Fulfilling the automotive promise

4 min read

With the US federal government nearly doubling fuel economy standards, electric and hybrid vehicle technology is set to play a vital role if lingering battery life and overheating issues can be resolved. Del Williams suggests that an innovative locking thread form that enhances connectivity may be key.

While automakers are using a range of technologies to improve fuel economy, electric vehicle (EV) and hybrid technology appeals to a growing number of eco-conscious consumers who want to eliminate or limit the need to 'fill the tank'. But with battery packs on EV and hybrid vehicles only storing the energy of about 1-2 gallons of gallons of motor fuel, more needs to be done to safely harness every milliamp of their electricity without overheating. To fulfil the promise of EV and hybrid technology 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 electric miles per gallon?" said Kevin Peacock, an application engineer for Stanley Engineered Fastening in Madison Heights, Michigan. "How far can you drive without [the engine]? Any losses in getting battery energy to the motor will compromise EV or hybrid range and viability." Yet traditional fasteners have difficulty maintaining electrical conductivity and connectivity with EV and hybrid 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 clamp load, according to Peacock. "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. 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 an innovative fastener called Spiralock, which is a brand of Stanley Traditional locking fasteners do not address a basic design problem with the standard 60-degree 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 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 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 hybrid 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-degree wedge ramp added at the root of the thread, which mates with standard 60-degree 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. "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, 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 hybrid systems. For EV and hybrid applications, the parameters will be changing on what constitutes a difficult to fasten joint, according to 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 aluminium to reduce weight in EV and hybrid applications, they should note that aluminium is a very stiff material that transmits vibration more rapidly across a structure than its steel counterpart," suggested Peacock. "In these cases, vibration resistance counts all the more to keep fasteners in place and maintain conductivity." The locking fastener with its 30-degree wedge ramp has been validated in published test studies at leading institutions including MIT, the Goddard Space Flight Center, Lawrence Livermore National Laboratory, and British Aerospace. In automotive, 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 and Titan Huygens space probe. Unlike traditional fasteners which depend on external locking features that can contribute to unwanted debris, stripping, or additional stack height, the locking fastener with its internal wedge ramp has no external locking feature add-on. Since it is free-spinning, a nut can be run all the way down by finger 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 hybrid design engineers can use a more space-efficient post to keep the battery terminals in place. "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," concluded Peacock. "That goal is within reach for designers now." Del Williams is a technical writer based in Torrance, California