Better efficiency with hydraulics

It's no secret that the power source for machine motion can greatly impact the amount of energy a machine consumes. Conventional wisdom holds that electric drives are more efficient than their fluid-power counterparts. But for certain tasks, hydraulics holds the upper hand. Equipment with a single, controllable hydraulic pump and accumulator can consume less power than electric motors and gearboxes in applications with a lot of back-and-forth motion, that move or hold heavy loads, or that have a large number of motion axes.

Consider a large robot that pours concrete into molds for sewer pipe, underground vaults, and catch basins. The machine builder, Hawkeye Group of Mediapolis, Iowa, switched to hydraulics because the electric motors initially powering the robot's two axes were ill-suited for the heavy and varying loads. Filled with concrete, the feeder mechanism weighs up to 10 tons, and Hawkeye's engineers found such loads would “trip out” and shut down the electromechanical drives. The system wasted electricity and could not always produce the required motion.

Such applications need extremely large electric motors to match the motive force of hydraulic motion arms. And adding to inefficiency in Hawkeye's casting robot, one motion axis moves the other's motion base as the robot arm follows various pouring contours. Thus, the weight of the motor on the upper axis becomes an additional energy-wasting burden on the base axis.

In comparison, a significantly smaller motor powers the hydraulic pump in the redesigned circuit, saving energy, and actuators weigh much less than the electric motors they replace. A similar weight advantage has resulted in hydraulics being the standard on heavy-duty construction and mobile equipment.

Hydraulics advantages

The power-saving benefits of hydraulics in the above application stem from the fact that a single electric motor and hydraulic pump can power many motion axes. And each axis, in turn, is a fraction of the weight of an electric motor-driven actuator of equivalent power. In addition, the hydraulic pump's motor can be sized to meet the average load the system must carry. This can be significantly smaller than in electromechanical systems, where the motor driving each axis must be sized for peak loads.

Hydraulic systems smooth out energy requirements with accumulators. These simple devices store energy in the form of fluid under pressure and release it when needed, making them useful tools in developing efficient circuits.

Most hydraulic accumulators use the compressibility of a gas - usually nitrogen - for storing energy. Basically, a hydropneumatic accumulator has a fluid compartment and a gas compartment, with a gas-tight element separating the two. For instance, bladder accumulators consist of a pressure vessel and an internal elastomeric bladder that contains the gas. The bladder is charged through a gas valve at the top of the accumulator, while a poppet valve at the bottom prevents the bladder from being ejected with the outflowing fluid.

When system pressure exceeds nitrogen precharge pressure, the poppet valve opens and hydraulic fluid enters the accumulator. The change in gas volume between minimum and maximum operating pressure determines the useful fluid capacity.

Thus, accumulators store energy when demands are less than average and transmit energy back into the system to satisfy peak requirements. Sufficient accumulator capacity ensures the hydraulic pump need not respond to sudden changes in the demand for oil.

Hydraulics also saves energy when machines must move slowly. That's because it can be inefficient to gear down the motion of electric motors via gearboxes. Overall efficiency depends on the efficiency of the motor and gearbox together. But gearbox efficiency specifications in catalogs are generally based on a single operating point and not entirely accurate for every application.

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