Keeping it together: fastener thread form designed to withstand diesel stresses

Automotive Industries, Feb, 2005

A special internal thread form developed by Spiralock Corp. is being used by diesel engine and vehicle manufacturers to help accommodate the extreme temperature and vibration of engine-powered equipment, while minimizing thread loosening, assembly and service costs.

"In the engine combustion process, joint fasteners may have to withstand exhaust gases up to 1,300 degrees F, components to 1,000 degrees F and continuous vibration over thousandths of an inch, with a force of acceleration up to six times that of gravity," says Todd Werner, a design engineer for Allentown, Pennsylvania-based Mack Trucks Inc. "A turbocharger's vibrational acceleration, for example, can exert up to 10,000 lb. of force on a joint."

Traditional methods of preventing joint loosening, such as nylon rings, adhesives and deformed threads, often can't accommodate the high temperature, high vibration diesel engine environment.

"At high engine operating temperatures, @on-coated fasteners will melt or burn, and adhesives aren't effective either," adds Sam Sutthiwan, a designer for Peoria, Ill.-based Caterpillar. "Split washers and standard thread-type fasteners tend to yield over repeated cycles of heating and cooling and require lock tabs to prevent them from losing their fastening ability. Prevailing torque fasteners can present assembly and service challenges, especially with stainless steel."

Threaded fasteners can face limitations in diesel truck engine applications, particularly those with high performance turbo requirements, as they can be susceptible to shock, vibration, loading, as well as thermal expansion and contraction.

The traditional 60 degree "vee" thread design was originally created to accommodate unavoidable problems such as drill, tap and die wear; hole and thread size variation; and errors in placement, thread finish, etc. However, they can be prone to self-loosening rotational movement, particularly in high vibration environments. Stripping or shearing can also occur since 60 degree threads position most of the clamp load on the first and second engaged thread, permitting subsequent male threads to "float" within the female threads. Testing has found the first two threads alone often carry as much as 80 percent of the load, enough to cause shearing or stripping.

Friction-induced thread galling, or fusing, can also create assembly and service problems when high-resistance prevailing torque nuts are used with high-speed assembly equipment and stainless-steal fasteners. Due to galling, prevailing torque nuts may "freeze" before fully tightened during assembly, leading to exhaust leakage, gasket failure, reduced engine performance and a reduction in fuel economy.

Through the geometry and physics of the thread itself, Madison Heights, Mich.-based Spiralock developed its alternative thread form designed to address fastener loosening and stripping under high temperature and vibration. Instead of the traditional 60 degree vee thread design, the Spiralock thread form is a 30 degree "wedge" ramp cut at the root of the female thread. 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[degrees] threads using nylon or adhesives. It allows for both thermal expansion and contraction without slippage.

The 30 degree wedge ramp not only eliminates sideways motion that causes vibrational loosening, the company says, but also distributes the threaded joint's load throughout all engaged threads. This claim, Spiralock says, is supported by a research study conducted by the Massachusetts Institute of Technology. Other studies show the load percentage on the first engaged thread with a Spiralock thread form is significantly lower, which further reduces possible bolt failure and improves product performance. 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 in a heavy truck model it rigorously tested the Spiralock thread form.

"During a particular engine durability test, the fasteners were exposed to temperatures as high as 1,300 degrees F, which is hotter than normal operating temperatures," says Werner. "The engine was then rapidly cooled every 12 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," Werner adds. "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."