The porting tool motor circuit was running slow and didn’t have much power, but the positioner motor circuit worked fine. The unloader valves in the A and B positioner circuit must be leaking oil to the tank when the porting tool circuit is energized. When the first valve was disassembled, the spool was found to be broken in half.

Tandem type spools are hollow and ported to allow pressurized fluid to flow past the blocked A and B ports and out to tank. This weakens the spool, and if they are cycled at a high rate, fatigue can cause them to break into two pieces. They can still cycle end to end, but when commanded to center, the two halves move in opposite directions and connect all four valve ports together.

In this case, the A and B ports were partially obstructed buy the spool halves. Some of the fluid dumped over the unloader valves, while some flowed to the porting tool motor.

However, their troubles weren’t over when technicians installed a new valve. They didn’t notice a 65-psi pilot pressure check inserted into the P port of the old valve, and neither valve would cycle at startup. They installed a line-mounted 65-psi check valve between the external pilot line connection and the first valve to solve this new problem.


Robert J. Sheaf Jr., is founder and president of CFC Industrial Training, a Div. of CFC Solar, which provides technical training, consulting, and field services to any industry using fluid power technology. Visit for more information.