Lubrication of industrial fans

Photo: Kuznietsov Dmitriy / Adobe Stock

Industrial fans are used to control air quality, temperature, and humidity, and to provide ventilation and maintain safe working conditions. They are essential in a wide range of industries including manufacturing, mining, agriculture, and construction. In terms of criticality, they can range from low to high priority, and if they go down, the process is shut down.
Let’s discuss the lubrication of the fan shaft rolling element bearings and when and why grease or oil is appropriate.

Bearing reference speed
The bearing reference speed is an important parameter for determining whether the bearing can be run in grease or oil. The reference speed is published by bearing manufacturers and in a simple sense it is approximately the speed when the bearing generates more heat than it can dissipate.

However, this is not an exact number, rather it is a basis for a calculation that considers the loads applied to the bearing and often the “adjusted” reference speed is lower than the published reference speed. To determine the adjusted reference speed, you can enter the bearing load and speed data into online programs offered by bearing manufacturers.

If the fan requires the bearing to run faster than the adjusted reference speed, then some form of cooling needs to be used to help remove the heat. Typically, that cooling comes in the form of an oil circulation system.

Grease lubrication
Grease lubrication is the most common method used to lubricate the fan shaft bearings. It offers several advantages in fan applications
• Grease forms reservoirs inside the bearing and the bearing housing and provides sustained lubrication over an extended period. This is particularly useful where fans may be difficult to access for frequent lubrication.
• Grease helps seal out contaminants such as dust, dirt, and moisture which can contribute to premature wear and failure of fan bearings.
• Grease simplifies the design of the overall system as there is not a need for extra equipment related to the delivery system.
• Grease can be delivered in several ways such as a lube tech with a grease gun, by single point lubricators or by multi-point lubrication systems.

Managing grease lubrication
Select a grease type that is suitable for the fan’s operating conditions, temperature range, load, and speed. Typically, an NLGI 2 grease is used, with a base oil viscosity ranging from ISO VG 100 to 150 mm2/s at 40C, with a mineral oil and a lithium or lithium complex soap. When reading the product data sheet of a grease, be sure that it states that the grease is intended for a fan application (or similar).

Determine the appropriate lubrication frequency based on the fan’s operating conditions including the fan’s environment. Calculation programs are available online at bearing manufacturers sites.

Apply the recommended amount (dose) of grease to each bearing. Bearing manufacturers publish formulas based on bearing type, size and where the grease will be applied (to the centre of the bearing or to the side of the bearing).
Smaller fans use unit ball or roller bearings that have integral seals. These seals can be “blown out” by excessive grease added at a single time. Bearings in split pillow blocks typically have labyrinth seal which generally allows the excess grease to escape so they are not as sensitive to over greasing. In either case, if you feel the bearing needs more grease, add the proper dose more often (don’t increase the dose itself).

Regularly monitor the fan’s performance and conduct inspections to identify any signs of inadequate lubrication, excessive heat, abnormal noise, or vibration.

Limitations of grease
When using grease there are limitations placed on the allowable bearing speed, which is based on the applied load and the type of bearing. These limits are published in bearing manufacturers catalogues. For example, a fan with a 3 15/16” shaft has the following speed limits for grease lubrication (when the applied load is considered normal):
• Self-Aligning Ball Bearing 3,600 RPM
• Spherical Roller Bearing and CARB 1,800 RPM
As discussed, as the loads change the allowable speeds change.

Oil lubrication
Oil lubrication is another, but less common method, used to lubricate shaft support bearings in industrial fans, particularly in high-speed and heavy-duty applications. Oil lubrication offers several benefits.
• Oil can remove heat (which grease cannot), making it suitable for applications with higher speeds, higher loads or when the process temperatures affect the operating temperature of the bearing.
• Oil lubrication is always a better “lubricant” as is more readily forms a full lubrication film than as only small amounts of oil are bled into the rolling contact area.
• Since different ISO grades can be chosen, the oil provided to the bearings can be more easily adjusted to accommodate the application condition than greases.

Considerations and steps for oil lubrication
Choose an oil that is suitable for the fan’s operating conditions, temperature range, speed, and load. Consult your lubricant supplier for the appropriate grade for the specific application conditions. Alternately, bearing manufacturers offer calculations for selecting the needed oil viscosity for a given bearing at a given speed operating at various temperatures.

Depending on the fan’s design and complexity, it may have an oil bath or require an external oil lubrication system. Ensure that the oil reservoir is adequately sized and designed to provide proper cooling.

Implement a condition monitoring practice for the oil system that includes checking the level, sampling, testing the oil condition, checking/changing the filtration, monitoring temperature of the oil at different points, monitoring the bearing temperatures, vibration levels of the bearings and fan system.

To this point, the reliability of the oil system is critical to the proper functioning of the fan system. Although most fan systems function well with grease as the lubricant, some fans operate at speeds and temperatures that require an oil circulating system. It is important to be aware of the limitations of grease and to know when it may be time to convert to an oil system to improve the reliability of the fan.
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Douglas Martin is a heavy-duty machinery engineer based in Vancouver. He specializes in the design of rotating equipment, failure analysis, and lubrication. Reach him at mro.whats.up.doug@gmail.com.