Machine Design / September 11, 1997 / Page 106
Couplings such as those on turbines sometimes present bolting problems, and shaft alignment is a primary concern. Current practice uses fitted bolts with interference fits, requiring precision machining. These are also difficult to assemble and disassembling large machine elements with interference fits is even more difficult. A bolting system for machines requiring large fitted bolts, such as steam turbines, generators, and steel mill machinery, eliminates interference fit components. Instead Expansion Bolts developed by Superbolt, Carnegie, Pa., consist of a tapered stud with threads on both ends. The stud slides into a sleeve with an internal taper. The sleeve has a slot which allows it to expand as the tapers are drawn together. Holes for the bolt have to be line-bored - not oval or tapered - to ensure good alignment. However, close diameter tolerances are not required. Expansion Bolts bridge relatively large gaps, for example, a 3-in.-diameter bolt can expand up to 0.040 in. For practical purposes, to ease assembly and disassembly, the bolts are 0.001 to 0.002 in. in diameter smaller than the hole diameter.
Expansion bolts begin to work as a Torquenut on the small end supplies the large force required to draw the tapered stud into the tapered sleeve. As the tapered sleeve expands, a large radial force is exerted in the coupling through-holes. A Torquenut on the large end creates additional clamping force on the joint and also eases removal of the tapered stud. Once the tapered stud is extracted, the sleeve collapses for easy removal. For disassembly and assembly, expansion bolts only need hand tools. And the loose fit sleeves, up to 0.010 to 0.020 in. in the through-hole, are easily inserted even in holes misaligned due to rotation. Tightening requires less than 5 main/assembly. Because the torque required is usually under 100 lb-ft. high-powered tools, bolt heaters, and high pressure hydraulic systems aren't necessary.
For precise alignment, a system needs just four Expansion Bolts and loose fit through-bolts with Torquenuts fasten through the remaining holes. They're reusable since the split sleeve springs back to the original size. Compared to the close tolerance sleeve in other systems, the loose fit split sleeve is much less likely to hang up during removal. Axial and radial force are generated simultaneously, making it easier to pull together coupling halves. A multi-jackbolt design compensates for small variations in the perpendicularity of the spotface to the hole centerline, eliminating coupling face machining where the nuts seat. Also, because generous sleeve expansion permits finish machining in advance, on-site finish machine is eliminated. |
