Abstract: Bolt connection is a commonly used method in parts assembly. Compared with welding, bonding, riveting and other processes, it has the characteristics of "high strength, simple operation and stable and reliable connection". The key technology of bolt connection lies in the control and detection of tightening. Based on the basic structure and control principle of servo electric screwdriver, this article discusses the basic concepts, technical requirements and commonly used control methods of bolt tightening.
Keywords: bolt, tightening, clamping force, control method.
introduction
Servo electric systems incorporate torque, position, and speed three-loop control, and are widely used in the transmission and control fields of industrial control equipment due to their high speed, high torque, and high precision. The MINI series of small-size, low-power servo motors launched by Tamagawa Seiki Co., Ltd. of Japan has unique applications in medical equipment, semiconductors, and multi-joint robots. As the general agent for the Tamagawa brand, Airtek has a unique advantage in applying its new products and technologies, and has successfully developed a screw tightening system using servo motors for precision control and drive, filling several gaps in the domestic market.
I. Basic Concepts of Tightening
1.0 The basic requirements for tightening are three points:
1) The connecting parts fit tightly; 2) They can withstand a certain dynamic load; 3) They have sufficient clamping (compression) force.
2.0 Key variables in the tightening process
2.1 Torque (T): The tightening torque applied, in Newton-meters (Nm);
2.2 Clamping force (F): The actual axial clamping (compression) force between the connecting bodies, in Newtons (N);
2.3 Friction coefficient (U): The torque coefficient consumed in bolt heads, threaded joints, etc.;
2.4 Angle (A): Based on a certain torque, the bolt undergoes a certain axial elongation or the connecting parts are compressed, requiring...
The required thread angle.
3.0 Bolt Main Parameters
3.1 Standard Torque: The torque required to tighten standard bolts under specified conditions, corresponding to different models, specifications, and materials.
Torque magnitude, measured in Newton-meters (Nm);
3.2 Preload: The axial tensile stress required to tighten the bolt, measured in Newtons (N).
3.3 Yield Strength: or yield stress, the tensile stress required to initiate plastic deformation in a bolt, measured in N/mm².
3.4 Tensile strength: or tensile stress, the minimum tensile stress that would cause the bolt to fail to tighten or even break, in N/mm².
II. Brief Analysis of the Tightening Process
1. The 541 rule (i.e., 50%, 40%, 10%)
See Figure A: Under normal circumstances, during the bolt tightening process, only 10% of the torque is actually converted into bolt clamping force. The remaining 50% is used to overcome the frictional force under the bolt head, and 40% is used to overcome the frictional force in the threaded pair. This is the "541" rule, which mainly reflects the relationship between clamping force and frictional force. However, if certain improvement measures are applied (such as applying lubricating oil) or if there are defects in the threaded pair (such as impurities, dents, etc.), this ratio will be affected and changed.
Figure A, Rule 541
2. Characteristics of bolted connections
Figure B: Characteristics of Bolted Connections
3. Factors affecting bolt tightening
According to Rule 541, whether a bolt is actually tightened (or whether the bolt preload is reached) depends primarily on the change in the coefficient of friction between the bolt and the connecting parts. So, what are some possible factors that could cause a change in the coefficient of friction?
1) Bolt head surface: Including plating, coating, etc., is it smooth and flat? Are there any unevenness or defects? Is there any rust or damage? etc.;
2) For threaded pairs: Are there any impurities or damage? What is the thread processing grade? etc.?
3) Connection method: There are two types: hard connection and soft connection, such as the application of washers and gaskets;
4) Material properties: mainly related to the hardness or rigidity of the material, and can also be broadly divided into hard connection and soft connection methods;
5) Improvement measures: such as applying lubricant or performing special treatment on the bolt surface.
III. Bolt Tightening Control Methods
1.0 Torque Control Method
1.1 Definition: A control method that immediately stops tightening when the tightening torque reaches a certain set control torque.
1.2 Advantages: The control system is simple and direct, and the tightening quality can be easily checked using a torque sensor or a high-precision torque wrench.
1.3 Disadvantages: Low control precision (preload error of about ±25%), and cannot fully utilize the potential of materials.
2.0 Torque-Angle Control Method
2.1 Definition: A control method that first tightens the bolt to a small torque, and then starts from that point and tightens it by a specified angle.
2.2 Advantages: The bolt axial preload accuracy is high (±15%), allowing for the acquisition of a large axial preload, and the values can be concentrated in one area.
The distribution is near the average value.
2.3 Disadvantages: The control system is relatively complex, requiring the measurement of two parameters: torque and rotation angle; and it is also difficult for quality control departments to find suitable methods.
Check the tightening results.
3.0 Yield Point Control Method
3.1 Definition: A method of tightening a bolt to its yield point and then stopping the tightening.
3.2 Advantages: The tightening accuracy is very high, and the preload error can be controlled within ±8%; however, its accuracy mainly depends on the bolt itself.
Yield strength.
3.3 Disadvantages: The tightening process requires dynamic and continuous calculation and judgment of the slope of the torque and rotation angle curves, and the control system...
It has high requirements for real-time performance and computing speed.
IV. Airt Servo Electric Screwdriver Series
4.1 Basic Components
Human-machine interface, control unit, I/O and communication interface, servo drive, servo motor, precision reducer, power supply unit.
4.2 Product Features
4.2.1 A precise and compact servo electric system is selected as the drive unit, and the torque and rotation angle can be set arbitrarily;
4.2.2 It has very powerful control and detection functions, which can complete the setting and indication of multiple tasks, multiple programs, multiple parameters, and multiple states;
4.2.3 User - friendly human-machine interface, easy to operate and learn, and can preset and save multiple parameters or modes determined by the test;
4.2.4 A series of screwdriver bodies with various specifications, installation methods and usage methods can meet the needs of different precision applications.
4.3 Technology and Process
4.3.1 An embedded ARM+DSP design scheme, which integrates torque and rotation angle control methods, enables the preload accuracy to reach a high level;
This is mainly due to the introduction of servo motor technology and the superior performance of Tamagawa's product technology and processes.
4.3.2 Powerful and practical programmable input/output functions and multiple communication interfaces, allowing for standalone use or online/networked operation .
Centralized control and monitoring greatly facilitate the system integration of different production lines, operating conditions, or automated equipment.
4.4 Upgrades and Expansions
1) Refine the product's specialized application areas, introduce more precise control methods, and form a multi-closed-loop, high-precision tightening system;
2) The graphical and dynamic settings and monitoring interface makes operation/use more intuitive, simple, and easy to understand;
3) Consider combining it with our existing industrial robots and work platforms to improve the efficiency of screw and bolt tightening processes/techniques.
It is more systematic and integrated, more precise, more stable and reliable, and more productive.
References:
[1] GB/T26547-2011 Test Methods for the Performance of Rotary Tools for Threaded Fasteners
[2] Quick Lookup Table of Standard Torque and Preload for Bolts
[3] Online resources: bolt tightening techniques, etc.
