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Abstract: The application development background and specification requirements at home and abroad for food － grade grease are introduced，and the components are analyzed． The selecting method for food － grade grease of rolling bearings is reviewed combing with use conditions for rolling bearings． It is believed that the environmentally friendly，biodegradable，food － grade grease is application development direction of greases for rolling bearings．
Key words: rolling bearing; food － grade grease; selecting method; development trend
To reduce friction and wear, lower temperature rise and noise, and improve the reliability and service life of bearings, different lubricants and lubrication methods are selected for rolling bearings according to their usage conditions. Bearings used in production and processing equipment such as food and drugs have higher requirements for lubricants. In addition to meeting normal lubrication performance, it is also necessary to ensure the safety of lubricants and prevent food safety issues caused by high-temperature evaporation and leakage. Therefore, bearings used in such mechanical equipment need to use specialized food grade lubricants.
Due to the fact that the DM n value of bearings used in food machinery transmission, processing, packaging and other equipment generally does not exceed 3 × 105 mm · r/min, and the working temperature is mostly -25-160 ℃, under low and medium load conditions, the operating environment is mostly humid and requires regular cleaning. Therefore, lubricating grease lubrication is often used.
With the rapid development of the food processing industry and further emphasis on food hygiene and safety, the demand for food grade lubricating grease for bearings is constantly increasing. Due to the small production and high prices of food grade lubricants in China, there are certain safety hazards in using vegetable oil and mineral oils such as petroleum jelly and paraffin as lubricants in the market; For rolling bearings with high mechanical stability, extreme pressure wear resistance, and service life requirements, the required high-performance food grade lubricating grease mainly relies on imports. Therefore, summarizing the performance characteristics and application of food grade lubricating grease for bearings provides a reference for better selection of lubricating grease.
1. Standard specifications for food grade lubricants
In order to ensure safety and limit abuse, various countries and regions have established specific regulations for food grade lubricants. The food grade lubricant approval and registration system established by the US Department of Agriculture (USDA) in 1962, which is widely recognized in the same industry worldwide, requires formulas to comply with the requirements of the US Federal Regulations FDA 21 CFR. Subsequently, the National Health Foundation (NSF) of the United States followed USDA's review requirements and classified authorized food grade lubricants as H1, H2 and H3 levels. The European Union, the Canadian Department of Agriculture and Food (AAFC)/Food Inspection Agency (CFIA), and the Australian Inspection and Quarantine Agency (AQIS) also certify or license food grade lubricants. Developed countries such as Europe and America have formed a series of food machinery lubricant products that can meet various performance needs. The management and certification systems are standardized, and manufacturers can provide certified lubricant products of different grades.
ISO 21469-2006 "Safety of Machinery and Hygienic Requirements for Lubricants in Contact with Subsidiary Products" specifies the hygiene requirements that need to be paid attention to in the design, production, use, and handling of lubricant formulations in the event of possible contact between lubricants and products. It is a voluntary standard and also has a certification process. The release of this standard marks the trend of internationalization in the certification of food machinery lubricants, and also demonstrates the unique position of food machinery lubricants.
The relevant technical standards successively introduced by China include GB/T 12494-1990 "White Oil for Food Machinery" GB 15179-1994 Food Machinery Lubricating Greases GB 4853-2008 Food grade white oil GB 23820-2009 "Safety of Machinery - Hygienic Requirements for Lubricants in Accidental Contact with Products" and GB 7189-2010 "Food grade Paraffin Wax". The production and application of domestic food grade lubricants are still extremely asymmetric with the development of the food industry, and the management is not standardized enough. The various testing items for lubricants only include routine basic tests such as kinematic viscosity, water, mechanical impurities, water-soluble acid-base, carbides, polycyclic aromatic hydrocarbons/UV absorbance, and do not involve safety, health, and environmental protection items; GB 23820, which is equivalent to ISO 21469, focuses on the pre-production approval process and does not involve the detection, limitation, and testing methods of toxic substances in the product; There is no recommended explanation for lubricant performance tests, such as high and low temperature resistance, oxidation stability, shear stability, extreme pressure and wear resistance, and service life.
2. Composition of lubricating grease
Grease is a gel like lubricating material, which is composed of base oil, thickener and additives under high temperature. It can also be considered as a binary gel dispersion system formed by highly dispersed dispersed phase as thickener and dispersion medium as base oil. In addition to meeting normal mechanical lubrication performance, food grade lubricating grease for bearings must also ensure hygiene and safety.
2.1 Base oil
The base oil must have good antioxidant properties, high and low temperature resistance, anti emulsification properties, and anti friction properties to extend the service life of bearings. Base oil is divided into mineral oil, synthetic oil, and bio (vegetable) oil.
1) Mineral oil is a super refined mineral oil, currently mainly refined by hydrogenation cracking method, which is food grade white oil. Its composition is relatively pure, with chain saturated hydrocarbons accounting for more than 99%, low water content, sulfur content less than 10 μ g/g, and no aromatic hydrocarbons.
2) The commonly used synthetic oils include poly alpha olefins (Poly Alpha Olefin), PAO), Silicone oil, perfluoropolyether, etc. Among them, PAO has excellent viscosity temperature characteristics, low-temperature fluidity, and lubrication performance, with a pure and single component, which is the future trend of application and development.
3) The advantages of vegetable oil are non-toxic and biodegradable, but it has poor oxidation stability, low-temperature flowability, and hydrolysis stability. Currently, research has been conducted to improve its performance through additives, which is also an application trend.
2.2 Thickener
The selection of thickeners must first require non-toxic and harmful properties, and then be selected based on the usage conditions and the dripping point, high temperature resistance, water resistance, etc. of the lubricating grease made from different thickeners. Composite aluminum soap, composite calcium soap, polypropylene, lithium soap (12-hydroxystearic acid), etc. are often used. From the perspective of better biodegradability, a new type of composite titanium soap has emerged; But some people suggest that in the future, food grade lubricants will mainly be composite calcium sulfonate based lubricants. The specific selection of thickener needs to be fully demonstrated based on the operating conditions of the bearing.
2.3 Additives
Additives are components added to improve certain properties of lubricating grease, such as antioxidants, extreme pressure additives, anti-wear agents, rust inhibitors, oiliness agents, etc. For food grade white oil used in heavy-duty and impact loaded rolling bearings, oil-based agents and extreme pressure agents are added to improve lubrication performance. Animal oil can be used as the oil-based agent, and materials approved by FDA or USDA, such as polyalkyl ethylene glycol, can be used as the extreme pressure agent. For food grade lubricating grease, the use of additives is strictly limited because additives often have certain toxicity and unsafe factors. In addition, high-performance lubricants, especially artificially synthesized high-performance food grade lubricating grease, have very few varieties and quantities of additives, and even no additives are added.
3. Selection of lubricating grease
Lubricating grease accounts for only 3% to 5% of the cost in equipment maintenance, but it has a significant impact on normal production. Choosing appropriate high-performance food grade lubricating grease is an important factor that cannot be ignored in ensuring safety, reducing maintenance costs, and improving efficiency. The selection of food grade lubricating grease for bearings should consider factors such as speed, temperature, load and environmental conditions, and lubrication method. On the basis of comparing the performance characteristics of various representative food grade lubricating greases at home and abroad (Table 1, all of which are NSF H1 registered products), the selection methods of lubricating greases are analyzed and summarized.
3.1 Speed
The spindle speed and bearing inner diameter are important criteria for selecting lubricating grease for bearings, usually represented by the value of DM n. According to the speed coefficient limit of different types of rolling bearings using lubricating grease, the general principle is that the higher the speed, the greater the viscosity of the selected grease (the smaller the cone penetration), to ensure the lubrication performance under centrifugal force.
Bearings for food processing machinery generally use 2 # or 3 # grease (viscosity). For high-speed beverage bottling machines, 2 # or 3 # grease from the series of products can be selected; In addition, the issue of the long service life of the lubricating grease during its 24-hour operation should also be considered. For the lubrication of long-life bearings, it is required to fully consider the high-temperature performance and oxidation stability of food grade lubricating grease when designing the formula, and choose synthetic base oil to extend the service life of food grade lubricating grease.
3.2 Temperature
The plasticity and similar viscosity of lubricating grease vary with temperature. Under high temperature conditions, the base oil evaporates, oxidizes, deteriorates, accelerates oil separation, and even becomes completely ineffective; Under low temperature conditions, the viscosity of lubricating grease increases or even completely solidifies, resulting in loss of fluidity and inability to lubricate. In addition, different food grade lubricants have significant differences in high temperature resistance due to different thickeners, so the selection of lubricants is very important. For bearings in high-temperature ovens and microwave ovens used in food processing, if the operating temperature exceeds 200 ℃, it is necessary to use ultra-high temperature resistant food grade lubricating grease, such as food grade fluorine grease; For high-temperature bearings with a temperature not exceeding 160 ℃, general high-temperature food machinery lubricating grease can be used for lubrication. The temperature of equipment such as freezers and freezers is below -30 ℃. If the lubricating grease loses its fluidity, it will cause difficulty in starting the bearings, resulting in abnormal damage to the equipment. Therefore, it is necessary to choose low-temperature resistant lubricating grease, such as Great Wall high-temperature food machinery lubricating grease, Krupp NH1 94-301, and other low-temperature resistant food grade lubricating greases.
3.3 Load
There are various types of food processing machinery, and the bearing capacity also varies greatly. Most food machinery carries medium to low loads, so the requirements for lubricating grease are also different. Lubricating grease with low viscosity (high cone penetration) should be appropriately selected for heavy loads, and it should also have high oil film strength and extreme pressure performance under high pressure. If the sugar squeezing industry's sugar squeezing machine bearings are subjected to heavy loads, it is necessary to require food machinery lubricating grease with high viscosity of base oil and high content of thickener, which has excellent extreme pressure performance and wear resistance, and can provide protection for heavy-duty or low-speed bearings.
3.4 Environmental conditions
Dust and corrosive media are strictly controlled in food processing, mainly in humid environments and water contact conditions. Fruit, vegetable, meat processing equipment, beverage production lines, etc. require frequent high-temperature alkaline flushing. Water mist and spray impact can cause the loss of lubricating grease. Therefore, in addition to good sealing performance, bearings must also ensure that the lubricating grease has good adhesion, water resistance, rust resistance, and alkali resistance. In the beer industry, the filling line requires special food machinery lubricating grease to have excellent ethanol resistance due to long-term contact between lubricating grease and beer. Some food processing equipment does not require continuous production, so rust prevention during equipment storage is highly valued. Therefore, it is necessary to choose food grade lubricating grease with excellent rust prevention performance or lubricating grease with good compatibility with sealing materials.
3.5 Lubrication method
According to research, the lubrication of equipment in the food and pharmaceutical industries generally adopts two methods: grease lubrication and centralized lubrication system lubrication, with significantly different requirements for the selection of food grade lubricants. When using grease lubrication, 2 # and 3 # food grade lubricating greases are usually chosen; When selecting lubricating grease for centralized lubrication equipment, 0 # and 1 # food grade lubricating grease can generally be selected based on the form, power, pipeline length, and thickness of the centralized lubrication equipment.
4. Conclusion
With the development of technology and the deepening of environmental protection concepts, the development direction of bearing lubrication is: 1) specially designed sealing structures, environmentally friendly and durable sealing materials, and lubricants without leakage; 2) Using special materials or surface treatments in a "non lubricating" manner; 3) Develop safe, environmentally friendly, and environmentally friendly lubricants. As a specialized grease, food grade lubricating grease must not only be used in food and pharmaceutical processing equipment, but also gradually develop towards using environmentally friendly, biodegradable, and food grade lubricants in bearings, gears, and chains of general mechanical equipment while ensuring lubrication performance, with the improvement of environmental protection concepts and the reduction of production costs. In recent years, Sinopec has developed a series of food grade lubricants and achieved certain results in their promotion and application, replacing imported products in large quantities. However, the evaluation and certification system for food grade lubricants in China has not yet been established, so it is urgent to establish an evaluation and certification system, strengthen supervision, and prevent food safety accidents. Domestic petrochemical, oil, and lubricant manufacturers should also follow the development trend of lubricants and invest funds in the research and development of more environmentally friendly food grade lubricants, especially the development of new processes and additives, to further promote the progress of domestic food grade lubricants.
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