Global Fastener News

PRODUCT NEWS: New Thread Form Takes Diesel Heat

John Wolz

An internal thread form is helping diesel-powered vehicle manufacturers combat extreme engine temperature and vibration.
“In the engine combustion process joint fasteners may have to withstand exhaust gases up to 1300� F, components to 1000� F, and continuous vibration over thousandths of an inch, with a force of acceleration up to six times that of gravity,” explained Todd Werner, a Mack Trucks design engineer. “A turbocharger”s vibrational acceleration, for example, can exert up to 10,000 lbs. of force on a joint.”
The new Spiralock thread design replaces the traditional 60� “vee,” which was created for drill, tap and die wear; hole and thread size variation; errors in placement; and thread finish problems. The traditional “vee” design is prone to self-loosening rotational movement with high vibration. Stripping or shearing can occur, since 60� 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% of the load, enough to cause shearing or stripping.
Friction-induced thread galling, or fusing, can create assembly and service problems when high-resistance prevailing torque nuts are used with high-speed assembly equipment and stainless steel fasteners. Due to galling, prevailing torque nuts may “freeze” before they are fully tightened during assembly, causing exhaust leakage, gasket failure, reduced engine performance and a reduction in fuel economy.
Madison Heights, MI.-based Spiralock Corp. introduced an alternative thread form designed to address fastener loosening and stripping under high temperature and vibration. The Spiralock thread form is a 30� “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 thread form is a mechanical solution to the loosening problem, it is more tolerant of cyclic temperature changes than 60� threads using nylon or adhesives.
The 30� wedge ramp eliminates sideways motion that causes vibrational loosening and distributes the threaded joint”s load throughout all engaged threads, according to Spiralock.
Web: spiralock.com \ �2004 FastenerNews.com