Being aware of the causes of seal failure and how to avoid them can extend seal life and lower overall cost of ownership. Highlighted in this article are common causes of seal failure and how they may be prevented.
Improper installation is probably the most common cause of seal failure. Using the right tools is critical to prevent seals from being installed in the wrong direction or becoming damaged during installation. Some seal materials are less robust than others, meaning they may be more easily damaged, which can lead to unexpected leakage.
If the cylinder design requires installing a seal in a difficult place, a specialized installation tool such as a lead-in chamfer should be used. It is also important to ensure there are no sharp corners in the seal grooves that could damage the seal.
Installing a seal in the wrong direction is another common cause of failure. Careful training is a potential fix, but if that is not sufficient, manufacturers should consider changing the design so the seal cannot be installed the wrong way or using a product that can only be installed one way. This is sometimes referred to as poka-yoke, the Japanese term for a mechanism that helps equipment operators avoid mistakes.
It is also important to realize that seals specified in an alternative material to the original may need to be installed differently even though they are fitted in the same piece of equipment.
Seals wear over time, but there are ways to prolong their life. Contamination is a common cause of wear. It may originate outside the system or be generated internally by moving gears or pumps. The contamination should be excluded using combinations of specialist seals.
Note that some hydraulic fluids are better at keeping contaminants away from sealed areas, so if seals are breaking down faster than expected, consider changing fluids.
In addition, higher quality bearings are less prone to being deformed under load, decreasing the type of metal-to-metal contact in the system that causes contamination from particles.
This type of seal failure involves behavior in high pressure levels that can potentially cause parts of a seal to break off and flow into the gaps in the hardware. Mitigation involves using backup rings or changing from an elastomer seal to a thermoplastic one. Redesigning the mechanism to make the gaps between moving parts smaller (minimizing the area a seal can extrude into) is another option.
Leakage at low pressure can be a sign of seals becoming less elastic, usually referred to as compression set. Rubber and polymer seals are more prone to this phenomenon. Seals are very dependent on their elasticity at low pressure, which is why low-pressure leakage is a good indicator of an issue. Workarounds include replacing the seal, increasing preventive maintenance, and switching to a material less susceptible to compression set.
Ideally, if two seals are next to each other, they should not seal in the same direction. However, this can occur under certain conditions, causing fluid to be trapped between the seals. Unchecked, the pressure will eventually rise to a level where the fluid bursts through and causes one of the seals to fail. The solution is a back-pumping design that allows fluid to be pumped back into the pressurized side of the system.
This type of issue is usually associated with O-rings, most often when a mechanism uses long strokes that are fairly fast. This often causes the O-ring to twist in upon itself. Designers can alleviate this problem by switching to a polymeric material or to a different geometry, such as a square or X-ring.
Failure due to chemical incompatibility is becoming more common as users of hydraulic systems change fluids to save money and make their process more fire resistant or less harmful to the environment.
It is often necessary to change seal types when switching fluids. Existing sealing compounds used may be incompatible with newer hydraulic fluids, especially fire-resistant and biodegradable ones at certain temperatures. Though the application may stay the same, a specialty material may be required to maintain seal life.
Another issue is changes in formulations of lubricants. Sometimes even though the base fluid remains the same, an additive causes seals to fail. The effect is not always predictable – the seal may wear out faster than expected or simply not perform as it should. That is why testing seal functionality is critical every time the hydraulic fluid is changed, even if the base fluid remains the same. Compatibility information is available from both seal and fluid suppliers, but testing with one’s own equipment at operating temperatures is highly recommended.
By now it should be clear seals can fail in many ways. But by paying attention to installation and testing, the odds that seals will function as expected can be greatly improved.
Larry Castleman is director of product development for Trelleborg Sealing Solutions Americas. Castleman has held a variety of engineering positions related to manufacturing, marketing and Research & Development in 25 years with Trelleborg Sealing Solutions.