Good Things Come in Small Packages
By Pat Jones, P. Eng.Fluid Power commentary pneumatics
Micro-hydraulic applications prove fluid power's power density is just as effective on the miniature scale.
When discussing the wide variety of applications for hydraulics, most discussions eventually lead to the large force and power transfer capabilities of hydraulics. Large industrial presses can have input power in excess of 750 KW (roughly 1000 HP) or more and generate forces in excess of 60 million newtons (15 million pounds) of force. However, at the opposite end of the spectrum, there exist miniature or micro hydraulic systems that are every bit as high performance and can work in some of the most demanding applications in the world.
As with most things engineering, necessity is the mother of invention. In the early 1970s, an oil field tool manufacturer contacted French hydraulic company, Hydro Rene Leduc, with a request for a small hydraulic pump that could operate down hole inside a drilling tool and micro hydraulics was born.
“Micro-hydraulics often offers the only solution where maximum force is required from a minimum power source within a small size envelope,” says Beth Ogden, a manager with Hydro Leduc USA. The company is still the market leader in micro hydraulic components, but others, such as Bieri Swiss, have also established a place in this niche market.
What defines miniature or micro hydraulics is the small envelope and power density available. The maximum outside diameter of the smallest pump with its housing is roughly 30 mm (1.18 inches) with a length of approximately 75 mm (3 inches). Pump displacements range from 12 mm3/rev (0.00073 in3/rev) to 2.2 cm3/rev (0.134 in3/rev) with operating speeds up to 5000 rpm and peak pressure ratings of 1000 bar (14,500 psi). This equates to a maximum input power range of roughly 34 watts (0.045 HP) up to 6.3 Kw (8.4 HP).
“The main challenge to producing micro hydraulic pumps, motors and related controls is manufacturing tolerances,” says Laurent Rosec of Precitech LLC, a Bieri Swiss representative. These become extremely critical when trying to achieve typical hydraulic component volumetric and mechanical efficiencies, but with miniaturized pumps and motors. If a typical industrial piston pump has a volumetric efficiency of 97%, this 3% leakage would represent 3 lpm (0.79 USgpm) in a pump trying to pump 100 lpm (26.4 USgpm) at rated pressure. For a micro hydraulic pump trying to pump 0.1 lpm (0.026 USgpm), even 90% volumetric efficiency represents an infinitesimally small amount of allowable leakage at 0.01 lpm (0.0026 USgpm).
Typically the pumps that drive these systems are fixed displacement wobble plate or radial piston designs. However, in the search for better system efficiency and higher operating pressures using the same power, variable displacement micro hydraulic pumps have also been developed that have pressure compensation and horsepower (torque limiting) control options. These are common controls on larger, commercially available pumps. When applied to micro hydraulics, pressure compensation allows for longer battery life while still providing maximum crimping force. When input power is fixed, horsepower limiting allows for higher maximum operating pressures when compared to using a pressure compensated pump.