Bearings function to reduce friction between moving parts by converting sliding friction into rolling friction. When metal contacts metal directly, a large amount of friction is generated. This friction accelerates surface wear, causes grinding, and gradually damages the product components.  

The load applied to a bearing can be either a radial load or a thrust load. Depending on the installation position within the mechanism, the bearing may be subjected entirely to radial force, thrust force, or a combination of both. For example, a car wheel bearing experiences radial load from the vehicle’s weight as well as thrust load when the car turns left or right.

The construction of a bearing consists of smooth metal balls or rollers rolling between the inner and outer metal rings. Below are the most common types of bearings. 

Ball Bearings

Ball bearings are the most common type, capable of handling both radial and thrust loads. They are also known as deep-groove single-row bearings or Conrad bearings. The inner ring is typically mounted to the rotating shaft, while the groove on the outer ring serves as the raceway for the balls.  

As the balls roll in the raceway, load is transmitted from the outer race to the balls and then from the balls to the inner race. The curvature radius of the raceway is usually between 51.5% and 53% of the ball diameter.

• If the raceway curvature is small, the balls fit more tightly, but rolling friction increases due to the tight fit.
• If the curvature is larger, fatigue life may decrease since stress concentrates at a small contact area, which over time can lead to cracks or deformation of the balls. 

Conrad ball bearings are generally used for light-load applications. Since the contact between the ball and the outer race is a single spherical point, the balls roll very smoothly. However, due to the small contact area, excessive point load can cause ball deformation, resulting in bearing failure. Thus, ball bearings are mostly used for light to medium-load applications. 

Straight / Cylindrical Roller Bearings

Cylindrical roller bearings use rollers with a cylindrical shape running in a matching raceway. These bearings provide low friction, high radial load capacity, and high-speed capability. Unlike ball bearings with point contact, cylindrical roller bearings have line contact between rollers and raceways, distributing the load over a larger area and allowing higher load capacity. 

To reduce misalignment, the roller length is not significantly greater than its diameter. Cylindrical roller bearings are commonly used in applications such as conveyor rollers, where large radial loads must be supported.  

In their construction, one ring has flanges on both sides to guide the rollers, while the other ring has no flange, allowing thermal expansion.  

• If one additional flange is added to the other ring, the bearing can handle unidirectional thrust load.
• With flanges on both sides, it can handle bidirectional thrust load. 

Tapered Roller Bearings

Tapered roller bearings are designed to handle both radial and thrust loads simultaneously. They are widely used in automobile wheel hubs due to their high load-carrying capability. 

In these bearings, both the rings and rollers have a truncated cone shape, enabling them to support axial and radial forces at the same time. The load capacity depends on the contact angle between the roller axis and the bearing. 

• The larger the angle, the higher the thrust load capacity.
• The typical contact angle ranges from 10° to 16°, but for heavy thrust load applications, it can reach up to 30°.

Tapered roller bearings are often mounted in pairs to increase radial load capacity. For heavy-duty applications, double-row or four-row tapered roller bearings can be used in a single unit. 

Spherical Roller Bearings

Spherical roller bearings consist of two rows of rollers, barrel-shaped (thicker in the middle and tapered at both ends). This geometry helps distribute load evenly across the contact surface, enabling them to handle extremely heavy loads. 

The rollers run between two raceways:

• One raceway is on the inner ring.
• The other is a spherical-shaped raceway on the inner surface of the outer ring.

Because of this spherical design, the bearing can self-align when the shaft experiences slight angular misalignment while still maintaining stable operation. The roller shape closely matches the raceway, making the bearing robust and highly load-resistant. 

In practice, spherical roller bearings are often mounted in symmetric pairs, rotating in opposite directions within the same housing to support loads in both directions. They are commonly used in gearboxes, rolling mills, crushers, and mining equipment, where both radial (perpendicular to the shaft) and axial (parallel to the shaft) loads must be supported. 

Needle Roller Bearings

Needle roller bearings use very small and long cylindrical rollers. Their main characteristic is the small roller diameter relative to its length, with a ratio typically between 1:2.5 and 1:10. 

• Advantage: They require very little radial space (from shaft center outward), making them ideal for applications with limited installation space.
• Limitation: Due to their small size, guiding the rollers is challenging, often resulting in higher friction. They are less suited for high-speed operation and are mostly used in low-speed or oscillating motion applications.  

To address this limitation, a cage is often used to hold and guide the rollers, preventing misalignment, reducing friction, and improving operational stability. 

Needle roller bearings are therefore suitable for compact designs where space is limited but high speeds are not required. 

Thrust Bearings

Thrust bearings are designed specifically to handle axial loads (parallel to the shaft). There are several types, with the most common being:

• Roller thrust bearings
• Ball thrust bearings

1. Roller thrust bearings

Roller thrust bearings are designed to handle large axial loads, often found in automotive gear systems between gears or between the housing and the rotating shaft. Helical gears in car transmissions generate significant axial forces, which these bearings accommodate.  

When operating, the rollers not only roll but also experience slight sliding relative to the raceway. This occurs because the roller’s center and ends have different radii, resulting in unequal surface speeds at contact points. To compensate, the rollers both roll and slip slightly, preventing severe wear. 

This sliding behavior is inherent to roller thrust bearings and also the reason why they cannot achieve the same high speeds as ball bearings. 

2. ball thrust bearings

Ball thrust bearings are designed for light-load, low-speed axial applications, such as swivel chairs, where they carry loads from the seat surface. They consist of two grooved discs with a row of balls in between. 

At higher speeds, centrifugal force acts on the balls, making sliding at the contact points between balls and raceways more pronounced. 

The above provides a summary of the differences between various types of bearings, each with its own advantages and suitability for specific applications. In industrial manufacturing, when selecting bearings, companies often consider key criteria such as load capacity, operational accuracy, and long-term durability. For applications that demand precise and reliable motion, ball bearings and ball screws are highly valued for their smooth rolling performance, low friction, and ability to maintain accuracy over an extended period.

In addition to conventional rotary bearings, THK Corporation (Japan) has developed advanced motion systems such as the Linear Motion (LM) Guide, which enables machine components to move smoothly and stably along a straight path, and the Ball Screw, which converts rotary motion into linear motion.  

Both products inherit the core principle of bearings—replacing sliding friction with rolling friction—but are applied at a higher level of engineering, serving industries that demand high precision, durability, and stability.  

At present, Temas is the official distributor of THK products in Vietnam, offering a wide range of LM Guides, Ball Screws, and other precision mechanical solutions. Customers can explore the complete portfolio of THK products here. 

(All images in this article are referenced from the University of Cambridge)