What Is a Bearing? Types, Functions and How to Choose One

A bearing supports and guides relative motion between machine components while reducing friction. Bearings are used in motors, spindles, gearboxes, rollers, conveyors, pumps, CNC machines and countless other mechanical systems.

What does a bearing do?

A bearing can support radial load, axial load or a combination of both. It also helps maintain shaft position, control friction and provide predictable rotational or linear motion.

Main bearing families

Deep-groove ball bearings

Common general-purpose bearings that handle radial load and moderate axial load in both directions. They are widely used in electric motors and machinery.

Angular-contact ball bearings

Designed to carry combined radial and axial loads. They are frequently used in machine-tool spindles and other applications where axial stiffness matters.

Cylindrical roller bearings

Use cylindrical rolling elements and provide high radial load capacity. Depending on the design, axial movement may be permitted or constrained.

Tapered roller bearings

Designed for combined radial and axial loads. They are commonly used where preload or adjustable internal clearance is required.

Self-aligning bearings

Allow limited angular misalignment between the shaft and housing and can be useful where mounting or shaft deflection creates alignment variation.

Linear bearings

Guide straight-line motion on shafts or rails. CNC systems may use recirculating linear guide blocks, round-shaft bushings or other linear bearing technologies.

Radial load and axial load

Radial load acts perpendicular to the shaft axis. Axial load acts parallel to the shaft axis. The load direction and magnitude are fundamental inputs when choosing a bearing type.

Important selection factors

Factor	Why it matters
Load magnitude and direction	Determines bearing family and size
Rotational speed	Affects heat, lubrication and permissible bearing design
Required stiffness	Important for spindles and precision motion
Misalignment	May require self-aligning geometry
Environment	Dust, moisture and contamination influence sealing
Temperature	Affects lubricant and internal clearance
Mounting fits	Influence internal clearance and ring creep

Open, shielded or sealed?

Open bearings provide direct access to the rolling elements and are suitable where external lubrication and clean conditions are controlled. Metal shields help reduce entry of larger contaminants with low friction. Contact or non-contact seals provide greater contamination protection but can influence friction and speed capability.

Bearing clearance and preload

Internal clearance is the available movement between rings before mounting. Fits, temperature and operating conditions change the effective clearance. Some precision applications use preload to increase stiffness and control movement, but excessive preload creates heat and reduces bearing life.

Lubrication

Correct lubricant type, quantity and relubrication interval are essential. Too little lubrication can cause metal contact and wear; too much grease can increase churning losses and temperature.

Bearings in CNC machines

CNC systems use bearings in spindles, servo motors, ball-screw supports, rotary axes and linear-motion assemblies. Bearing selection in these applications must consider accuracy, stiffness, speed and thermal behavior as well as static load capacity.

Common causes of early bearing failure

• Contamination
• Incorrect mounting force
• Misalignment
• Insufficient or excessive lubrication
• Incorrect fit
• Electrical current through the bearing
• Overload or shock load

Selection rule

Do not select a bearing by bore diameter alone. Use the manufacturer’s load ratings, speed limits, clearance options, lubrication guidance and application calculations for the required life and operating conditions.