Plain bearings are a type of bearing used in various industries such as automotives, vessels, and industrial machineries. They possess different characteristics from rolling bearings, such as being lightweight, compact, and excelling in impact resistance.
This article will provide explanations on the types of plain bearings, their respective characteristics, mechanisms, advantages, disadvantages, and differences from rolling bearings.
If you're interested in understanding the specific properties and functions of plain bearings or whether they are suitable for your company's products, please refer to this article.
Additionally, for consultation on how to utilize plain bearings or considerations for implementation, please feel free to contact us as Oiles Corporation, the top manufacturer of oil-less bearings boasting the world's No. 1 share in various components such as steering system bearings. We will provide proposals tailored to your needs with our high technology and abundant expertise, so please don't hesitate to reach out to us.
What are plain bearings?
Plain bearings support shafts with surfaces and reduce friction by interposing a lubricating film (such as oil or air) between them. Plain bearings include self-lubricating materials to reduce friction and wear, making them lubrication-free (oil-less) bearings. Let's now look at an overview of plain bearings below.
Plain bearings are a type of bearing
A plain bearing is a type of bearing that fits into a housing and that stabilizes the shaft by supporting it to facilitate smooth shaft motion.
A bearing allows the shaft to move in the correct position and it supports the load of adjoining parts that rotate, oscillate, or reciprocate. The lubricating film also serves to reduce friction on rotating components, thus preventing loss of power.
Materials for plain bearings include ferrous metals, copper-based metals, and plastics. Plain bearings are often used in automobiles, industrial vehicles, aircraft, generators, and molding machines.
Characteristics and the structure of plain bearings
A plain bearing is structured to support a shaft on a flat or curved surface. Although the surface and shaft of a plain bearing are in direct contact, friction is typically reduced by a lubricating film (oil, air, etc.) between them.
Thus, a characteristic of plain bearings is that they allow a shaft to rotate while reducing frictional resistance through the action of a lubricating film.
Furthermore, plain bearings can be classified into radial bearings (journal bearings) that support loads perpendicular to the centerline of the bearing and thrust bearings that support loads parallel to the centerline of the bearing.
Advantages and disadvantages of plain bearings
The advantages of plain bearings are as follows:
- A simple and compact structure
- A highly customizable shape
- Excellent impact resistance, load-bearing capacity, resistance to foreign matter, and corrosion resistance
- Less expensive than rolling bearings
In contrast, the disadvantages of plain bearings are as follows:
- Inferior to rolling bearings in speed resistance
- Higher coefficient of friction than rolling bearings
Applications of plain bearings
Specific examples of plain bearing applications are as follows:
- Automobiles and motorcycles
- Infrastructure (dams, water gates, rolling stock)
- Energy (hydropower and wind power)
- Plants
- Construction machinery (forklifts and hydraulic excavators)
- Vessels
- Molding machines and die components
- Machine tools
- Robotics
- Food/packaging
- Semiconductors
- Pumps and compressors
What are rolling bearings? Their differences from plain bearings
In addition to plain bearings, there are rolling bearings where balls or rollers are mounted inside the bearing. These rolling bearings have different characteristics and mechanisms compared to plain bearings. It's important to compare and consider both types of bearings to determine which is appropriate for your needs.
Let us explain the features and mechanisms of rolling bearings, as well as the differences from plain bearings.
What are rolling bearings?
A rolling bearing is a type of bearing where rolling elements (such as balls or rollers) are positioned between the outer rings and the inner rings. These rolling elements are spaced apart by a retainer, preventing them from contacting each other.
As the shaft rotates, the rolling elements also roll along with it, reducing friction and supporting the load. If the rolling elements are ball-shaped, it's called a ball bearing, and if they are cylindrical, it's referred to as a roller bearing.
There are various types of rolling bearings with rolling elements suited to different conditions such as load capacity and rotational speed.
Advantages and disadvantages of rolling bearings
Rolling bearings offer distinct advantages over plain bearings. Here are the merits of rolling bearings:
- Easier adaptation to high-speed rotation compared to plain bearings.
- Lower friction coefficient compared to plain bearings.
- Less shaft deflection.
Now, let's discuss the disadvantages of plain bearings:
- Susceptibility to contamination by foreign particles.
- Requires lubrication.
- Higher cost compared to plain bearings.
- Tendency to have inferior impact resistance and load capacity compared to plain bearings.
Differences between plain bearings and rolling bearings
Plain bearings and rolling bearings are both types of bearings, but they have different characteristics and mechanisms. Below is a comparison table outlining the features of plain bearings and rolling bearings.
| Plain bearing (general type) | Plain bearing (self-lubricating type) | Rolling bearing | |
|---|---|---|---|
| Lubrication | Requires lubrication | Can be used without lubrication | Requires lubrication |
| Size | Thin-walled Compact | Thin-walled Compact | Thick-walled and large |
| Load-bearing capacity Impact resistance | Superior | Superior | Inferior |
| Speed tolerance | Inferior | Somewhat inferior | Superior |
| Rattling | Occurs due to clearance | Occurs due to clearance | Almost none |
| Coefficient of friction | Slightly low | Low | Extremely low |
As a generally, both plain bearings and rolling bearings require periodic lubrication. However, oil-less bearings (such as oil-less bushings) within plain bearings can operate without the need for lubrication.
In terms of size, rolling bearings tend to be larger due to the presence of rolling elements.
Regarding load-bearing capacity and impact resistance, plain bearings tend to excel because they provide broad support through surface contact with the shaft. Rolling bearings, on the other hand, have point or line contact rather than surface contact, which may result in inferior load-bearing capacity and impact resistance compared to plain bearings. Additionally, rolling bearings generally have a lower friction coefficient due to their smaller contact area.
Rolling bearings are more suitable for high-speed rotation applications.
Plain bearings may be prone to rattling due to the clearance required, which is a disadvantage. In contrast, rolling bearings, while having vibrations due to the rolling of the rolling elements, typically have minimal rattling due to clearance, which is an advantage.
What are Oiles bearings?
Image from:Oiles Corporation
At Oiles Corporation, we offer "Oil-less Bearings," plain bearings that can be used with minimal lubrication or without lubrication altogether. Oiles Bearings utilize various methods such as oil-impregnation, solid lubricant incorporation, or alternative lubrication methods (excluding sliding oil) to operate without the need for frequent oiling or even without any oil supply (relying on air or moisture).
These bearings boast excellent wear resistance and anti-seizure properties, allowing them to be used under high load conditions compared to conventional plain bearings.
Below is a comparison of the characteristics of Oiles Bearings and rolling bearings offered by Oiles Corporation.
There are various types of Oiles Bearings depending on differences in structure and materials. The main types are as follows:
- Metallic bearings
- Multi-layered bearings
- Sintered bearings
- Plastic bearings.
- Plain bearing units with a self-aligning mechanism
Each type has its own characteristics based on these differences. Let's take a look at the types and features of Oiles Bearings.
Metallic bearings
Metallic bearings are type of bearings made of metal materials such as copper alloy, with inherent self-lubricating properties. For example, solid lubricant embedded bearings, which embed plug-shaped solid lubricants into copper alloys, are a notable example.
One advantage of solid lubricant embedded bearings is the ability to select bearings tailored to usage conditions (load, temperature, environment, etc.) by combining base metals with solid lubricants. Additionally, the use of metal materials provides superior mechanical strength and abrasion resistance.
However, a disadvantage of metallic bearings compared to plastic or multi-layered bearings is their higher weight for the same bearing size.
Multi-layered bearings
Multi-layered bearings are type of plain bearings constructed with two or more different materials layered together. Examples include bearings where white metal or aluminum alloy is welded onto a steel backing, or bearings where a layer of resin covers a metal substrate.
Multi-layered bearings, particularly those with white metal or aluminum alloy layers, are commonly used in environments with oil, such as automotive engines. Multi-layered bearings with resin layers have excellent lubricating properties in the sliding layer and sintered layers, allowing for oil-free operation.
By supporting resin and sintered layers with a steel backing, heat conductivity and mechanical strength are improved. The advantages of multi-layered bearings include superior heat resistance and mechanical strength compared to resin alone, as well as contributing to compact design by reducing thickness.
Sintered bearings
Sintered bearings, characterized by their porosity (small pores on the surface), are a type of plain bearing that can be used in a self-lubricating state by impregnating lubricating oil into the pores. There are also solid lubricant dispersion type sintered bearings, which are made by mixing solid lubricants with metal powder and sintering them.
The advantages of sintered bearings include their inherent self-lubricating properties, with the following specific features:
- Resistant to seizing
- Can be used with minimal or no oil lubrication
- Capable of oil penetration through the bearing wall
- Suitable for high-speed rotation
However, there are also disadvantages such as weaker mechanical strength, higher friction coefficients, and unsuitability for small-batch production. Our company, Oiles Corporation, offers a wide range of products that include sintered layers in the composite bearings.
Sources:
[1] Japan Powder Metallurgy Association, Sintered Machine Parts: Their Design and Manufacture, Gijutsu Shoin (1987), p. 327.
[2] Kawasaki, Keimin, Oil-less Bearings, Agne (1973), p. 99.
Plastic bearings
Plastic bearings are a type of plain bearing made from thermoplastic (which liquefy when heated and solidify when cooled) or thermosetting plastic (which solidify permanently after being heated once and do not liquefy upon reheating).
The advantages of plastic bearings include being lighter and more cost-effective than metal materials, rust resistance, and the freedom to shape them as needed. By containing fillers such as lubricating oil, solid lubricants, or metal soaps, they can improve lubrication and even be used without additional lubrication.
Additionally, their mechanical strength is enhanced by fillers such as woven cloth, glass fibers, or carbon fibers. They also exhibit excellent sliding properties even underwater.
Thermoplastic bearings and thermosetting plastic bearings each have their own characteristics:
Thermoplastic Bearings:
- High flexibility in shape due to injection molding
- Can be machined from molded parts or round rod
- Available in oil bearings, solid lubricant dispersion bearings, and fiber-reinforced bearings
Thermosetting plastic Bearings:
- High mechanical strength
- Available in laminated bearings with a substrate impregnated with thermosetting resin
- Consist of either a single layer or multiple layers (sliding layer + backing layer)
Plain bearing units with a self-aligning mechanism
A typical plain bearing bears the load of an adjoining component that rotates, oscillates, or reciprocates on the inner surface of its inner ring. This bearing unit has that functionality as well as a self-aligning mechanism that adjusts shaft deflection and tilt by the relative spherical motion of the outside of the inner ring and the inside of the outer ring that are in spherical contact.
As an example, the Oiles #500 Spherical Bearing has an inner ring of Oiles #500SP and an outer ring of high-carbon chromium bearing steel with a molybdenum disulfide coating.
This makes it lubrication-free and gives it an extremely long life, so it can handle higher loads than other lubrication-free spherical bearings.
In addition to the typical function of plain bearings, which supports the load of the opposing material parts through rotation, oscillation, or reciprocating motion on the inner ring's bore surface, this bearing unit features a self-aligning capability. This is achieved through the relative spherical movement between the inner ring's outer diameter and the outer ring's inner diameter, allowing adjustment for shaft deflection and misalignment.
For example, our product, the "Oiles #500 Spherical Bearing" by Oiles Corporation utilizes Oiles #500SP for the inner ring and employs high carbon chrome bearing steel treated with molybdenum disulfide coating for the outer ring. This design enables the bearing to operate without oiling and offers an exceptionally long life, with the ability to withstand heavy loads compared to other lubrication free spherical bearings on the market.
Furthermore, We at Oiles Corporation offers not only Oiles bearings but also "Oiles Air Bearings," which are high-rigidity, high-precision, and high-speed oil-free bearings designed with a proprietary porous sintered layer and integrated back metal structure.
The features of Oiles Air Bearings are as follows:
- •Achieves a friction coefficient almost zero
- •Reduces running costs
- •Allows for customizable performance
- •Simplifies processing due to the integrated structure of the back metal and sintered layer, enabling cost-effective production
A mechanical element other than a bearing that is required for a shaft to operate properly
In addition to bearings, another mechanical element, the flange, is also important for a device’s shaft to operate properly.
A flange is a ring or rim or a hub with blades that serves to stop leakage by, for example, filling a gap in a pipe fitting.
Some plain bearings have flanges. A flange with a hub and blades on the outside is common.
Attaching a flange to a bearing improves the ease of assembly and positioning stability. Some flanges can be secured by inserting bolts in the holes in the blades.
Source:“flange” in Concise Katakana Dictionary (4th ed.), Sanseido, (2010).
Conclusion
A plain bearing is a type of bearing that supports a rotating or reciprocating shaft on a surface. It has excellent impact resistance and seizure resistance, and it has the advantage of being compact and flexibly designed. Plain bearings have different characteristics than rolling bearings, so which bearing should be used needs to be carefully considered.
Oiles Corporation is a leading manufacturer of oil-less bearings, boasting the world’s No. 1 market share in bearings for a number of components, such as steering systems.
To consult us on ways to use plain bearings or if you are considering incorporating plain bearings in your products, please Contact Us at Oiles Corporation. With our advanced technology and abundant know-how related to bearings, we can suggest a variety of options.
