EVs and HEVs: Staying in Contact
An innovative locking thread form may be a key to improving conductivity, battery life and connectivity.
With the federal government nearly doubling car and light-duty truck fuel economy standards to the equivalent of 54.5 mpg by 2025, electric vehicle (EV) and hybrid electric vehicle (HEV) technology is set to play a vital role—if lingering battery life and overheating issues can be resolved.
While automakers are using a range of technologies to improve fuel economy, EV and HEV technology appeals to a growing number of eco-conscious consumers who want to eliminate or limit the need to fill their gas tanks.
But with battery packs on EVs and HEVs only storing the energy of about 1–2 gal of gasoline, more needs to be done to safely harness every milliamp of their electricity without overheating. To fulfill the promise of electric powertrains for automakers, electrical conductivity, connectivity, and battery life must be improved, and an innovative locking thread form may be key to achieving this.
“The challenge is, ‘What is your E-mpg, or electric miles per gallon?’” said Kevin Peacock, an application engineer for Stanley Engineered Fastening (Madison Heights, MI). “How far can you drive without gas assist? Any losses in getting battery energy to the motor will compromise EV or hybrid range and viability.”
Loss of Connectivity
Traditional fasteners have difficulty maintaining electrical conductivity and connectivity with EV and HEV battery terminals because they tend to lose clamp load. After extended car vibration and thermal cycling, traditional fasteners typically lose about half of their original clamping capacity load, Peacock said.
“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,” said Peacock.
Another serious problem: when EV and hybrid fasteners lose clamp load, their batteries lose electrical conductivity. Heat can build up due to the battery’s live current, and electric arcing can occur, which is a potential fire or explosion hazard.
To assure 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, automotive engineers are finding a solution in a unique fastener called Spiralock. Spiralock is a brand of Stanley Engineered Fastening which provides innovative fastening and assembly technologies to all market segments.
Vibration-Induced Thread Loosening
Traditional locking fasteners do not address a basic design problem with the standard 60° thread form: that the gap between the crest of the male and female threads can lead to vibration-induced thread loosening, inadequate clamp load, and overheating in critical EV and HEV 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 its tendency toward axial loading. Temperature extremes can also expand or contract surfaces and materials, potentially compromising joint integrity.
Engineers, however, have successfully attacked these challenges while also eliminating traditional lock feature concerns about debris, stripping, or additional stack height with the Spiralock locking fastener. It has been successfully used in automotive EV and HEV battery applications for about five years, and in aerospace battery applications for about a decade.
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.
Holding on to Clamp-Load Capacity
“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,” said Peacock. “On battery terminal posts, for example, there’s an increase in electrical current available to flow through the connection.”
The increase in retained clamp load and conductivity could help not only with EV and hybrid batteries but also with terminals connecting leads together. It could help with everything essentially from individual battery cells to large grounding terminals which pool many leads into one connection, to any electrical connections carrying high-current, high-capacity charges throughout EV or HEV systems.
For these applications, the parameters will be changing on what constitutes a difficult-to-fasten joint, said Peacock. Engineers might think that the removal of large, heavy gas-powered engines from these vehicles would reduce vibration and the need for specialty fasteners, but the opposite may be true.
“As automakers go from traditional steel to aluminum to reduce weight in EV and hybrid applications, they should note that aluminum is a very stiff material that transmits vibration more rapidly across a structure than its steel counterpart,” said Peacock. “In these cases, vibration resistance counts all the more to keep fasteners in place and maintain conductivity.”
The Spiralock fastener ramp has been validated in published test studies at organizations such as MIT, the Goddard Space Flight Center, Lawrence Livermore National Laboratory, and British Aerospace. In the auto industry, it has long been used in applications 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: from the main engines of NASA’s Space Shuttle; to the Saturn Cassini orbiter and Titan Huygens probe; to medical implants, artificial limbs, and heart pumps.
Unlike traditional fasteners which depend on external locking features that can contribute to unwanted debris, stripping, or additional stack height, the locking fastener has no external locking feature add-on.
Since it is free-spinning, a nut can be run all the way down using fingers with little resistance between meshing threads, so there is no chipping, debris, or dust. This makes for a cleaner battery manufacturing environment, and eliminates the potential for later debris-caused electrical arcing, if the debris were to remain within the battery case.
Because the locking thread form is integrated into the part itself and available from the first engaged thread all the way up, a lower battery terminal post is possible. This means that EV or HEV design engineers can use a more space-efficient post to keep the battery terminals in place.
More Growth Ahead?
While EV and HEV use has been growing, Peacock envisions greater growth as consumers get tired of paying $4–$5 per gallon for gas and as electrical conductivity, connectivity, and battery life improves.
“Regardless of battery type, the design challenge is to ensure that more current gets from point to point as efficiently as possible in EV and hybrid vehicles, without risk of fasteners coming loose throughout their service life,” said Peacock. “That goal is within reach for designers now.”
Production changeovers to the fastener are typically quick and seamless, often requiring just an exchange of traditional nuts, wire inserts or simply drilling out and re-tapping existing parts stock that have unreliable standard tapped holes.
Edited by Yearbook Editor James D. Sawyer from material provided by Stanley Engineered Fastening.
This article was first published as a digital exclusive for the 2013 edition of the Motorized Vehicle Manufacturing Yearbook.