Perhaps the most famous O-ring failure in history is that of the Challenger space shuttle, which ended the lives of seven NASA astronauts in January 1986. In the shuttle’s right rocket booster, a seal failed at the aft field joint that led to the spacecraft exploding. It was later found that the O-ring didn’t have the capability to perform correctly at lower temperatures, so it failed to seal fully.
Investigators theorized that cold temperatures at launch time and frozen water within the rocket motor’s joints resulted in O-rings that didn’t return to their correct shapes before launch. This O-ring failure resulted in the joint along the right rocket booster failing as heated; combustible gases caused blow-by and erosion. The resulting domino effect led to the Challenger’s explosion 28 seconds after launch.
While O-ring failures don’t normally result in loss of life, their failure can often damage machinery and cause production delays. Knowing how to improve an O-ring seal will also make an application safer for everyone involved. For those dealing with machinery or equipment that relies on O-rings to operate properly, it’s important to understand what makes them seal so they won’t fail unexpectedly.
Why Seals Fail
Before learning how to make an O-ring seal better, it’s important to understand why seals fail in the first place. Generally, seals stop working because they fail or wear out. Often preventative maintenance programs that utilize regular inspections will prevent failures by replacing old seals before they completely wear out and fail. However, sometimes seals fail for reasons other than natural wear and tear.
The primary culprit for seal failure is human error emanating from using the wrong seal for the application, as is what evidently happened to the Challenger space shuttle. This is especially true for seals used to join components. That’s why it’s always important to use seals intended for use with a specific component. However, many other things can cause seal failure. Understanding the symptoms and what to look for can help those tasked with correcting such problems.
The more common types of seal failure, along with ways to fix or prevent them, include:
Abrasion: Signs of abrasion include scrapes or loose particles on the surface of the seal. While this could result from high temperatures, poor surface finish, or a rough sealing surface, these also indicate abrasive particles. To fix or prevent:
- Lube seal internally
- Remove abrasive materials from production environment
- Use gland surface finish recommended by manufacturer
Chemical degradation: When material becomes discolored, blisters or cracks prior to a seal failure, it’s likely due to chemicals degrading the material. The seal’s physical properties may also be seen to degrade. To fix or prevent:
- Ensure the seal’s material is compatible with chemicals in the environment
- Utilize an O-ring made from an elastomer that’s resistant to these chemicals.
Contamination: Foreign material on the seal’s surface cross-section indicates some type of contamination. This may result from conditions within the facility or degradation due to a chemical reaction. To fix or prevent:
- Use the correct seal material
Excess compression: Signs include a compressed seal occurring within a mated surface area, though alternatively, this could result from an improperly cured elastomer within the installed equipment or high temperatures. To fix or prevent:
- Confirm compatibility with material
- Ensure the correct design is being used
- Use only elastomers for O-rings that set at lower compression
Extrusion: When a seal’s edges become ragged or tattered along the low-pressure side, this is a warning of imminent failure. Though eroded gland edges, excessive clearances, high pressures, low modulus, too much elasticity, wrongly sized seal,s or other reasons could be the cause, it’s usually due to extrusion. To fix or prevent:
- Minimize clearances and pressure
- Use seals with elastomers that feature a greater modulus
- Use standby O-rings made from polymers
Heat degradation: When a seal shows radial cracks or its surface material becomes shiny and softens, the seal’s elastomers are likely not compatible with the system’s thermal requirements. To fix or prevent:
- Assess whether the seal’s surface can be cooled
- Select elastomer with sufficient thermal stability for seal
Improper installation: Damage occurring due to improperly installing a seal, or using the wrong type or size, can lead to seal failure. Indications like cuts, gashes or nicks on seal parts are indicative of this issue. To fix or prevent:
- Certify modulus is correct
- Clean surfaces and components of residual material
- Ensure the seal has the proper gland design
- Make sure the seal fits
- Remove any sharp edges
- Use the correct elastomer
Over-compression: When the surface of a seal develops circumferential splits with compressed surfaces, or if it becomes completely flattened, it’s likely to fail. Though this could result from improper design, environmental chemicals, changes in thermal volume or the selected seal doesn’t have properties necessary to withstand the compression, the problem is likelier due to over-compression. To fix or prevent:
- Confirm seal is sufficiently durable
- Ensure seal’s material properties are compatible with production environment
- Make certain seal meets any compression requirements
Now that we understand some common reasons why a seal might fail, let’s look at how to improve an O-ring seal.
Ways to Make an O-ring Seal Better
O-rings are used in various industries, including aerospace, agriculture, plumbing, and transportation, to name but a few. These common ring-shaped seals fit into couplings, engines, pipes, pumps, shafts, and valves in various mechanical equipment. They’re used to prevent fuel, lubricants, oils, refrigerants, steam, water, and other liquids or gases from escaping. Understanding how to make an O-ring seal better starts with the installation process and, when done correctly, saves businesses time and money on maintenance and repair.
Many seal failures result from improper O-ring installation. To function properly, they must be free of foreign material and fitted correctly without forcing, while lubrication and added tape covering help extend the O-ring’s life. Correctly installed O-rings prevent leaks and lengthen the life of other components, whereas when poorly installed, they lead to damaged equipment and downtime, as well as additional maintenance and repair. To properly install an O-ring and ensure its proper functioning, a few guidelines should be followed.
Guidelines for how to make an O-ring seal better:
- Don’t stretch an O-ring excessively to prevent snapping or tearing once assembled.
- Ensure the O-ring is correctly sized; if it’s too small, the O-ring will break or tear, while if it’s too big it won’t provide an adequate seal.
- Mate O-rings with threaded parts, avoiding tears and nicks by wrapping masking tape around threads and lubricating O-rings so they slide easily into place.
- Slide O-rings onto shafts rather than rolling them, which will cause them to spiral and limit their functionality.
- Thinly coat O-ring surfaces with lubricant to increase functionality and extend its operational life, filling spaces and gaps where mated parts join with the O-ring.
- Care should be taken during installation when working around corners and sharp edges to avoid tearing or nicking while lubricating the O-ring will help it slide more easily into place.
As per many of these guidelines, lubrication is particularly useful in improving O-rings seal.
Using Lubricant to Make O-rings Seal Better
To properly function, O-rings must be lubricated. Not only will lubrication help the seal last longer, but it will also prevent breakdowns of machinery, motors, engines, or other devices the seal is meant to protect should it fail. Applying oil or grease will protect the O-ring from damage when used. Lubricants also protect components by minimizing friction, providing water resistance, or preventing environmental degradation. Lubricating O-rings also reduces the chance of leaking seals in vacuum or pneumatic applications.
There are three main methods for lubricating an O-ring:
- Applying lubricant directly and evenly on the O-ring with a brush or fingers.
- Dipping an O-ring directly into a lubricant-filled container.
- Dispensing lubricant directly into the O-ring’s packaging and spreading it evenly.
Various lubricants can be used when installing O-rings, with solvents, soap, water, polymers, petroleum distillates, and ester-based synthetic oils being the most commonly used. Some of these lubricants have led to concerns regarding safety and health, however, while certain lubricants can also damage rubber O-rings. Solvents contain volatile organic compounds (VOCs) that pose risks to human health while also posing a fire risk. Though soap and water are safer, their lubricating properties are inconsistent, and these solutions don’t work well in wet or damp environments. Meanwhile, any petroleum-based lubricants cause elastomeric rubbers to dry out or swell.
Since most O-rings and seals are made from synthetic rubber-like material such as ethylene propylene diene monomer (EPDM) rubber, neoprene, or nitrile, they’re best lubricated with other synthetic compounds. Ester-based and silicone-based lubricants top this list. Their properties include reducing friction, low volatility, stability in high temperatures and fluidity in lower temperatures, and waterproofing. Both synthetic lubricants are safe, posing neither risk of contamination nor health, and are also usable with products containing natural rubber.
As O-rings are used so widely, it’s important to be installed and maintained properly to function as designed. This includes ensuring they remain free of foreign substances and that they’re properly lubricated. By ensuring proper installation and maintenance, O-rings will continue to play a vital role in the operation and fabrication of appliances, aircraft, automobiles, engines, machinery, medical devices, motors, plumbing, pumps, and trucks, along with many other modern devices and equipment on which modern civilization depends.