Creating reliable and leak-free connections should always be a primary goal for any hydraulic system. There are many different types and styles of fittings from which to choose. Regardless of what fitting style you decide on, there are several factors to consider in making the proper selection. Utilizing the acronym STAMP – Size, Temperature, Application, Media, and Pressure is probably the best place to start. After all, selecting the proper fittings and tubes for a given situation is an important part of system design and STAMP is an easy tool for guiding you through this.
The tube’s outside diameter (OD) and wall thickness needs to be considered based on fitting style. With most connection types, there are limitations with respect to the tube wall thickness. The flare type fitting, for example, will have a maximum tube wall limitation. 24° flareless bite type fittings, on the other hand, would tend to more have a limitation on the minimum tube wall thickness. The proper selection of the tube OD and wall thickness depends on other factors such as; system pressure, flow rate, and service environment. See Sizing Tube to Maximize Hydraulic System Efficiency.
The operating temperature range for tube fittings depends on the material type, plating, and type of seal, if applicable. Typically, O-ring seals add leak-free reliability to your fitting connections, but depending on the temperature of your application a metal to metal seal may be needed. There are now metal seals available for O-ring face seal type fittings and also for SAE O-ring ports (see Parker's Seal-Lok Xtreme metal face seal fittings). The following table contains temperature ranges for the most common fitting materials and seals.
Material Temperature Table
The system environment influences the design and selection of all types of fittings as well. Protective coatings are usually applied to steel fittings to extend their useful life in corrosive environments. The most common finish for steel fittings is electroplated zinc. Zinc will corrode sacrificially, protecting the steel base metal from normal rusting due to the presence of oxygen, moisture and acidic gases. For highly corrosive environments, stainless steel or brass material can be a viable option. Additionally, special plating for steel fittings such as zinc nickel can be more tolerant to fertilizer, salt, and some detergent applications.
The type of fluid being conveyed or in immediate contact with the fittings is another important factor that should be considered when selecting the fitting and seal material. The Fluid Compatibility Chart below can be used as a guide. You can also download this same Fluid Compatibility Chart as a pdf.
Fluid Capability Chart
Hydraulic systems can be classified as either dynamic or static. Static systems are free of pressure surges, shocks, and vibration and do not exceed 30,000 operating cycles. Because of this, static systems can typically operate at a 3:1 design factor.
Dynamic systems are far more common, and are generally considered as being able to withstand a minimum of 1 million operating cycles without failure. The dynamic pressure rating of the fitting should be equal to or higher than the system pressure. Tube fittings are customarily rated based on a 4:1 design factor. The design factor is generally applied as a ratio of the ultimate strength of the material with respect to the dynamic pressure rating of the connection. The 4:1 design factor applies to “normal” operating conditions, with moderate mechanical and hydraulic shocks.
For more severe operating conditions, a higher design factor should be taken into considerations or a “Derating Factor” should be applied directly to the dynamic pressure rating of the fitting. The following table displays the “Derating Factor” for three different severity of service conditions: Normal, Severe, and Hazardous.
Example of derating the stated dynamic pressure of a fitting:
Dynamic pressure rating of the fitting = 5000 psi
Severity of Service: B (Severe hydraulic shock and mechanical strain)
Design factor for "B" severity of service = 6:1
Derating factor for design factor of 6 = 0.67
Derated Pressure = 5000 psi X 0.67 = 3350 psi
Caution: Applications in which failures could result in death, personal injury, or property damage should have a higher design factor applied.
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If you have any additional questions or comments, please post them and I’ll respond if warranted. If you want to talk to me directly, I can be reached at Parker Tube Fittings Division, 614.279.7070 or via email.
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Contributed by Burleigh Bailey, senior project engineer, Parker Tube Fittings Division
Additional related content about hydraulic tube, hose, and port fitting connections:
Sizing Tube to Maximize Hydraulic System Efficiency
How Many Times Can I Reassemble a Hydraulic Fitting?
Turn vs. Torque? How Making the Right Choice Keeps Your Hydraulic Fitting Connections Leak-Free
A Dollars and Sense Approach to Preventing Hydraulic Oil Leaks
Tube Routing Tips for Hydraulic, Pneumatic, and Lubrication Systems