Ball screw nut pair working principle

Ball Screw Nut pair is a commonly used transmission device for converting rotary motion into Linear Motion in CNC machine tools. It replaces the sliding in the screw nut pair with the rolling of the ball, with low friction and good performance.
1. Composition and working principle:

·Composition: It is mainly composed of screw rod, nut, ball and raceway (ball return device), nut seat, etc.
·Working principle: Arc spiral grooves are processed on the screw rod and nut. When they are assembled together, a spiral raceway is formed, and the raceway is filled with balls. The balls roll along the raceway and circulate in a circular motion through the ball return tube. Both ends of the ball return tube also play the role of blocking balls to prevent the balls from falling out along the raceway.
2. Features:
High transmission efficiency: The mechanical efficiency can be as high as 92%~98%.
·Small friction: mainly use the rolling of the ball instead of the sliding of the ordinary screw nut pair.
·Axial clearance can be eliminated: It is also due to the action of balls, which improves the rigidity of the system. Clearance can be eliminated after preloading.
·Long service life and high manufacturing cost: high-quality alloy materials are mainly used, and the surface is heat treated to obtain high hardness.
3. Anti-backlash of Ball Screw nut pair
·Double nut washer clearance adjustment:



The ball screw nut pair adopts a double nut structure (similar to the double lamella gear structure in the gear pair). By changing the thickness of the washer, the nut is displaced axially, so that the two nuts fit with the two sides of the screw rod respectively. When the table is reversed, the table will follow the CNC's motion commands in the reverse direction without lag due to the elimination of backlash.
·Double nut thread clearance adjustment:




The figure shows the structure of adjusting the preload with two lock nuts. Two working nuts are connected to the outer casing with a flat key, and the overhang of the one on the right is threaded. When the two lock nuts are rotated, it is precisely because the flat key restricts the rotation of the working nut that the working nut with the external thread can move axially relative to the locking nut. After the clearance is adjusted, tighten the two lock nuts. The structure is compact, the work is reliable, and the application is wide.
·Double nut tooth difference adjustment:
Cut out gears with the number of teeth Z1 and Z2 on the flanges of the two working nuts respectively, and Z1 and Z2 differ by one tooth.
which is:
z2-z1=1
The two gears are respectively meshed with the corresponding inner gear rings at both ends, and the inner gear rings are fastened on the nut seat.
When one of the nut Z1 turns over a tooth, the axial movement of the screw rod is S1, then:
4. Installation of ball screw nut pair
The ball screw nut pair bears mainly the axial load. Its radial load is mainly the dead weight of the horizontal screw. When installing, ensure a good fit between the hole of the nut seat and the working nut, and ensure the verticality of the hole and the end face, etc. At this time, the Bearing is mainly selected according to the size and direction of the load. In addition, the form of installation and configuration is also related to the length of the Lead Screw. When the lead screw is long, two support structures are used; when the lead screw is short, a single support structure is used.
Figure a: One end is fixed, one end is free: suitable for short lead screw and vertical lead screw.
Figure b: One end is fixed and one end is floating: one end bears axial force and radial force at the same time, and the other end is radial force. When the lead screw is heated and elongated, a small amount of axial floating can be made through one end.
Figure c: The form of support with fixed ends: usually a disc spring and an adjusting nut are installed at one end, which can not only apply a pre-tightening force to the ball screw, but also maintain the same pre-tightening force after the screw is thermally deformed force.