Ball Bearing Sizes: A Comprehensive Guide

Ball bearings are essential components in a wide range of mechanical devices, from household appliances to industrial machinery. Their primary function is to reduce friction and support rotational motion, ensuring smooth and efficient operation. One of the critical aspects KAYDON Thin Section Bearings of selecting the right ball bearing for an application is understanding its size. This article delves into the intricacies of ball bearing sizes, explaining their importance, how they are measured, and their impact on performance.

Importance of Ball Bearing Sizes

The size of a ball bearing directly influences its load capacity, speed, and durability. Choosing the appropriate size is crucial for the following reasons:

  1. Load Capacity: Larger ball bearings can typically support higher loads. This includes both radial loads (perpendicular to the axis) and axial loads (parallel to the axis).
  2. Speed: The size affects the maximum rotational speed of the bearing. Smaller bearings can usually operate at higher speeds compared to larger ones due to reduced inertia.
  3. Durability and Lifespan: The right size ensures optimal performance and longevity. Bearings that are too small for the load they bear will wear out quickly, while oversized bearings may add unnecessary weight and cost.

Measuring Ball Bearing Sizes

Ball bearing sizes are defined by several key dimensions:

  1. Inner Diameter (ID): This is the diameter of the hole in the bearing through which the shaft passes. It is crucial for ensuring a proper fit on the shaft.
  2. Outer Diameter (OD): This is the external diameter of the bearing. It determines the fit within the housing or the outer component that holds the bearing in place.
  3. Width (W): This is the overall width of the bearing. It affects the bearing’s ability to handle axial loads and its overall footprint.

These dimensions are usually expressed in millimeters (mm) or inches, depending on the region and the industry standards in use. For instance, a ball bearing might be specified as 25mm ID x 52mm OD x 15mm W.

Bearing Numbering System

Manufacturers use a standardized numbering system to indicate bearing sizes, simplifying the selection process. A typical bearing number consists of a series of digits and letters that represent the type, series, bore size, and any special features. For example, in the bearing number 6204:

  • 6: Indicates the type of bearing (deep groove ball bearing).
  • 2: Represents the bearing series (dimension series).
  • 04: Signifies the bore size, in this case, 20mm (multiplying the last two digits by 5 gives the bore size in mm).

Selecting the Right Bearing Size

Selecting the right bearing size involves considering the following factors:

  1. Load Requirements: Determine the maximum radial and axial loads the bearing will encounter.
  2. Speed Requirements: Identify the operating speed of the application and choose a bearing that can handle it without excessive heat generation or wear.
  3. Space Constraints: Consider the available space for the bearing in the assembly. Compact designs may require smaller bearings.
  4. Environmental Conditions: Account for factors such as temperature, humidity, and exposure to contaminants, which may influence bearing performance and size choice.

Impact on Performance

The correct bearing size ensures optimal performance by:

  • Reducing Friction: Properly sized bearings minimize friction, leading to smoother operation and reduced wear on both the bearing and associated components.
  • Enhancing Efficiency: Efficient bearings reduce energy consumption in motors and machinery.
  • Prolonging Lifespan: Adequate sizing prevents premature failure, thus extending the lifespan of both the bearing and the machinery it supports.


Understanding ball bearing sizes is fundamental for engineers and maintenance professionals aiming to optimize machinery performance and reliability. By carefully considering load capacities, speed requirements, and environmental conditions, one can select the appropriate bearing size to ensure efficient and durable operation. Whether for automotive applications, industrial machinery, or everyday appliances, the right ball bearing size is key to achieving smooth and reliable performance.