FEATURE

8Sealing Technology December 2006

IntroductionTo ensure successful long-term operation of a hydraulic sealing system, the two main components – seal and counterface – should be manufactured for maximum compatibility (Figure 1). In a hydraulic cylinder the seals are typically manufactured from PTFE, polyurethane or an elastomer. The counter surface, such as a piston rod, is made of coated or hardened steel.

Field reports have shown that the initial contact between counter surface and sealing ring is crucial to the life that may be achieved from a sealing sys-tem. During the first few hours of operation, direct contact between the seal and rod influences both components. The counter surface becomes less abrasive, and the expected lifetime of the initial seal is significantly less than subsequent seals fitted to the same rod. In this case the first seal has super finished the counter surface by removing the abra-siveness.

On the other side the sealing edge could be dam-aged within the first few strokes by the abrasiveness of the counter surface. This can cause a reduction of the sealing function or lifetime compared to a subsequent seal. In statistical terms, the abrasiveness of the counter surface during initial operation can cause changes in the performance of the seal with respect to leakage and lifetime. Hence an improved rod surface will extend the operating time.

An analysis of the character of seal counterfaces has been undertaken, and both familiar and some additional parameters have been studied in detail. In particular, additional parameters from the Rk group have been investigated.

Common recommendations for counter surface parametersThe parameters currently used to specify a rod surface for sealing applications are limited to

Rmax, Rz, Ra and Mr, with an example of recom-mendations shown in Table 1. The effectiveness of these parameters is well known and established in the market.

It is possible to use different definitions of Mr, as it is possible to use either the baseline Cref or

to definite the cutting depth. This difference in selecting the baseline can be crucial, because Cref = 0% takes into account single peaks in the counter surface (Figure 2). These peaks could have an initial effect on the soft-hard contact between the seal ring and counter surface of a piston. However, Cref = 5% ignores these peaks, and this parameter is recommended for a hard-hard contact as the peaks are deformed by the very first contact.

For this investigation sealing systems which showed a wide range in lifetime were exam-ined. The wide range occurred although the counter surface parameters were within the

Counter surfaces of hydraulic seals for heavy-duty applicationsBy Dipl.-Ing. Frank Steep and Dr.-Ing. Guido Wüstenhagen – Merkel Freudenberg Fluidtechnic GmbH, Hamburg, Germany

A detailed investigation of the effect of piston rod surface texture on seal per-formance has been carried out. This has shown that the use of Rk parameters to control the presence of abrasive peaks on the surface can provide reduced wear and improved seal life.

Rmax Ra Material proportion Mr

Sliding surface ≤2.5 µm 0.05–0.3 µm

(Optimal Ra = 0.2µm)

Mr > 50% to max. 90% for cutoff c = Rz/2 and reference line Cref = 0%

(Optimal Mr = 80%)

Ground of the groove

≤6.3 µm ≤1.6 µm

Sides of groove ≤15 µm ≤3 µm

Table 1. Examples of recommended counterface surface parameters from a seal manufacturer’s catalogue.

Figure 1. The contact between seal and rod as a tribological system.

FEATURE

9Sealing Technology December 2006

recommended limits of Ra, Rmax and Mr. Initially the reasons for the variation were ana-lyzed by studying additional parameters, par-ticularly the Abbott graph and the Rk group (Figure 3). The surfaces showed almost simi-lar values, except for wide variations in the Rpkx parameter, highlighting the importance of this parameter. The surfaces analyzed com-plied with current recommendations, but seal life varied significantly.

Additional surface parametersFigure 4 contains proposals for additional sur-face parameters that should be used in con-junction with the current parameters. The Rk value is an excellent indicator for the general character of a counter surface quality. The Rpk and Rpkx values indicate the initial abra-siveness. The values have been defined to an uncritical limit regarding abrasiveness. In par-ticular, Rpkx will change during the first hours of operation. The valley parameters Rvk and Rvkx indicate the ability to build up a dynamic lubricant film. In particular, Rvkx is of minor interest, and the recommended values have been set to a quite high value. However, Rvk is of major interest, and has been recommended to achievable values. The parameters of the Rk group are real values and not mean values.

Practical experienceThese parameter recommendations have been implemented in partnership with OEMs on uncoated hardened rods, with benefits to both overall seal life and a reduction in the variabil-ity of seal life.

Test rig investigations have also been car-ried out. To compress testing time, the condi-tions have been more arduous than are used in practice. The test arrangement used a single seal without guide ring and permanent pres-sure. This arrangement removed any potential influence to the counter surfaces by the guide rings.

To prove the influence of parameters such as Rpk and Rpkx on the wear of the seal and vice versa, the first test surfaces had values outside the recommendations (Figure 5). The surfaces were measured before testing, after the first test and then after the second test.

Seal wear and Rpkx changed significantly, but the values of the other parameters remained constant. This indicates the interdependency of wear and Rpkx. Even if Rpkx shows only a small number of peaks in the surface, it could be a strong hint with regard to the general appearance of the surface. Taking into consid-eration that the area of a surface measurement covers only a minor part of the full surface, a few Rpkx peaks might be a small proportion of many on the full surface area of the rod.

To help prove the connections between wear and Rpkx, an additional rod with surface

Figure 2. The calculation of Material Ratio (Mr) by Cref = 0% (follow the numbering 1 to 5).

Figure 4. A proposal for recommended surface parameters, including both those that have been in use for some time and the new peak parameters.

Figure 3. The derivation of additional surface parameters of the Rk group.

FEATURE

10Sealing Technology December 2006

Standards Update

ISO valve emission standards are publishedBy David Harrison – Harrison Engineering Consultants Ltd, UK

A feature in Sealing Technology, February 2004, discussed valve fugitive emissions standards. ISO 15848 has recently been issued, but what support will it receive from industry?

Part 1 of ISO 15848, which covers ‘Classification system and quali-fication procedures for type testing of valves’, was issued earlier this year. The US voted against Part 1. Part 2 of ISO 15848, which covers ‘Production acceptance tests of valves’, was issued in October 2006. The UK voted against Part 2, on the basis that production tests should be carried out at the working pressure of the valve rather than the 6 bar specified by the Standard.

Within the user industry Shell has continued to be active in qualify-ing valves to its MESC SPE 77/312, which is based on ISO 15848.

BP has recently released valve standards, which for low emission appli-cations require that, ‘Evidence shall be provided that the proposed arrangement has demonstrated low emission performance, e.g. by type testing to ISO 15848 Class C, ISA 93.00.01 with not more than one packing adjustment, or VDI 2440.’ Some engineering contractors are now calling for valves to meet ISO 15848 requirements in their valve specifications.

A number of valve manufacturers have experienced difficulty qualify-ing valves to ISO 15848, in particular high-pressure valves using lip type energized PTFE seals, when trying to meet Class B requirements. There is evidence that the permeability of the PTFE seal alone may exceed the Class B emission requirement at high pressures. The current ISO stan-dard leak rates are not dependent on pressure, and this area may need to be revisited in a revision of the Standard.

Overall, it is early days in the application of the Standard. CETIM reports that it is active in ISO and Shell valve tests for fugitive emissions, while some valve manufacturers have reported limited requirements, to date, for valves meeting the ISO Standard. The next year or two will be interesting in seeing how widely ISO 15848 is adopted by the industry.

Contact: David Harrison, Harrison Engineering Consultants Ltd, London, UK. Tel: +44 20 8941 2525, Email: dave.harrison.eng@ukgateway.net

parameters within the common parameters, but outside the new Rpkx recommendations, was tested. During the first run more wear occurred than during the second; the surface had changed from the first run to the second run. After the first run, all parameters, includ-ing Rpkx, were within the recommended values. During the second run none of the parameters changed. Further testing is required to con-firm these results.

SummaryHere Rpkx has been shown to be a strong indi-cator for the wear and lifetime of a hydrau-lic seal. A proposal for additional parameters from the Rk group has been put forward for

application on hardened steel surfaces. This proposal should increase the performance of sealing systems by providing both longer and more consistent seal life.

Contact: Dipl.-Ing. Frank Steep, Merkel Freudenberg Fluidtechnic GmbH, Industriestrasse 64, D-21107 Hamburg, Germany. Tel: +49 40 7530 6409, Fax: +49 40 7530 66409, Email: Frank.Steep@freudenberg.de, Web: www.freudenberg-ds.de

This feature is an edited and updated version of a paper presented at the 14th International Sealing Conference (ISC) in Stuttgart, Germany on 10–11 October 2006. The ISC was organized by the

Fluid Power Association of the VDMA, the German Engineering Federation. For more information or copies of the proceedings (price 140), contact: Fachverband Fluidtechnik im VDMA, Lyoner Straße 18, D-60528 Frankfurt am Main, Germany. Tel: +49 69 6603 1318, Fax: +49 69 6603 2318, Email: ralf.stemmjack@vdma.org, Web: www.sealing-conference.com

Editor’s comment: This paper, which provides some interesting and useful information, was one of two that discussed this subject area at the conference. Many of the detailed peak and valley parameters discussed have been developed by the surface texture equipment manufacturers and the motor industry, for the specification of surface texture for engine cylinder bores, to pro-vide rapid run-in and minimum overall wear. It appears that this additional level of detail can offer similar benefits for seals once all the pro-viders in the supply-chain are equipped to mea-sure the parameters correctly.

It also helps to explain something that puzzled us during work on surface texture some time ago. In most cases the seals did not cause any measur-able change in surface texture, even though the rod would look visibly different. Measurements of these peak parameters would have been very useful, but at that time even obtaining rods with a consistent Ra was difficult. It also shows that repeatedly using a test rod, even though it may retain the same Ra value, may be unrepresenta-tive of seal performance on a new rod of similar Ra and manufacturing method.

This is current development work, and some data are updated from those reported in the con-ference proceedings.

Figure 5. Results of test rig runs to investigate the effect of Rpkx.