poor combination accuracy that the phenomenon of unbalanced load reproduction occurs
5. The disadvantages of the screw rod are also particularly obvious, it cannot self lock and has reversible transmission