What is the difference between plain bearings and ball bearings?

Bearings are crucial components in mechanical transmission systems. Their primary function is to support rotating shafts and reduce friction between them and other components. There are numerous types of bearings, and their varying structures and materials determine their performance under various operating conditions. Plane bearings and ball bearings are two widely used bearing types, each with distinct structural differences. Although both serve rotating systems, they differ significantly in their operating principles, application scenarios, and performance.
This article will comprehensively and in-depth explain the differences between plane bearings and ball bearings from multiple perspectives, including structural principles, friction characteristics, load capacity, application scenarios, manufacturing materials, and lifespan maintenance, to help readers gain a comprehensive understanding of these two types of bearings.

Differences in Structure and Operating Principles
1. Plane Bearings (also known as Sliding Bearings)
Plane bearings are a type of bearing structure that operates based on sliding friction. They consist of the contact surface between the bearing housing and the shaft journal, where a lubricating film (air, oil, or solid lubricant) forms, enabling the shaft to glide smoothly across the bearing surface. Its main characteristic is the absence of rolling elements (such as steel balls or rollers). Instead, it relies primarily on the lubricant film formed between the rubbing surfaces to reduce direct contact and frictional resistance. During operation, the shaft and bearing surfaces are in constant contact, requiring high-quality lubrication to minimize wear.
2. Ball Bearings (a type of rolling bearing)
A ball bearing operates on the principle of rolling friction and consists primarily of an inner ring, an outer ring, a cage, and rolling elements (steel balls). The inner ring rotates with the shaft, while the balls roll between the inner and outer rings, significantly reducing friction.
A key characteristic of ball bearings is that the rolling elements rotate between the two rings, converting sliding friction into rolling friction. This results in a lower coefficient of friction and, therefore, higher efficiency. Ball bearings offer a compact structure, high speed, and low temperature rise, making them suitable for a variety of applications requiring high speeds.

Differences in Friction Types and Efficiency
1. Plane Bearings: Sliding Friction
Sliding friction has a relatively high coefficient of friction. Under poor lubrication conditions or when the lubricant is ineffective, direct metal-to-metal contact can easily occur, leading to severe wear or galling. Therefore, plain bearings are more dependent on lubrication and require a constant supply of oil or coating to maintain performance.
2. Ball Bearings: Rolling Friction
The coefficient of friction of rolling friction is much lower than that of sliding friction, typically 1/10 or even less. This makes ball bearings easier to operate and more efficient. At high speeds, they far outperform plain bearings.

Differences in Load Capacity and Load-Bearing Methods
1. Plain Bearings Have Stronger Load Capacity
Due to their large contact area, plain bearings can theoretically withstand higher loads than ball bearings. Especially under low-speed, high-load conditions, plain bearings are more stable and less susceptible to damage from shock loads.
Plain bearings have surface contact, resulting in a large contact area and lower pressure per unit area, making them more capable of withstanding heavy loads and shock loads.
2. Ball Bearings Suitable for Medium and Light Loads
Ball bearings use point contact. While they offer low friction and smooth operation, their load-bearing area is limited, making them primarily suitable for carrying medium and light radial loads and certain axial loads. Under high loads, their rolling elements may fail due to excessive pressure. 

Speed Adaptability
1. Plane Bearings Suitable for Low-Speed Applications
Due to the high heat generated by sliding friction, planar bearings are not suitable for high-speed applications. At high speeds, their lubricating film is easily damaged, causing direct wear on the contact surfaces and ultimately shortening bearing life.
2. Ball Bearings Suitable for High-Speed Operations
Ball bearings are more suitable for high-speed applications due to their low coefficient of friction and low heat generation. They are widely used in high-speed rotating components such as motors, fans, and automobile wheels, ensuring smooth operation and low noise.

Differences in Manufacturing and Installation Complexity
Plane bearings:
Relatively simple structure and low manufacturing cost;
However, installation requires high concentricity, clearance, and lubrication conditions;
Maintaining a precise fit between the shaft and bearing is essential; otherwise, problems such as eccentric wear and seizure can easily occur.
Ball bearings:
High manufacturing precision, high degree of standardization, and excellent adaptability;
Easy installation and relatively simple maintenance;
Most bearings come pre-assembled, ready to use right out of the box, reducing on-site assembly time. 

Service Life and Maintenance Cycle
Planar bearings:
If lubricated adequately and operating conditions are mild, their service life is considerable.
However, if lubrication is insufficient or impurities enter the bearing, wear will accelerate, necessitating frequent maintenance.
A lubrication system or regular oiling is generally required to extend their service life.
Ball bearings:
The inherently sealed structure of the bearings ensures a good seal, and the retainer effectively isolates dust and impurities.
Under good sealing conditions, the maintenance-free cycle is long.
The service life is primarily determined by load, speed, lubrication, and temperature control, and is generally stable.

Differences in Application Scenarios
Planar bearings are commonly used in:
Heavy-duty industrial machinery (such as metallurgical equipment and ship shafting);
Harsh environments with high temperatures or high dust content;
Low-speed, heavy-load bearings;
Some applications requiring a large shaft diameter and short bearing length.
Ball bearings are commonly used in:
High-speed motors, fans, and generators;
Home appliances, automobiles, and office equipment;
Transmission structures requiring fast response and lightweight design;
Applications requiring low noise and low friction operation. 

Comprehensive Comparison and Selection Recommendations
When choosing between plain bearings and ball bearings, consider the following factors:
Does the bearing need to withstand high loads?
If so, consider plain bearings first; if the loads are less severe, ball bearings are more suitable.
Does the bearing require high speeds?
Ball bearings clearly outperform plain bearings at high speeds.
Are lubrication conditions adequate?
Plain bearings rely on lubrication; if this is not possible, consider ball bearings.
Are low noise and efficiency important?
Ball bearings, with their low friction and low noise levels, are more suitable for quieter environments.
Are there any cost or size constraints?
Plain bearings are low-cost and compact, but require precision machining; ball bearings are highly standardized and easy to maintain.

Conclusion
Plain bearings and ball bearings are two different types of bearings, each with unique advantages and applications. Plain bearings rely on sliding friction, have high load capacity, and are suitable for low speeds and heavy loads; ball bearings, on the other hand, use rolling elements to reduce friction, operate smoothly, and are suitable for high speeds and light loads.