Application of Vacuum Hydrocarbon Cleaning Technology in the Bearing Industry

Comparison of Traditional Bearing Cleaning Agents and Hydrocarbon Cleaning Agents

Traditional bearing cleaning agents include gasoline, kerosene, and aqueous cleaning agents. Gasoline has good cleaning effects, and the bearings are easy to dry after cleaning, but it is highly volatile and extremely flammable, posing significant safety risks and serious harm to human health and the environment. Kerosene is safer, but its cleaning performance is not as good as gasoline, and it is not a dedicated cleaning agent; it has complex components and a flash point around 42°C, which leads to a longer drying time. Aqueous cleaning agents have poor cleaning effects on organic contaminants and tend to leave residues of surfactants on the surface of the bearings after cleaning, which are not easy to dry. They also have poor rust prevention properties, so they are generally not used for cleaning finished bearings.

Hydrocarbon cleaning agents, like gasoline and kerosene, belong to the category of hydrocarbon cleaning agents. Broadly speaking, all three are hydrocarbons within organic compounds. The main component of hydrocarbon cleaning agents is high-purity alkanes, which are more suitable components distilled from petroleum through a distillation tower and then processed through hydrogenation, desulfurization, and de-aromatization to obtain saturated alkanes. These are then formulated with stabilizers and other preparations. Hydrocarbon cleaning agents have a strong ability to remove grease, and their cleaning effect and drying efficiency are much better than those of kerosene and aqueous cleaning agents. Their flash point is between 42 and 80°C, they are not easily volatile, and they are safer, making them an ideal cleaning agent.

Main Features of Hydrocarbon Cleaning Capability

The boiling point of hydrocarbon cleaning agents is high, and their natural drying properties are not as good as those of traditional halogenated solvents, which require drying equipment. They have a low flash point and need to take fire protection measures during cleaning, drying, and distillation regeneration. If ultrasonic cleaning is adopted, a cooling water device must be installed to prevent the temperature of the hydrocarbon liquid from becoming too high. To overcome these shortcomings and better utilize hydrocarbon cleaning agents, it is necessary to improve the process equipment, which also increases the one-time investment cost of cleaning equipment.

During the mechanical processing, heat treatment, and assembly processes, the surface of bearing parts is attached to many inorganic and organic contaminants, including machine tool cooling fluids, heat treatment oils, cuttings, and dust. Hydrocarbon cleaning agents require good solubility with various components of contaminants. Petroleum hydrocarbon compounds are a mixture of aliphatic hydrocarbons (cyclic) and aromatic hydrocarbons. Components such as grease and oil are easily dissolved in aliphatic hydrocarbons. When contaminants contain water-soluble components that are not easily soluble in hydrocarbon compounds, alcohol can be used for dissolution. Aromatic hydrocarbons have a strong odor and lack safety, so aromatic hydrocarbons are generally not included in the hydrocarbon cleaning agents currently in use.

Hydrocarbon Cleaning Under Vacuum Conditions

Ultrasonic cleaning technology has been widely used in the bearing industry. When ultrasonic waves propagate in the cleaning liquid, they produce positive and negative alternating sound pressure, which impacts the cleaning parts. Nonlinear effects generate sound streams and micro-sound streams; ultrasonic cavitation produces high-speed micro-jets at the solid-liquid interface. All these effects can destroy contaminants, remove or weaken boundary layers, increase stirring and diffusion effects, and accelerate the dissolution of soluble contaminants. However, when there is a lot of residual gas in the hydrocarbon cleaning agent, ultrasonic waves attenuate quickly in the air, and the presence of bubbles increases the loss of ultrasonic propagation. On the other hand, the more residual gas in the liquid, the smaller the cavitation threshold, the more likely cavitation will occur. But as the gas diffuses into the bubble during the growth process of the cavitation bubble, the impact of the shock wave when the cavitation bubble collapses will be reduced, thus weakening the ultrasonic cleaning effect. Therefore, by evacuating the closed ultrasonic cleaning workstation's liquid tank to reduce the pressure inside the tank, a large amount of residual gas in the cleaning liquid is precipitated, thereby enhancing the ultrasonic cleaning effect. Therefore, hydrocarbon cleaning equipment is generally equipped with a vacuum degassing device, with a vacuum degree of about -0.06 to -0.04 MPa.

The vapor phase cleaning of hydrocarbon liquid involves continuously passing hydrocarbon vapor through the cleaning tank. The temperature of the hydrocarbon vapor is relatively high, and when it encounters bearings with a lower temperature, a phase change occurs. The heat generated by the phase change of the hydrocarbon vapor increases the temperature of the bearings. As hydrocarbon vapor is continuously supplied, the temperature of the bearings and the hydrocarbon liquid continuously rises, and the hydrocarbon liquid's dissolution power for contaminants also continuously increases with the rise in temperature. Moreover, the vapor phase cleaning is always carried out with clean hydrocarbon liquid, so the cleanliness level of the bearings after vapor phase cleaning is generally very high. Moreover, vapor phase cleaning is carried out in a vacuum tank, which isolates oxygen, and the safety factor is high.

Vapor phase cleaning and vacuum drying are carried out in the same tank. The process of vapor phase cleaning for bearings is also the process of heating the bearings. Vapor phase cleaning is carried out under low vacuum conditions. Depending on the type of bearings being cleaned, after a certain period of cleaning, the supply of hydrocarbon vapor to the drying tank is stopped, the liquid in the tank is discharged, and then the drying tank is evacuated to a high vacuum. The vacuum degree in the drying tank drops sharply, but the temperature remains high, and the residual hydrocarbon cleaning agent enters the boiling state in an instant, becoming hydrocarbon vapor that is sucked away by the vacuum pump, thus achieving complete drying of the bearings.

Hydrocarbon vapor is generally produced by distillation in a distillation machine. Under atmospheric pressure, the boiling point of hydrocarbon cleaning agents is relatively high (150 to 190°C). However, if the liquid tank is evacuated, the boiling point of the hydrocarbon cleaning agent will be significantly reduced. When the vacuum pump reduces the pressure in the distillation tank to 10 mmHg, heating the hydrocarbon cleaning agent to about 80°C will cause it to boil violently and produce a large amount of vapor. Some of the hydrocarbon vapor can be used for vapor phase cleaning, and the rest can be condensed through a cooling system for reuse, while high-boiling-point oil and impurities remain in the distillation device and can be cleaned regularly. Hydrocarbon vapor used for vapor phase cleaning can also be drawn into a gas-liquid separation device through a vacuum pump, condensed, and reused, thus achieving green cleaning.