By Robert J. Sheaf, Jr.

A large snow plow truck was brought to Turner Hydraulics in Carlisle, Pa., the day after a snow storm dumped several inches of snow on the east coast. The operator complained about the pump pressure line failing and noted that it would be fine if he only ran the salt spreader circuit. However, if he moved any of the plow circuit cylinders to a bottomed-out position, the pressure hose from the pump would fail. None of the other hoses on the truck circuit had failed; the pump pressure hose was the only one that did. He tried to prevent any cylinder that he moved from bottoming out, but he could not always judge its position correctly and, consequently, would cause another hose to fail.

When Turner’s technician looked in the cab for a service manual hoping to find a hydraulic circuit, he found three new hoses, four old ones that had failed, and four 5-gal buckets of hydraulic oil. However, no documentation was found.

The technician, Mike Armstrong, crawled under the truck and found a Parker Hannifin load sensing, pressure compensated model PAVC65 piston pump. He also determined that the directional valve bank was equipped with a load sense line going down through the frame to the pump. There were three manual spools that controlled the plow and two proportional spools controlled by the salt spreader control electronics. Armstrong assumed the valve bank would have a closed-center circuit because pressure compensated piston pumps require a minimum backpressure to ensure adequate lubrication. He also determined that there wasn’t a safety relief anywhere in the system.

The operator said the owner of the truck had been troubleshooting a different problem on his truck and had removed the pump and valve stack. Upon doing so, he found hose material in one of the directional valve sections. He corrected the problem, but now they were having this bursting pressure hose problem. He also told them that when they first started up the system after finding the hose material, they heard a loud “thump,” but it did not return, and they could not find anything wrong.

The customer felt the problem was still in the valve bank, but Turner Hydraulics was pretty sure something was wrong with the pump. The illustration shown is of the Parker pump shown in its Catalog HY13-1553/ NA,EU, page A18.

Robert J. Sheaf Jr., is the founder of Certified Fluid Consultants (CFC) and President of CFC-Solar Inc. CFC-Solar provides technical training, consulting, and field services to any industry using fluid power technology. Visit www.cfc-solar.com for more information.