Honing Is a Life-Saver for Aerospace Component Maker

Sonic Industries, part of the Sargent Aerospace & Defense Company, uses a Sunnen SV-1000 honing system to achieve performance-critical ID tolerances and finishes on life-saving fuse pins, reducing cycle times by 75 percent.

Knowing when to ‘hold’em’ or when to ‘fold’em’ takes on a new meaning when referring to the fuse pins that are designed to hold a jet engine on a wing but automatically break away in emergency situations to allow the engine to separate from the wing and prevent catastrophic structural failure and fires.

This is the type of performance dilemma faced by the highly-engineered, precisely-manufactured aerospace components produced by Sonic Industries (Torrance, CA).

Sonic is a leading manufacturer of high-strength, safety-related fasteners and precision round bar components, and is a subsidiary of the Sargent Aerospace & Defense (Tucson, AZ) group that has supplied specialty fasteners, self-lubricated lined bearings, precision engine rings, and hydraulic controls for more than 30 years on both commercial and military platforms.

Sargent, in turn, was founded in 1920 and is a part of the Dover Industrial Products’ Mobile Equipment Transportation Group (Downers Grove, IL).

As a premier global supplier of custom-engineered precision products and services to the aerospace and defense industries, virtually every major commercial jetliner, military fighter, bomber, submarine and cargo plane includes Sargent products.

The ubiquity of Sonic fuse pins in today’s commercial and military aircraft, and the importance of their proper ‘hold-or-fold’ performance, make the design and manufacture of these small parts as important to air travel safety as the integrity of a wing or soundness of an engine.

The fuse pin, also known as a shear pin, affixes the engine onto the wing via the pylon – the structural component connecting the jet engine to the wing spar.

When necessary, it allows the engine to break away under an impact load in the event of a crash or other hard landing, protecting the fuselage from engine fire caused by a dragged engine. Fuse pins serve a similar function for landing gear assemblies.

Located in a structural assembly nicknamed the “doghouse fitting,” fuse pins attach the landing gear to the wing and are designed to ‘fail’ in the event of an extreme hard landing, allowing the main landing gear to safely break away from the airplane and prevent rupture of the fuel tanks inside the wing box.

Fuse pins are made of steel and stainless steel alloys, including 318 and 15-5, with various diameters and lengths up to 23 in.

The pins start as a bar forged to specific geometry and are gun-drilled, then bored to a rough preliminary hole size. They are then heat-treated and tested to establish the shear value, and the entire lot is processed to final machining and finish grind on the OD.

The heads are finish machined with slots or hexes. The pins are then bored to a specified size and honed to establish the critical ID size, geometry and surface finish required for proper performance.

Previous versions of fuse pins were designed with a notch that would act as a ‘weak spot’ and facilitate them breaking on impact. However, a cylindrical pin with a notch is more vulnerable to excessive corrosion and fatigue damage.

Therefore, fuse pins were re-engineered without the notch, making ID tolerance and finish the critical factors in their ‘hold-or-fold’ performance.