When specifying a hydraulic power system, a careful analysis of the application is required before selecting the various equipment. An open-loop hydraulic power unit is used to supply fluid power to various hydraulic actuators such as cylinders, rams and fixed-speed motors. Its pump typically runs at a constant speed producing a fixed fluid pressure to the system’s control valves. If no motion is demanded by the valves and actuators, the fluid is returned to a holding tank.
When specifying the hydraulic power unit, the maximum working flows and pressures must be defined before sizing components. The volume of fluid flow used in normal machine operation depends on of the number and types of cylinders, motors and transmissions in simultaneous operation at the required operational speed. The pressure is determined by calculating the necessary force needed to perform the designed operations of each actuator. The fluid flow necessary to meet system speed requirements must also be calculated. Many calculators and formulas are available on the Internet and in books dedicated to the subject.
Several types of pumps may be considered. These pump types can range from simple rotational gear pumps to more sophisticated bent axis pumps. The style of pump is primarily determined by the required flow and pressure. System actuators and the desired operation of the actuators dictate if a variable displacement device is required. The rotary gear pump style is the most common type of pump in use. The simple design leads to a rugged, cost-effective component that should offer many years of trouble-free operation.
Pump mounting locations are most often near the fluid reservoir. A top mounted pump allows for easier maintenance access. However, it also requires the pump to lift the fluid with suction before pressurizing it and may involve considerable labor to gain access to the interior of the tank for cleaning purposes. Some alternative pump locations can be alongside the tank reservoir where the pump inlet is below the minimum fluid level and below the tank reservoir to assure the pump is never starved for fluid.
ISO 4413 is the main industry standard describing best practices of design for hydraulic systems. The standard defines several roles for the reservoir (tank). The tank must dissipate the heat generated during normal operations unless other temperature control methods are employed such as a heat exchanger. Obviously, the tank should be able to hold all of the system fluid under normal system operation while maintaining sufficient levels to avoid starving the pump. There must be adequate room for thermal expansion. The returning fluid should be slowed to allow the release of trapped air and for contaminates to settle. Some method of separation between the incoming fluid and the pump intake should be provided, as with baffles or tank geography. Some method of access for cleaning should be made available.
A general rule of thumb used in sizing an hydraulic reservoir is three to five times the per-minute flow rate. Reservoir sizing can also be greatly affected by system characteristics and the type of actuators used. An hydraulic elevator using a single action cylinder needs a tank capable of holding nearly all of the system fluid when the elevator car is at the lowest point of travel without starving the pump when the car is at the highest point.
A pressure relief valve is used to limit the maximum system pressure by allowing the pressurized fluid to return to the tank during periods when the system is not using the pump's full volume of flow. When designing the hydraulic system, size the pump to the nearest maximum flow and pressure required for proper operation. When the pump is oversized, the pressure relief valve will be in constant operation, wasting energy and creating heat.
The hydraulic filtration system should be included as part of the power unit. When the filter system is located on the pressure side of the pump, it protects against contamination for all of the equipment downstream from the pump. Filtration pore sizes can be quite small—2 microns or smaller is not uncommon. Disadvantages of a pressure-side filtration system are pressure drops across the filtration system as the amount of contaminants build up; filters and housings must be capable of withstanding the maximum pressure produced by the pump; and any contaminants are passed through the pump before reaching the filter. Return-line filtration keeps air and contaminants out of the tank before reaching the pump. Return-line filtration is subjected to lower pressures than pressure-side. With either option, a differential pressure sensor should monitor the filter condition and alert personnel to replace the filter element.
Hydraulic power units often offer additional features such as a location to mount the control valve stack and locations to add instrumentation. At the very least, a low-level sight gauge and a system pressure gauge should be installed. Additional instruction may include a fluid level sight tube or an electronic level gauging device. Electronic pressure transducers allow data collection and a quicker detection of pressure drops or spikes. A temperature sensor can monitor changes in system performance over time.
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