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|Possible applications for microhydraulics include the power for
compression, crimping, bolting, tensioning, shearing and other types of
Linear actuators enabled by fluid power systems and used for controlling flow are an important part of the equation to reduce or limit equipment size. Design engineers, plant managers and other MRO specialists require smaller and smaller fluid power components in order to fit actuators in and around many types of process equipment and machinery.
Some solutions for the reduction of equipment size involve building on the accomplishments of previous designs by either scaling them down or combining old technologies in new ways. But reducing equipment size is not always as simple as making parts smaller.
Outside the Flow Box
Smaller parts require new, sometimes more complex manufacturing techniques. Decreasing part size also changes mechanical and performance properties as the mass-to-power ratios change. New design vulnerabilities — including susceptibility to shock and vibration, fluid impurities, temperature and manufacturing imperfections — begin to emerge as parts get smaller using traditional approaches to hydraulic actuation.
As with many types of problems, a solution sometimes emerges when traditional methodologies are abandoned in favor of radically new approaches to problems. One such approach to smaller linear actuators with greater performance capabilities is microhydraulics, which is a concept that uses annular floating seal valves for regulating fluid flow.
Microhydraulics refers to hydraulic systems regulated by fluidics rather than conventional valves. At the heart of the technological breakthrough is the discovery that o-ring seals can be engineered to double as valves. The demands on an o-ring, however, must be given special consideration.
As a seal under high pressures, most materials, such as n-butyl rubber, behave as butter, extruding into the interstitial space between a piston and its cylinder wall. This gives rise to possible nibbling, a cause of o-ring failure.
In the instant an o-ring transitions from high-pressure seal to an opened annular valve, the pressure surge is phenomenal, with fluids reaching near sonic velocities. This gives rise to the Bernoulli Effect, which subjects a flexible o-ring to madding contortions, including slamming it back against its seat and momentarily disrupting flow.
Meticulous attention to seal plate and back-up ring designs obviate Bernoulli, allowing fluidic regulation of hydraulic circuits and actuators. When applied to flow control, the annular floating seal valves enable creation of small, lightweight systems that fit within the palm of a hand. Despite their size, these small systems are able to easily generate 10,000 PSI pressures with flowrates of one half liter per minute. The feasibility of even higher pressures awaits validation.
|Figure 1. In a cylinder with annular floating seal valves, the barrier
stops the flow of fluid from the reservoir to the accumulator, so that
the fluid displaced by the advancing ram is forced back through the ram
cap into the ram chamber to create a regenerative circuit.
Furthermore, traditional valves, pumps and other convention hydraulic components are eliminated from the new hydraulic systems. Instead, machined manifolds contain all the necessary porting.
The elimination of valves, pumps and other components results in fewer parts that have the potential to fail, which naturally has the advantage of a smaller footprint for a system that can do the same work as traditional hydraulic actuators. Fewer parts also lead to less downtime and improved system reliability. Less downtime and greater reliability can also translate into less overall cost to build and maintain such a system for flow control applications.
|The barrier and ram cap assemblies control the shift of the system in
and out of the regenerative mode. It is a totally automatic process
that is regulated by the resistance encountered by the ram.
The new cylinder design based on annular floating seal valves uses two mechanical subassemblies — the barrier and ram cap. With very small parts, the subassemblies work together as a regenerative hydraulic circuit that rapidly advances a cylinder’s ram.
The barrier and ram cap assemblies control the shift of the system in and out of the regenerative mode. It is a totally automatic process that is regulated by the resistance encountered by the ram. In early designs, the cylinder’s bulkhead separates the pump from the actuator. Later designs have the pump and actuator in a common manifold.
|As the ram engages its work, resistance and pressure build. At a
pre-determined pressure, the floating seal valve in the barrier
squeezes off the surface of the seal plate, thereby opening the barrier
The ram is driven by the cross-sectional area of the ram rod. As the ram engages its work, resistance and pressure build. At a pre-determined pressure, the floating seal valve in the barrier squeezes off the surface of the seal plate, thereby opening the barrier gate. Fluid on the rod side of the ram is now expressed into the accumulator, and the ram is driven by the cross-sectional area of the ram cap.
|In addition to cylinders, annular floating seal valves can be
configured to perform the functions of check valves, spring-loaded
valves and bi-directional valves, such as would be required for this
handheld crimping tool.
The fluid used within the cylinders and other tools enabled by annular floating seal valves contains high water content. The fluid is four times less compressible than oil, which eliminates the sponginess and related challenges associated with other oil-based hydraulic fluids.
The water-based fluid has a low viscosity that reduces frictional losses. The high surface tension also keeps the water-based fluids from leaking. Additives provide lubricity, rust prevention, anti-freeze and other desirable properties. Microhydraulic systems are suitable for hazardous and hostile environments. The technology also minimizes the environmental risks of accidental spillage of fluid, fugitive leaks and catastrophic disasters. Small liquid volumes, no connections and water-based fluids inherent in the technology also represent a green alternative to larger, traditional hydraulic fluid-based systems.
Robert McPherson is co-founder, chairman and CEO of LatchTool Group LLC, a manufacturer and supplier of microhydraulic systems. Prior to his current position with LatchTool Group, Mr. McPherson was vice chair, EVP and COO of a high tech NASDAQ company for over a decade and served as president and CEO of a medical diagnostic subsidiary. Mr. McPherson also held business and corporate development positions in a medical device company and spent a decade in product management for two Fortune 100 companies. Mr. McPherson earned a bachelor’s degree in Chemical Engineering from Cornell University. He can be reached at email@example.com.
Joshua Hoyt, Ph.D., is the chief technologist for ZIBA Design, an award-winning industrial design firm, and a consultant to LatchTool Group. Prior to joining ZIBA, Dr. Hoyt was the head of new product development for Flextronics. He is also the founder and CEO of Gearhead Associates, an engineering consultancy located in Portland, Ore. Dr. Hoyt assembles highly focused engineering teams on an ad hoc basis, which allows Gearhead Associates to quickly respond to client requests without delays and additional overhead. He has extensive research and development experience in the design of remote and portable sensing equipment, medical diagnostics and underwater robotics. Dr. Hoyt received his bachelor’s and master’s degrees from the Massachusetts Institute of Technology and his Ph.D. from MIT/Woods Hole Oceanographic Institute (WHOI). He can be reached at firstname.lastname@example.org.