A technician at a plant that uses large three-axis milling machines explained that one of the machine’s hydraulic power units had foam coming out of the reservoir’s filler breather and the HPU was quite noisy. He observed the foam oozing out when a large cylinder retracted, and noise was normal. However, when the same cylinder extended, noise increased but the foaming stopped. The pump became very quiet when it compensated at its prescribed pressure.

The technician knew that all return lines should be below the oil level to prevent foaming. With this in mind, he drained the tank and inspected all the lines, making sure none were missing. After verifying that all were present, he checked to make sure all connections were tight.

After consulting with a hydraulic repair shop, he was told an air leak must be present in the intake, case drain, or shaft seal area. He recently had the pressure-compensated vane pump rebuilt by the same hydraulic repair shop. He figured it must be one of the lines leaking so he replumbed both the intake and case drain lines, but the problem still persisted. He inspected the shaft seal area, thinking it might be the cause, but it looked new and was dry on the outside.

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 www.cfc-solar.com for more information.

Any idea what's be causing the problem?