Plain Bearings Vs Ball Bearings

Plain Bearings Vs Ball Bearings

Plain Bearings Vs Ball Bearings: Choosing the Right Bearing for Your Application

Bearings are indispensable components in mechanical products, playing a crucial role in supporting rotating shafts. Based on the nature of friction within bearings, they can be classified into two main types: rolling-element bearings (commonly known as ball bearings) and plain bearings. Each type has its own unique characteristics and performance advantages and disadvantages, requiring comprehensive consideration when selecting the appropriate one for a particular application.

1. Comparison between Plain Bearings and Ball Bearings

Plain Bearings Vs Ball Bearings

Plain Bearings Vs Ball Bearings: Structural and Operational Differences

The most significant difference between plain bearings and ball bearings lies in the presence of rolling elements. Ball bearings contain rolling elements (balls), which support rotating shafts by rolling, resulting in point contact. The more rolling elements there are, the more contact points are established. On the other hand, plain bearings lack rolling elements and rely on smooth surfaces to support rotating shafts, resulting in surface contact. This structural difference determines that ball bearings operate through rolling motion, while plain bearings operate through sliding motion, resulting in entirely different frictional characteristics.

Load-Carrying Capacity: Plain Bearings Vs Ball Bearings

Overall, due to the larger bearing surface area, plain bearings generally have a higher load-carrying capacity than ball bearings. Additionally, ball bearings have limited ability to withstand impact loads, although hydrodynamic plain bearings, when fully liquid-lubricated, can withstand significant impact loads due to the cushioning and vibration-absorbing effect of the lubricating oil film. At higher speeds, the centrifugal force acting on the rolling elements in ball bearings increases, necessitating a reduction in their load-carrying capacity (which can lead to noise issues at high speeds). For hydrodynamic plain bearings, their load-carrying capacity increases with higher speeds.

Friction Coefficient and Starting Friction Resistance: Plain Bearings Vs Ball Bearings

Under normal working conditions, the friction coefficient of ball bearings is generally lower and more stable compared to plain bearings. However, the lubrication of plain bearings is more susceptible to external factors such as speed and vibration, resulting in a wider range of variation in the friction coefficient. During startup, plain bearings experience higher resistance due to the absence of a stable oil film, but hydrodynamic plain bearings exhibit minimal starting friction resistance and low operational friction coefficients.

Applicable Operating Speeds: Plain Bearings Vs Ball Bearings

Ball bearings, constrained by the centrifugal force acting on the rolling elements and bearing temperature rise, are not suitable for very high speeds and are generally used in medium to low-speed applications. Similarly, hydrodynamic plain bearings, due to bearing heating and wear, also have limitations on operating speeds. However, fully liquid-lubricated plain bearings demonstrate excellent high-speed performance, especially when air is used as the lubricant in hydrostatic plain bearings, enabling speeds of up to 100,000 rpm.

Power Loss Comparison: Plain Bearings Vs Ball Bearings

Due to their lower friction coefficient, ball bearings generally have minimal power losses, which are lower than those of hydrodynamic plain bearings. However, improper lubrication and installation of ball bearings can significantly increase power losses. Hydrodynamic plain bearings exhibit lower friction power losses, but for hydrostatic plain bearings, the presence of an oil pump can result in higher overall power losses compared to hydrodynamic plain bearings.

Service Life Comparison: Plain Bearings Vs Ball Bearings

Ball bearings are subject to material spalling and fatigue, typically designed for a service life of 5 to 10 years or replacement during major overhauls. The bearing shells of hydrodynamic plain bearings suffer from severe wear and require regular replacement. Theoretically, the service life of fully liquid-lubricated plain bearings is infinite, but in practice, stress cycles, especially in hydrodynamic plain bearings, can lead to fatigue failure of bearing shell materials.

Rotational Precision Comparison: Plain Bearings Vs Ball Bearings

Due to their small radial clearance, ball bearings generally exhibit higher rotational precision. Hydrodynamic plain bearings operate in boundary or mixed lubrication regimes, resulting in unstable operation and significant wear, leading to lower precision. Fully liquid-lubricated plain bearings, with the presence of an oil film, offer higher precision, with hydrostatic plain bearings providing even higher rotational precision.

Other Comparisons: Plain Bearings Vs Ball Bearings

Ball bearings use minimal amounts of oil, grease, or solid lubricants, with higher quantities required at high speeds, and require high oil cleanliness standards, necessitating sealing. However, they are convenient to replace, and shaft repair is generally unnecessary. For plain bearings, except for hydrodynamic plain bearings, lubricants are typically liquids or gases, requiring large quantities, high oil cleanliness standards, and regular replacement of bearing shells, and sometimes shaft repair.

2. Selecting Between Plain Bearings and Ball Bearings

Due to the complexity and diversity of real-world operating conditions, there is no unified standard for selecting between plain bearings and ball bearings. Ball bearings, due to their low friction coefficient, low starting resistance, responsiveness, high efficiency, and standardization, are generally preferred and widely used, making them convenient for use, lubrication, and maintenance in most applications. However, plain bearings have unique advantages and are typically applied in situations where ball bearings are not feasible or advantageous:

  • Restricted Radial Space or Partial Installation: In cases where radial space is limited or partial installation is required, plain bearings are preferred due to their compact design.
  • High Precision Requirements: Applications with high precision requirements often favor plain bearings, particularly hydrostatic plain bearings, due to their ability to dampen vibrations.
  • Heavy Load Applications: In heavy-load applications, plain bearings are preferred due to their ability to withstand significant loads without overheating or fatigue, such as in rolling mills, steam turbines, aircraft engine accessories, and mining machinery.
  • Specialized Conditions: Special conditions such as very high speeds, extreme impact, and vibration, or operation in water or corrosive environments may necessitate the use of plain bearings.

When selecting between ball bearings and plain bearings for a particular machine or equipment, it is essential to weigh the advantages and disadvantages of each type and consider the specific engineering requirements. Historically, large and medium-sized crushers often used plain bearings made of Babbitt alloy due to their ability to withstand significant impact loads and wear. In contrast, smaller jaw crushers typically use ball bearings for their high transmission efficiency, responsiveness, and ease of maintenance. With advancements in ball bearing manufacturing technology, many large jaw crushers now also employ ball bearings.

In conclusion, the choice between plain bearings and ball bearings depends on various factors such as load, speed, precision requirements, operating environment, and maintenance considerations. Both types of bearings offer distinct advantages and should be selected based on the specific needs of the application, ensuring optimal performance and longevity.

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