Abstract: This paper analyzes the principle of ultrasonic cleaning and studies methods to reduce the gas content in the cleaning fluid based on the characteristics of bearing parts. Simultaneously, addressing the drawbacks of hydrocarbon solvents, such as difficulty in drying and poor safety, the feasibility of vacuum drying technology is analyzed, and the vacuum working pressure for the vacuum drying process is determined to be below 13.3 kPa, and the heating temperature to be around 110℃.
Keywords: ultrasonic cleaning; automotive bearings; automatic cleaning
Foreword
With the rapid development of my country's economy and the improvement of people's living standards, automobiles are gradually transitioning from a luxury item owned by a few to a part of ordinary people's lives. China's huge automobile market has attracted the participation of international automotive giants and state-owned and private capital, making the automotive industry a pillar industry and significantly improving the level of scientific and technological development. However, various automotive parts need to be cleaned during automobile production and maintenance. Bearings are known as the "joints of a car," and the level of development of a country's bearing industry often represents and restricts the development level of the automotive industry and other related industries. However, automobile bearings operate under extremely harsh conditions. During machining and transportation, their surfaces accumulate many inorganic and organic substances. If not cleaned properly, this can seriously affect the quality of the finished product or the smoothness of the car's operation, and may even cause wear and corrosion of the bearing's inner and outer rings or balls, leading to malfunctions and causing considerable inconvenience to people's lives and work. In the past, to ensure transportation safety, a large number of bearings were replaced or scrapped to ensure the operation of high-quality bearings. However, with the continuous progress and development of modern science and technology, to ensure that normal transportation operations are not affected, maintenance points are often established to perform necessary regular cleaning and repair of bearings, in order to extend their service life, reduce financial losses, and thus obtain maximum economic benefits. Therefore, the cleanliness of bearings is one of the key issues of bearing quality. Although bearing cleaning cannot directly improve the precision, lifespan, or other performance indicators of bearings, cleanliness directly affects these indicators and is a necessary condition for maintaining bearing precision, lifespan, and other performance characteristics. With the increasing demands for bearing quality, bearing cleaning technology has become extremely important.
Fuzzy control technology has become an important branch of intelligent control technology, representing an advanced algorithmic strategy and a novel technique. Since E.H. Mandani, a British engineer, first used a fuzzy controller based on fuzzy set theory for steam engine control in 1974, fuzzy control technology has experienced widespread and rapid development over the past 30 years. Currently, fuzzy control is widely applied in metallurgical and chemical process control, industrial automation, intelligent home appliances, instrumentation automation, and computer and electronic technology applications. It demonstrates strong application value, particularly in traffic intersection control, robotics, manipulator control, aerospace flight control, automotive control, elevator control, nuclear reactors, and home appliance control. Dedicated fuzzy chips and fuzzy computers are now available for selection. my country began researching fuzzy controllers in 1979 and has achieved significant results in the definition, performance, algorithms, robustness, circuit implementation methods, stability, and rule self-adjustment of fuzzy controllers. The renowned scientist Qian Xuesen pointed out that fuzzy mathematics theory and its applications are related to my country's national strength and destiny in the 21st century.
Since 2003, China's nearly 20-year compliance activities have reached the national compliance phase (2003-2011). Due to the late start of cleaning equipment manufacturing in China, there were only a few small cleaning equipment manufacturers nationwide in the 1990s. These companies not only had rudimentary production and processing equipment, but also lacked a comprehensive understanding and mastery of new cleaning processes that could replace ODS cleaning agents. Therefore, it was necessary to begin using new cleaning equipment that utilizes non-ODS cleaning agents.
With the rapid development of the bearing industry and increasingly stringent quality requirements, phasing out ODS cleaning technology is imperative. This project aims to study a bearing cleaning process using the latest ultrasonic technology and non-ODS hydrocarbon solvents as cleaning media. Through experiments, the optimal process parameters will be identified and applied to the development of new cleaning equipment to achieve high cleaning levels, lower labor intensity, environmental friendliness, and reasonable equipment prices. In conclusion, this project has significant practical value and broad market demand.
Research and application of ultrasonic cleaning technology in my country began in the 1950s, almost simultaneously with the development of other countries. At that time, the core component of ultrasonic cleaning equipment was a magnetostrictive transducer, and the ultrasonic power supply was a vacuum tube device. By the 1970s, this transducer was gradually replaced by high-efficiency and easily manufactured piezoelectric transducers, and transistor and thyristor ultrasonic power supplies emerged, greatly improving equipment efficiency while significantly reducing size. However, most of the equipment produced was benchtop, single-tank ultrasonic cleaning machines, and the cleaning media used were mostly water-based cleaning solutions. With technological advancements, the maturation of ultrasonic cleaning technology, and the development of new environmentally friendly cleaning agents, more and more companies are beginning to adopt new multi-tank ultrasonic cleaning technology to replace older cleaning methods and equipment.
1. Process Analysis of Automotive Bearing Cleaning
Ultrasonic cleaning technology is now widely used in the bearing cleaning industry. When ultrasound waves propagate in the cleaning fluid, they generate alternating positive and negative sound pressure, impacting the parts being cleaned. Simultaneously, due to nonlinear effects, acoustic flow and micro-flow are generated; while ultrasonic cavitation produces high-speed microjets at the solid-liquid interface. All these effects can break down contaminants, remove or weaken the boundary layer, increase agitation and diffusion, and accelerate the dissolution of soluble contaminants. Table 2.2 shows the residual percentage of various industrial cleaning methods.
However, when there is a large amount of residual gas in the hydrocarbon solvent, the presence of bubbles increases the ultrasonic propagation loss because ultrasonic waves attenuate rapidly in air. On the other hand, although more residual gas in the liquid results in a lower cavitation threshold and a greater likelihood of cavitation, the increase in gas diffusing into the cavitation bubbles during their growth reduces the intensity of the shock wave when they collapse, thus weakening the ultrasonic cleaning effect. However, by evacuating the sealed ultrasonic cleaning station's storage tank, the pressure inside is reduced, causing a large amount of residual gas to be released from the cleaning liquid, thereby enhancing the ultrasonic cleaning effect. Therefore, hydrocarbon cleaning equipment is generally equipped with a vacuum degassing device, and the vacuum degree of vacuum degassing is relatively high.
The pressure is low, approximately -0.06 to -0.04 MPa. Vacuum degassing is based on Henry's Law, which states that under isothermal and equilibrium conditions, the solubility of a gas in a liquid is directly proportional to the equilibrium partial pressure of that liquid, i.e.:
Where PA is the equilibrium partial pressure of gas A in the gas phase when the system is in gas-liquid equilibrium, XA is the solubility of gas A in the liquid phase (average mole fraction in the liquid phase), and HA is the Henry's constant for the gas, which depends on the properties of the solute and solvent and the temperature of the system.
When the temperature remains constant and the pressure decreases, it's equivalent to raising all the air bubbles in the liquid to a position close to the liquid surface without evacuation. From thermodynamics, we know P1·V1=P2·V2, indicating that as pressure decreases, the gas volume increases, and consequently, buoyancy increases. When the buoyancy is sufficient to overcome the initial resistance, the air bubbles can rise within the liquid by buoyancy. During the ascent, the pressure continuously decreases, the volume further expands, the buoyancy continuously increases, and the rising speed becomes faster and faster until they reach the liquid surface. Vacuum degassing can be used to degas the entire cleaning tank for vacuum cleaning. Alternatively, the cleaning solution can be degassed separately. To improve production cycle time and save on equipment manufacturing costs, this paper proposes a process method for separately vacuum degassing hydrocarbon solvents before ultrasonic cleaning. Before entering the cleaning tank, the cleaning solution is first collected in a degassing tank for vacuuming, and then ultrasonic cleaning is performed after degassing.
Rinsing is essentially the process of diluting the contaminated cleaning medium remaining on the bearing surface. Before the bearing is removed from the cleaning tank, it is essentially in a contaminated cleaning agent. After removal, the surface will inevitably carry away or retain contaminated cleaning medium, which needs to be removed by rinsing. Therefore, rinsing is generally not used alone, but rather as an optional step after ultrasonic cleaning.
Steam cleaning with hydrocarbon solvents involves continuously passing hydrocarbon vapor through a cleaning tank. The high temperature of the hydrocarbon vapor causes a phase change when it encounters the cooler bearing. The heat generated by this phase change raises the bearing temperature. As the hydrocarbon vapor is continuously supplied, the temperature of both the bearing and the hydrocarbon solution rises. The dissolving power of the hydrocarbon solution for dirt also increases with temperature. Furthermore, since clean hydrocarbon solvent is always used for steam cleaning, the bearings typically achieve a very high level of cleanliness after steam cleaning. Additionally, steam cleaning is conducted in a vacuum tank, isolating oxygen and ensuring a high level of safety.
Steam cleaning and vacuum drying are carried out in the same tank. The process of steam cleaning the bearing is also the process of heating the bearing. Steam cleaning is carried out under low vacuum. Depending on the type of bearing being cleaned, after a certain time, the flow of hydrocarbon steam into the drying tank is stopped, the liquid in the tank is discharged, and then a high vacuum is drawn into the drying tank. The vacuum degree in the drying tank drops sharply, while the temperature remains high. The residual hydrocarbon solvent will instantly enter a boiling state and become hydrocarbon steam, which is then sucked away by the vacuum pump, thereby achieving complete drying of the bearing.
2. Automatic control process design for the cleaning machine
The main contaminants on automotive bearings include iron filings, oil stains, and dust. These adhere to the outer and inner surfaces of the bearings, forming heavily soiled deposits that can severely damage them. Some of these contaminants are only lightly attached to the surface, while others are stubborn stains adhering to the inner and outer rings and balls, causing adverse effects. Hydrocarbon solvents are non-polar organic solvents with strong oil cleaning capabilities, good penetration, and no risk of metal rusting or corrosion. They are ideal cleaning agents to replace ODS substances for cleaning the housing and rotor. According to the cleaning process flow, ultrasonic cleaning and vacuum drying are performed. Parts are quantitatively loaded into cleaning baskets for batch cleaning. To ensure the required cleanliness, the cleaning process flow for this equipment is determined as follows: ultrasonic rough cleaning after vacuum degassing—ultrasonic fine cleaning after vacuum degassing—rough rinsing—fine rinsing—steam cleaning + vacuum drying.
The ultrasonic cleaning system mainly consists of a cleaning solution tank and an ultrasonic generator. The cleaning tank is divided into two ultrasonic cleaning tanks and two rinsing tanks. The cleaning tanks need to be made of high-strength stainless steel that is resistant to general chemical corrosion. This equipment uses OCr19Ni9 (SUS304) stainless steel, which has good corrosion resistance, heat resistance, low-temperature strength, and mechanical properties. It also has good hot workability such as stamping and bending, exhibits no heat treatment hardening, and is non-magnetic. However, due to ultrasonic cavitation corrosion, the tank walls always experience some degree of wear. Although this wear is relatively small, it can affect the changes in some ultrasonic cleaning parameters over time. Therefore, a wall thickness of 3mm was selected in the design.
The installation of ultrasonic transducers is a crucial aspect of ultrasonic cleaning tanks. There are two methods for installing ultrasonic transducers in the cleaning tank. One method involves fixing individual transducers to the bottom or side of the tank. This method requires polishing the surface of the tank in contact with the cleaning fluid to reduce cavitation corrosion. The other method involves bonding multiple transducers together onto a radiation plate and sealing them into a box-like transducer assembly, called a transducer unit. This assembly can be immersed in any position within the cleaning tank containing the cleaning fluid to achieve optimal cleaning results. This method is called immersion installation. In this case, the transducer is an independent component, and immersion transducers are easier to maintain and can be quickly replaced. Therefore, the immersion installation method is adopted in the design. Based on the analysis of ultrasonic principles, a 1500W, 40kHz BNEN 8500 ultrasonic generator is selected for the cleaning machine. Each ultrasonic generator comes with a box-shaped transducer containing 18 transducers.
The ultrasonic transducer is mounted at the bottom of the cleaning tank. Because hydrocarbon solvents have a slight odor, ventilation slots are designed along the upper edge of the cleaning tank to maintain a good working environment. These ventilation slots are connected to the cleaning machine's air duct via end vents, allowing for the immediate removal of airborne hydrocarbon solvents. The rinsing tank has a similar structure to the ultrasonic cleaning tank, except that it is shorter due to the absence of ultrasonic transducer transducers at the bottom.
3. Design of the control system for the automotive bearing cleaning machine
3.1 Design of Pneumatic Control System
The pneumatically driven operation in the design of the bearing parts cleaning machine is as follows: conveying and hooking of the material transfer arm in the cleaning zone, conveying, lifting, and hooking of the material inlet and outlet arms in the drying zone, and movement and sealing of the drying chamber sealing cover. Additionally, various types of pneumatic valves are required in the liquid pipeline. Since the pneumatic control methods for vacuum degassing devices I and II are the same, the pneumatic control methods for vacuum drying chambers I and II are also the same.
The air source processor, consisting of a water separator, air filter, pressure reducing valve, and oil mist atomizer, plays a role in filtering, pressure regulation, and oil atomization in the pneumatic system. AC represents cylinders; analysis of the cleaning machine's operation process shows that double-acting cylinders are needed to complete the control requirements. Double-acting cylinders refer to cylinders whose two chambers can be respectively input with compressed air to achieve bidirectional movement, and are widely used. Among them, AC3 is a double-stroke cylinder, and AC7 is a rodless cylinder that drives the forward and backward movement of the drying chamber sealing cover. SC is a speed control valve; SR is a pressure reducing valve; SOL1, 3, 4, 6, and 11 are three-position five-way solenoid directional valves, SOL2, 5, 12, and 23 are two-position five-way dual-control solenoid directional valves, and the rest are two-position three-way solenoid directional valves; V represents pneumatic valves, among which V17 is a pneumatic rotary valve.
3.2 Cylinder Selection
The cylinder is selected according to the needs of the host machine, and standard cylinders should be selected as much as possible during the design process.
(l) Selection of installation method: The installation method is determined by factors such as installation location and purpose of use. The fixed installation method for cylinders adopted in this project is: front flange (MF1 type) and rear flange (MF2 type).
(2) Output force of cylinder: Based on the magnitude of the force required by the working mechanism, the thrust and pull on the piston rod are determined by considering the cylinder load rate, thereby determining the cylinder inner diameter.
For a conventional double-acting cylinder, when the rodless chamber is the working chamber, the theoretical tensile force F0 is:
In the formula, D is the cylinder diameter (m).
P — Cylinder working pressure (Pa).
When the rod chamber of a conventional double-acting cylinder is the working chamber, the theoretical pulling force F0 is:
In the formula, d represents the piston rod cylinder diameter (m).
Taking into account the friction between the piston rod and the piston itself, the actual output force needs to be corrected by the cylinder efficiency. If the cylinder dynamic parameters are required to be high and the operating frequency is high, the load factor is generally taken as η = 0.3~0.5, with a smaller value for high speed and a larger value for low speed. If the cylinder dynamic parameters are not required to be high and the operating frequency is low, and the motion is basically uniform, the load factor can be taken as η = 0.7~0.85.
(3) Cylinder stroke: The cylinder (piston) stroke is related to the application and stroke ratio of the working mechanism. In most cases, full stroke should be used to avoid the piston from colliding with the cylinder head. Especially for clamping mechanisms, in order to ensure the clamping effect, a stroke margin of 10~20mm must be added to the calculated stroke.
(4) Cylinder movement speed: This is mainly determined by the needs of the driven working mechanism. When a slow and stable speed is required, a pneumatic-hydraulic damping cylinder or a throttling speed regulation cylinder should be used.
4. Conclusion
The principle of ultrasonic cleaning was analyzed, and methods to reduce the gas content in the cleaning fluid were determined based on the characteristics of the bearing parts being cleaned, thereby improving the efficiency of ultrasonic cleaning. To address the drawbacks of hydrocarbon solvents, such as difficulty in drying and poor safety, the feasibility of using steam cleaning and vacuum drying was studied and analyzed, and it was determined that the vacuum working pressure during the vacuum drying process should be below 13.3 kPa.
