Articulated booms have become the workhorses of many industries because they can reach areas that would be inaccessible otherwise. For example, attaching a basket to the end of an articulated boom allows workers to inspect the underside of a bridge for assessing the condition of the understructure. Attaching a pair of forks allows lifting materials over or around obstacles in a crowded warehouse or storage yard. In construction, piping mounted alongside the articulated boom enables pumping concrete directly to a work site. This saves a tremendous amount of time over dumping concrete into a trough and transporting it in small batches to the work site.
But no matter what the industry, the number of joints in articulated booms and their reach have been limited by weight. Much of this weight is attributed to the hydraulic cylinders that provide power for movement. Reducing weight of the hydraulic cylinders, then, would make it possible to expand the capabilities of an articulated boom. Reducing the bore (and, therefore, weight) of the cylinder is one way of accomplishing this. But unless hydraulic system pressure is increased substantially, the cylinder will be unable to generate enough force to maintain an equivalent load rating. Furthermore, designing a hydraulic system to operate at higher pressure can add substantially to its cost.
Gross vehicle weight is important because laws restrict the weight-per-axle that a vehicle can carry and the number of axles allowed on specific roads. And because adding length to a boom would increase its weight, a longer articulated boom would have to be made of more expensive, higher-strength materials to avoid having to add another axle to the vehicle. Incorporating an additional axle to what is generally used today would require special licensing for the vehicle. But special licensing adds to the cost of a project and almost certainly leads to production delays. Therefore, the ideal articulated boom would have a longer reach or more joints, yet weigh no more than what is commonly used.
Widely used welded cylinders made of high-strength steel exhibit a high strength-to-weight ratio, but a new type of cylinder holds potential to more than double the strength density of hydraulic cylinders. Having a geometry similar to standard tie-rod cylinders, the new cylinder design uses a barrel made of carbon fiber-reinforced plastic (CFRP) wrapped around and bonded to a thin stainless steel sleeve.
Lighter cylinders improve design
Designed and manufactured by Lingk & Sturzebecher Gmbh, Stuhr, Germany, the CFRP cylinders have been made in a variety of sizes, shapes and configurations for use in robots, helicopters, foundation jacks, and, of course, articulated booms. Carsten H. Müller, president and owner of Lingk & Sturzebecher, revealed that his company recently won a contract with Airbus Industries to supply CFRP cylinders for use in commercial aircraft. Furthermore, he said his cylinders also are used in the M55 concrete pump manufactured by Putzmeister, Aichtal, Germany. At 55 m (180 ft), the M55 has the highest vertical reach in the world for an articulated-boom concrete pump.
Müller explained that as stroke increases, the weight of the barrel becomes a higher percentage of a cylinder's overall weight. He mentioned that in a steel cylinder, weight of the cylinder's end caps remains constant as stroke increases, and weight of the piston rod increases with stroke. But, according to Müller, because the barrel weighs so much more than the rod per inch of stroke (especially if the cylinder has a large bore and relatively small-diameter rod), as much as 75% of a cylinder's weight could be attributed to the barrel.
Müller continued by saying that a CFRP barrel weighs only a fraction of what a steel barrel of the same bore weighs. Furthermore, he said the CFRP barrel also exhibits a much higher tensile strength than steel, so a thin-walled CFRP barrel can withstand high pressure. He cited 350 bar (5800 psi) as the maximum rated working pressure for CFRP cylinders used in the concrete pump pictured at the bottom of this page. He also explained that the stainless steel liner provides an ideal surface for the piston seal to achieve low leakage and long life.
How much lighter are the CFRP cylinders? That depends, of course, on bore, stroke, rod diameter, and other considerations. But Müller revealed that the average CFRP cylinder weighs about 60% less than the steel cylinder it typically replaces, such as in mobile equipment. He admitted, though, that CFRP cylinders cost more than conventional cylinders, but that this can be justified by the weight savings. In general, he said, a CFRP cylinder may cost twice as much as a conventional cylinder, but weigh half as much. The weight saved by using the CFRP cylinders allows other improvements in the equipment design, so the CFRP cylinders add only a small increment to overall manufacturing cost but make a huge contribution to improved design. And as with most new technologies, Müller expects the cost differential to shrink as production volume increases.
CFRP cylinders are now part of Parker Hannifin GmbH, Kaarst, Germany. For more information, visit www.parker.com/lightraulics.