The anatomy of a large hydraulic power system

Feb. 21, 2002
Large hydraulic-system basics, shock-absorber lessons, air-hydraulic pumps, and more are included in this hydraulics reference guide.

The Lifco power unit, built for an 800-ton upstroking press, uses Denison valves and pumps.

The design and construction of a large central hydraulic system involves some obvious and not-so-obvious considerations. To ensure optimum performance, reliability, and durability, pay close attention to both fundamentals and details.

Before the details can be worked on, basic principles should be followed. Here is a review of common design features that should be included in every large hydraulic system:

•An L-shaped or overhead reservoir that is higher than the pumps helps maintain positive pressure at pump inlets.
•Drip trays to contain any fluid lost during maintenance or modifications.
•Grating over drip trays creates dry, slip-free surfaces for personnel to walk on and protects any electrical or hydraulic conduit.
•Rubber hoses and mounting pads isolate vibrations from the pump/motor groups and the rest of the structure, and reduces noise.
•Ball valves isolate individual pumps from the central headers, easing maintenance and troubleshooting.
•Light color aids inspection of small details and encourages good housekeeping.

Besides the fundamentals, there are other not-so-clear points to central hydraulic system design that deserve mentioning.

An important feature is the inlet header to the pumps. The header should be large or connected at both ends. Headers supplied from only one end risk pressure loss and force flow toward the other pumps that may prevent enough oil from reaching pumps at the far end of the header. The header should also be as direct as possible with as few changes in direction or diameter as possible. This ensures minimal pressure losses to the pumps being fed. Applying positive air pressure in the reservoir may help propel fluids to the pumps.

Similarly, the case-drain header that takes oil back to the reservoir should not be too small. Pumps sharing an undersized case-drain header run the risk of excessive case pressure. This could cause shaft-seal leakage or pump malfunction.

Some systems include case-drain flowmeters on each pump for maintenance. The intent is good, but meters create a restriction (back pressure) in the pump-case drain line. Added together, resistance created by flowmeters and the common case-drain header, coupled with lack of sufficient positive pressure at the pumps' inlets create a pressure imbalance across the pump pistons. Under these conditions, the piston slipper pads may not stay on the swash plate, and it could lead to failure similar to that caused by cavitation. This can happen even with positive pressure at the pump inlet. There should never be case-drain flowmeters, unless they are a full flow, zero-resistance type.

Ball valves should be equipped with limit switches that interlock with the electric-motor starters. Vacuum switches at pump inlets and pressure switches in pump cases are even more helpful. Without them, case-drain passages clog and fail while the pump is running.

Hydraulic systems should have fittings for auxiliary gauging points to help in troubleshooting. They let technicians tap into the system to check pressure even with the system running, and also facilitate fluid sampling and bleeding air from the system.

Finally, and perhaps most important, oil should be conditioned with filters and coolers. At least 80% of machine downtime is attributed to poor fluid conditioning. Systems that are well designed and built with adequate filtration will provide long, trouble-free service.

This information provided by Nate Andrews of Lifco Hydraulics Ltd., St. Catharines, Ontario, Canada and Denison Hydraulics, Marysville, Ohio.

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