Rod seals on hydraulic cylinders may have the toughest job in fluid power. They have to contain fluid — often pressurized to several thousand psi — while letting the rod effortlessly move in and out. And they have to minimize friction, heat buildup, and wear despite fast-moving rods and extreme forces.
Engineers at major seal manufacturers have met this challenge with designs that harness pressure to prevent leakage and use advanced materials that resist abrasion, extrusion, harsh fluids, and temperature extremes.
As well-engineered as these seals are, however, they are only as good as the components they work with — in this case the cylinder rod. Too smooth and surfaces won’t hold a lubricant film, and seals overheat and fail. Too rough and seals quickly wear and tear apart.
Surface quality, therefore, plays a major role in proper sealing and dramatically affects seal life and performance. Engineers at Simrit, Elgin, Ill., have identified inspection parameters that accurately evaluate rod-surface finish in hydraulic applications and developed guidelines for manufacturers and users. These factors could lead to modernized standards for counter-surface preparation that extend the life of seals and cylinders.
Traditionally, sealmakers have relied on three factors to measure surface finish and define sealing requirements: average roughness, Ra, the maximum peak-to-valley roughness, Rmax., and average peak-to-valley roughness,Rz. Although easy to measure and based on industry standards, these traditional factors are insufficient to accurately evaluate surface quality. Here’s why:
• Ra does not provide the needed detail of a surface. Two surfaces with entirely different roughness profiles can have the same Ra value.
• Rmax is measured across a standard length, so it does not indicate a skew toward peaks or valleys.
• Rz uses measurements across five equal segments of standard length, so it can miss problem areas because averaging masks extremes.
Although these factors are important, they do not adequately classify surface abrasiveness and how well it holds lubricant. And as demands for better hydraulic performance, life, and warranty expectations continue to increase among users, manufacturers need more useful guidelines for good surfaces. To do this, more-extensive definitions of surface finishes are required. Here’s how.
New surface definition
Simrit engineers set out to further define surface roughness using the Abbott-Firestone curve because it provides detailed information about a surface over a defined length, along with a complete profile depth. (See the sidebar for more background.)
A straight line divides the Abbott-Firestone curve into three areas. Important parameters include:
• Rk, the core roughness depth.
• Rpk, the mean height of the peaks on the roughness core profile.
• Rvk, the mean depth of the valleys on the roughness core profile.
These help to clearly define the surface’s abrasiveness and lubricant-carrying ability.
Additional values include:
• Rpkx, the full peak height,
• Rvkx, the full valley depth.
These indicate the presence of abrasive peaks and excessively deep valleys.
Today’s digital and laser-inspection equipment can readily measure these parameters. This, in turn, lets engineers determine the material ratio, Mr (in essence, the ratio of material to empty space in the region of interest), which works best with a particular seal.
The “Surface parameters” graphic shows Mr1 and Mr2 are the smallest and largest material ratios of the roughness core profile, respectively. But neither, on its own, provides the material ratio of the bearing area (called the kernel), which indicates the structure of the profile a majority of the seal will run against.
And even with sophisticated measuring machines, evaluation of the Abbott-Firestone curve leaves room for interpretation. For instance, engineers can consider all the measured area, Cref, or elect to ignore the extreme peaks and valleys. In other words, calculations may or may not include all the measured area when determining the material ratio. The same holds for the cutoff depth used to determine the material ratio. With a bearing raceway, engineers might prefer a generally smooth surface and choose Rz/4 as the cutoff depth. Seals require valleys to hold lubricant, so in this case a cutoff depth of Rz/2 might be the preferred choice.
For example, in an automotive engine cylinder, initial peaks that remain after manufacture wear away quickly during break-in, and typically have no effect on long-term performance. In this case, peaks could safely be ignored, permitting an Mr1 at Cref = 5% and a cut-off depth of Rz/4.
For seals, however, Simrit recommends taking into account the full peak height, as this is critical to seal wear (Mr1 at Cref = 0%). In addition, the cutoff depth should be Rz/2, as it more accurately indicates the position of the kernel surface.
The examples (see the graphic, “Same profile, different interpretation”) show how to determine the material ratio, and how using different definitions can provide completely different values for Mr on the same surface.
Increasing the number of factors from three to nine more accurately defines surface texture and, as a result, can extend a seal’s life. The new technique might require more-advanced measuring equipment, but making the additional measurements takes no extra time.
The ultimate goal of developing these new testing parameters is to help engineers better understand — and anticipate — how seals will behave in a given application. For common shaft materials used in reciprocating hydraulic devices, we recommend the following limits on the parameters:
Ra = 0.05 to 0.30 µm
Rk = 0.25 to 0.70 µm
Rmax = 0.00 to 2.50 µm
Rpk = 0.00 to 0.50 µm
Rpkx = 0.00 to 0.50 µm
Rvk = 0.20 to 0.65 µm
Rvkx = 0.20 to 2.00 µm
Mr = 50 to 90% at Cref = 0% and Rz/2 cutoff depth.
In-house testing shows that following these new parameters reduces part wear and extends system life. The concept can also be applied to other sealing systems, although the values will change depending upon the specific application — for example, rotating rather than reciprocating motion
The Abbott-Firestone curve, also called a bearing-area curve, describes an object’s surface texture. Surface profiles resemble white noise, so they can be analyzed using statistical methods.
In 1933, E.J. Abbott and F.A. Firestone introduced statistical methods to surface topography. According to Stachowiak and Batchelor (Engineering Tribology, Elsivier Science Publishers, 1993), they proposed a bearing-area curve to represent the surface profile.
It is constructed by plotting the measured surface height over a length, then passing an infinitesimally thin plane — parallel to the base — through the surface, starting at the highest peak and ending at the lowest valley. The length of intersections with the material along the plane is measured, summed, and plotted as a proportion of total length; and the procedure is repeated through a number of slices. In other words, it plots the bearing area at different heights above the object’s base.
Integrating the height distribution (assuming it is Gaussian) results in a classical cumulative probability density function of the surface profi le’s height. The curve starts from the highest projection, with 0% of the material, and ends at the lowest depression, where it includes 100% of the material.
The Abbott-Firestone curve is a good tool for assessing the functional properties of surfaces because it can distinguish between diff erent surfaces with the same value of Ra or other height characteristics.