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What Is a Slew Bearing and How Does It Work?

What Is a Slew Bearing and How Does It Work?

Undercarriage of an excavator

What is a slew bearing and how does it work?

Reading time: 5 min

One of the most impressive features of an excavator is the ability of its operator cab to spin 360-degrees atop the machine’s undercarriage. This incredible engineering feat, which makes excavators remarkably efficient, is enabled by an inconspicuous, yet indispensable component known as the “slew bearing”.

A slew bearing, sometimes called a slew ring, slewing ring, turntable bearing or slewing bearing, is a specialized type of rolling-element bearing designed to handle axial loads, radial loads and tilting moments simultaneously.

While slew bearings are central to an excavator’s design, they’re used in other heavy equipment as well. In fact, slew bearings can be found in machines as diverse as cranes, drilling rigs and even wind turbines.

In this guide, we’ll delve into the mechanics of slew bearings, how they work and their particular maintenance requirements. By understanding the importance of slew bearings in heavy equipment and planning for their maintenance needs, operators and fleet managers can maximize the efficiency and longevity of their machines.

How Does a Slew Bearing Work?

Like traditional bearings, slew bearings work by utilizing bearing races, a set of rings that rotate around each other. Between the bearing races, bearing balls reduce the amount of friction between the races—allowing for the smooth, frictionless rotation of the rings.

In the slew bearings used by heavy equipment, it’s common for the inner race, or ring, to have a gear mounted to it. This allows for the slew bearing to be controlled and driven by a set of gears connected to the machine’s hydraulic system.

In excavators, the slew bearing connects the lower half of the machine to the top half and facilitates the rotation of the operator cabin. The slew bearing’s unique design allows the top half of the machine to rotate 360-degrees in either direction and even multiple times around the center point of the swivel.

The slew bearing must be strong enough to handle the weight of the machine and any load it carries, keeping everything stable during operation. Acting like a giant pivot point, it allows the equipment to turn from side to side with ease, making construction tasks, such as digging and placing loads, much easier.

Mechanics of a Slew Bearing

Although slew bearings perform a complex function in heavy equipment, their design is somewhat straightforward.

What Components Make Up a Slew Bearing?

Slew bearings are made up of a few main components:


  • Rings: Slew bearings are made up of two rings (races), an inner and outer ring. Each ring is connected to one half of the machine, and the rings can rotate independently of each other. One ring, typically the inner ring, features a gear mechanism that interfaces with the machine’s hydraulic system.
  • Rolling Elements: Between the two rings, rolling elements enable the rings to glide and rotate around each other, reducing friction and maintaining clearance between the rings. Commonly, the rolling elements are bearing balls, but other designs exist as well.
  • Seals: The space between the rings of a slew bearing must remain heavily lubricated in order for the mechanism to function properly. Seals on either side of the slew bearing ensure lubrication does not escape the bearing and prevent the ingress of debris into the bearing.


While not part of the slew bearing itself, swing drives are another important component that interface with and enable the function of the slew bearing.

Swing drives are a kind of hydraulically-driven component that feature a swing motor and drive the rotation of an excavator’s cab. Without swing drives, excavator cabs would be stationary and unable to rotate about the undercarriage.

Slew Bearing Maintenance

As the backbone of heavy equipment’s rotational prowess, a well-maintained slew bearing is crucial for ensuring the efficiency, safety and longevity of your machine.


Common Ways a Slew Bearing May Become Worn or Damaged

Like any mechanical component, slew bearings are subject to wear and tear over time, especially when subjected to heavy loads and harsh environmental conditions.

Here are some common ways a slew bearing may become worn or damaged:


  • Heavy Loads: Constantly bearing the weight of heavy equipment and any additional loads can cause stress and wear on the bearing’s internal components.
  • Harsh Environment: Exposure to dust, debris, water and other contaminants can lead to corrosion and abrasion, accelerating wear.
  • Improper Lubrication: Insufficient or inadequate lubrication can increase friction, leading to premature wear and heat build-up.
  • Misalignment: If the equipment experiences misalignment, it can put uneven stress on the slew bearing, causing localized wear.
  • Impact or Overloading: Sudden impacts or overloading can cause excessive stress and deformation on the bearing, leading to damage.


Signs a Slew Bearing is Worn or Damaged

While slew bearings are designed to take a beating, they are still susceptible to wear over time. Detecting early signs of wear or damage is crucial to prevent catastrophic failures and potential safety hazards.

Here are some of the main signs your excavator’s slew bearing is worn or damaged:


  • Unusual Noises: Grinding, clicking or rumbling noises during rotation may indicate issues with the slew bearing.
  • Vibration: Excessive vibration or jerky movements while rotating can be a sign of bearing wear.
  • Increased Friction: Difficulty or stiffness in rotating the equipment can indicate inadequate lubrication or wear in the bearing.
  • Abnormal Movement: If the equipment exhibits wobbling or tilting during rotation, it may signal a damaged slew bearing.

The Importance of Slew Bearing Maintenance

Failing to maintain or replace a damaged slew bearing can have severe consequences for the rest of the machine and operations. Some of these consequences include reduced performance, unexpected downtime and safety risks.

The best way to avoid these consequences is to implement a maintenance program that properly addresses the needs of your machine’s slew bearing.

Here are some maintenance best practices for slew bearings:


  • Regular Inspections: Conduct routine visual and functional inspections of the slew bearing, checking for signs of wear or damage.
  • Lubrication: Ensure proper and timely lubrication per the manufacturer's guidelines to reduce friction and wear.
  • Environmental Protection: Implement measures to shield the bearing from dust, debris, water and other contaminants.
  • Load Monitoring: Avoid exceeding the equipment's load capacity and distribute loads evenly to minimize stress on the bearing.
  • Alignment Checks: Periodically verify the alignment of the equipment to prevent uneven stress on the slew bearing.
  • Timely Replacement: Replace worn or damaged slew bearings promptly to avoid further complications.

Slew Bearings for Non-Construction Purposes

While slew bearings are synonymous with heavy construction equipment, their versatility extends far beyond the construction industry.

These novel components find themselves being used in a diverse range of equipment and machinery, each benefitting from the smooth rotation and stability that slew bearings provide. Here are some examples of other machines that utilize slew bearings:


  • Cranes
  • Wind Turbines
  • Solar Tracking Systems
  • Aerial Work Platforms


Certain differences exist in the design and size of slew bearings, tailored to the specific needs of each application. For example, the slew bearings in wind turbines must support the massive weight of massive turbine blades, while those in aerial work platforms prioritize compactness and precise movement.

Slew Bearings Common Questions

What Does Clearance Mean in Relation to Slew Bearings?

In the context of slew bearings, "clearance" refers to the gap or space between the rolling elements (balls or rollers) and the raceways (inner and outer rings) within the bearing. 

This clearance is crucial for the smooth operation of the bearing. It allows for thermal expansion and contraction of the components during operation, ensuring the bearing maintains proper functioning and reduces the risk of damage due to excessive friction or jamming.

What Size Slew Bearing Do You Need?

Selecting the appropriate size of a slew bearing for your machine involves several essential considerations. Some important points to consider include:


  • Load Capacity
  • Operating Conditions
  • Rotational Requirements
  • Space Constraints
  • Quality and Reliability


If you’re unsure about the appropriate size of slew bearing for your machine, consider consulting with experts or the equipment’s manufacturer.

Final Thoughts

As the silent pivot point that enables smooth rotation and precise positioning, the slew bearing empowers machines to conquer the toughest construction and excavation tasks.

By understanding the mechanics of slew bearings, what causes them to wear prematurely and their maintenance requirements, you can safeguard your machine’s optimal performance and protect your operators.

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