The sealing package in a hydraulic cylinder is comprised of three components: the rod-seal, the back-up rings and the wiper. All three of these parts must work soundly together in order to provide leak-free, high-performance. With these different components, there are a number of potential problems that may occur. Focusing specifically on possible causes of rod leakage in a hydraulic cylinder applications, there are five critical areas of focus:
- Damaged rod
- Damaged seal
- Improper rod surface finish
- Incorrect gland dimensions
- Over-aggressive wiper
Wiper aggressiveness is one potential hazard that is often overlooked. The reasonable mind would think that the more aggressive the wiper, the better it is doing its job. This seems to make sense, however studies on mechanisms of cylinder leakage prove that the design features that cause wipers to be too aggressive are actually the reason for leakage. This is best explained by Parker’s Engineered Polymer Systems Division Application Engineer, Jeff Olsen:
“In a rod sealing system, such as that shown in [the image above], it may not be immediately clear how a wiper could be the cause of leakage. The answer has to do with a very thin oil film that the rod seal leaves on the rod as it cycles. This thin oil film provides beneficial lubrication at the dynamic interface between the sealing components and the rod. The thickness of the film is typically on the order of only a few microns; thin enough to be undetectable by sight or touch. The magnitude of this film is a function of the aggressiveness (sealing force) of the rod seal, rod surface finish, oil viscosity, and rod speed.”
Achieving a balance of sufficient sealing force to prevent contamination ingression with just the right amount of hydrodynamic oil film is not always an easy task. Yet, this balance is critical in producing a high-performance, leak-free cylinder.
Testing and results
In a controlled environment, Parker engineers tested multiple design solutions against a basic wiper design (a). They noted the following modifications:
- A radius at the leading edge of the wiper lip (b)
- Decreasing the wiper lip interference with the rod (c)
- Making the lip thinner and more flexible (d)
Parker engineers used axi-symmetric FEA models to examine and compare the force footprints of each of these concepts. The results were intriguing. The first design, with a rounded lip, created a concentrated force near the tip of the lip with an even more aggressive force gradient. The second design, a trimmed version of the baseline wiper lip, had less interference but the FEA model actually calculated a higher contact force and stress gradient. Lastly, the third design, which had a longer and thinner lip, was able to pass all tests with no rod wiper leakage. The thin, flexible, sharp lip-wiper allowed the microscopic oil film to remain after the retract stroke, but was aggressive enough to prevent contamination ingression and leakage.
Optimizing the rod wiper design for a given rod sealing package is a key factor to producing a leak-free hydraulic cylinder.
For a more in-depth look on the testing of each wiper design, contact the experts at Parker Engineered Polymer Systems Division.
Original article written by Jeff Olsen, Senior Application Engineer, Engineered Polymer Systems Division, Parker Hannifin Corporation.