M O B I L E
H Y D R A U L I C S
In contrast, traditional long-stroke cylinders face several hurdles. Buckling is probably the main problem. It can be overcome in some applications using a pulling cylinder. Hydraulic stiffness and hydraulic capacitance can also have an unfavorable effect on dynamic behavior and controllability. Longer hydraulic cylinders usually face difficulties at high pressure, as it reduces the system natural frequency. Thus, advantages normally associated with hydraulics — compactness and high power density — diminish with increasing stroke length. The Hydraulic Infinite Linear Actuator solves these problems. HILA provides long strokes at high system pressure, with a higher stiffness compared to conventional hydraulic cylinders. Each HILA cylinder has a much shorter body, so a sufficiently high natural frequency can be achieved at high system pressures. And bulk modulus increases slightly with increasing pressure. These are favorable factors in terms of actuator control design. The compact and lightweight system, built with robust and cost-effective standard components, is well suited for tough and demanding environments, especially in mobile applications, where there presently are no alternative solutions. Strokes as long as 7 to 8 m (23 to 26 ft) are possible, depending on the piston rod diameter, for pulling loads in horizontal applications. Even longer strokes can be implemented in vertical applications, such as in elevators. As the HILA technology needs small cylinders and low oil volumes, it is possible to design compact and long stroke electrohydraulic actuators (EHA). Symmetrical piston areas simplify the design. HILA technology also lets several active actuators to be positioned and locked on the same piston rod, which can be advantageous in certain applications. This means that several processes can be performed in parallel instead of serially, without the need for a dedicated actuator for each element. Here’s a look at two such applications. One involves timber vehicle aerodynamics and logistics; the other shows how HILAs can flexibly and optimally position row units on agricultural planters, to secure productive and sustainable farming. More-efficient timber trucks
There is a great need in forestry transport to reduce fuel consumption and limit variable costs and carbon dioxide emissions. One solution is to develop larger vehicles that carry greater loads and reduce transport cost per mile. There are likewise opportunities to improve vehicle aerodynamics, as air and rolling resistance are two of the most important factors that hurt fuel economy. Truck manufacturers put a lot of effort into reducing
The HILA combines two short-stroke cylinders with two coordinated clamping mechanisms, one in each cylinder. Simple logic valves control the motion. The compact and lightweight system permits strokes of more than 25 ft. Or several actuator bodies can move along a stationary rod, for use in timber transport and agricultural seeding applications. | courtesy of Gustav Näslund, GN Tech
air resistance through better aerodynamic design of the cab and chassis. For the trailers, no similar development has taken place. Complicating matters, a timber trailer is like two completely different vehicles, loaded and unloaded. When unloaded, several points experience strong turbulence, specifically around the vertical stakes that support the logs; and around the horizontal banks — the structural cross-members that hold the stakes in place and on which the logs sit. Simulation studies show that the biggest aerodynamic problems for unloaded timber trucks are turbulence under the trailers, and air resistance due to stakes and banks that stand upright in the air stream. Bundling stakes and banks together at one point would significantly reduce air resistance, depending on vehicle speed. The trend is toward timber vehicles that drive longer distances at higher average speeds, due to fewer and larger sawmills and pulp mills. As air resistance increases with the square of the speed, the need for consolidated banks and stakes is even more important for fuel economy. The same holds for aerodynamic resistance and turbulence that banks and stakes create on unloaded timber train carriages. While unloaded timber trucks drive at maximum speeds of around 100 km/ hr (60 mph) in the U.S. and Canada, freight trains travel at even higher speeds. Studies show that covering unloaded coal wagons can reduce air resistance by more than 40%. HILA advantages
HILA systems can address these issues. With HILA technology, banks and stakes on timber vehicles can be individually positioned, automatically, with high locking force. The system can also significantly increase vehicle flexibility and open up more combinations that facilitate cost-effective and environmentally friendly logistics for both timber and complementary goods flow. Key functions on towing vehicles and timber trailers include: The vertical stakes and horizontal banks on unloaded timber trucks cause significant aerodynamic drag when traveling at highway speeds. Bundling them together at one point reduces air resistance and can cut vehicle fuel consumption by around 5%. | courtesy of Gustav Näslund, GN Tech
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FLUID POWER WORLD
8 • 2021
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