Water Lubricated Bearing: Working Principle, Materials, Applications and Selection Guide

Water lubricated bearings are widely used in equipment that operates in or around water, including marine propulsion systems, hydro turbines and large water pumps. Instead of relying on oil or grease at the bearing interface, these bearings use water for lubrication and cooling.

The idea sounds simple, but designing a reliable water-lubricated bearing is not simply a matter of replacing oil with water. Water has a much lower viscosity than lubricating oil, so bearing performance is closely related to shaft speed, load, running clearance, material properties, water quality and shaft condition.

This becomes especially important during start-up and low-speed operation, or when the water contains sand and other abrasive particles.

This guide looks at how water lubricated bearings work, where they are used, how different materials behave and what engineers should consider when selecting a bearing for a marine, hydropower or pump application.

Water Lubricated Bearings at a Glance

What Is a Water Lubricated Bearing?

A water lubricated bearing is a sliding bearing that uses water between the shaft and bearing surface for lubrication and cooling.

When the shaft rotates, water is drawn into the running clearance. Under suitable speed, load and geometry, pressure develops within the water film and helps separate the shaft from the bearing surface. This reduces direct contact and allows the bearing to carry the shaft while keeping friction and wear under control.

Water-lubricated bearings are commonly found in stern tubes, propeller shaft systems, rudder systems, hydro turbines, vertical pumps and other water-handling equipment.

Water-Lubricated Does Not Mean Self-Lubricating

These two terms are sometimes used together, but they describe different things.

Water-lubricated refers to the lubrication method. Water is the working lubricant and coolant.

Self-lubricating refers to the friction characteristics of the bearing material itself.

A material used in a water-lubricated bearing may also contain self-lubricating components. This can help during start-up, shutdown and other periods when a complete water film has not yet formed. However, a self-lubricating bearing is not automatically a water-lubricated bearing.

That distinction matters when choosing materials and comparing bearing products.

How Does a Water Lubricated Bearing Work?

During normal operation, the rotating shaft draws water into the clearance between the shaft and bearing. The resulting pressure in the water film supports the shaft and reduces direct contact between the two surfaces.

The basic process is:

Shaft rotation → water enters the clearance → water-film pressure develops → surfaces separate → friction and wear decrease

At sufficient shaft speed, a stable hydrodynamic film can carry much of the bearing load.

The more difficult periods are usually start-up, shutdown and very low-speed operation.

What Happens During Start-Up?

Before the shaft reaches operating speed, there may not be enough hydrodynamic pressure to maintain a complete water film. Parts of the shaft and bearing can therefore operate under mixed or boundary lubrication.

This is where the friction and wear characteristics of the bearing material become particularly important.

A bearing used in equipment that starts several times a day may therefore need different material properties from one used in a machine that starts once and then runs continuously.

The same applies to temporary interruptions in the water supply. A material may tolerate a short dry-start period, but this should never be interpreted as unlimited dry-running capability.

Water Is Also the Coolant

Water does more than reduce friction. It carries heat away from the bearing.

If the water supply is restricted, both lubrication and cooling can deteriorate. Local temperature can rise, friction can increase and bearing wear may accelerate.

For this reason, water flow cannot be separated from bearing design. Bearing diameter, length, speed, load, groove arrangement and operating temperature all affect the amount and distribution of water required.

What Materials Are Used for Water Lubricated Bearings?

There is no single material that works best in every water-lubricated bearing.

A stern tube operating in seawater has different requirements from a vertical pump handling sediment-laden river water. A hydro turbine guide bearing also sees a different combination of load, speed and operating cycle.

Material selection should therefore start with the operating conditions rather than with the material name.

Rubber and Elastomeric Bearings

Rubber and elastomeric materials have been used in water-lubricated marine bearings for many years.

Their elasticity helps absorb vibration and impact, while the material can provide good sliding behavior under suitable water-lubricated conditions. These properties make elastomeric bearings particularly useful where shaft vibration, shock or small alignment changes have to be accommodated.

Performance still depends on the formulation. Load capacity, water absorption, temperature resistance and abrasive wear can vary considerably between different elastomeric materials.

Engineered Polymer Bearings

Engineered polymers offer another route to water-lubricated bearing design. Depending on the formulation, they can combine low friction with wear resistance, corrosion resistance, dimensional stability and relatively high load capacity.

Techemer uses several material systems because the requirements of clean water, heavy load and sediment-laden water are not the same.

TSTN

Techemer TSTN is an elastoplastic alloy containing self-lubricating microcrystals and extreme-pressure particles.

The material is intended to balance low friction and wear resistance, particularly in demanding applications where the bearing may experience heavy load, frequent start-stop cycles or periods of boundary lubrication.

Typical applications include pump guide bearings, stern tube bearings, rudder bearings and hydropower bearing systems.

Techemer material data gives a typical dynamic surface pressure of up to approximately 10 MPa and a water-lubricated PV value of up to approximately 100 MPa·m/s.

These figures describe material capability under specified conditions. They should not be used as universal design limits without considering bearing size, speed, temperature, water conditions and the complete shaft-bearing system.

PREM

PREM is an engineering elastomer used by Techemer in water-lubricated pump and marine shaft applications.

It is particularly useful where the application calls for higher load-bearing capability together with stable operation in water. Typical applications include vertical pump bearings and stern tube bearing systems.

As with any bearing material, the final selection should be based on actual bearing pressure, shaft speed, water conditions, dimensions and installation arrangement.

hRBR

Clean water and sediment-laden water place very different demands on a bearing.

Techemer hRBR is intended for applications where sand, silt and other abrasive particles are present in the lubricating water. Its combination of wear resistance and elasticity makes it suitable for sediment-laden pumps, hydropower equipment, dredgers, working vessels and other severe water applications.

This is an area where simply choosing the hardest material can be misleading. A bearing and shaft work as a pair. How particles move through the clearance, whether they become trapped, and how they interact with the shaft surface can matter as much as the hardness of the bearing itself.

Ceramic Water Lubricated Bearings

Ceramic bearings are useful in selected applications where wear, corrosion, chemical exposure or temperature makes conventional polymer solutions difficult to use.

Techemer ceramic bearing systems can incorporate SiC bearing surfaces, metallic housings, hard shaft sleeves and elastomeric cushioning structures.

Typical applications include pumps, hydropower equipment and equipment handling corrosive or abrasive media.

Ceramic is not automatically the better choice simply because it is harder. Impact loading, alignment, shaft pairing, installation and vibration still have to be considered.

How to Choose the Right Water Lubricated Bearing Material

A practical material choice starts with the operating environment.

This table is a starting point, not a material specification.

Two pumps of the same size can require different bearings if one handles clean reservoir water and the other carries suspended sand. The same applies to vessels, turbines and other water-lubricated machinery.

Where Are Water Lubricated Bearings Used?

Stern Tube and Propeller Shaft Bearings

A water lubricated stern tube bearing supports the propeller shaft while seawater or supplied water lubricates and cools the bearing.

These bearings have to deal with more than shaft rotation. Propeller loading, shaft alignment, vibration, seawater exposure and low-speed maneuvering all influence their performance.

The shaft sleeve is equally important. A good bearing material paired with an unsuitable or damaged sleeve can still result in rapid wear.

For this reason, stern tube bearing selection should consider the bearing and shaft sleeve as one tribological system rather than two independent components.

Rudder Bearings

Rudder bearings operate under a different lubrication regime.

The rudder stock moves slowly and oscillates through a limited angle instead of rotating continuously at high speed. A full hydrodynamic water film may therefore be more difficult to maintain.

High static load, shock, seawater exposure and boundary lubrication become more important considerations. Material elasticity and low-speed friction can be particularly relevant in this type of application.

Hydro Turbine Guide Bearings

Hydro turbine guide bearings control radial shaft movement and help maintain rotor alignment.

The bearing must operate reliably under continuous radial loading while dealing with the actual water quality at the station. Where river water contains significant sediment, abrasive wear can become one of the main design concerns.

Clearance, dimensional stability, shaft surface condition and vibration also deserve close attention.

Water Lubricated Pump Bearings

Water-lubricated bearings are common in vertical, horizontal, mixed-flow and axial-flow pumps.

Vertical pumps are particularly demanding because several guide bearings may support a long shaft line. Small errors in alignment can accumulate, while vibration and uneven loading may accelerate bearing wear.

Water quality also changes the problem significantly. A guide bearing working in clean water is not necessarily suitable for the same pump operating in sediment-heavy river water.

Which Water Lubricated Bearing Is Suitable for My Application?

The application provides the first clue, but the final decision should always be based on operating data.

Water Lubricated Bearing vs Oil Lubricated Bearing

Water lubrication and oil lubrication solve the same basic problem in different ways. Neither system is automatically better.

Water is attractive where machinery already operates in water and where eliminating lubricating oil from the bearing interface is desirable. Oil has much higher viscosity, however, and provides different film-forming characteristics.

The choice should therefore be based on equipment design, operating environment, load, speed and maintenance strategy rather than on one advantage alone.

Advantages of Water Lubricated Bearings

One of the clearest benefits is the elimination of lubricating oil at the bearing interface. This is especially useful in marine and water-handling equipment where oil leakage is undesirable.

Water also provides direct cooling. With a suitable bearing material, the same medium can both lubricate the sliding surfaces and remove frictional heat.

Polymer, elastomeric and ceramic bearing materials also provide corrosion-resistant options that are useful in freshwater and seawater environments.

Water lubrication can reduce some oil-related maintenance as well, although it should not be described as completely maintenance-free. Shaft condition, bearing clearance, alignment, wear and water supply still require attention.

Limitations Engineers Should Consider

The main challenge comes from water itself: its viscosity is low.

At normal operating speed, a properly designed bearing can develop a useful hydrodynamic film. At start-up or very low speed, that film becomes much weaker. The bearing material then plays a larger role in controlling friction and wear.

Sediment creates another challenge. Sand and silt can enter the bearing clearance, disturb the water film and act as abrasive particles.

Material water absorption, shaft alignment and running clearance also matter. An otherwise suitable material can perform poorly if the installed clearance does not account for its dimensional behavior in water.

Why Do Water Lubricated Bearings Fail?

Premature bearing wear is rarely caused by one isolated problem.

A better way to investigate a failure is to look at the complete system:

bearing material + shaft/sleeve + water + load + speed + clearance + alignment

The wear pattern itself often provides useful information. Localized wear may point toward alignment or load distribution, while widespread abrasive scoring can indicate contamination in the water.

How Does Sediment Affect Water Lubricated Bearings?

Sediment deserves special attention because it changes the lubrication mechanism.

Particles entering the bearing clearance can disturb the water film and create three-body abrasive wear between the bearing and shaft sleeve. The severity depends on particle size and concentration as well as bearing material, shaft hardness, clearance and water-groove design.

A harder bearing is not always the safest solution.

In some conditions, a more compliant material can accommodate particles and reduce damage to the mating shaft surface. This is one reason hRBR is used by Techemer for abrasive water applications such as sediment-laden pumps, hydropower equipment and dredging systems.

The final choice still depends on the actual sediment. "Dirty water" is not enough information for engineering selection; particle size, concentration and operating conditions are much more useful.

How to Select a Water Lubricated Bearing

A bearing drawing tells us its size. It does not tell us whether the bearing will work.

For a useful engineering evaluation, the following information should be considered.

1. Application

Start with the equipment: stern tube, rudder, hydro turbine, vertical pump, horizontal pump, dredger or another water-handling system.

2. Shaft and Bearing Dimensions

Shaft diameter, bearing ID, OD and length establish the basic geometry and projected bearing area.

3. Load

Radial and, where relevant, axial loads determine how heavily the bearing is working.

4. Shaft Speed

Speed strongly affects water-film formation. Low-speed and oscillating systems spend more time in mixed or boundary lubrication.

5. Water Conditions

Freshwater and seawater are only the beginning. Sediment concentration, particle size, temperature and chemical contamination can change material requirements.

6. Start-Stop Conditions

A continuously running pump and a machine that starts dozens of times per day may require different bearing characteristics.

Any expected dry-start period should also be specified.

7. Shaft or Sleeve Condition

Material, hardness, surface finish and corrosion resistance all influence bearing wear.

8. Running Clearance

There is no universal clearance for every water lubricated bearing. Diameter, material, water absorption, temperature, load, speed and installation method all affect the required value.

9. Water Grooves

Grooves distribute cooling and lubricating water and can help move contaminants through the bearing. At the same time, grooves reduce the available bearing contact area.

Their number, shape and location should therefore be chosen for the application.

10. Material

Only after these operating conditions are understood does material selection become meaningful.

Key Engineering Data for Bearing Selection

These factors interact. Increasing clearance, for example, changes more than fit; it also affects shaft support and water-film behavior. Bearing design should therefore be treated as a system rather than a checklist of independent values.

A Real Water Lubricated Bearing Application: Qinhuai New River Pump Station

Large pumping stations are a good example of why water-lubricated bearing design goes beyond material selection.

At the Qinhuai New River Water Conservancy Hub, Techemer supplied a complete water guide bearing set for an open vertical pump.

The project used Techemer's Dx hot-bonding process to integrate the bearing lining with its metal backing.

For a pump of this scale, material performance alone is not enough. Bonding quality, bearing geometry, shaft alignment, installation and water flow all contribute to the final result.

This is also why laboratory material data should not be used as a substitute for complete bearing design.

Techemer Water Lubricated Bearing Solutions

Techemer develops water-lubricated bearings for marine, hydropower, pump and severe water applications. Different material systems are used because operating conditions vary considerably.

TSTN is used where low friction, wear resistance, load capacity and start-stop performance need to be balanced.

PREM is suited to selected pump and marine shaft applications requiring stable load-bearing performance.

hRBR is aimed at sediment-laden and abrasive water, including pumps, hydropower and dredging applications.

Ceramic bearing systems are available for selected severe-service applications where wear, corrosion or chemical resistance becomes the dominant requirement.

The choice between them should be based on operating data rather than product preference.

What Should You Send to a Water Lubricated Bearing Manufacturer?

For a replacement bearing, the existing drawing is the best place to start. For a new design, operating data is equally important.

The more complete this information is, the easier it is to evaluate material, clearance, bearing structure and shaft compatibility.

Frequently Asked Questions

How does a water lubricated bearing work?

Shaft rotation draws water into the running clearance and can generate a hydrodynamic film that separates the shaft from the bearing surface. During start-up and low-speed operation, the bearing may temporarily operate under mixed or boundary lubrication.

Can a water lubricated bearing run dry?

Some materials can tolerate a limited dry start or temporary interruption in water supply. That does not mean they are suitable for continuous dry running. Dry-start capability should always be confirmed for the specific material and operating conditions.

Can water lubricated bearings work in seawater?

Yes. Water-lubricated stern tube, propeller shaft and rudder bearings are widely used in marine systems. Bearing material, shaft sleeve and surrounding components must be compatible with seawater.

What happens when the water contains sand?

Sand and other suspended particles can disturb the water film and cause abrasive wear. Material selection, shaft hardness, clearance and groove design become particularly important in sediment-laden water.

Why does a water lubricated bearing wear out quickly?

Common causes include insufficient water, sediment, misalignment, incorrect clearance, poor shaft condition, excessive load and unsuitable material. The wear pattern should be examined before replacing the bearing with the same design.

What clearance should a water lubricated bearing have?

There is no universal clearance. It depends on bearing diameter, material, temperature, water absorption, shaft speed, load and installation method. Clearance should be specified for the actual bearing and application.

What is the difference between water-lubricated and self-lubricating bearings?

Water-lubricated describes the lubrication method. Self-lubricating describes the friction characteristics of the bearing material. Some water-lubricated bearing materials also have self-lubricating properties, but the two terms are not interchangeable.

Is a water lubricated bearing better than an oil lubricated bearing?

It depends on the application. Water lubrication is well suited to many marine, hydropower and pump systems and eliminates lubricating oil at the bearing interface. Oil lubrication has different film-forming characteristics and remains appropriate for many other machines.

Need Help Choosing a Water Lubricated Bearing?

A bearing that fits the shaft is not necessarily a bearing that fits the application.

Load, speed, water quality, sediment, shaft material, running clearance and start-up conditions all affect how a water lubricated bearing performs.

If you are selecting a bearing for a stern tube, rudder, hydro turbine, vertical pump, horizontal pump or sediment-laden water system, send Techemer your existing drawing and operating conditions.

Our engineers can review the application and help determine the appropriate material, bearing structure, clearance and shaft pairing.