Ball screws are ideal for converting rotary motion to linear motion or linear motion to rotary motion. A ball screw consists of a threaded rod and a nut. Its function is to convert rotary motion into linear motion. When choosing a lead screw, what are our construction reference criteria? Let's take a look at the following:
1 Lead screw parameters:
1 The diameter is determined by the load. The output torque is included in the motor performance parameters. If it is also equipped with a reducer, calculate how much thrust is required under actual operating conditions. The ball screw sample (required by the manufacturer) has load parameters (that is, thrust parameters, generally expressed as dynamic load and static load, just look at the former), and select the nominal diameter of the ball screw.
2 Determine the linear speed and rotational speed of the ball screw. When the motor is determined, the output speed is known. Consider the maximum line speed you need. Multiply the speed of the motor (divide by the reduction ratio if a reducer is installed) by the lead of the lead screw, which is greater than the required value.
three. Determine the length of the ball screw according to actual needs (total length = working stroke + nut length + safety margin + installation length + connection length + margin). If protection is added, such as a sheath, the expansion ratio of the sheath should be considered (generally 1:8, that is, the maximum elongation of the sheath divided by 8).
4 Determine the accuracy of the ball screw according to actual needs. Generally, it is enough to choose below C7 for machinery, and C5 is more suitable for CNC machine tools (the corresponding domestic standards are generally P5~P4 and P4~P3).
If the nut structure and structure are different, it is not recommended to replace the nut according to the different structure and size.
6Ask the manufacturer of the selected product for price, payment terms and delivery time
7Select and determine the installation method (end). This mount can be designed or purchased by yourself. A standard mounting base is available. Pay attention to the stress state when designing. It is best to use 7000 or 3000 series bearings, because the lead screw is mainly subjected to axial force when working, and radial force should be avoided as much as possible.
8 Considering the guide elements and installation capacity, it is recommended to use ball linear guides for guides that match the ball screw. Of course, the configuration of linear axis and linear bearing is a relatively economical choice.
9 Draw the ball screw diagram according to the conditions determined above (mainly the end installation dimensions, which should be specified in detail, and provide the geometric dimensions and tolerances).
2DN value:
D: the nominal diameter of the ball screw, that is, the diameter of the center of the ball (mm);
N: ball screw limit speed (RPM);
The limitation of the DN value is not that the smaller the diameter, the higher the speed, because for the first determined feed speed, when the lead screw speed calculated by the feed speed is too high, the speed can be reduced by increasing the lead to meet the DN value requirements.
three. Ball screw specification and model selection
1 Determine the positioning accuracy
2 Determine the lead of the screw according to the motor and speed requirements
three. Check the nut size, determine the travel of the screw shaft end and related data
4 Determination of average axial force and velocity from load and velocity distribution (acceleration and deceleration)
5 Determining the preload by the average axial force
6 Expected life, axial load and speed determine dynamic load rating
7 The basic dynamic load rating, lead, critical speed and DMN value limits determine the lead screw outer diameter and nut form
8 Outer diameter, nut, preload, load determination, stiffness (machine design)
9 Ambient temperature and total nut length determine thermal variation and accumulated lead
10 The screw is rigid, the preload is determined by thermal deformation
11 The maximum speed, temperature rise time and screw size of the machine tool determine the drive torque and size of the motor