Characteristics of ball screw

Main parameters of ball screw

When it comes to the selection of ball screw, we need to first talk about its common parameters, and then we can start from 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 in these specifications, the manufacturers generally only prepare 16~50 goods, that is to say, most of the other diameters are futures (see single production, delivery time is about 30~60 days, Japanese products are about 2 to 2.5 months, European and American products are about 3 to 4 months).

Nominal diameter and load are basically proportional, the larger the diameter of the larger the load, the specific value can consult the manufacturer's product sample. Only two concepts are described here: dynamic rated load and static rated load, the former refers to the rated axial load in the state of motion, the latter refers to the rated axial load in the state of rest. Refer to the former when designing. It should be noted that the rated load is not the maximum load, and the smaller the ratio between the actual load and the rated load, the higher the theoretical life of the lead screw. Recommended: The diameter should be 16~63.

2. Lead

The lead refers to the distance that the screw rotates once and the nut moves in a straight line. Common leads are (unit: mm) : 2, 4, 5, 6, 8, 10, 16, 20, 25, 32, 40, and the parameters related to the lead are the nut movement speed and the linear thrust provided by the ball screw.

The larger the lead, the faster the linear motion speed under the same speed, the specific calculation relationship is: v=ri. Where v is the moving speed of the nut (unit: mm/s); r - the rotating speed of the lead screw (unit: r/s); i -- lead (unit: mm).

Relationship between lead and screw thrust: F= (2πTn) /i. Where F -- screw thrust (unit: N); T -- torque provided by the motor (unit N·m); n -- transmission efficiency (the transmission efficiency of ball screw is generally 85% to 95%); i -- lead (in m).

Step 3: Length

There are two concepts of length, one is the total length and the other is the length of the thread. Some manufacturers only calculate total length, but others need to provide thread length. There are also two parts in the thread length, one is the full length of the thread and one is the effective stroke. The former refers to the total length of the thread part, the latter refers to the theoretical maximum length of the nut moving in a straight line, the length of the thread = effective stroke + nut length + design margin (if you need to install a protective cover, but also consider the length of the compressed protective cover, generally calculated by 1/8 of the maximum length of the protective cover).

When designing the drawing, the total length of the lead screw can be roughly accumulated according to the following parameters: Total length of the lead screw = effective stroke + nut length + design margin + support length at both ends (bearing width + lock nut width + margin) + power input connection length (if the coupling is used, it is roughly half the length of the coupling + margin). In particular, it should be noted that if your length is very long (greater than 3 meters) or the length-to-diameter ratio is very large (greater than 70), it is best to consult the manufacturer's sales staff in advance to produce, the overall situation is that the maximum length of domestic manufacturers' conventional products is 3 meters, special products are 16 meters, foreign manufacturers' conventional products are 6 meters, special products are 22 meters. Of course, it is not to say that domestic manufacturers can not produce longer, but the price of fixed products is more outrageous. Recommended: try to choose the length below 6 meters, more than the rack and pinion more cost-effective.

4. Nut form

There are many kinds of nut forms on the product samples of various manufacturers, and the first few letters in the general model indicate the nut form. According to the flange form, there are about round flange, single cut flange, double cut flange and no flange. There are single nuts and double nuts according to the length of the nut (note that single nuts and double nuts have no load and rigidity difference, this point do not listen to the speech of the manufacturer's sales staff, the main difference between single nuts and double nuts is that the latter can adjust the preload and the former can not, and the price and length of the latter are roughly 2 times of the former). If the installation size and performance allow, the designer should try to choose the conventional form when selecting, in order to avoid the delivery of spare parts during maintenance. Recommended: Double nuts for frequent movements and high-precision maintenance, and double nuts with single side for other occasions. Recommended: Try to choose internal circulation double cut flange single nut.

Step 5: Precision

Ball screw, according to domestic classification, accuracy levels are P1, P2, P3, P4, P5, P7, P10, Japan, South Korea, as well as Taiwan Province of China using JIS grade, that is, C0, C1, C2, C3, C5, C7, C10; European standards are IT0, IT1, IT2, IT3, IT4, IT5, IT7, IT10.

In general, like our company to purchase are Taiwan's ball screw, cost-effective, followed by Japan.

The accuracy is expressed as follows: no matter how long your ball screw is, take a section of 300mm, the error is within the accuracy represented by the grade, and the accuracy represented by each grade is as follows.

Wechat Image_20230625095039.jpg

In general, ordinary machinery uses C7, C10 level, CNC equipment generally uses C5, C3 level (C5 more, most of the domestic CNC machine tools are C5 level), aviation manufacturing equipment, precision projection and coordinate measuring equipment generally use C3, C2 accuracy.

In addition, C7, C10 grade is generally manufactured by rolling method, and C5 grade and above are manufactured by grinding method.

In summary, the precision grade of the ball screw commonly used in non-standard design is C7 () rolling method manufacturing or some people call it conversion), and the accuracy grade of the ball screw has higher requirements, C5 (grinding method manufacturing) is also sufficient. Of course, I still want to say that we should analyze specific problems.

6. Preload level

Also called preloading, about preloading, we do not need to understand the specific preloading force and preloading method, only need to select the preloading level according to the manufacturer's sample. The higher the preload level, the tighter the fit between nut and screw; On the contrary, the lower the grade, the looser.

The principle to follow is: large diameter, double nuts, high precision, large driving torque, when the application of the screw appears above, the prepressure level can be selected higher, and vice versa.



Type selection

After understanding the main parameters of the screw, we can select the type according to our own requirements.

The first step: according to the above "classification of ball screw" mentioned in the various screw application scenarios, determine the type of screw suitable for their own working conditions; At the same time, it is also possible to determine the precision level of the screw (generally C7) and the preloading level;

Step 2: According to the size of the load, determine the shaft diameter of the ball screw;

Step 3: Determine the lead according to the moving speed required by the load; After determining the lead, the torque to be provided by the drive motor is determined according to the relationship between the thrust and the lead.

The details are as follows: The object is moving vertically up and down, the weight is 60Kg, and the required moving speed is 1m/s.

1) If you choose the servo motor as the drive, the rated speed is 3000r/min=50r/s, according to the formula: v=ri, determine the lead is 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, 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 1kW servo motor is 3.18N·m, which meets the requirements.

Above, the model of the ball screw is basically determined, and finally, according to the stroke you need to use, and the screw installation method mentioned above, the length of the screw is determined.