The Development and Structural Design Methods of Screw Tightening Mechanisms

Abstract

Automatic screw tightening machine is a machine in modern industry that improves workers' work efficiency and is widely used in many fields. Automatic screw tightening machine is an auxiliary equipment for many large devices, which can greatly reduce the labor intensity of workers. At present, the structure of automatic screw tightening machines is complex, with high technological content and difficulty in localization. The automatic screw tightening control realizes fully automatic control of the screw tightening process, and can accurately control a series of actions such as separating, transferring, and tightening screws according to the pre programmed program by the operator. The controller can perform comprehensive detection of the power supply, including displaying parameters such as voltage, frequency, power factor, output current, and automatically compensating for them. At the same time, the controller can be equipped with automatic step-by-step compensation function. When a hole is not screwed, the machine automatically re sends the screw through detection to ensure the quality of the workpiece. And it also achieves the purpose of using screws of different sizes.

Keywords: Automatic screw tightening, Assemble, Machine design.

Introduction and Research Status

Screw tightening mechanism is an automated equipment designed to improve the manufacturing level of automotive components for the production of automotive glass lifters. Adopting a modular multi axis design, the torque adjustable screwdriver is driven by a servo motor and transmitted through a precision universal joint; There is a reliable screw automatic sorting and branching supply system, with a wide range of screw diameter adaptability and strong universality, which can meet the needs of flexible production. Mainly responsible for designing screw mechanisms and screwdrivers. Screw size range: (diameter) φ 4- φ 8, various types of screws with length less than 25mm, it is a non-standard equipment. At present, research on screw locking machines is in a leading position abroad. China's research on screw locking machines started relatively late and there is still a big gap between China and developed countries in the world. However, the country has provided great support in this regard and is currently developing rapidly.

The automatic screw tightening machine largely liberates manpower, shortens tightening time, improves assembly accuracy, effectively enhances overall product production efficiency, promotes profit growth for enterprises and factories, and even promotes the expansion of enterprise and factory scale to a certain extent. Moreover, the automatic screw tightening machine can achieve personalized design, with different specialized models designed for different industries, and is also more intelligent.

At present, the production and manufacturing of automatic screw tightening machines in China are mainly distributed in the Pearl River Delta and Yangtze River Delta regions. Objectively speaking, although these regions have a large number of factories, their scale is relatively small, their technology is relatively backward, and there is a phenomenon of competition homogenization. Subjectively speaking, there is a lack of technical personnel reserve in the region, coupled with the short development time of automatic screw machines in China, there are some professional technical personnel and experience gaps. Insufficient accumulation of technical experience can lead to designers and enterprise personnel being insensitive to their own electronic products, afraid or unable to choose to use them, resulting in certain limitations in the structural design of automatic screw machines, which greatly limits the improvement of their technical level. Moreover, many enterprises have not truly achieved independence in many core technologies. It is difficult to reduce the cost of screw machines by simply relying on purchasing foreign equipment.

In terms of development trends, with the competition in major markets, the increase in labor costs, and the transformation and development of the processing and manufacturing industry, the mid to low end market of electronic products cannot escape elimination, or it may change towards the mid to high end market, which poses new challenges for companies in the automatic screw machine related industry. China is an industrialized country with great market development potential. The rapidly developing high-tech industry of automatic screw tightening machines is bound to move towards a stronger future.

Overall Plan Description

By analyzing the required functional structure, the automatic screw tightening machine mainly consists of a discharge system, a locking system, and a control system. The discharge part, also known as the screw supply machine or screw arrangement machine, and the locking part, also known as the screw tightening machine. It is a small non-standard automation equipment that arranges chaotic and disorderly screws in order, and then sends the screws to the electric screwdriver nozzle through a certain screwing method, and finally accurately and quickly tightens the screws. It is widely used in the assembly industry and can be applied to various specifications of screws.

The main content of automatic feeding includes automatically conveying screws to the distribution hopper, arranging them in an orderly manner through vibration, and delivering them individually to the working head of the screw tightening unit. The control part is designed with a PLC control system, whose main purpose is to coordinate the actions of various mechanisms, and determine the next step of work by judging whether the screws are in place and whether the tightening is completed. Due to the fact that the automatic feeding and control system for screws was completed by other students, the structural design of this automatic screw tightening machine mainly focuses on the locking mechanism.

Transmission scheme

The common tightening methods for automatic screw tightening include pneumatic transmission and hydraulic transmission. The following analysis and comparison will be based on the two files.

Hydraulic transmission mode

A certain form of transmission that uses liquid as the main working medium for energy transmission and control is hydraulic transmission. During its operation, hydraulic transmission is divided into hydraulic transmission and hydraulic transmission based on the differences in the form and mode of energy transfer. Hydraulic transmission uses energy changes caused by the pressure generated by the compressed liquid. And hydraulic transmission mainly refers to a transmission form that can achieve energy conversion by using liquid drive. Hydraulic transmission is a transmission method that uses liquid kinetic energy to achieve energy conversion, such as hydraulic torque converters and hydraulic couplings. Hydraulic transmission technology is often used in industrial machinery design, which can optimize the structure of machines, reduce machine quality, save mechanical materials, reduce manufacturing costs, alleviate labor intensity, improve work reliability and speed.

Pneumatic transmission method

Pneumatic transmission is a technology that uses compressed air as a power source to drive and control various types of industrial equipment to achieve automation and mechanization of industrial production processes. With the continuous development in the field of automation, the product quality of industrial machinery automation has gradually improved, which has led to the widespread application of their driving methods in various fields. Among them, pneumatic technology is one of the leaders in these driving methods. The most common driving method is the pneumatic automatic device with low production cost and relatively simple structure, which plays a particularly important role in industrial automation.

Compared with the advantages of pneumatic transmission, its unstable operation caused by external loads, high working pressure, and noise generated during exhaust are acceptable for application in automatic screw tightening machines.

Selection of Driving Form

According to the process of motion, motor control can be divided into driving stepper and driving servo, and the following requirements need to be met: ① Drive for linear motion. ② Has a certain load. ③ Accurate positioning.

We can choose between stepper motors and servo motors.

Stepper motors generally do not have overload capacity, but they are subject to inertia torque during operation. Therefore, when selecting a stepper motor, a larger torque is chosen to overcome it. However, such a large torque is often not required during operation, which often leads to torque waste. In contrast, the overload capacity of servo motors is about three times that of stepper motors.Therefore, servo motors are more in line with the requirements of screw tightening devices on the market.

The AC servo electric motor has a pair of stator windings with a phase space difference of 90 degrees, namely the excitation winding and the control winding. Essentially, it is an AC asynchronous motor with similar working principles. If the AC servo motor power supply is fixedly connected to the excitation winding, when the control voltage is at the initial level, the rotor stops working and the motor has no torque. When a control voltage is applied to the control winding and the current to the winding is different, a two-phase rotating magnetic field is generated, and the rotor moves under its action.

Characteristics of AC servo motors: AC servo motors have a simple structure, good heat dissipation effect of electronic windings, and low motion inertia. In addition, DC servo motors are structurally no different from low-power DC motors. In the early development of servo motors, DC motors were mainly used, but the control accuracy that DC motors can achieve is not high. Therefore, DC motors are generally not used in high-precision situations. The excitation winding of DC servo motors is usually controlled by armature, in addition to magnetic field control.It is necessary to compare the differences between brushed DC servo motors and brushless DC servo motors.

Table 2: Comparison between brushed DC servo motors
and brushless DC servo motors

Automatic screw tightening requires a high-precision motor, which needs to move smoothly at low speeds. The motor needs to have a certain load capacity. Based on these points and the analysis of the above motors, an AC servo motor is selected as the driving component.

Selection of lifting cylinder

Regarding the design of the lifting power source for the electric screwdriver, the electric screwdriver selected in this design belongs to the downward pressure starting type, so it is necessary to ensure high work efficiency and stable working speed during operation. Electric screwdrivers, universal joint devices, and clamps all have a certain weight load, so they require a certain ability to withstand the load.The pressure control operation force is above a moderate level, the movement speed is fast, the adaptability to the environment is good, the structure is simple and easy to maintain, suitable for medium distance manipulation, the service life is long, and the price is also cheap,more suitable for application in screw tightening devices.

A cylinder is mainly composed of a cylinder barrel, piston rod, sealing ring, front end cover,rear end cover, and magnetic ring. It is a metal component that uses compressed air to make the piston perform linear reciprocating motion in the cylinder body. It is a pneumatic actuator that converts the pressure of the working fluid into mechanical energy in pneumatic transmission. According to the function of the cylinder, it can be divided into two types: linear motion and reciprocating swing. In this design, the electric screwdriver performs reciprocating linear motion in the vertical direction, so a linear motion type cylinder is selected. The cylinders that perform reciprocating linear motion can be further divided into single acting cylinders, double acting cylinders, special cylinders, combination cylinders, fixed cylinders, and pin type cylinders. The diameter of the cylinder barrel is an important parameter for cylinder selection. The selection calculation process is usually as follows.

Axial load of cylinder:

Average speed of cylinder:

Theoretical output force:

In the above formula,S represents cylinder stroke, usually determined according to actual work needs. m represents the overall quality of the screw tightening mechanism. is the coefficient of friction. m represents the load rate, usually 0.5.

The cylinder diameter selected needs to be greater than the diameter calculated by formula 4. Due to the frequent operation of electric screwdrivers, the cylinder piston rod needs to continuously descend and ascend, so a non rotating cylinder is usually selected in this mechanism. Finally, the cylinder model can be determined based on the SMC cylinder manual and the calculated cylinder diameter.

Universal Joint

The universal joint has the function of changing the direction of axis movement, and it will not move rigidly in one direction during operation, which meets the requirement of improving the flexibility of automatic screw tightening machine tightening alignment.

The universal joint changes from linear contact to surface contact through the torque transmission part, increasing the contact area and enabling the transmission of greater torque. This design enables the coupling to withstand significant torque during screw tightening, ensuring the stability of the connection.

Figure 1: Universal Joint

The basic steps for selecting a coupling are as follows.Select a suitable type of coupling based on the size of the transmitted load, the speed of the shaft, the installation accuracy of the two connected components, and taking into account the characteristics of various types of couplings.

1. The magnitude and nature of the torque to be transmitted, as well as the requirements for buffering and vibration reduction functions. For example, for high-power heavy-duty transmission, gear couplings can be used; For transmission with severe impact loads or requirements to eliminate shaft torsional vibration, high elasticity couplings such as tire couplings can be selected.
2. The magnitude and direction of the relative displacement between the two axes. When it is difficult to maintain strict and precise alignment of the two shafts after installation and adjustment, or when there will be significant additional relative displacement between the two shafts during operation, a flexible coupling should be selected. For example, when the radial displacement is large, a sliding block coupling can be selected, and for connections with large angular displacement or intersecting axes, a universal joint can be used.
3. Reliability and working environment of Couplings that are usually made of metal components and do not require lubrication are more reliable; The performance of couplings that require lubrication is easily affected by the degree of lubrication and may contaminate the environment. Couplings containing non-metallic components such as rubber are sensitive to temperature, corrosive media, and strong light, and are prone to aging.

In addition to the above factors, it is also necessary to consider whether the torque of the coupling itself can meet the requirements for use.

Calculation torque of universal joint

Tn represents nominal torque of universal joint(It is a theoretical calculation value under given conditions, namely the coupling speed, n≈ 10r/min. The bearing life Ln=5000h, axis angle=3 ° , and the value when the load is stable.). Tf represents fatigue torque of universal joint. T represents theoretical rotation of universal joint. Kn represents Speed correction coefficient.Kh represents Bearing life correction factor for universal joint. K represents two axis angle correction factor.

Ka represents load correction factor. When the load is uniform and the work is stable, Ka=1.0; When the load is uneven and subjected to moderate impact, Ka=1.1～1.3; When subjected to significant impact loads and frequent forward and reverse rotations, Ka=1.3 ～ 1.5, Ka>1.5 under extreme impact load and frequent forward and reverse rotation. For universal joint with high rotational speed, large bending angle, or length exceeding the specified limit, in addition to the calculations mentioned above, their rotational flexibility must also be verified.

Discussion and Conclusion

This article takes screw tightening as the starting point, analyzes the precautions required in the screw tightening mechanism, and analyzes multiple components. Firstly, determine the overall mechanism scheme and compare it between hydraulic transmission and pneumatic transmission, and obtain that pneumatic transmission has a better role and more convenient economic conditions in this mechanism. At the same time, relevant analysis was conducted on the selection and calculation of the diameter of the pneumatic cylinder, and the precautions for this link were pointed out. In addition, relevant analysis has been conducted on the issue of couplings, pointing out the precautions and criteria for selecting couplings.

There are still several shortcomings in this design, such as the analysis of the jamming situation in the discharge. Due to lack of experience, there are many unreasonable aspects, and optimization is needed in the calculation and selection. Only by understanding the relevant shortcomings through practice can we continuously optimize the structure. Learn to start from functionality and choose various types of institutions reasonably based on requirements. From calculation to selection, everything is interrelated, and each level must be strictly controlled in order to formulate the final plan, determine the size and shape. Considering the economic perspective, this aspect still has research value.

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