Can Plain Bearings Replace Rolling Element Bearings in All Applications?
Traditionally, there were very clear “battle lines” drawn between what applications rolling element bearings were suitable for and what applications plain (or journal) bearings could be used for. The discussion around plain vs rolling element bearings has evolved significantly in recent years. Conventional thinking dictated that complex parts required complex ball, roller or needle bearings. That paradigm is now beginning to shift due to the advent of new bearing materials such as ToughMet®. With advantages in load, corrosion, space, weight and an extremely low coefficient of friction, as well as cost savings to be had, more and more engineers are replacing rolling element bearings with plain bearings. Craig Clydesdale explores the various plain bearing advantages and analyses whether they really are a viable replacement for rolling element bearings.
Simplicity in Design
Until recently, rolling element bearings enjoyed an unrivalled dominance in many applications and products. The traditional view of plain bearings was that they were ineffective and unable to take the same load. High-load and high-speed applications were considered the domain of the rolling element bearing. However, we are increasingly seeing plain bearings replacing rolling element bearings in a wide range of products including turbines, pumps, and equipment for agriculture and construction, as design engineers recognise that the simplicity of plain bearing design offers significant benefits.
One advantage that attracts project managers, design engineers, and buyers alike is, of course, cost. Tooling of complex, multi-component rolling element bearings is expensive in comparison to plain bearings. The simplicity of design, ease of tooling and simple construction of plain bearings lends itself to being a much more cost-effective solution. This also makes them far lighter than rolling element bearings. This is a boon to many design engineers who are constantly looking to save weight wherever they can. We are seeing more and more engineers from the automotive and aerospace sectors switching to plain bearings for this very reason.
Because rolling element bearings are complex, multi-component parts, they have a much larger section area than plain bearings. They consist of four main parts: the outer race, the rolling element, the cage, and the inner race. Plain bearings, in contrast, are essentially comprised of a single layer of metal, plastic, a composite material, or a combination thereof. Removing the necessity to accommodate such a large part saves a significant amount of space and weight, both of which are valuable in many applications.
These are just some of the key plain bearing advantages that are influencing modern equipment design.
It’s Good to Keep in Contact
The much larger wetted area of plain bearings was traditionally seen as a disadvantage. A larger contact area meant friction, and friction was certainly not a trait you would want associated with high-speed applications. However, with the advent of self-lubricating and oilless plain bearings, this issue has largely been overcome. The larger surface-contact area can now be viewed as an advantage, allowing designers to save space and cost by using smaller plain bearings that can accommodate greater loads.
Because of these properties, plain bearings also better withstand shock loads, which often cause fatigue damage and brinelling in rolling element bearings. An apt analogy would be stiletto heels versus flat-soled shoes; worn by the same person, the stiletto heels will cause damage on some flooring, whereas flat-soled shoes distribute the load more evenly. The same principle applies to load distribution in plain bearings compared to rolling element bearings.
Shaft misalignment is another factor that can often put rolling element bearings at a disadvantage. Because the load is concentrated on a narrow contact area, shaft misalignment can increase wear and shorten bearing life unless the bearing has been specifically designed to compensate. Misalignment is far less of an issue for plain bearings, as they distribute loads more evenly across the larger contact surface.
For engineers considering rolling element bearing alternatives, these characteristics can make plain bearings an effective solution in many demanding applications.
Silence is Golden
Noise levels are always a concern that designers need to address. Rolling element bearings can be noisy. This is a simple fact. They become even noisier as they wear or as lubricant migrates. Misalignment causes vibration, which translates into noise and is then amplified by adjacent parts within the assembly.
Plain bearings have no moving parts. There is nothing to create audible noise. The layered structure of many modern plain bearings absorbs vibration, resulting in quieter operation and reduced shaft interaction.
AgriHubs
Another industry in which we are seeing growing demand for plain bearings – traditionally the domain of rolling element bearings – is agriculture. Agricultural applications are heavy-duty by nature and require components capable of withstanding extremely high loads, shock loads, limited lubrication, and highly corrosive environments.
One of the biggest issues faced by rolling element bearing manufacturers is dirt and moisture contamination, together with lubricant migration. The constant challenge is to improve seal design to guard against these problems. It has often been said that rolling element bearings are only as good as their seal; once the seal is compromised, the bearing requires replacement. This is particularly relevant in AgriHubs.
Oxfordshire-based bearing manufacturer Bowman International sought to address this challenge by replacing the rolling element bearings within agricultural hubs with its unique heavy-duty BowMet® range of spinodal bronze plain bearings.
BowMet® bearings were specifically developed in 2015 for heavy-duty applications that conventional bearings struggled to handle. Their load, heat, speed, corrosion resistance, and low coefficient of friction combine to deliver exceptional performance in harsh operating environments. By replacing rolling element bearings with BowMet® bearings, dirt contamination becomes far less of a problem, as debris can pass through the unit without causing significant damage.
A typical farm requires 20–30 days of tillage during spring and 10–20 days during autumn. It is generally accepted that farmers will replace AgriHubs at least once a year, and often once every season. Considering that a typical harrow or seeder may contain between 60 and 120 hubs, replacement costs quickly become significant.
With this in mind, Bowman International’s engineering team sought to extend hub service life by replacing conventional rolling element bearings with heavy-duty BowMet® bronze plain bearings.
Managing Director Paul Mitchell said:
“Replacing traditional rolling element bearings with BowMet® bronze bearings indicates a step change in the design concept of AgriHubs. It can significantly increase the life expectancy of the hubs.”
Conclusion
It would be reductive—and indeed incorrect—to suggest that plain bearings can replace rolling element bearings in every application. There remain many situations where rolling element bearings continue to be the best solution. However, when considering plain vs rolling element bearings, engineers are increasingly recognising the simplicity of design, cost-effectiveness, reliability, and reduced maintenance offered by plain bearings.
Understanding when to use plain bearings depends on the specific operating environment, load requirements, maintenance expectations, and available installation space. As advanced materials continue to evolve, plain bearings are becoming one of the most effective rolling element bearing alternatives across a growing number of industrial applications. This comparison of plain bearings vs ball bearings demonstrates that bearing selection should always be application-specific, making a thorough bearing selection guide an essential part of the engineering design process.
