SpecMate: If It Flows, It Should Go

A Short Primer on the Mechanical Decisions That Improve Hydraulic Control System Design

"Watch for internal pressure spikes that can cause damage to system components. A certain amount of force can be trapped between the cylinder and valves and can generate a harmful pressure spike in the system."

The logic in your hydraulic control scheme is rock solid, you say. But, if the system has some aggravating leaks or flow constrictions that you didn't anticipate, your control scheme may not work as intended. And, oh yes, you have the primary responsibility for troubleshooting, if things go awry at a customer site.

It might make some sense then, to spend a little time understanding some of the mechanical decisions that get made in the selection and design process of the components themselves. It might also alter your ideas about the control scheme or about component placement on your machines.

"Plumbing a fluid power circuit is a field of its own, in my opinion," says Bud Trinkel of CFPE Hydra-Pneu Consulting (www.fluidpower1.us). "I always call the experts in to do the job. There are many types and brands of conductors and fittings to use, and often it is the users preference that determines which one is used."

Trinkel offers a handful of recommendations, among them never specifying taper pipe threads on hydraulic circuits. They're OK for air since pressure is low.

He says it's wise to specify tubing with radius bends up to 2 in. instead of pipe and elbows, and support the tubes/pipes generously in rubber-mount-type supports.

"Use SAE straight threads or flange fittings, since they have O-ring seals," adds Trinkel, "and specify hoses to isolate or allow movement between stationary and moving parts, not for general plumbing. Hose installation is another whole field of endeavor and can be used to advantage only when applied correctly."

One big piece of advice about that: "No matter whose hose assembly system you are using, always, always, specify that manufacturer's fittings on that manufacturer's hose and crimp them in that manufacturer's machine," cautions Scott Kane, applications engineering manager, Parker Hannifin (www.parker.com). "Mixing and matching components is a sure recipe for problems."

So, if you've dealt with this properly, what can affect the performance of the system during normal operation? Heat generated in a hydraulic system by moving parts and fluid velocities is a key variable. "This heat, if excessive, will create greater wear on the components, as well as break down the integrity of elastomeric seals and flexible hose assemblies," says Kevin Dugan, GPP program coordinator for Parker Hannifin. The heat can be dissipated by proper sizing of the oil reservoir or adding an oil cooler, but, says Dugan, "a lot of engineers do not consider that you can also remove heat through proper sizing of the hydraulic lines."

This leads to another factor: line velocities. "Excessive line velocities create a more turbulent flow of the hydraulic fluid as well as increase the frictional heat generated by the resistance to flow of the components," says Dugan. "This condition also can cause inner tube washout of the flexible hose assemblies." There are recommendations for maximum line velocities in both tube lines and hose assemblies.

Vibration is inherent in most hydraulic systems and works against the integrity of the connections, hoses, tubes and adapters. Proper system connector routing, component and tube support, and minimizing system shock can minimize the effects of vibration, says Dugan.

Watch for internal pressure spikes that can cause damage to all working components of the system. "When moving load forces in a system stop, a certain amount of force can be trapped between the cylinder and valves of a system. This trapped energy can generate a pressure spike in the system, says Dugan. The relief valve on the circuit will release this pressure, but, since the relief valve activation is not instantaneous, "there is a moment when the pressure exceeds the relief setpoint and creates a harmful pressure spike," cautions Dugan. "Selecting quality hydraulic components can minimize the effects of pressure spikes."

Hydraulic system contamination is a troubleshooter's nightmare, adds Dugan. Consider a floating contaminant inside a system that intermittently causes a valve spool to stick. If the system is not acting up when the troubleshooter arrives, you will have a difficult time diagnosing the problem.

The effects of contamination can be minimized by ensuring new oil is added with a good clean filtration system, consistent replacement of pressure and return filters, annual cleaning of the suction strainers and regular planned maintenance.

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