AEROSPACE | APPLICATION


.. and a closeup of its machinery


due to the eccentric platform load. Radial forces tend to bend the mast cross-section out of plane and to deform it, forcing the round shape to look like an ellipse. This causes mast systems to jam a lot more towards the end of their useful life. CP&A installed instead a four-rail system, with, again, the guide wheels and rails equally spaced around the circular cross- section of the mast. A four-rail system eliminates radial loads - there is always a wheel at a position that can counteract that stress - and allows the loads to be transferred tangentially to the tube; the offset load does not therefore locally deform the tube out of plane. Richard Phillips, mechanical engineer


at CP&A, says “One of the biggest design decisions we had to make was the that of the mast rail system. By using four rails we were able to create a more favourable load path and direct the loads to transfer from the platform to the mast tube in the most favourable way possible. We optimized the load path, which lead to a minimum weight design and has the added benefit of being easier to maintain.” “Reducing the weight of the mast


system was important; this allowed a larger platform, while maintaining the weight of the previous stackers. Maintaining the weight was very important for structural considerations related to the building.” Notably, even with the weight of a fourth


rail the mast tube did not need to be as thick, since the load path from the guide


26 | February 2025 | www.hoistmagazine.com


rollers to the mast tube is now tangential to the mast tube - a twisting force rather than a crushing or bending one. As Phillips said, when the platform rotates, it wants to twist the mast tube with a torque action, and tube sections are very strong against torsion. He explains the advantage with a kitchen-sink example: “An easy experiment is to squeeze an empty can of soda and see how easy it is to create a bulge. Compare that to twisting the can: it takes much more effort to damage the can by twisting. Chances are you’ll only be able to break it by gripping too hard and unintentionally squeezing it.” The two-rail tube of the original crane


was 1/2in (13mm) thick and the three-rail tube was 3/8in (9.5mm) thick. CP&A was able to get the four-rail tube thickness down to 3/16in (4.7mm), thus, saving weight despite having more rails than the other systems. Another concern about the older cranes,


says Phillips, was that the running surface of the bar rail was pitting. Pitting occurs when there is repeated loading and the contact stresses between the guide rollers and running surface are high enough to start peeling off the top layer of the running surface.


“The pitting phenomenon was difficult to


solve,” he said. “Hardened tool steel offers great pitting resistance; however, it is very difficult to weld and machine.” The issue was solved by avoiding welding altogether. Instead, new hardened steel


runners were bolted into place. Reasoning that only the running surface needs to be hardened, CP&A chose to bolt hardened rail runners onto a machined surface on a channel section. The rail runners were made from tool steel and hardened with a gas nitride process after machining. “Gas nitriding is a great way to harden


steel and maintain dimensional control,” Phillips explained. “The use of tool steel was only possible because the steel was not being welded. For bolted rails you just undo the bolts and replace the rails. Welded rails are much harder to replace; when they have too much wear the whole mast would need to be replaced.” The stacker cranes that were being


replaced used A514 (also referred to as T-1) steel, which requires a special weld procedure. T-1 steel is much harder than standard structural steel; for the existing cranes the rails had a hardness of about 300 Brinell (145ksi), while, for comparison, the replaceable tool steel rails were hardened up to 475 Brinell (240ksi), so they are much less prone to pitting. The crane also features a series of


complex features such as collision avoidance and operating in a hazardous zone while carrying people. So, Aerospace lifting is a varied scenario, with small- and large-scale requirements. At all scales though reliability is all- important. As with aircraft themselves, coming down to earth unexpectedly would be very bad news indeed.


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