Bearing Report: Extreme temperatures impact bearings
Bearings are a vital component in rotating industrial equipment, and often represent a significant capital investment. Bearing performance can be affected adversely by excessive heat generation. However, proper bearing selection and appropriate lubrication can minimize heat generation to help keep industrial equipment running and to reduce potential downtime.
Most bearing manufacturers aim to engineer low operating temperatures into their bearing designs. However, these efforts may be wasted if installers don’t choose a bearing suitable to the application and its surrounding environment, or if its lubrication maintenance isn’t appropriate.
With so many factors to consider, engineers need to understand why certain bearings are suitable for heavy-load applications, what type of performance can be expected, and the impact of environmental temperature on performance. When the correct bearing is chosen, the result can mean higher efficiency for the entire machine, minimizing energy losses and reducing operating costs.
Selecting the proper bearing is essential for reliable performance in challenging applications, such as wind turbines and rolling mills. These applications demand bearings that can accommodate dynamic misalignment and slide-roll, while maintaining a low operating temperature.
Spherical roller bearings (SRBs) are often the most suitable bearings for applications of this type, because they’re engineered to support combinations of radial and axial loading, even under significant dynamic misalignment conditions. However, many factors influence how well a bearing will perform in an application, which is why it’s vital that the correct bearing is chosen.
Bearing selection is the process of evaluating the suitability of bearings for specific applications. While many bearing types exist—from needle to ball, and tapered to spherical—just as many mechanical factors exist to influence the best choice of bearing.
Characteristics, such as stiffness or allowable deflection, lubrication requirements, static and dynamic misalignment, speed capability, load capability, and desired service life, all affect the choice of the best bearing. Thorough and proper analysis is most essential to ensure proper bearing performance.
• Environmental factors: Surrounding environmental conditions, such as low and/or high temperatures, dust and dirt, moisture and unusual mounting conditions, can affect a bearing’s performance adversely. Therefore, both mechanical and environmental factors may affect the choice of a bearing and its performance.
The first step in choosing a bearing is identifying the roller element type. Each type has advantages and disadvantages that are design-specific and affect factors, such as the load and speed the bearing can tolerate in the application, as well as the predicted durability and service life.
The second step in the selection process is to address the constraints of the available space. This is done by considering the minimum shaft diameter, the maximum housing bore and the available width.
The third step is to perform a bearing system-life analysis to determine which alternative best achieves the predicted service life in the application. Key contributors to performance and service life include ability to carry combination radial and axial loads, high load capacity, speed, and misalignment capability.
Bearing selection is completed once the bearing design options are defined. These include the cage type, bearing flange configurations (for cylindrical and tapered roller bearings), radial internal clearance, precision level and lubrication. These options are chosen based on the application’s speed, temperature, mounting and loading conditions, and help achieve optimum bearing performance and life.
• SRBs for gears: Gear applications often place great demands on a bearing system. SRBs are best suited to applications that carry significant radial or axial loads, such as spur-single and double-helical geared systems that generate combined loads. They also offer similar speed capabilities to other bearings, such as tapered roller bearings.
SRBs for gear-drive systems require specific features that must be considered in the installation and mounting arrangement. After selecting a suitable cage, the optimum radial internal clearance (RIC) has to be determined.
Defining the RIC is particularly important in an SRB system because the unitized design can easily lead to the outer ring being misaligned relative to the inner ring, cage and roller assembly. Only the inner or outer ring can be interference-fit with the shaft or housing to prevent relative motion, fretting wear and excess heat in operation.
It’s equally important to maintain a bearing’s performance with proper lubrication and monitoring. In any power transmission system, the friction generated translates into heat, producing a change in temperature or a thermal gradient. Gear contacts and bearings generate heat, which affects not only the mechanical elements, but also interacts with the lubrication and the environment to create a stable, thermally balanced system. The resultant stabilized operating temperatures for the gear and bearing positions determine the effectiveness of the lubrication, and its impact on bearing and gear function.
Successful bearing and gear performance depends on generating a sufficiently thick lubricant film to separate the rolling/sliding contact surfaces. As the operating temperature increases, the viscosity decreases, making the lubrication film thinner.
In general, more viscosity is good, but too much viscosity generates heat and results in higher operating temperatures, cancelling any positive benefits. If a bearing temperature issue is identified, it’s important to review the OEM’s lubrication specifications to confirm they are being followed and are appropriate for the current operating conditions.
The environmental temperature around a power transmission system is equally important because it establishes the stabilized base temperature for the system. Each type of bearing has unique geometric and manufactured features that influence the amount of friction that occurs during operation. This, combined with factors discussed previously about how an application can influence heat generation, underscores the fact that a power transmission system is a complex assembly that interacts with its surrounding environment. The more complex the system, the more important it is to assess the application characteristics, to select the proper bearing type and features, and to optimize the lubricant type.
When a bearing system is selected, installed, and maintained to run at lower temperatures, the results should be lower operating costs, improved reliability, increased uptime, and reduced energy losses. The time and resources spent assessing application needs, selecting the correct bearing types and modifications, and determining the impact of surrounding environments can lead to enhanced system performance and total cost savings.
This is an edited article provided by The Timken Company. For more information on bearings and other products, visit www.timken.com.