When turbo mounting nuts vibrate loose and fall off, this can lead to engine failure and unscheduled downtime that can upset production schedules and delivery commitments.

To keep its industrial engine customers productively on the job, Columbus, IN -based Cummins Inc., sought the ideal solution to secure exhaust manifold joints when developing its Off-Highway Tier 3 QSX engine.

“Our industrial customers rely on our engines to get the job done, often for decades, so our engines must be up to the task,” said Dale Gibby, a Cummins mechanical development engineer at the company’s Columbus Technical Center in Columbus, IN. “Our goal was to design the QSX engine for rock-solid, real-world reliability in industrial applications ranging from farm tractors, road graders and rock crushers to generators, air compressors, and drill and concrete pumpers.”

To assure engine reliability in the field, Gibby was tasked to keep the QSX engine’s exhaust manifold joints secure despite the severe vibration, joint fatigue and temperature extremes present in industrial field use.
Standard fasteners were less than satisfactory due to their susceptibility to vibration, thermal distortion, loosening due to thermal cycles, and wide variability in initial pre-load.

“For the exhaust manifold joint application, we avoided adhesives and nylon plugs because the high temperatures involved would burn them off,” explained Gibby. “We avoided split washers and deformed threads because their stress concentration is a big concern for fatigue failure. Split washers, in fact, can cause more problems than they solve if they gouge surfaces and start fatigue cracks.”
A tall spacer and a tall capscrew can add stretch in the bolted joint, which can help in some joint applications, according to Gibby. But for many engines, including the QSX engine, there is not enough
package or under-the-hood space for this approach so it was not an option, he said.

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. 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 innovative Spiralock locking fastener. This re-engineered thread form adds a unique 30° wedge ramp 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 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; to medical implants, artificial limbs and heart pumps.

Gibby decided to pro-actively test the innovative Spiralock locking fastener, head-to-head against standard fasteners in an abusive, test-to-fail, thermal cycling test. The test compared how long typical stainless steel nuts vs. Spiralock stainless steel nuts stayed securely fastened on the exhaust manifold joints of a QSX industrial engine. Four nuts each were tested, with the Spiralock nuts silver plated for more consistent pre-load.

“We concluded that the Spiralock locking fasteners will survive under-torqued installations where standard nuts would fail,” said Gibby. “The result is more reliable engine performance that allows Cummins customers to get their jobs done in the toughest of conditions. Those with an engine in the field for many years will especially benefit.”
www.spiralock.com