KEY SPECIFICATIONS Operating conditions—Cylinders must meet the requirements of the design specification, such as force, maximum pressure and mounting configuration, but consideration for operating conditions must also be heeded. Cylinders must also withstand extreme temperatures, humidity and even salt water for marine hydraulic systems. Also, when ambient temperatures rise to more than 300° F, standard Buna-N nitrile rubber seals may fail and will instead require synthetic rubber seals, such as Viton. When in doubt, err on the safe side and choose a cylinder design capable of more of than you will ask of it. Fluid type—Most hydraulic systems use a form of mineral oil, but applications using toxic synthetic fluids—such as phosphate esters—require Viton seals, which will not break down or swell in the fluid. Once again, Buna-N seals may not be adequate to handle some synthetic hydraulic fluid, although the gentler synthetics, such as PAO-based stock, will be fine. Hydraulic systems using high water-based fluids may require stainlesssteel construction, as well as PTFE (Teflon) seals, especially if no glycol is used in the fluid.
Seals—Seals are the most vulnerable component of a hydraulic system. Properly chosen and applied seals can reduce friction and wear, lengthening service life, while the incorrect type of seal can lead to downtime and maintenance headaches as a result of failures. Every manufacturer likes to use a different seal style, so it is important to replace them with a similar type and material when rebuilding. Cylinder materials—The type of metal used for cylinder head, cap and bearing can make a significant difference in performance and reliability. Most cylinders use bronze for rod bearings and medium-grade carbon steel for heads and bases, which is adequate for most applications. But stronger materials, such as 65-45-12 ductile iron for rod bearings, can provide a sizable performance advantage for tough industrial tasks. The type of piston rod material can be important in wet or high-humidity environments (like marine hydraulics) where stainless steel may be more durable than the standard case-hardened carbon steel with chrome plating used for most piston rods. A new option for rod surface treatment is nitriding, which is an oxidation process to increase the surface hardness of metals, and in the case of cylinders, makes for excellent corrosion resistance.
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FLUID POWER WORLD
6 • 2016
6/17/16 8:42 AM
Fluid Power World Handbook 2016