Flangeform® is a unique captive sheet metal fastener for high strength attachments in thin gauge material. Originally developed for automotive and appliance applications, it provides torque and push/pull out values that PSM International says greatly exceed industry standard rivet or clinch fasteners. Installation time can also be much less when used in secondary operations or integrated with automated processes. Here PSM International uses a case study to illustrate how all of these advantages were used to add value to a large solar power project in the United States.
Utility scale solar farms typically use PV panels mounted on tracker or racking structures. Bolted joints enable assembly in the field but require construction personnel to handle massive quantities of individual fasteners. Captivating one half of these connections speeds up the erection process and minimises lost hardware. Mechanical performance is critical since wind loads can be dynamic. Hardware must also resist corrosion in a harsh outdoor environment. Project developers and other stakeholders often require their suppliers to provide performance warranties of up to 20 years.
Application
This solar farm will generate 200MW of renewable power when completed in Q4 2013. It will occupy nearly 2,000 acres of land in the Southern California desert on an ideal site with especially high solar irradiance and grid access. A Power Purchase Agreement is in place with a major utility company. A single-axis tracker structure will be used to mount thin film PV panels. This system is a bespoke design that includes rails roll formed from HSLA steel. Two of these include captive fasteners. More than 14 million bolted joint connections must be assembled on site during the construction process. One of the rails with fasteners is 1mm thick, about 250cm long, with 5 nuts and 2 studs pre-installed. Overall system design makes pull through values for these studs especially important. The other section with captive fasteners is 1.5mm sheet, about 60cm long, with 2 nuts. Torque performance is the key requirement for these nuts. Stainless hardware is required throughout the project and future disassembly for maintenance is a must. Potential galling between mating fasteners was also a concern since both would be made from a similar grade of A2 stainless steel.
Fastener solutions considered
The tracker designers initially believed industry standard rivnuts and studs would meet their structural requirements if fasteners and sheet thickness were both sized properly. A design was completed and sent to roll formers for proposals. Sub-contractors roll forming the rails would need to install these fasteners as a secondary operation – manually, and one at a time.
PSM proposed Flangeform to a number of roll formers bidding on the project as a way to save on labour. Proposals were submitted that included installing all the fasteners required in each rail with a single press stroke. Additional automation in the form of a system that bowl fed five nuts simultaneously was also presented.
Why Flangeform was selected for production
The tracker design team found Flangeform provided two structural advantages. Prototype testing confirmed that pull out requirements for the studs would be higher than planned and could not be met with standard rivstuds. Flangeform studs exceeded the new requirements with no modifications.
Improved torque performance in thin material also enabled them to downsize the two nuts in the short section one thread size, and reduce the steel thickness from 1.78mm to 1.5mm. The nut face profile on these two nuts was easily modified by PSM to help locate mating parts and aid in field assembly. Flangeform exceeded expectations again and was specified for exclusive use on both applications.
All three Flangeform fasteners were specified with a coating to resist galling. Using A4 material was investigated, but rejected because of its higher cost. A zinc-rich, chrome free, dry film coating with proprietary chemistry was chosen as an economical alternative. The remaining nut was open to roll former choice; either Flangeform or rivnuts were approved for use. Volume on this was significantly higher than the other two fasteners. Two roll formers received orders for the rails needed to complete the project. Both of them elected to use Flangeform exclusively based on compelling labour savings. Custom bowl feed and press tooling was quickly designed, fabricated and commissioned by PSM. Mass production started Q1, both roll formers quickly achieved or exceeded the Flangeform installation rates they had projected.
Having spent a decade in the fastener industry experiencing every facet – from steel mills, fastener manufacturers, wholesalers, distributors, as well as machinery builders and plating + coating companies, Claire has developed an in-depth knowledge of all things fasteners.
Alongside visiting numerous companies, exhibitions and conferences around the world, Claire has also interviewed high profile figures – focusing on key topics impacting the sector and making sure readers stay up to date with the latest developments within the industry.
