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Rivetless Fastening Takes Off
Nut plate assembly eliminates rivets to simplify Pratt & Whitney's jet engine assembly
Michael C. Gabriele, Contributing Editor -- Design News, August 14, 2005
As it ramps up production and testing of its GP7000 engine for the Airbus A380 double-decker jet, Pratt & Whitney (P&W), is already enjoying the benefits of reduced fabrication time and enhanced assembly productivity through the use of a rivetless metal joining system designed by Textron Fastening Systems.
P&W, a unit of United Technologies Corp., selected the Textron rivetless nut plate for the assembly of the GP7000 fan case. The joining system, which meets National Aerospace Standard Military (NASM) 25027 requirements for torque out and push out, consists of an expanding mandrel and a retainer; together these eliminate the metal flaring typically required for riveted nut plates to meet the NASM 25027 requirements. A locking clip and a nut complete the assembly. (For a look at how the assembly goes together, go here.)
Mark Costa, design analyst and fan case designer for P&W, describes the rivetless nut plate as a "very benign" joining technique that has met all specifications and is delivering ease-of-assembly advantages. "The biggest benefit for us is the reduced amount of drilling, counter sinking, inspection of rivet holes and installation that we need to do," Costa says. "The installation is very quick. As the mandrel pulls through it seats itself. Our technicians enjoy working with this product."
Costa recalls that the Textron fastener proved to be the best choice to marry the aft acoustic liner, which bridges the P&W forward fan case with the General Electric aft fan case. The GP7000 engine family is the result of a strategic alliance between the two aerospace manufacturers, combining the GE90 and PW4000 engine families.
Originally, designs called for a bonded-in liner on the P&W forward fan case and a bolted liner in the aft GE fan case. However, the need to maximize the acoustic area of the engine drove the design to a common bolted-in liner, which was able to bridge both fan cases.
Textron had touted its rivetless product and demonstrated a prototype design in May 2003 at P&W's Middletown, CT, facility. Coordination between P&W and Textron refined the design to meet P&W requirements. P&W engineers determined that the Textron method "made sense for this engine," and it prevailed as the joining technique of choice, Costa says.
The first engine set of development hardware was delivered and assembled at P&W's Middletown facility in December 2003 and was tested in early 2004. Steven Stalcup, Textron Fastening Systems, Aerospace, district sales manager, who interfaced with P&W engineers on the fastener system, says the benefits provided by this joining method add up for manufacturers, particularly when it comes to eliminating the amount of drilling done in assembly operations.
There are 145 rivetless nut plates used in each GP7000 engine—56 in the case and 89 in the "splitter," or air fairing, located inside the fan case. Textron's rivetless design requires only a single drill hole for each fastener. By comparison, a riveted joining system would need an additional 290 drill holes—two additional holes required to align and fix every nut plate at each attachment point on the fan case. In addition, Stalcup says the rivetless design eliminates the countersinking and deburring that would be necessary for the additional drill holes.
Advantages from the Textron metal joining technique also extend to maintenance operations throughout the engine's life, according to Stalcup. Because of its rivetless design, there is no need for maintenance technicians to drill out rivets in order to remove and replace nuts. Instead, mechanics only need to unclip and replace worn nuts with the Textron design. This benefit also reduces the risk of scarring the aircraft skin caused by the slip of a drill when working in tight locations. In addition, it eliminates the risk of oversizing rivet holes in the fan case as well as other potential rejection criteria in maintenance inspection, Stalcup points out.
The fastener components—designed by Textron engineer Ray Toosky—are produced at Textron's aerospace products center in Santa Ana, CA. The components typically are made of A-286 stainless steel, but can be offered in other metal grades as well, depending on the application.
Specified for aluminum sheets with a minimum thickness of 0.062 inch, the rivetless nut plate is installed using Textron's Cherry G704B assembly tools. Stalcup explains that an operator inserts the mandrel into a pre-drilled hole and actuates the tool. As the fastener sleeve is pulled into the hole, the mandrel radially expands the retainer in the screw hole and pushes outward, holding the retainer in place as a result of the expansion. Once the attachment point has been established on the aluminum sheet, the mandrel is ejected.
Four GP7000 engines will power a single Airbus A380 jetliner. Costa says the engine has an outside diameter of 125 inches, a fan-tip diameter of 116 inches, a length (flange to flange) of 187 inches, and a takeoff thrust of 70,000 lbs. He notes that the GP7000 has the largest production fan diameter of any jet engine ever produced by P&W. Flight testing for the engine is slated to begin early next year, Costa says.
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| For more information on rivetless nut plates, visit Textron Fastening Systems. | ||
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