As the federal government is set to nearly double car and light-duty truck fuel economy standards to the equivalent of 54.5 mpg by 2025, electric vehicle (EV) and hybrid electric vehicle technology will play a vital role—if lingering battery life and overheating issues can be resolved.
Battery packs on EV and hybrid vehicles only store the energy of about 1-2 gallons of gasoline. In addition, energy losses can come from the fasteners that hold the battery packs together. Traditional fasteners have difficulty maintaining electrical conductivity and connectivity with EV and hybrid battery terminals because after extended car vibration and thermal cycling, they typically lose about half of their original clamp load, according to Kevin Peacock, application engineer for Stanley Engineered Fastening, Madison Heights, Mich.
In addition, heat can build up due to the battery’s live current, and electric arcing can occur, which is a potential fire or explosion hazard.
“Inside EV and hybrid batteries, whether lithium or acid-based, several packs are typically linked to each other in a series. If a connection is weakened by losing clamp load, you lose not just one battery cell but the whole series of battery cells,” cautioned Peacock.
The Spiralock locking thread may resolve many of these issues, as it can safely harness every milliamp of hybrid vehicle electricity without overheating. It can help ensure adequate clamp load and joint integrity in critical areas from the battery pack and battery terminals to the battery box itself, while improving connectivity and battery life.
Traditional locking fasteners, which have a 60° thread form, feature a gap between the crest of the male and female threads that can lead to vibration-induced thread loosening, inadequate clamp load, and overheating in critical EV and hybrid battery joints. Stress concentration and fatigue at the first few engaged threads is also a problem, along with an increased probability of shear, especially in soft metals, due to a tendency toward axial loading. Temperature extremes can also expand or contract surfaces and materials, potentially compromising joint integrity.
The Spiralock locking fastener overcomes these challenges while also eliminating traditional concerns about debris, stripping, or additional stack height.
What makes this re-engineered thread form unique is its 30° wedge ramp added at the root of the thread, which mates with standard 60° male thread fasteners. The wedge ramp allows the bolt to spin freely relative to female threads until clamp load is applied. The crests of the standard male thread form are then drawn tightly against the wedge ramp, eliminating radial clearances and creating a continuous spiral line contact along the entire length of the thread engagement. This continuous line contact spreads the clamp force more evenly over all engaged threads, improving resistance to vibrational loosening, axial-torsional loading, joint fatigue, and temperature extremes.
The free-spinning design allows a nut to be run all the way down by finger with little resistance between meshing threads, so there is no chipping, debris, or dust, which in turn eliminates the potential for later debris-caused electrical arcing.
“Since the re-engineered thread form has up to 30% more retention of clamp load underhead pressure than traditional threads, the actual faces of the battery terminal are pressed together for better conductivity,” explained Peacock. “On battery terminal posts, there’s an increase in electrical current available to flow through the connection.”
The increase in retained clamp load and conductivity can also help with terminals connecting leads together as well as from individual battery cells to large grounding terminals which pool many leads into one connection, and to any electrical connections carrying high current, high capacity charges throughout EV or hybrid systems.
The Spiralock fastener has been validated in several published test studies including MIT, the Goddard Space Flight Center, Lawrence Livermore National Laboratory, and British Aerospace. It has been used in automotive applications for five years, ranging from ring gears, torque converters, and chassis assembly to exhaust manifold joints and axle, turbine, or transmission housings, and for diesel engine applications. It has also been used in extreme fastening applications with virtually no chance of recall: in the last decade, it has been used in the main engines of NASA’s Space Shuttle, the Saturn Cassini orbiter and Titan Huygens probe; as well as medical implants, artificial limbs, and heart pumps.