Unweighting a crane to increase payload limit
The payload capacity of a crane depends on the strength and stiffness of the materials used to make the arm. It also depends, ironically, on the arm’s weight. That is, the greater the arm’s weight, the less payload it can bear. Conversely, you can increase the payload capacity of the crane by reducing the mass of its arm. In other words, the crane arm is an ideal application for composites.
The Manitowoc Co., a crane manufacturer located in Shady Grove, PA, US, recognized this advantage and decided to target the stinger or fly jib — the final segment of an articulating crane arm — on one of its truck-mounted cranes, replacing the traditional 24-ft (7.3m) steel structure with one of carbon fiber composite.
Sammy Munuswamy, senior principal engineer, global engineering and innovation at Manitowoc, says the company is “in the business of building lifting experiences for our customers around the world.” And a quality “lifting experience,” in Manitowoc’s view, should be one where the tool (crane) facilitates the jobs to be done at a variety of jobsite environments, including buildings, roadsides, heavy construction sites and more. “Cranes are getting lighter,” Munuswamy says, “and we need materials to meet that expectation. The stinger section was identified as an ideal candidate for conversion into a carbon fiber light-weight structure since the outermost crane arm components generate the highest bending moments on the crane. Therefore, reducing weight in such members brings the most tangible benefits.”
The stinger was developed by Manitowoc in collaboration with Riba Composites Srl (Faenza, Italy), which has extensive experience designing and manufacturing large composite structures. Munuswamy says one of the challenges the company faced was the reality that cranes, as a cost-sensitive, low-volume product, do not allow for expensively engineered structures. In addition, the composite stinger is a drop-in replacement for its predecessor.
Because the carbon fiber stinger works as a component retrofit compatible with existing cranes, Riba's engineers exploited all the available design space, maximizing the moment of inertia and the geometric properties of the stinger. The result is a hybrid structure where steel and composite match to take advantage of the specific properties of each material. The junction between steel and composite relies on bonding and bolts, which allow an effiient solution.
Andrea Bedeschi, general manager at Riba, says the composite stinger is hand-laid, using carbon fiber prepreg and autoclave cure. The carbon fiber, standard-modulus 12K and 24K tow, is supplied by Mitsubishi Chemical Carbon Fiber & Composites Inc.. The resin is a toughened epoxy. Riba performed NDT evaluation of the stinger; physical load, stability and structural performance testing was done by Manitowoc.
The composite stinger is 35% lighter than its steel predecessor and, says Munuswamy, increases payload capacity 12-15% more than the steel version in some specific boom configurations. The composite stinger also is more expensive than its steel predecessor, but Munuswamy says this is more than compensated for by increased jobsite efficiency and transportability.
Will Manitowoc expand carbon fiber use to other crane components? “This [the stinger] is leading us in that direction,” Munuswamy says. “The stinger was the first step.”
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