In the previous article, Bearing Maintenance in Milling Operations: An Overview and Important Application Considerations (Milling Journal, First Quarter 2026, pp. 4-10), a brief introduction to the complex world of mill bearing application and selection was provided. Various bearing applications in flour mill operations were presented. A review of bearing manufacturer’s in-depth explanations of bearing design and application was encouraged. This article will examine differences in bearing design for several mill applications identified in the previous article.

Application-Specific Bearing Requirements

Bearings of varying specifications and designs are used in flour mill operation. Table 1 identifies equipment found in wheat and other grain milling and processing operations, including their speed and load related to horsepower ranges. Bearing design must accommodate speed and load requirements in each specific application. As shown, bearing requirements of a rotary lock bearing compared to a centrifugal fan bearing in a pneumatic system are markedly different.

Every bearing has a limiting speed that is dependent on bearing type, lubrication method, applied load and heat displacement capability. Excessive friction, increased wear and lubrication breakdown occur beyond designed speed limits. Additionally, centrifugal forces increase at high speeds, impacting rolling element stability, cage dynamics and lubrication film thickness. Exceeding design centrifugal forces can damage rolling elements, leading to uneven load distribution and accelerated wear.

Load Considerations and Failure Risk

Bearings are designed to carry specific loads, including axial, radial and combined loads under both dynamic and static conditions with varying degrees of misalignment. Exceeding design load specifications, even for a short time, can lead to material fatigue, pitting and flaking, excessive heat generation and premature bearing failure. Achieving a long service life of selected bearings hinges on accurate and complete assessment of bearing load. Overlooking or underestimating any one load component can lead to early bearing failure, often accompanied by collateral equipment damage. Repeated bearing failures on a piece of equipment suggest improper assessments of bearing loads and/or installation flaws that should be addressed in root cause failure analysis (RCFA).

Operating Conditions and Equipment Variability

Table 2 presents milling equipment applications that can be found in a 12,500± cwt flour/day milling operation (725 ± MT wheat/day). Equipment, material and flow rates are identified for comparison purposes. The two elevators are operating under different conditions. Bucket Elevator No. 1 handles a higher load on an intermittent basis under widely varying environmental conditions outdoors (temperature, relative humidity, rain, sleet, ice and snow), while Bucket Elevator No. 2 handles a smaller load on a continuous basis in a narrower range of environmental conditions indoors.

Drag and screw conveyors may have different bearing needs, as the axial and radial load requirements are different despite having similar transfer rates and power requirements. Bearing needs for high-speed and high-horsepower equipment such as hammer mills and centrifugal fans may be quite similar. Smaller and slower-speed equipment such as rotary air locks require less robust bearings than those found in centrifugal fans.

Bearing Specifications and Design Variables

Table 3 provides basic specifications for the applications identified in Table 2, including dimensions, static and dynamic loads, maximum speed and maximum operating temperatures. The specifications presented are a small portion of bearing characteristics and options available for industrial use. Beyond the simple dimensions of bore, outside diameter and width (bore x OD x W), load characteristics and operating limits such as speed and temperature, other design requirements, including bearing tolerances, housing type, lubrication accommodation and mounting methodology, must be identified for the machine application.

These characteristics are often identified by various prefixes and suffixes, some of which are universally known (ISO standard suffixes like C3, K and W33) for bearing 22324, while others are specific to bearing manufacturer. A summary of common 22324 configurations is shown in Table 4.

Load Differences Across Equipment Types

The difference in bucket elevator capacity is reflected in bearing static load being lower for the elevator used in the mill carrying less load and supporting less equipment mass. As might be expected, a bearing on a rotary lock is required to carry less static and dynamic load than might be used in a centrifugal fan, requiring bearings like those shown for the hammer mill, also operating at high speed and horsepower.

The number of bearings used in roller mills is quite significant, and they have a high dynamic load requirement due to grinding forces. The static and dynamic load characteristic needs of the six-section sifter bearing are very high due to its motion, as well as the axial force, or thrust, required to support the sifter’s mass containing in-process stock.

Bearings in Drag Conveyor No. 2, carrying mill feed (30,000 lbs./hr.), have greater load capacity than those in Drag Conveyor No. 1 conveying more wheat (66,000 lbs./hr.). The difference may be the result of conveyor length and conveyor size due to density differences between wheat (45-48 lbs./ft³) and mill feed (18-24 lbs./ft³). Bearings may appear to be the same from the grease zerk (fitting) perspective; however, their designs and specifications can be markedly different based on the application.

Housing, Mounting and Clearance Requirements

These additional design characteristics accommodate bearing housing and mounting requirements, as well as clearance specification needs that vary from machine to machine. Common bearing housings include flange bearing housings used on the end plate of a screw conveyor or pillow blocks used on centrifugal fans, hammer mills or bucket elevator drive shafts. Additional housing alternatives such as split pillow block or outboard housings may also be available.

Cylindrical-bore bearings can be press-fit onto a shaft and utilize mechanical retention methods like circlips (snap rings), locknuts, washers, end caps and clamping collars. Tapered-bore bearings can be secured using adapter or withdrawal sleeves.

Internal bearing clearances may need to be specified as greater than normal to allow for thermal expansion and/or misalignment compensation. Alternatively, bearings may be self-aligning. Bearing cage types, a specific position of lubrication grooves and holes for relubrication, or sealed designs and seal types, including material designation, may be required.

Market Variability and Specification Challenges

To understand the challenges of bearing selection, a search was conducted on a vendor website (QBL) to identify a replacement sifter bearing, SKF 22324, using only the bearing size (120 mm x 260 mm x 86 mm). The results shown in Table 4 identify bearings from a distributor as well as four other bearing manufactures offered by the distributor. The average, minimum and maximum prices are shown by distributor/manufacture and overall for the 101 bearings identified in the search.

The differences in average, minimum and maximum price are significant, which may or may not be reflected in the product value. What is not shown is the variations in prefixes and subfixes associated with the common bearing number. The 22324 bearing is a spherical roller bearing designed for high radial and axial loads, such as in a free-swinging sifter application, and typically features a self-aligning design.

Selection Considerations and Outlook

Differences in bearing design for several mill applications were identified, showing a wide range of applications and demands. The original equipment manufacturer (OEM) often identifies bearings as part of its spare parts supply inventory by internal spare identification numbers with limited description of the bearing manufacturer design identification. Manufactures of the OEM recommended bearings are often available through the manufacturer and/or bearing distributions.

It is essential that compatible bearings meeting or exceeding OEM recommendations be considered as an alternative source. Failure to properly identify the bearing order from an alternative source may result in premature bearing wear and/or failure, causing additional equipment damage, production downtime, personal injury or catastrophic failure.

Alternative bearing selection requires careful study of the existing application and utilization of vendor resources, including engineering catalogues and industry professions. Careful planning and selection, followed by proper installation, pays dividends in savings. The next article will investigate additional design issues associated with proper installation of bearings.

Dr. Jeff Gwirtz is CEO of JAG Services, Inc., an international consulting company in Lawrence, KS; 785-341-2371; jeff@jagsi.com. He also is adjunct professor in the Department of Grain Science and Industry at Kansas State University, Manhattan.