Many coal powered electric generating plants receive their coal via river barges. The barge pulls into a lock, and a large vertical conveyor with multiple rectangular buckets is lowered into the front of the barge to unload the coal. To remove the coal from the barge, the conveyor is lowered until it reaches the bottom of the barge. A cable attached to the front of the barge then pulls it forward while the vertical conveyor continues to transfer the coal to a horizontal conveyor system.

When the vertical conveyor reaches the end of the barge, it raises and moves sideways. Then the barge drifts back to its original position, and the process of removing the next row of coal starts again. The barge is completely emptied in four back-and-forth passes.

We were called into a plant having a problem with the cable system that pulled the barge forward and then allowed it to drift back to its starting point. Several hydraulic motors failed from the system overheating, some from what looked like cavitation damage and continuous seal leaks.

The hydraulic circuit for the cable drive is shown. It has a pressure-compensated pump with a safety relief valve to limit pressure spikes. Plant personnel found the safety relief setting was set too close to the pump’s compensator, which created some of the heating problem. They adjusted the pressure setting to 150 psi higher than the pump, but the overheating problem continued. After technicians replaced the motor with a rebuilt one, flow from both pump and motor case drains was measured and found to be within their acceptable range.

This system had very little information in the service manual other than a bill of materials and the schematic. They had no idea how to set any of the adjustments other than what they were told about the safety relief setting.

Any idea what they should do next?

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.