A guide to bonded washers
In this detailed resource, we’ll explain everything you need to know about these essential components, from their construction and purpose to their diverse applications across industries. Bonded washers are crucial for creating secure and leak-proof seals, especially for hydraulics systems, providing reliability and peace of mind to engineers and technicians worldwide. We’ll cover:
What is a bonded washer?
A bonded washer, also known as a Dowty bonded washer or Dowty seal, is a type of washer typically made of two parts:
● Outer ring: This forms the base, and is a hard, metal ring, usually made of steel.
● Inner ring: On the outer ring’s inner surface is placed an elastomeric sealing element. Nitrile rubber, known for its resistance to oil, fuel, and hydraulic fluids, is a common choice. An EPDM bonded washer is another option.
What is its Purpose?
A bonded washer is used to create a tight, leak-proof seal around a screw or bolt. They’re used when you need a stronger seal than what ordinary washers provide. Bonded washers excel at sealing across a wide pressure range (both high and low) due to their clever design that combines the strengths of two materials:
1. Metal washer for strength and support: The outer metal ring provides rigidity and distributes pressure evenly. This prevents the elastomeric ring from over-compressing or extruding (squeezing out) under high pressure. Compare this to a regular washer – under pressure, it could bend or deform, compromising the seal. The metal ring in a bonded washer acts like a strong foundation, ensuring the seal maintains its shape.
2. Elastomeric ring for conformability and sealing: The inner rubber ring, made from elastomeric materials like nitrile rubber, is what creates the actual seal. These materials are naturally flexible and can conform to slight imperfections in the mating surfaces, ensuring a tighter seal. They also recover well from compression, allowing them to function effectively at low pressure as well.
Here's how these two components work together:
● High pressure: The metal ring takes the brunt of the pressure, preventing the elastomeric ring from being crushed. The elastomeric ring, in turn, fills any small gaps and creates a tight seal.
● Low pressure: The elastomeric ring's natural elasticity allows it to maintain contact with the mating surfaces even at lower pressures, preventing leaks.
This combination of strength, support, and conformability makes bonded washers a reliable choice for sealing across a wide pressure range.
Learn more about other washers to ensure a bonded washer is what you need. Our experts have put together The Complete Guide to Washers.
Where are bonded washers used?
Bonded washers’ reliable sealing across various pressure, find applications in a range of industries. In some situations, you might also consider using finishing washers. Learn more in our Guide to Finishing Washers.
Automotive
Engines, transmissions, fuel systems, hydraulic systems, and braking systems all require secure seals to function properly. Bonded washers handle the pressure fluctuations, vibrations, and exposure to oils and fuels typical of these environments.
Hydraulics and Pneumatics
These systems rely on pressurised fluids or gases to operate. Bonded washers ensure leak-proof connections in components like pumps, valves, cylinders, and piping, preventing performance issues and potential safety hazards.
Plumbing
Reliable seals are crucial in preventing leaks throughout a plumbing system. Bonded washers are used in pipe connections, tap assemblies, and other components where water tightness is essential.
Machinery and Equipment
Many types of machinery and equipment utilise bonded washers in assemblies involving fluids or gases. This could be anything from industrial gearboxes to medical devices, where leaks could compromise functionality or even safety.
General Applications
Beyond these specific industries, bonded washers find use in various applications requiring secure, leak-proof connections. This includes electrical enclosures, appliances, and any situation where a strong and reliable seal is needed.
Benefits of using bonded washers
The advantages of bonded washers include:
● Superior Sealing Power: Bonded washers excel at creating a tight seal due to their unique design. The rigid metal ring distributes pressure evenly, preventing the elastomeric core from deforming under high pressure. This core, made from flexible rubber-like materials, conforms to imperfections in mating surfaces, ensuring a leak-proof connection. This effectiveness translates to reliable performance in demanding environments with minimal risk of leaks.
● Vibration Absorption: Traditional seals can be susceptible to loosening or failure under constant movement. Bonded washers address this issue with their built-in vibration dampening. The inner rubber ring acts like a shock absorber, mitigating vibrations that could compromise the seal. This enhances the washer's reliability in applications prone to movement and vibrations.
● Effortless Installation: Unlike some sealing methods that require specific positioning or tools, bonded washers are a breeze to install. Their design ensures they self-centre during installation, minimising the risk of user error and simplifying the process. This saves time and effort during assembly.
● Long-Term Cost Savings: While bonded washers might have a slightly higher initial cost compared to plain washers, their durability and effectiveness lead to significant savings over time. Their resistance to wear and tear allows them to function well for extended periods without needing frequent replacements. This reduces maintenance costs and translates to a cost-effective solution in the long run.
● Adaptable for Various Applications: The combination of reliable sealing and easy installation makes bonded washers perfect for a wide range of applications. The choice of metal and elastomer materials can be tailored to specific needs, allowing them to handle diverse pressure ranges, temperatures, chemicals, and fluids. This adaptability makes them a go-to sealing solution across numerous industries.
If electrics are involved in your application, consider if you need shoulder washers, particularly in scenarios with dynamic movement. Learn more in our Guide to Shoulder Washers.
Understanding bonded washer measurements
Our own bonded washers are available in metric and British Standard Pipe (BSP) sizes to cater to different bolt and thread standards. The main difference between metric and BSP sizes for bonded washers lies in their measurement system:
● Metric: Measured in millimetres (mm). The size refers to the nominal diameter of the bolt the washer is designed to fit. For example, an M12 bonded washer has a centre hole diameter to fit a 12mm bolt.
● BSP: Not directly equivalent to inches. BSP uses a thread standard with designations like 1/4" BSP or 1/2" BSP. These designations indicate the nominal pipe or bolt diameter the thread is designed for.
The Discrepancy Between BSP Thread Sizes and Ruler Measurements
The BSP thread system can be a source of confusion because the designated size (like "1/2 inch BSP") might not match what you measure with a ruler. For example, a 1/2 inch BSP thread might actually measure closer to 3/4 inch, as seen below:
This difference stems from the history of BSP sizing. Originally, BSP sizes were based on the inner diameter of the pipe the thread was designed to seal. Think of it as the size needed to fit snugly inside the pipe wall.
However, modern pipes often have thinner walls for efficiency. This means the outer diameter of the pipe, which the threads are machined onto, is larger than the original pipe size used to define the BSP designation.
Here's the catch: People sometimes mistakenly assume the BSP size refers to the outer diameter of the threads themselves. This is where the confusion arises. The BSP size reflects the historic inner diameter of the pipe, not the actual diameter of the modern thread.
Bonded washer size lists
Here's a summary of the key differences between metric and BSP sizes::
|
Feature |
Metric |
BSP |
|
Measurement system |
Millimetres |
Nominal bolt diameter |
|
Refers to |
Bolt diameter |
Thread compatibility |
|
Example |
M12 bonded washer (for 12mm bolt) |
1/2" BSP washer (designed to fit 1/2" thread) |
Important to remember: A "1/2 BSP" washer is specifically made to seal a threaded connection with a nominal diameter of 1/2 inch, but the washer itself won't necessarily measure exactly half an inch.
Metric: Our metric bonded washers are compatible with fasteners in these sizes:
|
Code |
Metric |
|
490080 |
M8 |
|
490081 |
M10 |
|
490082 |
M12 |
|
490083 |
M16 |
|
490084 |
M18 |
|
490085 |
M22 |
|
490086 |
M24 |
|
492253 |
M20 |
|
492254 |
M26 |
|
492255 |
M30 |
BSP: Choose a bonded washer with the same BSP designation as the bolt or pipe thread you’re using. Our sizes include:
|
Code |
BSP |
Bolt |
|
490070 |
1/8 |
3/8 |
|
490071 |
1/4 |
1/2 |
|
490072 |
3/8 |
|
|
490073 |
1/2 |
13/16 |
|
490074 |
5/8 |
7/8 |
|
490075 |
3/4 |
1 |
|
490076 |
1 |
15/16 |
|
492250 |
1 1/4 |
1 5/8 |
|
492251 |
1 1/2 |
1 7/8 |
|
492252 |
2 |
|
Try before you buy
Request free samples now to make sure you’ve chosen exactly what you need. If you have any questions about bonded washers, our experts are always happy to advise you.
Questions?
Email us at sales@essentracomponents.co.uk or speak to one of our experts for further information on the ideal solution for your application 0345 528 0474.