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Foreword: [Ball screw] In the CNC tool machine, it replaces the traditional Aikem screw, the main function is to increase the positioning accuracy and extend the service life. To eliminate the backlash of mechanical operation, it is usually matched with a ball screw with pre-stress, but if the ball screw is not installed properly, it will not show its high precision and long life. This article mainly discusses the problems encountered in the installation of the ball screw and the prevention of the problem, and detailed the measurement procedure to help the user find out the cause of the abnormal use of the ball screw.

Causes and prevention of problems in ball screw installation: The following is a description of the causes of the three ball screw installations and their prevention methods.

1. The operation is not smooth:

[1] screw and nut processing problems: (1) improper processing of the reflow system position; (2) screw or nut steel ball groove Cao grinding roughness is not good; (3) screw or nut steel ball groove true round Degree is out of tolerance; (4) lead lead or nut lead lead error or pitch diameter is out of tolerance. ?

?[2] Overtravel: (1) The cause of the overtravel may occur in the machine setting, limit switch failure or crash. Over-travel can cause damage and dents in the return pipe, and even break, causing the steel ball to fail to operate normally: under these harsh operating conditions, the surface of the screw or nut bead may be peeled off. To reinstall, the ball screw must be re-repaired by the manufacturer and prevented from further damage.

[3] Eccentricity: (1) When installing the ball screw, the bearing support and the nut seat at both ends should be adjusted to the best condition of three points of concentricity. If installed under different conditions, the nut seat and the bearing seat will be produced. Radial load, when the amount of confidence is too large, will cause the screw to bend. Even in the case of confidence, if there is no obvious bending of the screw, the abnormal wear continues to occur, and the precision of the screw is rapidly reduced. At the same time, the ball screw and the motor should also maintain a natural concentric state to avoid bad radial moments. The higher the pre-pressure set by the nut, the higher the requirement for confidence accuracy.

[4]? Foreign matter enters the ball track: (1) If the ball screw is not installed with a wiper or the wiper is damaged, the impurities (iron scraps, etc.) or dust accumulation during processing will hinder the ball track, resulting in poor smoothness. Reduced accuracy and reduced service life.

[5] Damage to the recirculation system: (1) The reflow system is severely impacted during installation, which may cause dents and damage, resulting in a path that is not smooth when the steel ball enters the recirculation system.

[6] The nut and nut seat are not properly combined: (1) When the nut is installed, the connection with the nut seat is inclined or the confidence will cause an eccentric load, which makes the motor running current unstable.

[7] Damage to the ball screw during handling: (1) During the installation process, the nut should be prevented from coming off the screw thread, because the amount of steel ball will be scattered and there will be preloading changes, circulation system and Damage to the wiper. (2) The friction coefficient of the ball screw is small. During the handling and installation process, be careful to avoid the damage caused by the self-weight of the nut or the screw itself when it is placed vertically. If this is the case, it must be overhauled by the manufacturer to prevent further damage. ?

2. The gap is too large

[1] No preloading or insufficient preloading: (1) When the ball screw without preloading rotates vertically, the nut will slide due to its own weight; therefore, the ball screw without preloading will have considerable The backlash exists, so it can only be used for machines with small operational resistance, but it is less required for positioning accuracy. PMI determines the correct preload on different machine applications and adjusts the preload before shipment; therefore, it does work before you order the ball screw.

[2] Improper bearing selection and improper bearing installation: (1) Usually the ball screw must be matched with the bevel bearing, especially the bearing designed with high pressure angle is the better choice; when the ball screw is loaded with the axial direction, generally Deep groove ball bearings cannot eliminate their backlash by preloading, so the installation of such bearings produces a fixed amount of axial backlash, so deep groove ball bearings are not suitable for this purpose. (2) Two bearing nuts are fitted with spring washers or spacer rings to prevent the bearing from loosening during operation; (3) the bearing core and the shoulder locking nut are not perpendicular to the V-shaped shaft center. Or the parallelism of the locking nut faces in the two corresponding directions is not good. When the two are matched with each other, the bearing will be inclined; therefore, the locking nut V-shaped teeth of the shoulder of the screw must be processed at the same time to ensure the verticality. If it is processed by grinding, it is better. (4) If the bearing is mounted on the ball screw and the mutual fit is not correct, it will cause backlash in the case of axial load. This may be because the shoulder of the screw is too long or If it is too short, the backlash can be eliminated by using a spacer ring.

[3] The surface parallelism or flatness of the support base is not good: (1) Whether the surface of the joint element is ground or shovel, as long as the equality or flatness is out of tolerance, the repeatability of the position of the bed is poor. Therefore, a machine usually has a thin gasket between the support base and the mechanical body to adjust the assembly precision.

[4] The rigidity of the nut seat or the bearing housing is not good: (1) If the nut seat or the bearing housing is insufficiently rigid, the inertia force generated by the weight of the component itself, the mechanical load or the mechanical operation may cause elastic deformation, resulting in Skewed.

[5] Improper assembly of the nut seat or housing: (1) The nut fixing screw is loose due to vibration or lack of a spring washer; (2) The fixing screw is too long, so that the screw hole depth is too shallow, so that the screw of the fixing screw is The contact surface of the contact element cannot be attached, so that an effective lock force cannot be obtained. (3) The fixing screw is too short, so that the fixing screw cannot obtain an effective locking force. (4) The coupling assembly is loosened due to vibration or unused positioning pins. Solid pins can be used instead of spring pins for positioning purposes.

[6] Improper combination of motor and ball screw: (1) The coupling is not strong or the rigidity is good, which will cause a difference in rotation between the screw and the motor. (2) The looseness of the keys, or any improper matching between the keys, the keyway and the hub, will cause gaps in these components. (3) If it is not suitable for the gear-driven structure to be non-rigid, it can be driven by the gauge to prevent slippage.

[7] Fragmentation: (1) Steel ball rupture: The most commonly used material for steel balls is chrome-molybdenum steel. If a steel ball with a diameter of 3.175mm (1/8吋) is to be broken, it needs about 1400KG (3080 lbs) to 1600KG ( 3520 pounds). When the ball screw is actuated, there is a significant difference in temperature rise when there is no lubrication. This temperature rise may cause the steel ball to be broken or damaged, and the screw bead groove is damaged. Therefore, the lubrication of the design must be considered. If the automatic lubrication system cannot be used, the supplementary plan of the lubricant must be included in the maintenance manual.

?[2] Reflow system sag or fracture: The over-stroke of the nut is caused by the impact of the reflow system, causing the recirculation system to sag or break, thus hindering the circulation path of the steel ball, causing the steel ball to slip rather than roll, and finally causing the reflow system to break. The precautionary method can be equipped with a bumper at the front and rear ends of the screw to prevent damage to the recirculation system and the nut during overtravel.

[3] Ball screw shoulder break: (1) Improper design: The shoulder of the screw should avoid sharp angle design to reduce local stress concentration. (2) Screw journal bending: the perpendicularity of the bearing bearing surface and the locking nut V-shaped shaft axis is not good, or the locking nut surface of the two corresponding steering wheels is not equal, which will lead to the shoulder of the screw Bending or breaking; therefore, the yaw of the shoulder of the screw must not exceed 0.01 mm (0.0004 吋) before and after the locking nut is locked. (3) Radial force or repeated stress: The eccentricity caused by the installation of the screw will produce abnormal alternating shear stress and cause early damage of the ball screw. (4) The design of the shoulder size of the screw should avoid excessive difference from the cross-sectional area of ​​the screw.

[4] The effect of temperature rise on the ball screw: When the ball screw is running, it will affect the accuracy of the mechanical transmission system, especially the high-speed and high-precision machinery. The following factors affect the temperature rise of the ball screw:

[1] Effect of pre-pressure: In order to avoid the change of the mechanical transmission system, the nut can be achieved by increasing the rigidity of the nut, which means that the pre-pressure of the nut is raised to a certain level. Applying a pre-stress to the nut increases the frictional torque of the thread and increases the temperature rise of the screw when it is actuated. It is recommended that the pre-pressure be 1/3 of the maximum axial load, and the pre-pressure should not exceed 10% of the dynamic load to obtain the best life and low temperature rise effect.

[2] Effect of lubrication: The choice of lubricating oil directly affects the temperature rise of the ball screw. The ball screw must be lubricated with one of oil or grease. It is generally recommended to use bearing oil as the ball screw oil for lubrication. For grease, it is recommended to use lithium soap based grease. Oil viscosity selection is based on operating speed, operating temperature and load conditions. When the working condition is high speed and low load, it is best to use low viscosity oil; when low speed and high load, it is recommended to use high viscosity oil. Generally speaking, at high speeds, it is recommended to use a lubricating oil at 40 ° C with a viscosity index range of 32-68 cSt (ISO VG32-68) (DIN 51519). At low speeds, the recommended viscosity range for a lubricating oil at 40 ° C is 90cSt (ISO VG90) or higher. For high speed and heavy loads, forced cooling is required to lower the temperature, and cooling oil can be supplied by a hollow screw or cooling nut to achieve cooling.

[3] Effect of pre-tension: The screw shaft is elongated and deformed due to heat, which may cause the positioning accuracy to deteriorate. The amount of thermal elongation can be obtained by the formula, and the amount of thermal elongation can be compensated by pre-tensioning; and the target value of the pre-tensioning compensation is the negative T value indicated by the national surface. Excessive pre-tensioning will burn out the support bearings. Therefore, it is recommended to use a pre-tension value of less than 5 ° C, but if the diameter of the screw rod exceeds 50 mm, it is not suitable for pre-tensioning; if the diameter of the screw rod is large, a large pre-tensioning force is required, thus causing the supporting bearing to overheat and burn out. ?