The cylinder in the illustration represents one with such a small volume and that is so far from the directional valve that the lines to and from the cylinder hold more fluid than it does. Because fluid flowing out of the cylinder never reaches the directional valve, fresh fluid never cycles through the cylinder; it just flows into and out of the cylinder port repeatedly. The result is overheating, and all wear particles and contaminant ingested through the rod seal remain in the fluid until the cylinder fails prematurely.

isolated cylinders

Changing the lines to the dual flow path shown eliminates the problem. A tee fitting is installed at both valve work ports and both cylinder ports, and two pairs of lines are run as shown. Check valves facing opposite directions are installed as shown in each pair of fluid lines. When the valve shifts to extend the piston rod, fluid flows into the cap end of the cylinder through the left hand line. When the valve shifts to retract the rod, fluid flows out of the cylinder’s cap end through the other line in the pair. Because all fluid must flow into the cylinder through one line and out the other, fresh fluid is continually being fed to the cylinder while used fluid returns to be cooled and filtered. Improved cylinder operation and much greater longevity result.

The check valves can be installed anywhere in the lines, but the tee fittings should be as close as possible to the valve and cylinder ports.

This dual flow path circuit is even more important when the cylinder is mounted vertically with the rod up. Contamination lays on the piston and seals and can score the cylinder tube and cause premature seal wear when allowed to collect.

This information is excerpted from our eBook, Fluid Power Basics, which you can view for free on our website.