As the main part of the hydraulic cylinder, the piston rod is used in tough surrounding and corrosive conditions; Consequently, a high-quality protecting layer is essential. Currently, electroplating hard chrome is a widespread method. Due to its powerful performance and low cost, electroplated hard chrome is a common method for piston rod treatment.
Criteria of piston rod coating
1) Toughness
Toughness is an essential characteristic of piston rod coatings. Coatings demonstrating poor toughness or being not tough enough could not absorb much energy during the angular stone or hard grit strike the piston rod, surface damage then happens easily, and the hydraulic cylinder will in turn fail to work immediately due to coating delamination or flaking.
The impact test is a dynamic test in which a selected specimen is usually struck and broken by a swing pendulum. The most common tests of this type are the Charpy V-notch test and the Izod test which are described in ASTM E23. The principle difference between the two tests is the manner in the specimen is fixed.
2) Corrosion resistance
Due to the poor working environment, corrosion resistance is very important for hydraulic cylinder piston rod coating. The Salt spray test is a conventional standardized test method used to check the corrosion resistance of hydraulic cylinder piston rod coating; it is an accelerated corrosion resistance test and the appearance of corrosion products is evaluated after a period of time.
The apparatus for testing as shown in figure 4 consists of a closed testing chamber, where a salted solution, mainly a solution of sodium chloride, is sprayed by means of a nozzle. This produces a corrosive environment in the chamber and thus, parts in it are attacked under this severe corroding atmosphere. Tests performed with a solution of NaCl are known as NSS (neutral salt spray). Results are generally evaluated as testing hours in NSS without the appearance of corrosion products. Other solutions are ASS (acetic acid test) and CASS (acetic acid with copper chloride test). Chamber construction, testing procedure, and testing parameters are standardized under national and international standards, such as ASTM B117, DIN 50021, and ISO 9227. After the test duration, the sample could be rated according to the rusted surface area using reference standard as shown in table 1.
3) Wear resistance
As a power transmission unit, the piston rod needs to move forth and back frequently, at the same time wear happens during the coating surface slide against the cylinder sealing. Hence wear resistance is also an important requirement for piston rod lifetime. Surface hardness is the key parameter for wear resistance. Besides toughness, corrosion resistance and wear resistance, according to various industry standards and customers’ requirements, other criterion of piston rod coatings are listed in the table2.
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Post time: Sep-23-2022