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How piston-rod coatings affect hydraulic seals
Date:2015-05-25 10:49:32    Browsed:513    Source:www.othchina.net

  Key points:

  • Hard-chrome plating for cylinder rods has fallen out of favor.

  • Manufacturers have developed alternatives, including ceramic, nitrided, and nickel coatings.

  • The various coating affect seal friction, wear, and leakage in different ways.

  Seal performance

  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.

  Hard-chrome plating

  There are two knocks against hard-chrome-plated piston rods. One involves service life. The other shortcoming of hard-chrome plating relates to the environment.

  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.

  Alternative coatings

  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.

  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.

  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).

  Test results

  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.

  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.

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