The Plain Truth About Plain Bearings

Chris Corneliussen, GGB; Kimberly Evans, GGB

Self-lubricating bearings can offer good wear and low-friction performance in harsh conditions.

What do Boeing, John Deere, General Motors and the National Aeronautics and Space Administration (NASA) have in common? Aside from being industry leaders, they all use self-lubricating plain bearings. You may wonder how a self-​lubricating plain bearing can lubricate without the addition of external lubricants. The simple answer is that they work by having solid lubricant impregnated within the sliding layer of the bearing.

Self-lubricating plain bearings can provide comparable thermal and mechanical performance to externally lubricated needle and roller bearings across many points of measurement. By eliminating the need for external lubricants, they also offer versatility in a wide range of industrial applications.

A notable example of their versatility in the aerospace industry is the use of three anti-friction bearings in the drill spindle of NASA’s Curiosity rover. The spindle is one of the four components that allows the drill to operate in the harsh environment on Mars and has kept Curiosity exploring the red planet since 2012.

Self-lubricating bearings can offer good wear and low-friction performance in harsh conditions.

Three bearing segments serve as the primary suspension components for the drill spindle on the Mars Curiosity rover.

Your maintenance department might not have to travel 141 million miles to service your equipment, but that doesn’t mean plain bearings can’t also reduce your maintenance and labor costs while increasing equipment service life. These components are used virtually everywhere in industrial applications. However, the most common industrial applications for self-lubricating plain bearings include pivot points and slideways.

Three bearing segments serve as the primary suspension components
for the drill spindle on the Mars Curiosity rover.

Why Bearings Fail

When traditional bearings fail or are replaced would be the perfect time to evaluate your bearing needs as well as to determine what happened in the application that led to a failure. If bearings are properly designed and applied, the most frequent cause of failure is inadequate lubrication, which often results from seal damage or inadequate seal performance. Poor maintenance can also be a cause of inadequate lubrication.

The next most common cause of bearing failure is spalling on the raceway or rolling elements. This can occur as a result of fatigue stress or system deflections that lead to localized high-stress concentrations on the edges of the rolling elements and/or raceways.

Reducing Downtime

Plain bearings may even be able to assist in your efforts to reduce maintenance downtime. The slim, one-piece design allows substantial space and weight savings by decreasing the housing size. The straightforward installation into simple machined housings also helps to prevent fitting damage compared with rolling-element bearings. In contrast, industrial bearings are complex and require exact installation, making them better suited to applications requiring a precise shaft location and/or extremely low friction. In addition, the non-metallic plain bearings can offer improved corrosion resistance compared to standard rolling-element bearings.

Eliminating external lubricants can improve
machine performance and make your
operation more environmentally friendly.

Plain Bearing Benefits

One of the greatest advantages of plain bearings is the ability to reduce or eliminate the use of external lubricants. It’s important to note that some plain bearings do require external lubricants, although often in much lower quantities than traditional bearings. Employing bearings that are self-lubricating can also make your operation more environmentally friendly and save you money.

Other advantages include a higher load capacity with shock load resistance and a greater contact zone, improved bearing life under oscillatory movements, compensation for misalignment and vibration damping, quieter performance with the absence of moving parts, and reduced costs of shaft and housing surface finishes and hardness treatments.

Regarding the environment, new bearing materials are constantly being developed to comply with environmental regulations. These new innovations can also provide longer life under harsh conditions and enable more precision installations.

When Traditional Can Be Better

Despite the advantages of plain bearings, there are some instances when traditional bearings may perform better. The most common reason not to use plain bearings is the slightly higher shaft and/or housing clearance compared to rolling-element bearings. Roller bearings have a larger footprint than plain bearings, so it’s difficult to remove the roller bearing and drop in a plain bearing, as there will be a lot of empty space to fill. At this point, the bearing must be sleeved or the housing bore redesigned.

Another reason for not using plain bearings is that a bearing with two races will have one race installed on the shaft, meaning the shaft will not need to be designed and manufactured for a bearing surface. Some single-race needle bearings (and cylindrical bearings) will run on shafts.

So, while self-lubricating plain bearings are not a panacea for every application, they can serve as an alternative to lubricated bearings in a variety of applications, offering longer life, low maintenance, easy customization, high wear resistance and the ability to perform in harsh weather conditions, saving you time and money.

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