01 April 2013
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Harnessing the Californian sunshine

Originally developed for automotive and appliance applications, Flangeform is a captive sheet metal fastener for high strength attachments in thin gauge material. Gavin Ball describes how
an American solar power project has benefitted from its adoption.

Captive sheet metal fasteners provide torque and push/pull out values that 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.

A case study illustrates how all of these advantages were used to add value to a large solar power project in the US. 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 minimizes 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.

The West Coast USA solar farm will generate 200 MW of renewable power when completed later this year. It will occupy nearly 2000 acres of land in the Southern California desert on an ideal site with especially high solar irradiance and grid access and a Power Purchase Agreement is in place with a major utility company.

Bespoke design

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 1.0 mm thick, about 250 cm long,
with five nuts and two studs pre-installed. Overall system design makes pull through values for these studs especially important.

The other section with captive fasteners is 1.5 mm sheet, about 60 cm 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.

The tracker designers initially believed industry standard rivet nuts 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. 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 rivet studs.

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. Azincrich, 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 rivet nuts 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 earlier this year and both roll formers quickly achieved or exceeded the installation rates they had projected.

Author
Gavin Ball

Related Companies

PSM International Fasteners Ltd

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