Why Do Hydraulic Fittings Still Leak After Tightening?
If a hydraulic fitting still leaks after tightening, more torque may not solve the problem.
In connections that use a sealing washer, the seal forms where the washer meets the fitting surface. A worn washer, the wrong rubber compound, incorrect dimensions, or damage on the sealing face can all create a leak path.
Over-tightening can make things worse. Too much compression may deform the rubber or push it outside the area it is supposed to seal.
A bonded sealing washer, also known as a hydraulic dowty seal or bonded seal, combines a metal washer with a bonded rubber sealing ring. That structure gives the rubber enough compression to seal while the metal ring supports the tightening load.
Why Does a Hydraulic Fitting Still Leak After Tightening?
When a connection uses a bonded sealing washer, start by checking the washer and the surface it seals against.
Common causes include:
- Rubber that is cracked, hardened, worn, or permanently compressed
- A rubber compound that does not match the hydraulic fluid
- Incorrect washer ID, OD, or thickness
- A washer that does not sit correctly on the sealing face
- Scratches, corrosion, burrs, or dirt on the mating surface
- Rubber damaged by previous over-tightening
- Too little compression during installation
- Vibration, pressure changes, or repeated operation over time
The mating surface is easy to overlook. Even a new washer can leak if it sits against a scratched or uneven face.
Washer dimensions matter for the same reason. A part that looks close enough may not center properly, leaving the rubber outside the intended sealing area.
If a fitting keeps leaking after tightening, inspect both parts before replacing the washer with another part of the same size.
How Does a Hydraulic Dowty Seal Work?
A hydraulic dowty seal consists of a metal washer with a rubber sealing ring bonded to it.
When the fitting is tightened:
- The rubber ring compresses against the sealing face and blocks the fluid path.
- The metal ring carries the tightening load and controls how far the rubber is compressed.
The metal portion effectively acts as a compression stop. It keeps the rubber from being crushed or squeezed too far outside its working area.
That is the main difference between a standard washer and a bonded sealing washer.
| Feature | Standard Flat Washer | Bonded Sealing Washer |
|---|---|---|
| Distributes fastening load | Yes | Yes |
| Rubber sealing element | No | Yes |
| Provides fluid sealing | No | Yes |
| Controls rubber compression | No | Yes |
| Typical use | General fastening | Hydraulic fittings, plugs and fluid connections |
Put simply, a flat washer mainly handles fastening load. A bonded sealing washer adds a sealing function.
KCS provides Bonded® Washer solutions for applications that require rubber-to-metal bonding and fluid sealing.
Why Can Over-Tightening Make the Leak Worse?
A bonded washer needs compression to seal, but the rubber still has a working range.
Too much tightening can cause the rubber to:
- Deform beyond the sealing area
- Split or tear
- Be pushed outward from the metal ring
- Take a permanent set and lose its ability to recover
Under-tightening creates the opposite problem. Without enough compression, the rubber cannot fully contact the sealing surface or fill small irregularities.
If tightening does not stop the leak, inspect the washer and sealing face before applying more torque.
The fitting or equipment torque specification should remain the reference for installation.
Which Rubber Compound Should Be Used With Hydraulic Fluid?
Rubber compounds respond differently to hydraulic fluids, heat, and environmental exposure.
NBR, EPDM, HNBR, and FKM are used under different conditions, so the working fluid and temperature need to be known before the material is selected.
The table below is a starting reference. Final compatibility should be confirmed against the actual fluid and operating conditions.
| Fluid / Condition | Typical Material Direction | Check Carefully |
|---|---|---|
| Mineral hydraulic oil | NBR | EPDM |
| Phosphate ester fluid | EPDM, FKM | NBR |
| Water-glycol fluid | NBR, HNBR | Actual fluid formulation |
| High-temperature conditions | FKM | Standard NBR |
| Outdoor / weather exposure | EPDM | Standard NBR |
For example, asking only for an EPDM bonded washer does not give the supplier enough information for a hydraulic application.
A useful inquiry should include the working fluid, operating temperature, pressure condition, washer dimensions, metal material, and application environment.
For a high pressure sealing washer, pressure is not the only specification that matters. Rubber compound, washer dimensions, sealing surface, and installation conditions also need to match the application.
What Should Buyers Confirm Before Ordering?
Before moving into sampling or mass production, confirm these five areas with the supplier.
Washer Dimensions
Provide the required ID, OD, thickness, and fitting dimensions.
The washer has to sit and center properly around the connection. If the dimensions are off, compression may become uneven even when the nominal bolt or thread size looks correct.
Working Fluid and Rubber Compound
Name the actual oil or fluid instead of specifying only "for hydraulic use."
This gives the supplier enough information to evaluate the rubber compound against the application.
Metal Ring Material
Confirm the required metal according to the fitting, corrosion exposure, and operating environment.
Different applications may require different metal materials rather than one standard specification.
Dimensional and Bonding Consistency
One good sample is not enough for a production order.
ID, OD, thickness, rubber geometry, and rubber-to-metal bonding need to remain consistent from batch to batch. If the rubber separates or shifts from the metal ring, the sealing area changes with it.
Actual Installation Conditions
Pressure, vibration, temperature changes, outdoor exposure, and repeated servicing all affect the washer requirement.
For a non-standard fitting, send a drawing or physical sample instead of selecting from a size chart alone.
KCS Enterprise has focused on rubber-to-metal bonding, rubber sealing components, and bonded washer manufacturing since 1983. For customized requirements, KCS can evaluate the washer based on drawings, samples, dimensions, material requirements, and application conditions.
Where Are Hydraulic Bonded Sealing Washers Used?
Bonded sealing washers are commonly used where a fitting, bolt, or plug also needs fluid sealing.
Typical applications include:
- Hydraulic fittings and port plugs
- Banjo bolt connections
- Pumps and valves
- Compressors
- Oil and fluid line connections
- Industrial machinery
- Construction and agricultural equipment
- Automotive sealing points
- Engine and gearbox applications
The same basic washer structure can be used under very different working conditions. That is why size alone should not determine the final specification.
For non-standard dimensions or material requirements, KCS can evaluate a custom bonded washer from the actual fitting, drawing, or sample.
Need a Hydraulic Sealing Washer for Your Application?
If you are sourcing hydraulic dowty seals, hydraulic sealing washers, bonded sealing washers, rubber-bonded washers, high-pressure sealing washers, or custom bonded washers, send KCS your drawing, sample, dimensions, working fluid, material requirements, and operating conditions.
Contact KCS Enterprise to discuss the right Bonded® Washer for your application.
FAQ
What is the difference between a hydraulic dowty seal and a bonded sealing washer?
In common market usage, both terms usually refer to the same type of sealing component: a metal washer combined with a bonded rubber sealing element. You may also see the terms bonded seal, dowty washer, hydraulic sealing washer, and rubber bonded washer.
Why does a new bonded sealing washer still leak?
Check the sealing surface, washer dimensions, rubber material, and installation compression. Scratches, burrs, incorrect sizing, the wrong rubber compound, or over-tightening can cause leakage even when the washer itself is new.
Can I use an EPDM bonded washer with mineral hydraulic oil?
EPDM is generally not the preferred choice for mineral hydraulic oil. NBR is commonly considered for mineral-oil applications. Final material selection should be confirmed against the actual working fluid and operating temperature.
Can over-tightening damage a bonded sealing washer?
Yes. Excessive tightening can deform, split, or displace the rubber sealing element. Once the rubber moves outside the intended sealing area, additional torque can worsen the leakage.
Can bonded sealing washers be customized?
Yes. ID, OD, thickness, rubber compound, metal material, and other requirements can be evaluated for customization. For non-standard fittings, send KCS a drawing or sample so the washer can be matched to the actual sealing area.
