Critical Juncture

Amid the big issues surrounding diesel engines today–fuel economy, lubes, emissions, to name a few–it’s easy to overlook the simplest details. Like the fasteners that literally hold everything together. Considering that your engine has to harness the heat energy of a blast furnace while keeping the vibrational equivalent of a jet fighter’s acceleration in check, it’s no small job.

“In the engine combustion process, joint fasteners may have to withstand exhaust gases up to 1,300 F, components to 1,000 F, and continuous vibration over thousandths of an inch, with a force of acceleration up to six times that of gravity,” say Todd Werner, a design engineer for Mack Trucks. “A turbocharger’s vibrational acceleration, for example, can exert up to 10,000 pounds of force on a joint.”

There are steps you can take to help prevent a joint coming loose, like nylon rings, adhesives, and deformed threads. The trouble is, they probably won’t stand up in the high-heat, high-vibration environment under your hood today. Split washers and standard-thread type fasteners can yield after repeated cycles of heating and cooling and require lock tabs to prevent them from losing their fastening ability. Prevailing torque fasteners can create assembly and service challenges, especially with stainless steel.

A better alternative may be to use a different thread design altogether. The traditional 60-degree “vee” design was originally created to accommodate unavoidable problems such as drill, tap, and die wear; hole and thread size variation; and other maladies brought on by imprecise manufacturing methods. With the vibrations and heat from today’s engines, though, those older fasteners can work loose. Stripping or shearing can also occur since 60-degree threads put most of the clamp load on the first and second engaged thread, allowing subsequent male threads to “float” within the female threads.

Through the geometry and physics of the thread itself, Madison Heights, Mich.-based Spiralock offers a thread form designed to address fastener loosening and stripping under high temperatures and vibration. Instead of the 60-degree “vee” thread design, the Spiralock thread form is a 30-degree “wedge” ramp cut at the root of the female thread. Spiralock says the shape of the thread form, in and of itself, does not produce a stronger thread. But the fact that the design distributes the load over a greater length on the bolt tends to produce a stronger joint and one which is less susceptible to stripping and fatigue failure.

Under clamp load, the crests of the threads on any standard male bolt are drawn tightly against the wedge ramp. Since the Spiralock thread form is a mechanical solution to the loosening problem, it’s more tolerant of cyclic temperature changes than 60-degree threads using nylon or adhesives, the company says. The 30-degree wedge ramp not only eliminates sideways motion that causes vibrational loosening, it also distributes the threaded joint’s load throughout all engaged threads. Tests show the load percentage on the first engaged thread with a Spiralock thread form is significantly lower, which further reduces possible bolt failure. Using this thread design, bolts spin freely until clamped to a final torque-retaining position, which eliminates galling during assembly and needed service repair.

When Mack found that a mechanical crimp nut wasn’t meeting its installation and service requirements in mounting a turbocharger to the exhaust manifold, it tested the Spiralock thread form. The fasteners were exposed to temperatures as high as 1,300 F, which is hotter than normal operating temperatures. The engine was then rapidly cooled every 12 minutes for 3,000 hours. Upon inspection every 250 hours, the fasteners maintained joint integrity without losing torque for 15,000 cycles.

“After their adoption, none have failed in the field to my knowledge,” says Mack’s Werner. “They’re not only self-locking but also re-usable during service without damage to the nut or stud.”

The fasteners are now used on every Mack turbocharger mount across its vocational truck line and on the EGR valve mount on the company’s highway truck line. Caterpillar and International Truck and Engine also use Spiralock fasteners for turbocharger mounts and exhaust manifold joints.
‘
After pre-production dynamometer and field testing, International replaced standard flange nuts with stainless steel Spiralock fasteners. “We’ve solved the joint integrity problem, but also avoided potential assembly issues with prevailing-torque-style fasteners and improved assembly production,” says International engine designer Gary Svidron.

“Since the Spiralock fasteners spin freely until it’s time to tighten them, they take significantly less time to tighten than resistance-heavy locking methods such as prevailing torque nuts,” he says.

“With four fasteners per assembly on a volume of many thousands of engines per year, we’re saving a large amount of assembly time.”

For details, see www.spiralock.com or call 1-800/521-2688.