The linear axes of modern CNC machine tools are mostly driven by ball screw nut. The transmission accuracy of the ball screw nut pair directly affects the running precision and machining quality of CNC machine tools. The related faults of the ball screw nut will cause the CNC machine tool to have faults such as lower positioning accuracy, excessive backlash, mechanical creep, excessive noise, and insufficient rigidity, which will affect the normal use of the machine tool. Therefore, timely removal of the fault caused by the ball screw nut pair is an important part of ensuring the normal operation and processing quality of the CNC machine tool.
1) Performance of ball screw nut pair:
The ball screw nut pair has excellent performance, mainly has high transmission reliability, is not easy to wear, and can be operated without trouble for a long time; the transmission is sensitive, the running precision is high, and it is not easy to generate creep; the transmission efficiency is high and the operation is stable, generally η=0.92- 0.96; easy to use, lubrication and maintenance; after the pre-tightening force is applied, the axial clearance can be eliminated, and there is no free travel in the reverse direction; the reverse transmission can be reversed; the disadvantage is that it cannot be self-locking, and a balancing device is required for vertical installation.
2) Features of the ball screw nut pair
a) The transmission form of the ball screw nut pair can be divided into screw rotation and nut rotation.
Screw drive transmission means that the screw rotates and the nut is fixed on the workbench. The screw rotates to push the nut, and the nut drives the table to reciprocate. This type of installation is common in most of us.
The nut rotation drive means that the lead screw is fixed, the nut is fixed on the worktable through the bearing, the servo motor drives the nut to rotate, and the nut moves axially along the lead screw, thereby driving the table to move. This type of transmission is generally used on machine tools with long lead screws to prevent vibration and deformation of the lead screw due to insufficient rigidity.
b) According to the ball screw nut nut structure can be divided into: inner circulation and outer circulation two ways.
The ball screw nut pair of the outer circulation type is composed of a lead screw, a ball, a ball return system, and a nut. The main difference from the inner circulation mode is that the two ends of the nut spiral groove are connected by a ball return pipe, so that the ball can return from the one end to the other end to form a closed circulation loop. The key to the internal thread type ball screw nut nut structure is to install a cylindrical cam type reverser in the side hole of the nut, and the reverser is milled with an s-shaped ball return groove to connect the adjacent two thread raceways. The ball enters the reverser from the threaded raceway, and the reverser is used to force the ball over the tip of the lead screw into the adjacent raceway for circulation.