What Is Hydraulic Length?

Meaning

• Pretend you're an inch tall. Jump into the hydraulic system's reservoir, swim the through the main lines of the hydraulic system and back to the hydraulic reservoir. That's one way to describe the length of the hydraulic system. The problem is that sometimes, the system may seem longer because of valves that are open or closed. Unless that "extra length" is compensated for, the hydraulic system will seem underpowered and ineffective.

Effects of an Increase

• When a power steering system's hydraulic oil level is low, it squeals. That's the system's reaction to an unexpectedly long hydraulic length. When you turn the wheel, if the power steering fluid level is low, the hoses are the same length and the pump is still the same, but the system's hydraulic system's length is longer. The steering wheel is harder to turn and, if the situation is allowed to continue, the steering pump will suffer damage, or the vehicle's steering may fail altogether.

Calculating Hydraulic Length

• Calculating the hydraulic length of a system means using a formula, called the Darcy-Wiesbach Equation, that compares the pressure extant in the system to the pressure the hydraulic system should exhibit. This equation uses a "dimensionless factor" called the Moody Friction Factor, which expresses the effect of friction on the flow in the system. This factor was calculated by Lewis F. Moody, a professor of hydrology at Princeton University, for pipes of varying shapes, sizes and configurations. The resulting diagrams, called Moody Diagrams, are curves that show the effects friction, interior roughness of a pipe and turbulence have on hydraulic length.

Uses

• For automotive designers, the hydraulic length of hydraulically assisted systems, such power steering and power brakes, is critical, as each element of the design affects the other elements within a system. Other applications where the hydraulic length of a system is critical, such as the hydraulic systems on the robots in an assembly line, are critical from a production standpoint. A production line that stands still costs thousands of dollars a minute. When the hydraulic oil in the system is low, the system will not operate at its intended pressure. For many automotive systems, such a discrepancy leads to an expensive failure.