What Is a Ball Screw?

Ball screw is an ideal product to convert rotary motion into linear motion, or convert linear motion into rotary motion.

The history of people using screws for transmission is actually not very long. Traditional screws have always been poorly positioned and easily damaged. Until 1898, people first tried to put steel balls between nuts and screws to replace sliding friction with rolling friction to improve its poor positioning and easy damage. In 1940, the ball screw was placed on the steering device of the car, and it was a huge revolution in the application of the ball screw, and gradually replaced the traditional Acme screw (ACME). Until recent years, the ball screw has become one of the most widely used components in the industry ^[1]

1. According to the national standard GB / T17587.3-1998 and application examples, the ball screw (has been basically replaced

Ball screw bearings provide a wide range of standardized products to suit various applications. It is widely used in machine tools, and the circulation methods of ball are circulating duct type, circulator type and end cap type. Preload method: positioning preload (double

There are two common circulation methods: outer circulation and inner circulation. When the ball is in contact with the lead screw during the cycle, it is sometimes called the outer cycle; when it is always in contact with the lead screw, it is called the inner cycle. The closed circuit of each ball cycle is called the column, and the number of leads contained in each closed circuit of the ball cycle is called the number of turns. Each nut of the inner circulation ball screw pair has 2 rows, 3 rows, 4 rows, 5 rows, etc., each row has only one turn; each outer loop has 1.5 turns, 2.5 turns, and 3.5 turns, etc. ^]

1. Small friction loss and high transmission efficiency ^[4]

The ball screw pair can be lubricated to improve wear resistance and transmission efficiency. Lubricants are divided into two categories, lubricating oil and grease. Lubricant oil, 90 to 180 turbine oil or 140 spindle oil. The grease can be lithium-based grease. Grease is added to the housing space of the threaded raceway and the mounting nut, and the lubricating oil is injected into the nut space through the oil hole in the housing.

Ball screw pair and other rolling friction transmission elements, as long as abrasive particles and chemically active substances are prevented from entering, these elements can be considered to work with almost no wear. But if dirt falls on the raceway, or if a dirty one is used

The main parameters of the thread

1) Outer diameter d (large diameter) (D) the diameter of an imaginary cylindrical surface that coincides with the crest of the external threadalso known as the nominal diameter

2) Inner diameter (small diameter) d1 (D1) the diameter of the imaginary cylindrical surface which coincides with the bottom of the external thread, and the calculated diameter of the dangerous section is used in the strength calculation

3) Middle diameter d2the diameter of an imaginary cylindrical surface where the tooth thickness and interdental width are equal in the axial section, which is approximately equal to the average diameter of the thread d20.5 (d + d1)

4) Pitch Pthe axial distance between two adjacent points on the generatrix of the middle diameter cylindrical surface

5) Lead (S)-the axial distance between two adjacent points on the same spiral line on the generatrix of the middle diameter cylindrical surface

6) Number of lines nthe number of thread spirals, generally for the convenience of manufacturing n4 The relationship between the pitch, lead, and number of lines: S = nP

7) Spiral rising angle -the angle between the tangent of the spiral on the medium diameter cylindrical surface and the plane perpendicular to the axis of the spiral.

8) Profile angle the angle between the two sides of the thread profile in the axial plane of the thread 9) Profile angle the angle between the side of the profile and the vertical plane of the thread axis. Symmetrical tooth profile

The main geometric dimensions of various threads (except rectangular threads) can be consulted in the relevant standards-the nominal size is that the outer diameter of the thread is approximately equal to the inner diameter of the pipe.

The self-locking condition of the screw pair is that the thread rising angle is less than or equal to the equivalent friction angle of the screw pair.

The transmission efficiency of the screw pair is: the ratio of the effective work to the input work when the nut rotates once.

The circumferential force required to overcome the axial force Q rising at a uniform speed

Ball screw failures are numerous and do not have a fixed pattern. Some failures are gradual failures, and there must be a development process, which becomes more and more serious as the use time increases; sometimes it is a sudden failure, usually without obvious symptoms, and it suddenly occurs. Adverse factors and external interactions. Therefore, the correct fault detection to determine the real cause of the fault is a prerequisite for rapid and accurate repair.

1) Detection and repair of clearance of ball screw nut pair and support system When the CNC machine tool has large reverse error, unstable positioning accuracy, and knife marks appearing in the quadrant, first check whether there is clearance in the screw system. The detection methods include: using a dial indicator with a steel ball placed in the central hole at one end of the screw to measure the axial movement of the screw, and another dial indicator to measure the movement of the table. Turn the screw forward and backward, observe the values reflected on the two dial gauges, and confirm the fault location according to the changes in the values.
a) Detecting and repairing the clearance of the screw support bearing, such as measuring the dial indicator when the screw rotates forward and backward, the pointer does not swing, indicating that the screw does not move. If the indicator of the dial indicator swings, it indicates that the lead screw is moving. The difference between the maximum and minimum measurement values of the dial indicator is the distance of the axial movement of the screw. At this time, we need to check whether the backing cap of the supporting bearing is locked, whether the supporting bearing has been worn out, and whether the preloaded bearing gasket is suitable. If there is no problem with the bearing, it only needs to be reconfigured as a preloaded washer. If the bearing is damaged, it is necessary to replace the bearing, refit it as a pre-loading washer, and then tighten the back cap tightly. The axial movement of the screw is mainly due to the accuracy of the bearing preloading washer. The most ideal state for the installation accuracy of the screw is that there is no positive and negative clearance, and the support bearing must have an interference of about 0.02mm.
b) The clearance and maintenance of the ball screw double nut pair have passed the inspection. If it is confirmed that the fault is not caused by the screw movement. Then we need to consider whether there is a gap between the screw nut pair. The detection method in this case is basically the same as detecting the screw movement. Use a dial indicator to measure the workbench connected to the nut. Turn the screw forward and backward to detect the maximum gap between the screw and the nut, and then adjust it.

The method is shown in FIG. 2. The thickness of the washer 4 is adjusted to cause axial displacement of the left and right nuts 1 and 2, thereby eliminating the gap between the ball screw nut pair and generating a pretension force. Because the structure of the screw nut pair is different, the adjustment methods are also different, which are not listed here one by one.
c) Inspection and maintenance of single nut pair For single nut ball screws, the gap between the screw nut pair cannot be adjusted. If a gap is detected in the screw nut pair. First check whether the threaded arc of the screw and nut has been worn. If the wear is severe, the complete set of screw nuts must be replaced.
If the inspection shows slight wear, you can replace the larger diameter ball to repair it. First, the maximum clearance of the screw nut pair is detected, converted into an increase in the ball diameter, and then a suitable ball is selected for reassembly. Such maintenance is relatively complicated, takes a long time, and requires a high level of technology.
d) The gap between the nut flange and the worktable is not fixed. This problem is generally easy to be ignored. Due to the long-term reciprocating movement of the machine tool, the loosening of the screws of the fixed method pull plate creates a gap. Remove the fault factor first to avoid taking a detour during repair.
e) Maintenance of faults such as unstable movement of ball screw nut pair and excessive noise.
The ball screw nut pair's uneven movement and excessive noise are mostly caused by poor lubrication, but sometimes it may be caused by the servo motor drive parameters not being adjusted well.

2) Bearings and lead screw nuts are poorly lubricated. If the machine tool generates noise and vibration during work, after detecting that there is no problem in the mechanical transmission part, the problem of poor lubrication must be considered first. After many years of operation, the screw nut is automatically lubricated. The system is often blocked and cannot be lubricated automatically. The problem can be solved by adding high temperature and high speed resistant grease to bearings and nuts. Grease can ensure the normal operation of bearings and nuts for several years.

3) Servo motor driving problems Some machine tools generate vibration and crawling during the movement. Often, there is no problem in detecting the mechanical part. No matter how the adjustment is made, vibration and crawling cannot be eliminated. After careful inspection, it was found that the servo motor drive gain parameters were not suitable for the actual operating conditions. After adjusting the gain parameters, vibration and creep can be eliminated.