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Bearing Design Considerations for Pumps and Compressors – Part 3

Bearing Design Considerations for Pumps and Compressors – Part 3

In the previous installment of this technical article, we explored the basics of bearing life calculations. Here we will examine two more bearing design elements: fits and material. The following is Part 3 of a four-part post. You can read Part 2 here and Part 1 here.

Bearing Fits 

Selecting the proper shaft and housing fits is critical in optimizing a radial ball bearing's performance and life. Improper bearing fits - too loose or too tight - can cause undesirable operating conditions and premature failure. Fits that are too loose lead to damage of the housing bearing seat, reduced rotational accuracy, and excessive wear, noise, and vibration. Conversely, fits that are too tight will give rise to a reduction in radial play, overheating, or unintended preloading. Overly tight fits also require very large forces to mount a bearing on a shaft (or remove it). The internal design of a pump or compressor should allow for the support of the bearing rings across their entire width and circumference. In doing so, the entire load carrying capacity of the bearing will be utilized. The housing and shaft fits must be selected so that there is no creep, or slippage, between the components. These fits are determined by the tolerances specified in the ISO-286 standard, along with the specified bore and outer diameter tolerances of the bearing. 

Bearing Material 

It is generally assumed the load ratings published by bearing manufacturers are based on the use of high-quality, heat-treated steel. This is the basis for using a life adjustment factor a2 = 1 discussed previously. The most common material used to produce load carrying bearing components (balls and rings) is AISI 52100 chrome steel. The chemical composition of this steel features a high carbon content and the inclusion of chromium. Using controlled processing and heat-treating methods, the finished bearing components possess high strength able to resist cracking and a hard surface to resist wear and subsurface rolling contact fatigue. Accordingly, 52100 steel exhibits good fatigue life in rolling element bearings. A disadvantage of using 52100 steel for bearing applications is its poor corrosion resistance. Consequently, bearing surfaces must always be protected with a coating of rust inhibitor oil to prevent oxidation. Bearings manufactured from 52100 chrome steel have a maximum operating temperature of 120°C (248°F). In applications requiring resistance to higher temperatures, it is possible to increase the maximum operating temperature of the bearing through heat stabilization of the components. This process involves a tempering treatment at a temperature corresponding to the desired operating temperature. This elevated tempering treatment does have a detrimental effect on the hardness of the steel, and as a result the load carrying capacity of the bearing is reduced. Table 3 lists common designations for specifying the heat stabilization treatment and the corresponding temperature ranges. These designations can vary from one bearing manufacturer to the next, so the supplier should always be consulted prior to selection. While bearings made from heat stabilized steel are an expensive solution, they offer enhanced resistance to high temperatures, reduced risk of failure, and extended operating life. Applications such as ovens, furnaces, kilns, blowers, and exhaust systems can benefit from using heat stabilized bearings.



Stainless steel is a common alternative for making bearing components, as it is more resistant to surface corrosion (due to a higher content of chromium). The chromium in the steel reacts with oxygen to form a thin, passive layer of chromium oxide on the material surface, rendering it resistant to corrosion. AISI 440C stainless steel has a high enough carbon content that it can be hardened using standard heat-treating methods. However, due to differences in the chemical composition between 440C and 52100 steels, 440C steel cannot achieve the same level of hardness as 52100 steel. As a result, the load carrying capacity of a stainless-steel bearing is 20% lower than that of a chrome steel bearing of the same dimensions.

In the next installment of Bearing Design Considerations for Pumps and Compressors, AST will discuss bearing lubrication. Look for Part 4, coming May 22, 2024.