Before selecting a hydraulic unit, define the work it must perform. Record required force, speed, stroke, duty cycle, and motion accuracy. Measure the heaviest load, not the average load. Include starting torque, shock loads, and seasonal temperature changes. ISO 4413 recommends identifying foreseeable risks, energy isolation needs, and safe operating limits in hydraulic systems.

Tip: Build a real operating profile. A cylinder moving slowly for eight hours needs different cooling and filtration than one cycling rapidly for ten minutes. The U.S. Department of Energy’s Industrial Technologies Program reports that hydraulic losses from throttling, leakage, and heat can exceed 50% in poorly optimized systems. That figure deserves attention. It is not a universal result, but it shows why pressure alone is a weak buying specification.

Specify pressure and flow together. Ask for continuous and peak values. Check oil viscosity at cold start and normal temperature. Confirm reservoir capacity, filtration level, noise limits, installation altitude, and maintenance access. ISO 4406 cleanliness codes help buyers state contamination requirements clearly. In practice, many purchasing errors begin with incomplete duty data. I have seen a system pass factory testing yet struggle on site because ambient heat and cycle frequency were underestimated.

Tip: Request test evidence under your actual conditions. Require pressure, flow, temperature, and leakage records. Leave room for uncertainty. A modest safety margin is useful, but excessive oversizing increases cost, heat, and energy consumption.