Improving Power Density andShock Load Resistance With
Packing more power into smaller spaces is a common goal for equipment designers. In today’s industrial andscape, manufacturers are always seeking to im- prove efficiency and productivity while reducingoperating costs. Components that are powerful, compact insize, and able to deliver the same capabilities as larger andmore complex products are in high demand. This is especiallytrue for actuators used on outdoor equipment such as snowplows and utility task vehicle (UTV) dump beds. For motioncontrol in such applications, designers have traditionallychosen between hydraulic and electromechanical technologies but have had to accept some tradeoffs for each. However,there is now a hybrid solution that offers the best features ofboth technologies without the drawbacks.
High Load Handling
Hydraulic systems amplify force according to Pascal’s law,which states that any pressure applied to a contained fluid isdistributed equally throughout that fluid. This enables a relatively small force to be multiplied many times throughout asystem, giving hydraulic force its ability to handle high loads.
But directing that fluid to do real work in hydraulic cyl-inder-based systems requires a complex assembly of hoses,pumps, valves, and reservoirs, which can be noisy and costlyto install, operate, and maintain. Adding to this is extensivefluid-handling and storage and leakage issues, which increasemaintenance costs and jeopardize workplace cleanliness. Moreover, hydraulic cylinders do not adapt well to digital control.
Clean and Compact
Electromechanical systems derive power from a motor,converting electrical energy into mechanical motion. Electromechanical actuators using electrical energy do not requirethe complex, space-consuming, and messy support structuresof hydraulic cylinders, and because the current can be controlled infinitesimally, they are much more suited to today’sdigital systems. As such, system designers are increasinglyspecifying electromechanical technologies over hydraulics.
Electromechanical actuators have had drawbacks as well.Although they can move heavy loads of more than 3,500 lbs.,this capability has required a larger envelope. Furthermore,they are more sensitive to shock loading, which can be encountered when an overloaded trailer shifts its load while being towed. Such high shock loading can bend the ball screw,extension tube or gearing of a conventional electromechanical actuator—especially if the shock happened while the actuator was fully extended.
Handling High Loads Without
the Mess and Overhead
For many applications, designers can now enjoy the powerof Pascal’s law in a compact footprint that can also withstandshock loading, all at a lower cost. Electro-hydraulic actuatorscontain powerful hydraulic systems in a housing similar tothat used in a conventional electromechanical actuator.
Here’s how the inner workings of an electro-hydraulic actuator in an extension cycle work. An electric motor rotatesclockwise, turning the gears which pressurize the hydraulicfluid. Valves open to draw fluid from both the reservoir andhead side, and control delivery to extend the rod. On retraction, the motor runs counterclockwise, reversing the operation, and returning the fluid to the reservoir and the oppositeside of the piston.
Replacing the gear and lead screw assemblies of conventional electromechanical actuators with a compact hydraulicsystem allows for handling greater loads—up to 4,800 lb.—within a comparably sized envelope.
This architecture enables electro-hydraulic actuators towithstand shock loads by redistributing the fluids throughoutthe internal channels, thus absorbing the energy.
Compressed Cycle Times
In addition to high-density load handling and shock resistance, electro-hydraulic actuators can achieve higher totalcycle speeds. Unlike conventional electromechanical actuators, which run at a relatively constant speed regardless ofload, electro-hydraulic actuators run at high speedsat low loads and lower speeds at high loads. If ahigh percentage of total cycle time is at less thanfull load, there is potential for a decreased overallcycle time.
The load on an actuator controlling the dumpbed of a UTV, for example, might be 4,800 lb. initially and thus limited to a slow speed of about aquarter of an inch per second. As the bed raises, theleverage of the actuator increases and the materialfalls from the bed, decreasing the load on the actuator and increasing the speed to around 4 in./sec.With a conventional actuator, the speed while raising the dump bed would remain relatively constant,resulting in a longer total cycle time.
High power density, shock load resistance, and shorter cycle times can be beneficial in a wide range of applications, including the dump beds mentioned above, agricultural sprayers, snow plows, mower lift decks, and many others.
Snowplows: When a moving plow blade strikes a stationaryconcrete obstacle, it can produce a sudden jarring that woulddestroy most linear actuators. Hybrid actuators cushion thesetypes of blows and continue operating as required. As a result,equipment designers are able to save on replacement costs.
Larger agricultural sprayers: Agricultural sprayers are increasingly designed with larger boom lengths, reaching 130ft in the U.S. (177 ft in Europe), which increase cost efficien-cies in the field. Large sprayer booms use actuators to foldthem into a stowed position for transport, which puts veryhigh loads on the actuators. Also, the bouncing of the boomsduring transport subjects the actuators to moment force. Suchforces could destroy conventional actuators quickly, but electro-hydraulic actuators can handle them with ease.
Mower deck lifts: As mower decks grow larger and incorporate new innovations for faster mowing, traditional actuatorsare fast exceeding their limits for these applications. Hybridactuators offer more reliable load holding, higher durability,and the capability to withstand higher transport speeds andhigh-pressure washdowns.
Other examples. Electro-hydraulic technologies can be ofbenefit in many other applications, including:• Turf care, and lawn and gardening, including golf coursesweepers, sleeve hitch kits, blade lifts, and mid-mount implement lifts.
• Marine applications such as hatch lifts, transom actuators,and mud motor lifts.
• Material handling applications such as pallet lifts, lift tables,conveyor deflectors, and scissor tables.
• UTV/truck applications such as tailgate locks, UTV plows,and bed lift dump boxes.
• Construction applications such as pavers, quick-attachbucket releases, and plow/blade positioning.
The Right Fit
Electro-hydraulic actuators are designed for extreme applications requiring high load capacity in a small footprint. Likeconventional electromechanical actuators, they are cleanerand quieter than traditional hydraulic systems and do notrequire a complex, space, and maintenance-consuming support infrastructure of hosing, valves, and other external assemblies. However, unlike conventional electromechanicalactuators, they are capable of withstanding shock loads andoffer the potential to reduce cycle times. Any application thatrequires high power density or shock resistance or integration with digital control systems is a candidate for electro-hydraulic actuation.
By Chad Carlberg
Designers have traditionally chosen between hydraulic and electromechanical technologies but have had to accept some tradeoffs for each.
However, there is now a hybrid solution that offers the best features of both without the drawbacks.