Bearing designs and applications employed in the grain milling industry were presented in “Bearing Maintenance in Milling Operations: Bearing Designs in Various Mill Applications” Milling Journal Volume 34 Issue 2 pages 18-22. As described, there is a wide range of applications and demands associated with various milling application depending on shaft speed, dynamic and static load. In this article an important bearing design and placement requirement will be presented.
An important bearing design consideration of “Fixed” and “Float” was mentioned in Table 3 “Example characteristics of bearings in selected milling applications”. The table identifies that a screw conveyor utilized a “Fixed” bearing design at the drive end while utilizing a “Float” bearing design at the discharge end. An alternative term for a “Fixed” bearing is a “Locating or Anchor” bearing while a “Float” bearing is called a “Non-locating or Expansion” bearing. Instead of making entirely different models of the same bearing, manufacturers identify how bearing rings (installation components) are fitted or alternatively provide specific internal flange or housing variations. The original equipment manufacturer (OEM) generally specifies the way the bearing is to be installed in the machine.
Shafts through the bearing and the bearing mounting often experience temperature variation. For example, the bearing supporting a very warm roll journal may be mounted on or in a cooler metallic surface of a different composition. Differences in linear expansion or contraction of the shaft and bearing housing support are expected. The “Fixed” bearing holds the shaft lengthwise in a fixed anchored position. Alternatively, the “Float” bearing allows for axial (linear) shaft expansion or contraction due to temperature changes. At least one bearing in the drive system must be fixed while others can be floating.
Generally, fixed bearing set up is achieved by two different approaches. In one approach the outer ring is fit tightly to the bearing housing while the inner ring is fit tight against the shaft for a tight interference fit. In the second approach axial locking, utilizes retaining rings, lock nuts or end plates to physically lock both inner and out rings in place. In either case, the shaft is not free to move due to thermal expansion/contraction or under radial forces or thrusts loads.
Float bearing set up is also achieved by two different approaches. In the first approach, external sliding fit is created by using a loose transition or clearance fit also known a sliding seat. The second approach allows internal displacement where rollers move across an un-flanged ring in NU or N-type cylindrical roller bearings. NJ-type bearings simply permit movement inside the bearing itself. In either case, the shaft is free to move due to thermal expansion/contraction or under radial forces or thrusts loads.
Fixed and float bearing arrangements can be found in any type of bearing housing used in grain milling equipment. Bearing block/housing assemblies are often used to hold roller mill bearings and are sometimes configures as split blocks. Pillow blocks often house bearings on drive shafts for fans and hammer mills in addition to fixed roller position of some roller mills. Flanged bearing housing are often used on the vertical surfaces of drag and screw conveyors. Take up housings may be employed on belt and drag conveyors to allow for tension adjustment.
Figure 1 shows a roller mill lay out for the purpose of explanation of fixed and float bearing assignment. The bearing on the drive side of the fast roll is allowed to float or move left to right or right to left as dictated by journal axial movement due to either thermal issues or physical force experienced. The shaft movement impacting the drive pully is deemed to have limited impact on pully or belt drive wear. The bearings on the transmission side of the roll pair however are fixed not allowing axial shaft movement. While not identified in this figure the transmission side may use a timing belt and pully assembly or and assembly of intermeshing gears to create the desired differential in the roll pair. Neither assembly alternative can tolerate misalignment increasing energy loss and accelerating wear.
Fixed bearing location employed by other pieces of equipment used in the grain processing industry often place the fixed bearing closest to the drive location. Table 1 shows the typical placement location of fixed bearings for common grain processing equipment is closest to the power source. This placement locks axial position of the shaft maintaining proper alignment while countering chain or drive belt pull and reaction forces. Properly aligned drives are energy efficient and minimize gearbox, coupling, sprocket/chain or pulley /belt drive wear.
Conclusion
Due to the many different bearing types and bearing type combinations there are many more alternative designs of bearing arrangements than can be reported in this brief article. Differences in drive orientation (horizontal, vertical), gear types (helical, spur, worm or bevel) etc. may require specialized bearing selection and placement. Further exploration of the alternatives is encouraged, suggesting the reader to consider viewing the web-site references identified at the end of this article. More importantly, the exact mounting sequence for your grain processing equipment’s drive assembly are identified by the original equipment manufacture and should be followed to achieve optimal performance and reduced maintenance cost. Exact torque, shimming and bearing seating instructions must be followed for the intended design and application.
Following OEM guidance for selection and bearing placement in a particular machine location is critical as is following bearing manufacture instructions for bearing installation. The next article will address general removal and installation guidelines.
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.
References
https://www.designworldonline.com/when-are-floating-bearing-arrangements-necessary/
https://medias.schaeffler.us/en/knowledge-center/rolling-bearings/design-of-bearing-arrangements
https://www.regalrexnord.com/regal-rexnord-insights/proper-positioning-is-key-to-bearing-performance
https://www.timken.com/wp-content/uploads/2016/10/Timken-Engineering-Manual_10424.pdf
https://ibtinc.com/learn/how-remove-replace-industrial-bearing/