Edited by Kenneth J. Korane
From aerospace equipment to earthmoving machines, hard-chrome plating on piston rods has been used for decades to protect hydraulic cylinders against corrosion and wear. Stricter environmental regulations and the demand for longer service life, however, have led cylinder manufacturers to develop alternative piston-rod coatings in recent years. But how do these new surface treatments affect seal performance, and is there an ideal coating for all applications?
Friction between a cylinder rod and its seals, and the resulting wear, has a crucial influence on the efficiency and service life of hydraulic cylinders. As long as the hydraulic pump generates sufficient power, seal friction is often overlooked with respect to the hydraulic system’s efficiency. In such cases the main concern is that the cylinder seals and wipers prevent leaks and keep dirt out of the system — even under harsh conditions such as when excavating a riverbed.
But the frictional properties of piston seals and piston-rod seals are becoming increasingly important to the operators of modern fluid-power systems. High static and dynamic friction accelerate wear, decrease the efficiency of the entire hydraulic system, and lead to undesirable stick-slip and high breakaway forces after prolonged rest — the so called “Monday-morning effect.” In addition, unsuitable surfaces can cause seals to squeak or creak and make equipment noisy.
There are two knocks against hard-chrome-plated piston rods. One involves service life. The surface quality or roughness of hard-chrome-plated piston rods changes over time. The surface is typically smoothed which, counterintuitively, increases friction on the piston-rod seals or causes microcracks and abrades the chrome tips on the rod surface. Long score marks also form across the entire running surface. These changes hurt the entire tribological system and can lead to equipment failure.
The other shortcoming of hard-chrome plating relates to the environment. Hexavalent chromium is toxic and heavily regulated by the U. S. Environmental Protection Agency (EPA). It is a human carcinogen and the EPA considers it a hazardous air pollutant under the Clean Air Act, a hazardous substance under the Clean Water Act, and a hazardous waste under the Resource Conservation and Recovery Act. By-products of the plating process cannot be discarded into wastewater
As a result, manufacturers are turning to rod plating and coating processes which do not pose a health hazard to workers or harm the environment.
The demand for piston-rod coatings which do not change roughness and structure even after extended service, and are environmentally benign, has led to a number of developments. But of ultimate interest to design engineers: How do these coatings perform in the field?
To provide an answer, Parker-Prädifa has conducted many short and long-term friction and leak endurance tests to determine the effects these alternative coatings have on piston-rod seals.
The following coated piston rods, among others, were tested in the Parker-Prädifa lab to gauge their influence on seal friction, wear, and leakage:
• Hard-chrome plated, from various manufacturers, with different degrees of roughness.
• Hard-chrome plated with PTFE surface treatment.
• HVOF (high-velocity oxygen-fuel) spray coated.
• Ceramic coated, with and without a sealing coat.
• Salt-bath nitride.
• Plasma nitride, with and without additional grinding.
• Chemically nickel coated.
• Melted compound coatings.
• Nitrided and nitrocarburated.
In all cases, technicians measured surface properties at the beginning and end of the test and documented the changes.
Dynamic tests were conducted using OD, B3, and BS type piston-rod seals. The OD is a unidirectional, PTFE buffer seal that vents trapped fluid pressure back into the cylinder. It rides on a single sealing point when the rod extends and, if pressure is trapped, the seals rocks on the return stroke and lets fluid pass under the seal and return to the system. The seal has a PTFE-filled-bronze cap and nitrile-rubber energizer. It’s considered a low-friction, long-life seal.
The BS is a nonsymmetrical rod seal with a knife-trimmed sealing lip and a secondary lip to enhance sealing and give a tight, stable fit in the gland. It’s made of polyurethane for long life and extrusion resistance. The B3 is a nonsymmetrical, polyurethane U-cup. It has a knife-trimmed, beveled lip and it resists wear, extrusion, and compression set. It does not have a secondary sealing lip.
Tests were performed on universal hydraulic-rod-seal test rigs based on ISO Standard 7986 for endurance test equipment. Test parameters were selected according to in-house and international standards, with endurance tests conducted according to DIN 7986.
For instance, for results shown in the accompanying diagram, operating pressures ranged from 0 to 200 bar, depending on the piston rod’s direction of motion. Temperature of the HLP 46 mineral oil was 65°C. The rod had a 36-mm diameter and 250-mm stroke, surface speed was 0.15 m/sec, and the test lasted for 500 km of travel (1 million load reversals).
The results detailed here, plus a large number of additional tests, show that the treatment or coating of the rod surface has a significant impact on the friction, leakage, and wear of piston-rod seals. While some results were positive, a number of alternative, initially “seal-compatible” piston-rod coatings increased seal wear after prolonged testing, with resulting premature leaks and failure.
he diagram illustrates the various friction forces with type B3 U-rings on alternative piston-rod coverings. Except for the thermal-spray, hard-coated piston rod, all U-ring seals tested with the different coatings tended to exhibit the same friction-force behavior and friction level on the piston-rod seal surfaces.
In the case of the thermal-spray, hard-coated piston rod, a secondary process follows spray coating of the surface. This leads to a homogenous structure that improves the friction behavior of the test seals. In this case, surface friction remained low even after longer periods with the hydraulic system at rest. This behavior was a consideration in the recent design of cylinders for harbor cranes exposed to saltwater.
All coatings have about the same roughness, but the ratio between peaks (Rp) and valleys (Rv) on the surfaces varied significantly. In endurance tests these differences had a major impact on U-ring leakage. After just short running periods, leaks significantly increased due to heavy wear of the U-rings on piston rods with ceramic coatings. As a result, several trials had to be stopped prematurely. In the case of the nitrided and Ionit-Ox (nitrocarburized and oxidized) piston rods, significant wear of the test seals was noted as well. However, only the cylinder with the nitrided rod leaked.
The PTFE-type OD seals presented a similar picture, but changes were more pronounced with this material.
Test results reveal there is no one, ideal plating or coating for all applications. Both standard hard-chrome plating and alternative techniques have advantages and disadvantages with respect to the operating performance of piston-rod seals.
Therefore, to design the best sealing system for a specific application, the total tribological system — the piston seal surface, lubricant, and seal — must be closely analyzed prior to its use in the field. It’s generally a good idea to consult with the application engineers at a major seal manufacturer for comprehensive advice and recommendations.