Tag Archive for: marine slewing bearing

What Is a Slewing Bearing?

A slewing bearing is a large rotating component that supports heavy loads while enabling rotation between two structures. These bearings handle axial loads, radial loads, and tilting moments simultaneously. They are essential in cranes, excavators, wind turbines, and other heavy machinery.

Most slewing bearings operate in air. Some operate in environments where water, moisture, or humidity is present. A smaller number operate fully submerged underwater. These underwater applications present unique engineering challenges that do not exist in conventional installations.

Why Underwater and Hydropower Applications Demand Special Slewing Bearings

Underwater and hydropower applications place extreme demands on slewing bearings. The bearing must function reliably in an environment that attacks every component of its design.

The pressure problem

Water pressure increases with depth. A tidal power plant positioned 15 meters below the water surface experiences pressure that is three times stronger than the pressure on a wind turbine. This pressure acts on every surface of the bearing and tries to force water past seals and into internal cavities.

The corrosion problem

Seawater contains chloride ions that penetrate metal surfaces and break down protective oxide layers. This triggers electrochemical corrosion. In the splash zone and tidal zone, corrosion rates can reach three to ten times the rate of fully submerged areas. Marine organisms such as barnacles and algae attach to bearing surfaces, increasing rotational resistance and creating conditions for localized corrosion.

The maintenance problem

Underwater maintenance is expensive and difficult. A seal that fails at the surface can be replaced with relative ease. A seal that fails 15 meters underwater requires specialized diving equipment, extended downtime, and significant cost. The patent literature identifies this as a key problem: seals in underwater slewing bearings cannot be replaced underwater, and the maintenance operation is expensive and requires frequent fixing of the assembled machine.

The load problem

Hydropower and tidal applications involve high loads. Tidal turbine rotor bearings must withstand the force of moving water. Hydropower gate bearings must support the weight of water-control structures. These loads combine with the constant pressure and corrosion of the underwater environment.

Design Challenges for Underwater Slewing Bearings

Sealing Challenges

Seals are the first line of defense against water ingress. In underwater applications, they face conditions that do not exist in air.

Multiple seal layers

A single seal is not sufficient for underwater service. Patent designs use multiple adjacent seals on the bearing face that contacts water directly. Three or more seals may be arranged in series to provide redundant protection. Adjacent seal lips can be oriented in different directions to block water intrusion from multiple angles.

Deflector elements

Advanced designs include a deflector mounted on the bearing ring. The deflector partially surrounds the seal lips and creates a narrow passage that blocks the flow of water carrying contaminating particles. This prevents particles from reaching the seal lips and extends seal life. Deflectors made from polyether ether ketone (PEEK) resist seawater and marine organisms.

Seal material requirements

Standard rubber seals degrade in seawater. Underwater bearings require specialized materials. Fluororubber seals offer superior resistance to seawater, ozone, and aging. The sealing surface itself may be made from stainless steel to resist corrosion.

The seal replacement problem

The most challenging design constraint is that seals cannot be replaced underwater. This means the seal system must be designed for the full service life of the bearing. Every component must be engineered for maximum durability because there is no opportunity for intervention.

Corrosion Protection Challenges

Every exposed surface of an underwater slewing bearing must resist corrosion. The bearing rings, gear teeth, mounting surfaces, and fasteners are all vulnerable.

Surface coating technologies

Thermal spray zinc-aluminum alloy coatings provide cathodic protection. The zinc acts as a sacrificial anode and protects the underlying steel even if the coating is scratched or damaged. Polytetrafluoroethylene (PTFE) composite coatings reduce friction while providing a corrosion barrier.

Material selection

Stainless steel or duplex stainless steel rings offer superior corrosion resistance compared to standard bearing steels. For rolling elements, ceramic materials such as silicon nitride eliminate corrosion entirely because they do not contain iron that can oxidize.

Cathodic protection

Impressed current or sacrificial anode cathodic protection systems can be used for submerged bearings. These systems shift the metal surface potential to a range where corrosion does not occur. They work in combination with coatings and seals to provide comprehensive protection.

Lubrication Challenges

Lubricant must protect the bearing from friction and wear while resisting water contamination. In underwater applications, lubricant selection and retention become critical.

Lubricant selection

The lubricant must resist water washout, maintain film strength under pressure, and protect against corrosion. Specialized greases with corrosion inhibitors are used. The lubricant must also be compatible with the seal materials and the operating temperature range.

Lubricant retention

The sealed chamber must retain the lubricant for extended periods. Every seal must perform its function without failure. The annular chamber between the inner and outer rings is protected by the seal system to prevent lubricant loss and water ingress.

Centralized lubrication

Large slewing rings may require centralized lubrication systems. These systems deliver measured quantities of grease to multiple points on the bearing at defined intervals. For underwater applications, the lubrication system itself must be protected from corrosion.

Load and Structural Challenges

Underwater slewing bearings must handle loads that are often higher than comparable surface applications.

Tidal turbine loads

A tidal turbine rotor bearing supports the weight of the rotor assembly and the thrust force from water flow. The three-row roller bearing design is used for this application because it handles high axial and radial loads simultaneously. The bearing must also withstand the cyclic loading from tidal flow reversals.

Hydropower gate loads

Hydropower gate bearings support the weight of large steel gates and the water pressure acting on them. These bearings must maintain precise positioning to control water flow accurately.

Structural integration

The bearing must integrate with the surrounding structure in a way that maintains alignment and prevents distortion. For underwater installations, the structural interface must also resist corrosion and marine growth.

Solutions and Design Approaches

Multi-Layer Seal Systems with Deflectors

The most effective solution combines multiple seals with a mechanical deflector. The deflector creates a physical barrier that blocks large particles and reduces water velocity at the seal lip. The multiple seals provide redundant protection. The innermost seal sees the cleanest environment because the outer seals and deflector have removed most contaminants.

Corrosion-Resistant Material Selection

Designers can choose from several material options:

• Stainless steel rings for maximum corrosion resistance
• Duplex stainless steel for higher strength and corrosion resistance
• Ceramic rolling elements to eliminate corrosion at contact points
• PEEK deflectors to resist seawater and marine organisms

Surface Treatment Combinations

No single coating provides complete protection. A combination approach is most effective:

• Thermal spray zinc-aluminum coating for cathodic protection
• Organic sealant over the thermal spray for additional barrier protection
• PTFE coating on sliding surfaces to reduce friction
• Corrosion-resistant fasteners and mounting hardware

Design for Maintenance-Free Operation

Because underwater maintenance is so difficult, the design must minimize the need for intervention:

• Seals designed for full service life
• Lubricant selected for long-term stability
• Corrosion protection systems that do not require replacement
• Materials that resist marine growth

Condition Monitoring

Where maintenance access is limited, condition monitoring becomes essential. Vibration sensors can detect early bearing damage. Lubricant sampling, where possible, can reveal wear particles and contamination. Temperature sensors can detect abnormal operating conditions. For fully submerged bearings, remote monitoring systems transmit data to surface operators.

Applications in Hydropower and Tidal Energy

Tidal Power Plants

Tidal power plants use slewing bearings to support and adjust rotor blades. A novel rotor blade adjustment bearing supports rotors of a tidal power plant positioned 15 meters below the water surface. The three-row roller bearing withstands extraordinary loads under water pressure that is three times stronger than wind turbine pressure.

Hydropower Gates

Hydropower facilities use slewing bearings to rotate and position water-control gates. These bearings must operate reliably for decades with minimal maintenance. The underwater environment accelerates corrosion, making material selection and sealing critical.

Underwater Turbines

Underwater turbines for tidal or river current energy use slewing bearings for blade pitch control. These bearings must resist corrosion, water pressure, and the constant flow of water carrying abrasive particles.

How LDB Bearing Supports Underwater Applications

LDB Bearing designs and manufactures high-quality slewing bearings for heavy machinery applications, including those requiring corrosion resistance. Products use verified 42CrMo forged alloy steel with induction-hardened raceways achieving 55–62 HRC.

LDB’s capabilities:

• Material options: Standard alloy steel with corrosion-resistant coatings for moderate environments
• Seal configurations: Heavy-duty and multi-lip seal options for moisture and water exposure
• Custom designs: Application-specific engineering for demanding environments
• Quality manufacturing: ISO 9001-certified manufacturing with documented inspection reports and full material traceability
• Global reach: Serving 73 countries with over 500,000 units in service

For underwater and hydropower applications requiring specialized corrosion protection and sealing beyond standard configurations, LDB offers engineering consultation to assess application requirements. The company’s experience in heavy machinery and renewable energy applications provides a foundation for addressing demanding operating environments.

Contact LDB Bearing today to discuss your slewing bearing requirements.