Helical grooved pins enhance design for disassembly

Spirol has developed a special helical grooved pin to solve a problem faced by a manufacturer of diesel fuel injection assemblies. This pin features wide grooves and low-profiled peaks that spiral around the body of the pin and create broad bearing surfaces. This eliminates skiving of the holes and deformation of the pin and host during installation, thereby preventing slivers from being introduced into the assembly.

The insertion force for Spirol's special helical grooved pin is approximately equal to the three-coil coiled pin that was previously used for this application and 25 per cent lower than that of a traditional grooved pin. A further benefit is that there is minimal variation in the insertion force, with the insertion and removal forces about the same. As the Spirol pin is solid, heat-treated 6150 alloy steel, its shear strength is approximately 30 per cent higher than that of the coiled pin previously used. This is resulting in reduced repair and replacement costs for the injector components.

Difficulties in disassembly

With the coiled pin previously used, the problems arose during disassembly. At the end of the life of the injector, its parts are disassembled, rebuilt, and then resold on the remanufactured market. During use, carbon would occasionally build up within the fuel injector and bind a nut of the injector casing to three aligned components within the assembly. When the injector casing was unscrewed from the base, the bound nut transmitted the torque to the pins in the aligned components. The torque was sufficient to exceed the shear strength of the pins. When the pins sheared, the pins in the tight-fit blind holes became lodged and the sharp edges of the broken pins occasionally marked the mating surfaces of the components. The result was expensive machining to repair the assembly and, at times, the components could not be salvaged and had to be discarded.

The manufacturer therefore approached Spirol to design a pin that had higher shear strength than the three-coil pin in order to withstand the torque generated during disassembly. The pin still needed to be configured such that it did not cause deformation of the host material or debris during installation. The special helical grooved pin has met all these requirements.

Full details of the design engineering service provided by Spirol Industries and literature on helical grooved pins are available directly from the company.