You might want to read this article - it's from the March 2007 edition of "Machinery Lubrication":
Recently, synthetic greases have found greater use in rotating equipment. Long considered the lubricant of choice for applications involving extreme temperatures, loads and speeds, synthetic greases are becoming a more viable option for bearings and other components facing medium-duty operating requirements.
There are two main reasons for synthetics' increased popularity. The first involves the availability of new products and formulations. Synthetic greases are currently offered with additional viscosities and consistencies, and even come in "green" or biodegradable varieties. The broader selection allows maintenance and lubrication professionals to consider synthetic greases for a wider range of applications.
The second reason is affordability. Synthetic grease costs have not increased as rapidly as conventional petroleum-based greases, thereby reducing the cost differential between the two grease categories. As a result, synthetics are now a more affordable option in some applications where their use was previously cost-prohibitive.
Grease Characteristics
Grease has traditionally been the most common lubricant for rotating machinery components such as bearings. It is composed primarily of a lubricating agent and a thickener. The thickener acts as a carrier for the lubricating component, holding the lubricant between its lattice-like fibers. As temperatures increase during operation, the oil bleeds from the grease and lubricates the contact and sliding surfaces within the bearings.
Conventional petroleum-based greases and synthetic greases employ thickeners made of a metallic substance, such as lithium or calcium. These can be formulated with additives to meet extreme pressures, excessive vibration or other challenging requirements.
The difference between conventional and synthetic greases can be found in the lubricating agent. Conventional greases employ a petroleum-based product, such as mineral oil. Synthetic greases use a synthetic lubricant, such as silicone. Consequently, they can withstand temperature extremes and maintain lubricating effectiveness better than conventional greases.
Synthetic vs. Petroleum-based
Petroleum-based greases are the predominant lubricant in most industrial applications. However, synthetics are slowly playing an increasingly important role.
The following operational requirements or environmental conditions may indicate the need for synthetic grease:
High Temperatures
Conventional petroleum-based greases may soften at high temperatures, leading to lubricant leakage. The upper temperature limit for conventional greases is approximately 285°F (140°C). However, synthetic greases exhibit better mechanical stability in these conditions. As a result, they are traditionally used in automotive engine and other automotive applications.
Low Temperatures, Low Torque
Synthetic greases also excel at the opposite extreme, performing in low-temperature, low-torque applications. A typical example is wheel bearings, which must rotate smoothly even at low temperatures. In contrast, conventional greases can become stiff and lose effectiveness at low temperatures. This can prevent bearing rolling elements from rotating, especially at startup and under light loads.
High Speeds
Machine tool spindles, industrial fans and other high-speed applications often require synthetic greases, which run quieter than standard greases at high speeds.
Outdoor and Harsh Environments
Because of their temperature and performance properties, synthetic greases are often employed in equipment operating in outdoor conditions. They can also be specially formulated to withstand exposure to excessive moisture.
As a result, synthetics have become the standard lubricating grease at many amusement and theme parks. They are used to relubricate the motors, wheels, chains, winches and oscillating equipment in various rides. Synthetics are also finding increased use in construction and mining applications.
Facilities with Centralized Lube Systems
Traditionally, most synthetic greases for industrial applications were classified as NLGI 2, according to the stiffness and consistency scale developed by the U.S. National Lubricating Grease Institute (NLGI). NLGI ratings range from NLGI Class 000 (very soft) to NLGI 6 (very stiff). Recently, synthetic formulations have been introduced with NLGI 1 and NLGI 0 ratings. These greases have a softer, smoother-flowing consistency than NLGI 2 greases and are more conducive to centralized lubrication systems, where grease is pumped to various machines and multiple lubrication points.
On occasion, synthetic greases may be appropriate for normal-duty applications. For example, facilities using synthetic greases in high-temperature or high-load applications can potentially extend the use of these greases to include some normal-duty applications. This allows facilities to reduce the number of lubricants in inventory, lowering stocking and procurement costs.
Grease Compatibility
When two grease types are mixed, the resulting combination will likely have a softer consistency than either of its component greases, potentially leading to lubricant failure. It is advisable not to mix greases, especially those with different thickeners.
Lubricant suppliers also recommend purging the old grease and cleaning the machines and lubrication systems before introducing a new grease. This helps prevent problems associated with mixing incompatible greases.
Flow Rates
Maintenance and lube technicians should consider lubricant flow rates when converting from conventional petroleum-based grease to synthetic grease. This is important in plants utilizing lubrication delivery systems that pump grease to multiple lubrication points. Synthetic grease flow rates can differ from petroleum-based products with similar viscosity. In some cases, it may be necessary for plants to upgrade lube systems and increase pumping pressure to deliver synthetic lubricant properly to the intended points.
Today's synthetic greases offer affordable, versatile lubricating solutions in an array of applications. Contact your rotating equipment manufacturer or lubricant supplier for lubricant selection assistance.
