Reducing vehicle weight

Whether it is for ground vehicles, planes, or satellites, lighter is better – and often required – not only for fuel efficiency but for tactical mobility.

No matter what the vehicle, every extra ounce of weight requires more fuel – which is costly, adds more weight, and may be difficult to supply in remote locations. Excess weight can even threaten a vehicle’s viability, whether for tactical objectives such as airborne transport to a drop zone, for launch out of Earth’s orbit, or for commercial or consumer adoption.

To meet the challenge of reducing vehicle weight, engineers are tackling the problem in an unexpected way. With an innovative fastener technology, they are eliminating the weight and complexity of secondary locking features in military, aerospace, and consumer vehicles, including a prototype of the Joint Light Tactical Vehicle (JLTV), a family of next generation military vehicles still in development.

The JLTV, for instance, is designed to offer the U.S. Army and Marine Corps. better light tactical vehicle payload, protection, performance, and fuel efficiency, while being light enough to be transported by helicopter or C-130 aircraft. Despite the shock and vibration of off-road driving and battlefield conditions, it must remain reliable.

Why Fasteners?
Fasteners may not be the first place engineers look t o reduce vehicle weight, but the typical car has about 15,000 fasteners, and most military and aerospace vehicles have even more.

While not every fastener has a nut, bolt, and locking feature, critical joints often use lock washers, lock wires, cotter pins, brackets, or other secondary locking features, which cumulatively add a significant amount of weight and complexity. The problem is, that when a fastener loses its clamp load it no longer does its job and safety, recall, or warranty issues occur. Locking devices are commonly added to retain clamp load, but do not always hold up under shock, vibration, or temperature extremes.

An overdesign belt and suspenders mentality has prevailed, for instance, in automotive design where putting eight bolts on an exhaust manifold is common when four might do. This has led to big, heavy, fuel-inefficient cars.

To reduce vehicle weight, assure joint integrity in critical areas, and prevent safety, warranty, or recall issues, vehicle engineers are finding a solution in a fastener called Spiralock. For example, the JLTV prototype with a fastener like the Spiralock could have a potential weight savings in the 20 lb to 40 lb range per vehicle.

A chassis on a typical military vehicle is held together with about 200 7/8" bolts, nuts, and lock washers. Eliminating the need for the 7/8" lock washer, which weighs about 0.065 lb each, could reduce weight by about 13 lb per vehicle. That is for just one size of fastener. There could be dozens of fastener types and sizes per vehicle that could safely eliminate their secondary locking features with the use of Spiralock.

Traditional locking fasteners do not address a basic design problem with the standard 60º thread form, which is the gap between the crest of the male and female threads can lead to vibration-induced thread loosening. 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 reducing component weight and enabling re-usability with the Spiralock locking fastener. This re-engineered thread form adds a 30º wedge ramp at the root of the thread which mates with standard 60º 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 Spiralock locking fastener has been validated in published test studies at leading institutions including MIT, the Goddard Space Flight Center, Lawrence Livermore National Laboratory, and British Aerospace. It has 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 and from medical implants to artificial limbs and heart pumps.

“The Spiralock fastener’s ability to retain clamp load under extreme shock, vibration, and temperature with less weight, complexity, and maintenance makes it a natural choice for military, aerospace, and consumer vehicles,” says Ray Genick, director of new business development and former VP of engineering, Emhart Teknologies. “It retains clamp load even with dissimilar exotic materials, such as composites and titanium, which is important in reducing vehicle weight.”

Weight savings also come from reducing fastener size or number with an equal or better safety margin compared to fasteners with the standard 60º thread form, according to Arlindo Marques, director of global engineering, Spiralock.

“The key is Spiralock’s full contact along the entire engaged length of the thread,” Marques says.

Spiralock
Madison Hts., MI
spiralock.com