Fluid power systems are noted for their high power density — permitting high force and torque output from relatively small components when compared to electromechanical systems. Microhydraulics makes it feasible to obtain a significant amount of force from a minimal power source within a very restricted space envelope. Thus, it can provide a straightforward solution to problems that are often beyond the limits of traditional mechanical options.
In many cases, these systems are ideally suited for wide-ranging applications like medical orthotic and prosthetic equipment, human-assist lifts, exoskeletons, hand tools, rescue robots, aircraft and missiles, race cars and oceanographic instrumentation.
Engineers might be tempted to simply downsize typical commercial components when the need arises to control motion and force in very small powered systems. However, the reality is a bit more complex because scaling laws are not intuitive, according to researchers Jicheng Xia and William Durfee of the University of Minnesota.
For example, they note that in a cylinder, force is proportional to area (L2) while weight is proportional to volume (L3). On the other hand, the thickness and weight of a cylinder wall required to contain a ﬁxed pressure goes down with bore size. Thus, the ﬁnal weight of a hydraulic system at small scale cannot be determined by proportionally scaling a large system.
Also, the fundamentals associated with pressure-driven ﬂow dictates that high pressures are required to permit high ﬂow rates through micro-sized channels. In laminar-flow conditions, an order-of-magnitude decrease in the hydraulic diameter of a channel increases by two orders of magnitude the pressure difference required to maintain a constant average ﬂow velocity.
Another critical barrier for increased hydraulic power density at reasonable efﬁciency is the seals. Surface effects such as friction drag of seals and viscous drag of gaps become signiﬁcant in small bores and that impacts overall efﬁciency. Too tight and friction dominates; too loose and the pressurized ﬂuid will leak past the seal. Cost and power consumption are also important considerations. Fortunately, a number of manufacturers have designed or re-engineered hydraulic components to work on a “miniature” scale.
As one example, Bieri Hydraulik, a unit of Hydac International based in Liebefeld, Switzerland, makes six standard versions of Type AKP micro-axial piston pumps designed with three or five pistons. For instance, the 5-piston Size AKP36 pump measures only 1.4 in. (36 mm) in diameter by 3.9 in. (99 mm) long. It features a displacement of 0.36 cm3/rev with 250 bar maximum pressure and 4,000 rpm max speed.
The Size AKP103 measures 1.9 in. (50 mm) in diameter x 3.8 in. (98 mm) long. It has 3 pistons, displacement of 0.1 cm3/rev, 500 bar max rated pressure, and runs at speeds to 5,000 rpm. A 5-piston version offers displacement of 0.3 cm3/rev.
The quiet-running units reportedly offer high volumetric efficiency even at minimum speed of 100 rpm. They are valve controlled on pressure and suction side, so are not suitable as motors. The small units are used in offshore and oil and gas applications, in metering systems, and general hydraulics systems with small displacements.
Hydro Leduc, Azerailles, France offers a complete range of fixed and variable displacement micro-pumps; micro-motors (speeds from 350 to 6,500 rpm); check, pressure-relief, solenoid and pilot valves; and complete micro-power packs for operating in widely varied working environments.
For example, its PB32 fixed displacement micro-pump has a body diameter of only 1.28 in. (32 mm) with displacement as small as 0.0007 in.3, maximum speed of 5,000 rpm continuous and 6,000 peak, and maximum pressure of 4,350 psi continuous and 5,075 psi peak. A slightly larger PB33 HP version has a 0.0027 in.3 displacement and a maximum continuous pressure rating of 13,050 psi and max peak of 14,500 psi (1,000 bar).
The Lee Company., Westbrook, Conn., makes an array of miniature, high-performance fluid control components, including Lee Plug® expansion plugs, solenoid valves, flow restrictors, safety screens, relief and check valves, and shuttle valves. The company’s flow controls, to cite one typical product, offer metered flow in one direction and free flow in the opposite direction. It’s also called a one-way restrictor. In the smallest size, diameter is only 0.18 in., yet nominal system pressure rating is up to 3,000 psi. Other similar-size products include poppet-style check valves that can flow one GPM at 25 psid and have a nominal system pressure rating up to 4,000 psi; and pressure relief valves that have cracking pressures from 20 to 100 psid and, in some versions handle nominal system pressures up to 5,000 psi. Some miniature check valve and restrictor models can even handle system pressures up to 8,000 psi.
Likewise, safety screens as small as 0.13 in. diameter help protect orifices, relief valves, and other sensitive hydraulic components. Critical components are often relatively immune to low levels of small-size contaminants, but a single large particle can cause sudden failure — possibly with catastrophic effects. While filters maintain fluid cleanliness during operation, safety screens provide an added level of protection. The units come with hole sizes from 0.0008 in. to 0.062 in., and high-pressure versions won’t burst or collapse at pressures of 7,500 psid, even if fully clogged.
SFC Koenig, North Haven, Conn., offers a range of plugs, flow controls, check valves and related components. The Expander plugs, for instance, reportedly seal drilled holes with excellent reliability. They come in sizes from 0.093 to 0.875 in. and are rated to 6,500 psi (450 bar) for push-type units and 7,200 psi (500 bar) for pull versions.
The company’s stainless-steel Restrictor units provide precise flow control in fluid systems and are available in sizes as small as 0.093 in. (4 mm) in expander and threaded styles, and handle pressures to 2,900 psi (200 bar). Orifice can be specified to achieve desired flow rates. Check valves, 0.216 in. (5.5 mm) diameter, handle forward or reverse flow, have a cracking pressure of 2 to 29 psi (0.14 to 2 bar) and maximum working pressure of 4,352 psi (280 bar).
Takako Industries, a member of the KYB Group based in Kyoto, Japan, claims to make the world’s smallest axial-piston pump. The square shape TFH-040 unit measures only 1.18 in. (30 mm) wide by 3.0 in. (77 mm) long and is rated for a maximum working pressure of about 2,030 psi (140 bar). Displacement is 0.4 cc/rev, input speed is to 2,000 rpm, with a flow rate of 0.8 lpm.
The unit is part of a family of micro-pumps which feature a hybrid drive system that combines the benefits of hydraulics with the controllability of an ac-servomotor and inverter to satisfy a broad range of specifications with a small-volume pump. Typical applications, according to the company, include a pump for valve controls, mold switching equipment for forming machines, hydraulic clamps, and crimping presses.