Solar Energy (Photovolatics)
It is widely accepted that the reliable aluminum PV racking system fastened joint must have a clamp load which is high enough to prevent the joint from either opening or slipping due to dynamic wind loading. This joint clamp load is frequently developed through the torque tightening of a bolt and nut. Unfortunately, much of the initial clamp load developed in PV racking system fastened joint is often lost due to non-rotational and rotational loosening of the bolt and nut after tightening. This presentation will briefly present each of these loosening mechanisms and describe general methods in used in the automotive and off-highway industries to address these problems.
Non-rotational loosening of the bolt and nut occurs when no relative movement occurs between the internal and external threads but a loss of joint clamp load still occurs. In general, non-rotational loosening or joint relaxation in bolted joints is caused by localized plastic collapse which occurs at each contacting interface within the joint. In aluminium PV racking systems, another source of plastic strain in the joint is electrical bonding devices or serrated surfaces which purposely pierce the non-conductive anodized aluminium outer surface to improve the electrical bond. As a result, a significant portion the clamp load developed through tightening can be lost after the fastened and bonded joint is placed in service, especially with very short grip lengths and if the bolts or nuts are not retightened.
Rotational loosening of the bolt and nut, more commonly referred to as self-loosening, occurs when the nut rotates under the action of external loading. Gerhard Junker is credited with explaining why and how rotational loosening (self-loosening) occurs. He showed that bolts and nut will self-loosen when relative movement occurs between the mating threads and the fastener bearing surface. Such relative movement will occur when the transverse force acting on the joint is larger than the frictional resisting force generated by the joint clamp load and the joint slips. PV racking bolted joints are prone to self-loosening because of the dynamic loading of the joints due to very high wind loads. Automotive and off-highway equipment manufacturers have long recognized these joint loosening mechanisms of dynamically loaded joint and as result, have developed standards and methods to either reduce the clamp losses or prevent the loosening altogether. It is believe that the application of these standards and methods to PV racking system bolted joint could improve the reliability of PV racking system bolted joints substantially.