Taming extreme forces

Extreme may be the new buzzword for dangerous sports and high-caffeine sports drinks, but for vehicle design engineers extreme has always been part of the job.

"The challenge is to keep critical joints fail-safe not only under expected use, but also under extreme use," said Frank Metelues, a design engineer at Dana Corporation, a supplier of axle, driveshaft, engine, frame, chassis, and transmission technologies to vehicle OEMs with $9.1 billion in worldwide sales last year.

"You've got to do this while hitting all the required pricepoints for performance, manufacturing, and warranty, and some drivers are always going to push the limits," continues Metelues, who works in Dana Corporation's Traction Technologies Group.

"Uphill towing or off-roading just compounds the extreme loading, vibration, and thermal forces engineers contend with."

While engineers are tasked to design vehicle components that reliably withstand extremes of use - from shock, torsional, axial, and vibrational loading to thermal resistance and lifecycles of use - they must always keep their eyes on the bottom line as well. That means issues such as the cost of performance, assembly, maintenance, and warranty must also be optimised. But until now, engineers have faced significant drawbacks using traditional fastening methods.

Because traditional fasteners are susceptible to self-loosening rotational movement, stripping, and shearing, their use in critical joints without additional locking methods isn't always appropriate. Testing, in fact, has found that the first two threads of traditional fasteners can carry as much as 80% of the load, permitting stripping or shearing, while subsequent male threads "float" within the female threads.

The standard methods used to "lock" traditional fasteners, however, have their own limitations. Locking adhesives, for instance, progressively lose effectiveness as temperature rises. In high volume, their use typically requires a large capital expense to purchase and program robot applicators. And when re-application is necessary, cleaning the threads of affected components takes added time and labour before re-application is possible.

Bolts secured with single-use, drypatch adhesive - activated when the bolts are tightened - can similarly add to assembly, maintenance, or warranty costs. This is because, once used, the bolts must be replaced for any necessary rebuilds or maintenance. Affected threads must also be cleaned before new bolts with drypatch adhesive can be applied, adding to time and labour costs.

Mechanical locking features such as brackets, however, frequently prove too costly and tedious to use on components with multiple bolts. If not properly fastened during assembly, maintenance, or rebuilds, they can also pose a quality assurance risk.

Fortunately, vehicle engineers are successfully attacking these problems with a variety of new technologies that help optimise design reliability and profit, even under extreme conditions.

One of the most interesting solutions is also the simplest - an innovative self-locking fastener called Spiralock. By its unique design, Spiralock is capable of resisting loosening even under loads and vibrations strong enough to break the fastener, can be reused many times, and is highly resistant to heat.

The differential assembly inside the front-axle Dana Corporation provides for truck and SUV OEMs is arguably one of the most important components in the vehicles, since it essentially delivers power to the tyres. Because the ring gear carries so much torque from the driveline via the transmission, however, its fasteners must be fail-safe against shock, torque, vibration, and temperature. This is true for the lifespan of the vehicle, whether used for many years or for extreme activities like off-roading.

"A loose fastener meshing with the ring gear can potentially affect steering, braking, and overall vehicle control," says Metelues. "It's a mission critical component and the fasteners securing it need to reflect that."

For Dana Corporation's high-torque, front-axle application, the company avoided mechanical locking features due to their cost and complexity. Instead, they traditionally added locking adhesive to secure bolts. Alternately, they used bolts with drypatch adhesive, activated when the bolts were screwed into threads. While the adhesive-locking methods were adequate for safety, they did have disadvantages.

"Locking adhesives lose bonding power at high temperature, which can be generated by extreme or extended use," explains Metelues. "Bolts with drypatch activated adhesive, in turn, are one-use items, which must be discarded during rebuilds or any necessary servicing. This can add to assembly or warranty costs."

In proactive design testing to boost reliability, performance, and assembly effectiveness, Dana Corporation compared the clamp force retaining ability of locking adhesive with that of Spiralock locking fasteners. What makes Spiralock unique is a 30º "wedge" ramp cut at the root of the female thread (while traditional fasteners use a 60º thread).

Under clamp load, the crests of the threads on any standard male bolt are drawn tightly against Spiralock's wedge ramp. This not only eliminates sideways motion that causes vibrational loosening but also distributes the threaded joint's load throughout all engaged threads, a claim supported by a Massachusetts Institute of Technology research study. The load percentage on the first engaged thread is significantly lower than traditional thread forms, which further reduces possible bolt failure and improves product performance.

In Dana Corporation's rigorous Impact-Durability Test designed to examine torsional fatigue, a minimum of 3,000 back and forth cycles at maximum vehicle torque load were carried out. "It was the equivalent of putting the vehicle in neutral and flooring the gas: first forward, then reverse, thousands of times," says Metelues.

A demanding Dynamometer Test also simulated extreme customer use over the life of a vehicle with varied torque speeds.

"In both tests, Spiralock locking fasteners demonstrated 15 to 20% better clamp retention than traditional locking adhesives," states Metelues.
"The bolts using Spiralock fasteners did not back out," adds Metelues. "The design distributes load more evenly than traditional threads which minimises thread yielding and deformation, while the wedge ramp helps prevent torque and axial loads from backing the bolts out. And unlike adhesive, whose locking effectiveness degrades at higher temperature, the design exhibits significant temperature resistance."

Todd Werner, a design engineer at Mack Trucks, one of North America's largest producers of heavy trucks, provides evidence of the Spiralock locking fastener's resistance to extreme temperature and of its re-usability. He found special benefits for high-temperature diesel engine applications.

"During a particular engine durability test, the fasteners were exposed to temperatures as high as 1300º F, which is hotter than normal operating temperatures," says Werner. "The engine was then rapidly cooled every twelve minutes for 3,000 hours. Upon inspection every 250 hours, the Spiralock fasteners maintained joint integrity without losing torque for 15,000 cycles."

"After their adoption, none have failed in the field to my knowledge," continues Werner. "They're not only self-locking but also re-usable during service without damage to the nut or stud. The Spiralock fasteners are now used on every Mack turbocharger mount across our vocational truck line and on the EGR valve mount on our highway truck line."

At ease with Dana Corporation's use of Spiralock locking fasteners in the front-axle, Metelues is now investigating their use in both rear axles and pinion gears in a number of trucks and SUVs.