Disadvantages of Hydraulic Control


Although hydraulic controls ofl’er many distinct advantages, several disadvantages tend to limit their use. Major disadvantages are thie following:

1. Hydraulic power is not so readily available as that of electrical powe:r This is not a serious threat to mobile and airborne applications but moist certainly afi’ects stationary applications.

2. Small allowable tolerances results in high costs of hydraulic comi- ponents.

3. The hydraulic fluid imposes an upper temperature limit. Fire amhydraulic system is used near a source of ignii- tion. However, these situations have improved with the availability (of high temperature and fire resistant fluids. Hydraulic systems are messs) because it is diflicult to maintain a system free from leaks, and there is always the possibility of complete loss of fluid if a break in the system occurs. 4. It is impossible to maintain the fluid free of dirt and contamination. Contaminated oil can clog valves and actuators and, if the contaminant is abrasive, cause a permanent loss in performance and/or failure. Contaminated oil is the chief source of hydraulic control failures. Clean oil and reliability are synonymous terms in hydraulic control. 5. Basic design procedures are lacking and difficult to obtain because of the complexity of hydraulic control analysis. For example, the current flow through a resistor is described by a simple law—Ohm’s law. In contrast, no single law exists which describes the hydraulic resistance of passages to flow. For this seemingly simple problem there are almost endless details of Reynolds number, laminar or turbulent flow, passage geometry, friction factors, and discharge coeflicients to cope with. This factor limits the degree of sophistication of hydraulic control devices. 6. Hydraulics are not so flexible, linear, accurate, and inexpensive as electronic and/or electromechanical devices in the manipulation of low power signals for purposes of mathematical computation, error detection, amplification, instrumentation, and compensation. Therefore, hydraulic devices are generally not desirable in the low power portions of control systems. The outstanding characteristics of hydraulic power elements have combined with their comparative inflexibility at low power levels to make hydraulic controls attractive primarily in power portions of circuits and systems. The low power portions of systems are usually accomplished by mechanical and/or electromechanical means.

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