In addition to fittings and connectors, the piston rod on a hydraulic cylinder can also contribute to leakage. A typical piston rod assembly in a cylinder consists of a primary seal, a backup washer to protect the seal from extrusion, and a wiper. All three of these components need to work together to prevent leakage. If any one doesn’t do its job, leakage will result.

A piston rod must to carry a thin film of hydraulic fluid on its surface as it extends for lubrication to reduce friction. Just how much fluid depends upon a variety of factors, but could be crucial in a zero-leak application. If the film is too thick, the wiper will shave off enough oil as the rod retracts to form a drop of oil.

A relatively simple option to decrease the likelihood of hydraulic leakage is the use of multiple glands. A venting hole or leakage oil line must be arranged between the glands for the oil to be returned to the cylinder. Another option is a dual-action scraper on the cylinder rod. While these solutions increase the intial cost of components, the cost advantages of longer service life and a leak-free hydraulic system greatly offset the initial cost differential.

Unclogged arteries

When selecting hose for your hydraulic system, it is imperative to pay attention to details. A hydraulic hose consists of an inner tube that carries the fluid, a reinforcement layer, and a protective outer layer.

Common factors contributing to hose failure (and ultimately leakage) include:

• flexing the hose to a radius smaller than the recommended minimum
• operating the system at too high a temperature or pressure
• intermixing hose and end fittings from different manufacturers, and
• twisting, pulling, or kinking the hose.

The inner tube must have some flexibility and needs to be compatible with the type of fluid it will carry. One detail you cannot overlook is that the fluid may be compatible with the hose, but the additives in the fluid may not. Additives in hydraulic fluid can extend the life of the fluid, help reduce the effects of corrosion, and even enhance the fluid’s performance. However, they can also affect hose life and seal life if the additives are not compatible with the hose or seal material.

A challenge issued

Manufacturers of hydraulic components have risen to the challenge and are now offering products that use zero-leak technology. For hydraulics to survive against other forms of power transmission, distributors and end users must follow suit. By using common sense, following fitting, seal, and hose specification guidelines set forth by new ISO standards, and adhering to proper installation instructions, you can ensure your fluid power system will operate efficiently and leak-free for years.

Causes of seal failure
Contamination 21.4%
Installation damage 14.3%
Extrusion 10.8%
Wear and abrasion 9.6%
Cold start-up 6.8%
Compression set 5.3%
Excessive heat 4.7%
Fluid incompatibility 4.3%
Surface finish 3.9%
Aging, weathering, or both 3.2%
Dithering 2.9%
Gland overfill 2.4%
Other* 10.4%

*Other causes include low temperatures, air entrainment, stability, dieseling, fluid jet erosion, and stress fatigue cracking.

Failure analysis finds sealing solutions
Leakage control depends more on sealing devices than any other component in a hydraulic system. That’s why the analysis of seals and sealing systems at the initial design stage is critical. Equally important to a new design is the evaluation of any sealing device or system after it has either completed its life cycle or failed prematurely by allowing leakage.

Since 1991, we have conducted more than 1000 seal failure analysis projects. These represent sealing devices submitted by a wide variety of customers over a range of industries including those from 23 other manufacturers. A summary of the data compiled from these projects reveals the most frequent causes of leakage -- seal failure. The table shows the causes and their frequency of occurrence from the data.

All of these types of failures are preventable! Steps to prevent these leaks must occur during the initial design and, of course, during manufacturing/ assembly of the machine. In the design phase, the engineer must realize that no two applications are exactly the same. Therefore, all design and application parameters must be identified and evaluated for each seal within a sealing system. At this stage an ounce of prevention is worth gallons of cure. Too often designers are tempted to take short cuts in this vital process. Don’t do it! Instead, pay close attention to application requirements, operating environment, and operating expectations. Each of these major categories is further broken down into steps that must be considered in detail to arrive at the proper seal selection.

In manufacturing and assembly, clean and careful are the two most important guidelines. As can be seen in the chart, contamination and installation damage represent over onethird of the failures evaluated. Proper design and clean components manufactured to specification and assembled carefully using proper tools and techniques are the ingredients that produce a hydraulically powered machine with no leaks.

Submitted by E. Wayne Walden, director of engineering and P. Dennis Koontz, vice president, HPS, Inc.- Tech Center, Chambersburg, Pa.