Rolling Element Bearings

Some of the most critical components on a fan are the bearings.  Although there are many types of bearings, we will focus on only three – the single row ball bearing, the cylindrical roller bearing, and the spherical roller bearing.  The bearing designs, load ratings, maximum safe operating speeds, seal choices, and many other items will dictate the choice of bearing for a given application.

Rolling element bearings are basically two races separated by a set of balls or rollers.  The balls/rollers are held apart from each other by a cage or retainer, which can be metallic or synthetic.  The cage not only spaces the balls apart, but also keeps the lubrication evenly distributed on the various parts of the bearing.

The first type of bearing we will discuss is probably the most common – the ball bearing.  These bearings come in an almost infinite combination of sizes, materials and accessories.  In most cases, fans will use a pillow block mounting, by which the bearing is mounted using two bolts through the feet, one on each side of the bearing housing.  These bolts are perpendicular to the shaft.  Other common arrangements include flange mount housings, where the mounting bolts are parallel with the shaft.

Ball bearings can turn in either direction.  Some very low speed bearings may be provided with back-stop devices that will only allow the bearing to turn in one direction, but most fan applications do not require them.

Cylindrical roller bearings, as their name suggests, have small cylinders as the rolling element.  These elements can only roll along a single axis (picture a soda can on its side), and the contact surface is the side of the roller against the bearing race.  This type of bearing requires more precise alignment.  With poor alignment, one end of the roller contacts the race and not the other, leading to quick failures.

Spherical roller bearings have rolling elements shaped more like a wine barrel.  These bearings result in more contact area for the same size bearing, so they can carry more loads.

All of these bearing types can come in single row or two row configurations.  The two row configurations can carry more loads, but in many cases require more precise alignment.

All rolling element bearings rely on either grease or oil lubrication.  Oil lubrication systems have special requirements (heat rejection rates, coolant flow rates, heat exchangers and pumps, controls, etc.), so our focus will be only on grease lubrication.  The fluid film between the rolling elements and the races is typically less than one micron (0.00003937”), and the pressure in the contact area can be as high as 500,000 psi.  When the lubricant is squeezed into the contact area, a phenomenon happens known as elastohydrodynamic lubrication.  The lubricant momentarily turns into a solid and elastically deforms the rolling element and the race.

Under these conditions, any contaminate in the lubricant will cause fatigue failures in the metallic components.  This is the reason that clean lubricant is vitally important to the life of the bearing.  In addition to contaminated lubricant, the presence of water in the lubricant will cause a number of problems.  For one, it does not exhibit the same elastohydrodynamic response to pressure, so it allows metal-to-metal contact, resulting in quick failures.  It also can allow rust to form, further decreasing the bearing life.

The loads that bearings can carry, how those loads are calculated, how the loads applied affect the life of the bearing, and many other variables will be the topic of discussion in a future newsletter.

For now, we also want to remind the end user that bearings can deteriorate while not in use.  When a fan is in storage, or out of service for an extended period of time, make sure to periodically rotate the shaft several times by hand (typically once per month).  This will redistribute the grease, and renew the lubrication film between the rolling elements and the races.