Test methods and results
Dynamic tests on Parker OD, B3, and BS type piston rod seals were performed with piston rods having different coatings in a large number of short- and long-term friction and leakage endurance tests on universal hydraulic rod seal test rigs. The OD is a unidirectional, PTFE buffer seal that vents trapped fluid pressure back into the cylinder. It has a PTFE-filled bronze cap and nitrile-rubber energizer and is considered a low-friction, long-life seal.

     Test parameters for DIN 7986 endurance tests

Operating Pressure 0 to 200 bar (depend on extent or retract)
Oil temperature 65° C             
Duration 500 km (1 million load reversal)
Rod diameter 36 mm
Surface speed 0.15 m/sec
Fluid HLP 46 mineral oil
Endurance test rig based on ISO 7986

The B3 is a nonsymmetrical, polyurethane U-cup with a knife-trimmed, beveled lip. It resists wear, extrusion, and compression set and has no secondary sealing lip. The BS is a nonsymmetrical rod seal made of polyurethane for long life and high extrusion resistance. It has knife-trimmed sealing lip and a secondary lip to enhance sealing and give a tight, stable fit.

In all cases, rod surfaces were analyzed at the beginning and end of the test and changes documented. Test parameters were selected according to in-house and international testing standards, with endurance tests conducted according to DIN 7986.

The graph illustrates the comparison of the friction forces with type B3 U-rings on alternative piston rod coverings. Except with the thermal-spray hard coated (TSHC) piston rod, all U-ring seals tested tended to exhibit the same friction force behavior and friction level on the piston rod seal surfaces with different coverings. The TSHC piston rod used a post spray coating process that produced a homogenous structure with the resulting positive effect on the friction behavior of the test seals. Surface friction remained low, even after the hydraulic system did not operate for long periods time. This positive behavior was considered and implemented in the design of cylinder systems for harbor crane systems, which often come into contact with salt water.

Graph shows friction measured on B3 U-rings mated piston rods seal with various coatings. Values are based on pressure at 20 MPa at a temperature of 60° C.

All coatings exhibit about the same roughness values, with the ratio between peaks and valleys on the surfaces varying significantly. In endurance tests these differences had a major impact on the leakage behaviour of the U-rings. After just very short running periods, a significant increase of leakage due to heavy wear of the U-rings was noted on the piston rods with ceramic coatings. As a result, several of the trials had to be stopped prematurely. In the case of the nitrided and nitrocarburized piston rods, significant wear of the test seals was noted as well. Leakage, however, occurred only on the nitrided rod.

The PTFE type OD sealing edge rings presented a similar picture, but the differences between the various tribological systems were more pronounced in this case.

An ideal plating or coating?
The results of the tests reveal that there is no ideal plating or coating for all application scenarios. Both the standard hard-chrome plating and the alternative techniques exhibit advantages and disadvantages with respect to their impact on the operating performance of piston rod seals.
Therefore, to design the optimal sealing system for a specific application, the total tribological system — the piston seal surface, the lubricant, and the seal — must be subjected to a detailed analysis prior to its use in the field.

Thomas Papatheodorou is manager of technical services at Parker Hannifin’s Seal Group - Europe, Bietigheim-Bissingen, Germany. For more information, visit bit.ly/OH52MC.