We have come to the last installment of our discourse on proper lubricant selection for rolling-element ball bearings. We hope the previous posts have provided you with a clear understanding of all the variables involved in this critical process. In this final AST blog installment we will examine how environments can affect the ideal choice.
Examples of Applications
Typical environments include factory and industrial sites, dirt and contaminants, humidity, and wash down areas. Typical properties of a grease that is suitable for the majority of motor applications:
- NLGI Grade 2
- Mineral or Synthetic base oil
- Thickener formulation that provides durability against mechanical shear forces
- Low noise properties
- Corrosion resistance
- Operating Temperature Range of around -20°F to +350°F
High speed operation - The DN value, bearing bore diameter in mm X RPM, can be used to determine if the bearing is operating at high speed. DN values over 1.5 million warrant a high speed lubricant. Or, a safe rule of thumb is - if the bearing operates at over 70% of permissible speed value listed in the catalog, a lubricant for high speed should be selected. High speed greases typically have base oils with lower kinematic viscosity. At high speeds, higher viscosities lead to excess heat generation. Also, the stiffness of the grease should be considered. A grease that has channeling properties is often desirable. Channeling greases are more easily pushed out of the way by the rolling element as the bearing rotates, and stays out of the way. This results in less churning and less temperature gain. Greases that are non-channeling, or slumping, flow back into the ball path and can result in the generation of excess heat.
High Temperature - A high temperature grease should be considered for bearings that continually operate at temperatures above 300-350°F. At higher temperatures the lubricant is subject to thermal degradation. This may be the most challenging situation for lubrication engineers. There are many options that include a variety of base oil and thickener formulations. Oxidation and thermal properties of the grease components - base oil, thickener, additives - must be taken into consideration. However, always remember the base oil is the component of the grease that is primarily responsible for lubrication. The correct base oil viscosity is the factor that determines if there is an EHD film.
Extreme Environments can include marine use, salt water, aerospace with exposure to fuel and the hard vacuum of space. In vacuum applications, outgassing is often a consideration. PFPE, or perfluoropolyether, oils and greases are often the solution. They have low vapor pressure and many are formulated with a thickener and additive package that is highly resistant to chemicals. These are often selected for use in aerospace and aviation applications. This family of products can be very expensive.
Regulatory Environments such as food processing, medical, and pharmaceutical may require the use of lubricants that have been approved for use applications.
The United States Department of Agriculture (USDA) created the original food-grade designations H1, H2 and H3. The approval of a new lubricant and its registration in one of these categories depends on the list of the ingredients.
H1 lubricants are food-grade lubricants used in food-processing environments where there is the possibility of incidental food contact.
H2 lubricants are food-grade lubricants used on equipment and machine parts in locations where there is no possibility of contact.
H3 lubricants are food-grade lubricants, typically edible oils, used to prevent rust on hooks, trolleys and similar equipment.
Deciding whether there is a possibility of contact is tough, and many have erred on the side of safety with respect to selecting H1 over H2. Since September 30, 1998, the National Sanitation Foundation (NSF) took over for the USDA as the USA organization issuing registration of food-grade lubricants.
Failure Modes / Improper Selection
Engineers often fail to consider the three important factors of temperature, speed, and loads and don’t realize the impact these factors have on the lubricant. If they have not properly analyzed the operating conditions, they can realize too late that they have exceeded the operational characteristics of the grease. Equipment operated in environments the lubricant was not designed for can result in equipment failure, and in an attempt to determine why it failed, the OEM discovers that a different grease will not only have solved the problem but also expand the usefulness of the equipment.
One of the most common mistakes is not knowing that a grease engineered for certain conditions can greatly increase life. Designer simply selects a bearing with a standard factory supplied lubricant. Although these lubricants are a good choice for most applications, they may not be suited for certain environments.
In addition to temperature, speed, and loads, designers must consider other operating factors and environmental conditions that may impact lubricant performance and life. These include oscillatory movement, vibration, and shaft orientation (vertical versus horizontal). Environmental conditions include extreme temperatures, moisture and humidity, or the hard vacuum of space. Water entry and particulate contaminants can also affect the efficacy of the lubricant.
Bearing failure is generally the result of wear from the ball/raceway contacts. If the lubricant fails, the load bearing EHD film that prevents contact between the metal surfaces breaks down. When this happens, the high asperities of the raceways and balls come in contact and break off and metal particles enter the lubricant. As wear progresses, the lubricant becomes a mixture of metal wear particles and degraded lubricant. This leads to deterioration of the components and ultimately bearing failure.
Selection of the right lubrication is essential for peak performance and extending the life of rolling element bearings in electric motors and gearboxes. Engineers who consider all the lubrication selection factors discussed here will maximize the life of their bearings and machinery, therefore saving money, time, and manpower and make operations more efficient and more reliable.