Timken has been a leader in the advancement of bearing technology for over a century with expert craftsmanship, world-class production facilities and a continuing investment in programs that ensure our products are synonymous with quality and reliability.
This guide to bearing fundamentals is not intended to be comprehensive but can serve as a useful resource for those who want to improve the performance and reliability of their machines.
There are many types of bearings, each having a unique internal design with inherent advantages and disadvantages. All bearings consist of a cone (inner ring), cup (outer ring), rollers (rolling elements) and cage (roller retainer), with all components having different sizes and geometries based on the needs of the application. Some common bearing types include:
- Tapered Roller Bearings: Rollers having tapered angles allow bearings to efficiently control a combination of radial and thrust loads. In this design, the extensions of the raceways and the tapered surfaces of the rollers come together at a common point on the axis of rotation, providing true rolling motion.
- Spherical Roller Bearings: These bearings utilise two rows of barrel-shaped rollers and a cup raceway that is spherical in form to tolerate misalignment and high loads in heavy-duty equipment. Like tapered roller bearings, they can handle a combination of radial and thrust bearings. Critical stresses are often present when managing high radial loads, and spherical roller bearings can outperform other designs where a stronger solution is needed.
- Cylindrical Roller Bearings: Contoured cylindrical rollers give these bearings the ability to handle higher radial load capacity for a given size compared to other bearing types (their ability to handle thrust loads is limited, however). There are full-complement and one-, two- and four-row designs to meet various application requirements.
- Thrust Bearings: Thrust bearings include ball, crossed roller, cylindrical, tapered and spherical designs for managing thrust (heavy axial loads) in industrial and automotive applications. All types have large rolling elements for maximum capacity, and for some thrust bearings, contoured rollers are used to ensure uniform, full-length contact between the rollers and raceways under heavy loads.
- Ball Bearings: Ball bearings are used extensively in auxiliary applications that have light loads and/or high-speed conditions. There are radial, thrust and angular contact ball bearings to suit different demands as well as precision ball bearings for applications where very tight tolerances and running accuracies are required. In general, ball bearings cannot handle the higher loads rolling element bearings are designed for.
Table 1 ranks different bearing types on various performance characteristics and can help you narrow down your options. A qualified friction management expert can also assist you in building an application model that identifies the most appropriate bearings for your specific challenge.
The next step is to assess bearing size constraints including the bore, outside diameter (O.D.) and width. This is done by defining the minimum shaft diameter, maximum housing diameter and available width for the bearing in the application. At this point, bearings may be selected that fit within the defined size envelope.
To ensure smooth operation, it is also important to evaluate known environmental conditions (e.g., ambient temperature and the cleanliness of the environment) as well as application requirements (e.g., bearing setting, lubricant type, flange arrangements).
Again, seek out an expert who can perform bearing life calculations that account for all these factors and more, and who can provide you a comprehensive analysis of your bearing requirements. As many equipment owners have learned, an optimal bearing solution can save thousands of dollars a year by avoiding frequent repairs and replacements.
Mounting, fitting, setting and installation procedures must be carefully followed to achieve proper bearing performance. Practices differ for different bearings, but there are many similarities that apply to all bearings. These similarities are summarised below.
Most bearings are mounted on a shaft and into a housing, wherein the shaft and housing have shoulders to back the rings. The purpose of the shoulders is to positively establish the axial location and alignment of the bearing.
Figure 1 shows various types of backing shoulder designs with the conventional method using a shoulder that is machined on a shaft or in the housing. Other times, snap rings are used as the shoulder. It is essential that a shoulder be square with the bearing ring and of the right diameter to provide adequate backing of the bearing raceway.
Note: When you receive a bearing shipment, do not remove products from their packaging until they are ready for mounting, so they do not become corroded or contaminated.
Generally, bearing rings mounted on a rotating shaft or in a rotating housing should have an interference fit (also known as a press fit, where the friction between two parts serves as the only means of fastening).
A loose fit may allow the ring to creep or turn, leading to premature wear of the mating surface and backing shoulder that can result in damage to the bearing, shaft or housing. In most cases the stationary ring can be a loose fit to facilitate installation.
Fitting practices vary according to several other factors including the precision class of the bearing, bearing configuration (e.g., single- or double-row bearings), type and direction of load (e.g., continuous or alternate rotating), operating conditions (e.g., shocks, vibrations, overloading and high speeds), shaft and housing section and material, as well as ease of mounting/dismounting and setting conditions among other considerations.
Figure 2 shows roller bearing shaft and housing fit selection that conforms to accepted industry standards. The bars designated g6, h6, etc., represent shaft/housing diameter and tolerance ranges to achieve various fits required for different load and ring rotation conditions.
Often in the manufacture of bearings, it is standard practice to assemble rings and rolling elements with a specified internal clearance that compensates for the effects of interference fits and thermal expansion of bearings, shafts and housings. For some bearings, this can also provide the desired internal contact angle after mounting.
Internal clearance is measured by either gauging radially or axially. Radial internal clearance (RIC) is accepted as the typical setting characteristic for most bearings because it is more directly related to bearing fits. Meanwhile, tapered roller bearings and angular contact ball bearings are the exception as setting is usually measured in the axial direction.
Achieving the correct setting is important to meeting anticipated bearing operating conditions, thereby providing optimum system performance. A qualified professional can help you specify setting requirements for your bearings.
Cleanliness is essential for bearing installation. Always ensure that shafts are clean, free from nicks and burrs, straight and of proper diameter to avoid bearing performance issues. Adequate tools must also be used to properly fit the bearing inner rings onto the shaft and outer rings into the housing to avoid damage—consult your manufacturer or supplier about tools and kits that can facilitate fast, precise installation of bearings.
Note: When installing a new bearing, do not remove the lubricant or preservative applied by the manufacturer. The preservatives used on almost all bearings are fully compatible with commonly used oils and other lubricants. Leaving it in place will protect the bearing from corrosion.
With so many options, it can be difficult to choose the best lubricant for your bearings. The successful use of bearing oil or grease depends on the physical and chemical properties of the lubricant as well as application and environmental conditions.
At the design level, the first consideration is whether oil or grease is optimal for the application. Most bearings (roughly 80%) use grease, which in general is easier to use than oil. Bearings that are initially packed with grease require only periodic relubrication to operate efficiently. Meanwhile, in applications where heat must be carried away from the bearing or in very high-speed applications, oil must be used.
There is no simple, all-inclusive guideline to follow for proper lubricant selection given the countless combinations of bearing types and operating conditions that can be experienced. It is advisable to consult your lubricant supplier or equipment/bearing maker for specific questions about lubrication needs for a certain application.
Table 2 can help you understand which grease types may be optimal for your bearings (this information is not intended to replace the specifications by the equipment builder, however). Talking to an expert about grease selection is also a good time to review proper lubrication practices and ensure your employees have received appropriate training and education.
Where to Turn
While there is much to know about bearings, keeping the simple things in mind can often avoid problems that lead to downtime, delays and equipment damage in the worst-case scenarios. By understanding the basic bearing requirements of your machine, you can have a more productive conversation with an expert who is more likely to lead you to a successful outcome.
Many companies offer training programs and helpful resources that can boost your knowledge of bearings and hands-on maintenance. For those wanting to continue their education, a call to your local bearing supplier is a great place to start.