A four-station molding operation had a major problem: The customer complained that the pump failed at least once a month and that the frequency was increasing. He demanded a pump that would last longer.
The molding machine is controlled by a hydraulic system consisting of a pressure compensated, variable displacement vane pump rated for 30 gpm at 1500 psi. The system runs on water/glycol fluid, and the clamping cylinders have cast iron piston rings.
At the first station, four cylinders clamp a mold for 30 to 45 seconds at full system pressure. At the next two stations, the cylinders are extended but pressure is relieved for 60 to 90 seconds. At the last station, the cylinders are retracted and pressure is relieved. The system has six sets of clamp cylinders that rotate though each station. The rest of the stations are idle allowing the operator to remove product.
The failed pump was replaced and a flow meter installed after the pump while the system was shut down. Then, flow readings were monitored during production. Initially, the pump operated continuously at 22 to 25 gpm and 1500 psi, indicating that the pump was operating correctly. However, internal leakage in the rest of the system caused so much bypass that the pump was never able to compensate. It could never return to a no flow condition, which is required by this particular pump design to distribute wear evenly on the internal surfaces.
Monitoring the pump for the next eight days showed that pressure remained at 1500 psi but flow began to increase to 25 to 28 gpm, depending on which portion of the duty cycle the system was in. Once system flow reached pump capacity, pressure began to decrease, and the operators complained about rejected product because the clamp cylinders did not hold. A new pump was installed and production continued.
When the removed pump was inspected internally, it showed very little evidence of failure except the beginning stages of contamination wear. In reality, most of the pump’s useful life remained, indicating that the true problem lay in other areas of the system. To trace the source of the problem, each station was analyzed, and flow and pressure values were documented for each component. As a result of this analysis, the customer realized that the true problem in the circuit was bypass flow on the cylinder’s cast iron piston rings and in each valve spool. This created alternate flow paths for the fluid to return to the reservoir without doing any work. The heat generated was masked by a high ambient temperature and extremely oversized heat exchanger. Therefore, while pump capacity was sufficient to meet operating demands, it could not compensate for flow loss due to internal leakage.
The solution was to modify every cylinder with Viton lip seals, replace the worn directional valves and, as an additional enhancement, install a permanent flow meter, flow control and pressure gage. A kidney loop system was also installed to downsize the heat exchanger and add continual flow through a finer micron filter element. With these, the system has operated without a pump failure for over a year, enabling the plant to maintain normal production without unscheduled downtime.