WHY PROPELLER SHAFT BEARINGS USE NBR RUBBER – AND WHY ORDINARY RUBBER IS NOT ENOUGH

Open almost any catalogue for water-lubricated propeller shaft bearings and you will see the phrase “NBR rubber bearing”.
That is not a marketing slogan – over time, NBR has become the de-facto standard material for marine shaft bearings.

So what exactly is NBR? How is it different from “normal” rubber, and why is it the preferred choice inside the stern tube?

What is NBR and how does it differ from regular rubber?

When people say “normal rubber” they usually mean natural rubber (NR) or general-purpose synthetic rubbers used in tyres, mounts and simple seals.

NBR – Nitrile Butadiene Rubber – is a synthetic rubber made from butadiene and acrylonitrile.
Thanks to this chemistry, NBR offers:

• Much better resistance to oil, grease and fuel than NR
• Good mechanical strength and stability in water and salt solutions
• A wide range of adjustable hardness and elasticity, ideal for engineered parts

For shaft bearings, NBR compounds are further filled and reinforced to:

• Improve abrasion resistance
• Withstand seawater, sand and mud
• Keep their elasticity after long service

The shaft bearing’s job: a much harsher life than most rubber parts

A stern-tube bearing has to survive conditions that are far tougher than most rubber components ever see:

• It is continuously submerged in seawater or brackish river water with salt, mud and sand.
• It carries the full weight of the shaft and propeller, with load changing as speed, sea state and cargo change.
• Lubrication is by water only, so the fluid film is thin and quickly drops into boundary lubrication if conditions are not ideal.
• Heat generated at the contact zone is hard to remove if the water supply is restricted.
• Hard particles – sand grains, small stones, shells, marine growth – can get trapped in the bearing grooves.

Put a soft, low-grade or oil-sensitive rubber into this environment and the result is predictable:

• The bearing quickly becomes pitted and flaky.
• The rubber loses elasticity, hardens and stops damping vibration.
• Cracks and tears appear when shock loads occur.

In short, the bearing needs a material that is mechanically strong, abrasion-resistant and chemically stable in this environment – which is exactly what NBR provides.

Why NBR outperforms ordinary rubber in shaft bearings

Better abrasion resistance in sandy, muddy water

Seawater around the propeller is rarely clean.
Every minute, hard particles pass through the bearing with the cooling water: sand, silt, shell fragments.

NBR’s high tear strength and abrasion resistance mean:

• the grooves are less deeply scratched;
• a properly vulcanised NBR surface wears evenly instead of breaking off in chunks;
• shaft-to-bearing clearance stays within a usable range for longer.

That translates directly into longer bearing life and less scoring on the shaft surface.

The right mix of stiffness and elasticity

For a shaft bearing, rubber that is:

• too soft will deform, letting the shaft wobble, accelerating wear and misalignment;
• too hard will transmit vibration and noise like a rigid plastic or bronze bush.

NBR can be formulated in the 60–80 Shore A range – a sweet spot for marine bearings:

• soft enough to absorb vibration and knocking, protecting the hull structure;
• stiff enough to carry shaft and propeller loads without excessive indentation.

Other rubbers at similar hardness often become much less elastic and more brittle than NBR, which leads to cracking.

Stable in seawater, oils and normal operating temperatures

Even though the bearing is water-lubricated, the stern-tube area still sees:

• oil and grease from the sealing system,
• heat coming from the rotating shaft,
• seawater containing salt and anti-fouling / anti-corrosion chemicals.

Here NBR has clear advantages:

• it swells far less in oil than natural rubber;
• typical NBR grades comfortably cover −20 to 80–90°C, which spans the operating range of most marine shaft lines;
• it is less prone to blistering or turning soft and mushy when exposed to oil-contaminated water.

Conventional rubbers, by contrast, tend to swell, soften and lose strength in oil, and their surfaces easily blister or crack when locally overheated.

Manufacturing-friendly for precision bearing designs

Marine shaft bearings are not simple rubber tubes. They have:

• complex water groove patterns,
• tight tolerances on concentricity, roundness and surface finish,
• rubber layers that must be uniform in thickness around the circumference.

NBR’s processing behaviour and vulcanisation stability make it well suited to:

• moulding or hot-bonding into metal or composite shells,
• machining grooves after curing (turning, milling, grinding),
• repeatable, mass-production with small dimensional tolerances.

That makes it easier for manufacturers to guarantee that the finished bearing will match the drawing and run smoothly once installed.

Balanced cost and lifecycle performance

Compared with premium high-temperature or special oil-resistant elastomers, NBR offers:

• a much better price/performance ratio,
• easy availability and supply,
• a long track record in marine applications.

Compared with cheap general-purpose rubber, NBR may cost a little more per kilogram, but:

• it survives many more operating hours, reducing replacement frequency;
• it greatly reduces the chance of shaft and seal damage, which are expensive to fix.

Once you include dock time, pulling the shaft, machining and re-assembly, NBR bearings are almost always cheaper over the life of the vessel than “saving” a bit of money with low-grade rubber.

What happens when you don’t use NBR?

Using non-optimised rubber in a stern-tube bearing often leads to:

• rapid wear and deep grooving, with chunks of rubber missing;
• rubber that hardens over time and no longer damps vibration;
• severe swelling and softening in the presence of oil and grease;
• a real risk of bearing burn-out and shaft seizure when lubrication is poor.

The resulting repair bills and lost operating days quickly wipe out any initial cost saving.

Propeller shaft bearings operate in one of the harshest environments on the vessel.
In that environment, NBR stands out by combining:

• high abrasion resistance,
• the right balance between stiffness and elasticity,
• good stability in seawater, oil and temperature,
• and good processability at a reasonable cost.

That combination is why most marine bearing manufacturers have standardised on NBR – to extend bearing life, protect the shaft and cut operating costs throughout the vessel’s life.