Industrial fans and blowers are a critical part of many industrial processes. Thus, it is essential that they meet the demands of higher performance and lower total cost of operation. Bearings and related components are important factors in improving fan productivity. By combining knowledge from key areas – bearings and housings, lubrication, seals and service – SKF is now providing a standardized solution for upgrading fans.

Bearing failures in industrial fans are costly, since this equipment is often critical to the production process. A breakdown might cause a total stop in production, which can result in losses of thousands of dollars. Therefore, the extension of bearing service life in industrial fans and blowers is a high-priority issue.

Previously, every fan upgrade was customized for each individual application. Now, however, SKF engineers have come up with a complete solution for upgrading industrial fans, making it possible to standardize the procedure. The upgrade is specially designed to reduce operating temperatures, and includes optimized bearings and housings and a new circulating oil lubrication system. The effects of the lower operating temperatures are increased reliability, lower maintenance costs and extended service life.

Demanding conditions
Productivity demands in industry often require that fans and blowers perform at their limit. Higher rotational speeds mean higher operating temperatures. In addition, many fans are used to evacuate hot gases, with temperatures ranging from 100 to 600 degrees Celsius (212 to 1,112 degrees Fahrenheit). Some of this heat is transferred to the bearings via radiation and conduction in the fan shaft. Operating under high temperatures can lead to bearing failure, which in turn causes unplanned stops, unnecessary downtime and production losses.

High operating temperatures also have a negative impact on bearing lubrication, as they shorten the service life of the base oil. Degradation of the lubrication means increased friction in the bearings, inadequate lubrication and possible bearing failure, which leads to an increased risk of unplanned stops. More than one-third of all premature bearing failures are caused by lubrication problems.

Temperature-reducing components
A complete solution for reducing bearing temperature in industrial fan applications has been developed through SKF’s combined knowledge of bearings and housings, lubrication, seals and service.

A fan upgrade starts with an assessment – a functional analysis of the equipment in which current problems are evaluated – and suggests how the upgrade should be carried out. SKF can take care of the whole process, from assessment to installation of new equipment, taking full responsibility for the functionality, should this be required by the customer.

The SKF fan upgrade comprises a number of components, each with its own contribution to lowering bearing operating temperatures. A new, circulating oil lubrication system reduces operating temperatures and the need for maintenance, the SKF self-aligning bearing system generates lower friction and specially designed housings are optimized for use with circulating oil lubrication. A condition monitoring system is also part of the total offering, providing early warning of potential bearing failure. The full value of the solution is achieved when the complete fan upgrade, including all components, is installed.

Automated lubrication system
Lubrication is an essential part of preventive maintenance and can be handled in different ways, depending on the application. Some commonly used methods are manual grease lubrication, centralized grease lubrication, oil bath and circulating oil lubrication. The high bearing temperatures present in hot gas fans are often such that a circulating oil lubrication system is required, as neither grease lubrication nor oil bath lubrication are suitably efficient under these conditions. In this application, the lubrication system is an important means of removing thermal energy. Years of experience have resulted in the development of a new standard solution for circulating oil units for industrial fans (Figure 1).

Figure 1. Circulating Oil Unit

The SKF circulating oil lubrication unit provides cool, clean oil to each bearing and is the most important component in the temperature-reducing solution. Circulating oil lubrication systems are complex equipment and all components (Figure 2) are important to provide a good result.

Figure 2. Working Principle of Circulation Oil Units

A large tank provides enough time for the oil to rest and de-air. During operation, it also has the advantage of enabling the oil to cool before it is used again. The tank also contains a heater that warms up the oil at start-up, in order to maintain the appropriate viscosity at all times. A high-capacity filter cartridge removes polluting particles from the oil. The oil lubrication unit also includes pressure sensors that give a warning when the pressure is too high and the filters need to be changed. If the pressure exceeds a certain level, a bypass option will open automatically. A manual bypass valve enables for change of filters during operation.

A common problem in oil lubrication units is excessive oil circulating back to the tank. Typically, systems use an overflow valve to handle this problem, but this leads to energy losses, increased oil temperature, component wear and oil degradation. In the SKF system, the solution is to minimize the amount of excessive oil circulating back to the tank by optimizing the amount of oil in the system. Thus, energy and oil are saved, resulting in reduced environmental effects. Some variants are available for each standardized oil lubrication unit. For example, a cooler and a control unit for alarm functions and monitoring are optional, and a standby pump could be used in critical applications. As the oil is handled in a well-controlled, closed system, lubricant leakage is minimized and the effects on the environment are reduced. For even larger fans, SKF has the Flowline solution, a complete oil circulation system family designed to fulfill various requirements.

Oil viscosity
The basic function of lubrication is to create a very thin but adequate oil film between the rolling elements, raceways and cage(s) so that metal-to-metal contact is prevented at all times. It is important that this oil film has the right thickness. The thickness is determined by the bearing operating temperature and the viscosity of the selected oil. Oil viscosity varies with temperature; a higher temperature means a lower operating viscosity. Therefore, the most important parameter when choosing a lubricant is the viscosity it has at the bearing operating temperature.

The effectiveness of the lubricant is primarily determined by the degree of surface separation between the rolling contact surfaces. If an adequate lubricant film is to be formed, the lubricant must have a given minimum viscosity when the application has reached its normal operating temperature. The condition of the oil film is described by the viscosity ratio, k.

k = v/v1

where k = viscosity ratio

v = viscosity of the lubricant at operating temperature [mm2/s]

v1 = rated viscosity at operating temperature, depending on the bearing mean diameter and rotational speed [mm2/s].

(For calculation guidelines, please use the SKF general catalog.)

An oil type and viscosity is recommended having a k value between 1 and 4 at bearing operating temperature. This normally results in an oil viscosity ranging from 68 to 220 mm2/s (ISO VG 68 to 220) at a reference temperature of 40 C (104 F). If the k value is below 1, the surfaces are not fully separated, and there is a risk of metal-to-metal contact. A k value of more than 5 will increase the friction in the bearing and, consequently, the operating temperature.

Self-aligning bearing systems
The bearing system in industrial fans has to accommodate misalignment, shaft deflections and thermal expansion of the shaft. In conventional applications, a self-aligning bearing system (Figure 3) is used, consisting of two self-aligning ball bearings or two spherical roller bearings. This system has to be specially designed to accommodate thermal expansion of the shaft. One of the bearings is locating and the other is non-locating. The non-locating bearing moves axially on its seat in the housing. When it moves, it generates considerable friction, which in turn induces vibration, axial forces in the bearing system and heat – all of which can significantly reduce bearing service life.

Figure 3. This is a conventional self-aligning bearing system comprising two spherical roller bearings. The bearing to the right is “axially free” in the housing.

The SKF self-aligning bearing system (Figure 4) overcomes this problem. It incorporates a self-aligning bearing in the locating position and a CARB toroidal roller bearing in the non-locating position. The CARB bearing is a self-aligning radial bearing with an inner ring that moves independently of the outer ring – similar to a cylindrical roller bearing. The total bearing system accommodates misalignment and shaft deflection, while the CARB bearing accommodates shaft elongation within the bearing, virtually without friction and without inducing internal axial loads.

Figure 4. This is an SKF self-aligning bearing system comprising a spherical roller bearing and a CARB toroidal roller bearing. The CARB bearing accommodates shaft elongation within the bearing, virtually without friction.

Since there are no induced internal axial loads, the solution works with lower friction, keeping operating temperatures down. The SKF self-aligning bearing system also operates with reduced vibration and noise levels and enables safer, more optimized designs. This, in turn, leads to extended relubrication intervals and extended bearing service life.

In an SKF self-aligning bearing system that consists of a spherical roller bearing and a CARB bearing, both bearings belong to the SKF Explorer performance class. SKF Explorer bearings provide a substantial improvement in key operational parameters, which adds to bearing service life.

Figure 5. This is a comparison of self-aligning bearing systems in a large industrial fan. At left: Conventional design with two 22244 CC/C3W33 spherical roller bearings. Note the high vibration levels, intermittent peaks and increase in operating temperature. At right: The SKF self-aligning bearing system equipped with a C2244/C3 CARB bearing in the non-locating position. Note the low vibration levels, lack of intermittent peaks and lower operating temperature

Bearing housing
The SKF assortment of standard housings incorporates designs that specifically address temperature issues. The most suitable for industrial fans are the standard SKF SONL plummer block housings (Figure 6) and the standard SKF SAF pillow block housings. They are suitable for high-speed and high-temperature applications.

Figure 6. This is a principal sketch of a SONL bearing housing equipped for circulation oil lubrication incorporating a CARB bearing.

The SONL housing has a deep reservoir that holds a large volume of oil, providing an efficient way to remove thermal energy. The SONL housing is suitable for both oil bath and circulating oil lubrication and has a well-proven, specially designed oil seal to avoid leakage.

The SAF pillow blocks are available in standard variants for circulating oil lubrication. These are designed with large drain holes to provide increased oil flow capabilities and an optimized cooling effect. When used with an oil lubrication system, the SAF pillow blocks should be equipped with a LER contact oil seal to avoid leakage.

SNL plummer block housings, modified for oil lubrication, are another good alternative, with their high-performance oil seals and heat-transfer capabilities.
Using one of these SKF bearing housings with the SKF self-aligning bearing system dramatically reduces bearing operating temperatures.

Conclusion
Bearings in fans situated in high-temperature environments often operate at their limits. Thanks to specialist knowledge within a number of key areas, SKF can deliver a complete solution for upgrading industrial fans. The results – increased reliability, extended bearing service life, extended lubricant life and reduced maintenance – are obviously beneficial from a business perspective, but they are also important for sustainability.

Summary
Fans are crucial components in many systems and applications and are often the workhorses in many industries. As productivity requirements grow, increasing demands are put on their performance and reliability. SKF, with its thorough understanding of bearing systems, has developed a standardized solution for fan upgrades. This solution, which brings in the Group’s expertise in bearings and housings, lubrication, seals and service, means that fan upgrades can provide improved operation, extended bearing service life, longer lubricant life and reduced maintenance costs.

About the authors:
Hans Lindh is the business leader for industrial fans, Industrial Division, at SKF in Sweden, and Emma de Laval is a technical communication specialist at the SKF Group in Sweden. For more information, visit www.skf.com. Also, check out Evolution, SKF’s business and technology magazine, at http://evolution.skf.com.