A brief discussion on the design and basic installation of ball screws

　In recent years, more and more ball screws have been used in various machines to meet the needs of high-precision and high-performance equipment, making ball screws one of the most widely used transmission components. In CNC machinery, ball screws enhance positioning accuracy and extend their service life. Ball screws are gradually replacing sliding screws in traditional machinery. Ball screws usually incorporate preload to eliminate backlash during operation, but improper installation can compromise their high precision and long lifespan. This article mainly discusses problems with ball screws and their prevention, detailing diagnostic procedures to help users identify the causes of malfunctions.

　　Causes and Prevention of Ball Screw Problems

　　The following describes three main causes of ball screw problems and their prevention methods.

　　Excessive Backlash

　　1. No preload or insufficient preload
　　A ball screw without preload, when vertically positioned, will rotate and slide down due to its own weight. A screw without preload has a certain amount of backlash and can only be used in machines with low operating resistance. However, the main concern is the lack of positioning accuracy requirements.

　　2. Excessive torsional displacement
　　(1) Improper heat treatment, too thin a hardened layer, uneven hardness distribution, or too soft material of the steel balls, nut, and screw shaft. Standard hardness values are: HRC62~66, HRC58~62, HRC56~62.
　　(2) Improper design—excessively high slenderness ratio. The smaller the slenderness ratio, the higher the rigidity. The slenderness ratio must be below 80. If the slenderness ratio is too high, the screw shaft will sag due to its own weight.

　　3. Improper selection of support bearings
　　Ball screws usually require angular contact bearings, especially those with a high-pressure angle design. When a ball screw is subjected to axial loads, ordinary deep groove bearings will produce a certain amount of axial clearance, making them unsuitable for this application.

　　4. Improper installation of support bearings
　　(1) If the bearing is not tightly fitted to the ball screw, axial load will cause clearance. This may be due to the screw shoulder being too long or too short.
　　(2) Poor perpendicularity between the bearing support surface and the locking nut V-shaped tooth axis, or poor parallelism between the two corresponding locking nut surfaces, will cause the bearing to tilt. Therefore, the locking nut V-shaped tooth on the screw shoulder and the bearing support surface must be machined simultaneously to ensure perpendicularity. Machining by grinding is preferable.
　　(3) Use two locking nuts with spring washers to secure the bearing and prevent loosening during operation.

　　5. Insufficient rigidity of the nut seat or bearing seat
　　If the nut seat or bearing seat is not rigid enough, its own weight or the machine load will cause it to tilt.

　　6. Improper assembly of the nut seat or bearing seat
　　(1) Due to vibration or lack of fixing pins, components may loosen. Use solid pins instead of spring pins for positioning.
　　(2) The fixing screws are too long or the screw holes in the nut seat are too short, preventing the fixing screws from tightening.
　　(3) Due to vibration or lack of spring washers, the nut fixing screws may fall off.

　　7. Support seat surface flatness or planarity exceeds tolerance
　　Regardless of whether the mating surface is ground or otherwise processed, if its flatness or planarity exceeds the tolerance range, the repeatability of the screw's positioning accuracy will be poor during worktable movement. Therefore, in a machine, shims are usually used between the support seat and the machine body for adjustment.

　　8. Improper connection between motor and ball screw
　　(1) An insecure or non-rigid coupling will cause a rotational difference between the screw and the motor.
　　(2) If gear transmission is unsuitable or the drive structure is not rigid, conventional belts can be used for transmission to prevent slippage.
　　(3) Loose keys, or any improper combination of keys, keyways, and equipment, will create clearance between these components.