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Diesel Fuel Injector

A manufacturer of diesel fuel assemblies required a pin that had higher shear strength than the 3-coil Coiled Pin that was being used in order to withstand the torque generated during disassembly.



The customer currently uses a 3-coil Coiled Pin to align three components within a diesel fuel injector. The pins are pressed into holes in the top and bottom of the center component, with a slip fit in the two mating components. The components are assembled, and then carefully placed into the injector housing.

The pins position the individual parts such that a fuel hole is aligned amongst each of the components. Fuel passes through these holes to the end of the injector and exits the injector nozzle to flow into the cylinders of an internal combustion engine.

In addition to aligning the components, it is imperative that the pins do not cause the host material to bulge and that debris is not generated during installation. Traditional Solid Pins have been discounted as a viable solution in the past for several reasons. Grooved Pins require too high an insertion force and distort the face of the component around the hole preventing a clean mating surface between the parts. Knurled Pins have the potential of introducing slivers into the assembly if the knurls shear off during installation. Finally, Straight Dowel Pins require extremely tight tolerances to ensure a proper fit, and drastically increase the total installed cost of the components.

The 3-coil Coiled Pin provides the customer with a pin that can be installed without generating any debris, and allows standard drilling and reaming tolerances to be used in the tight fitting holes. Due to the fact that 50% of the pin is retained in the tight fitting hole and there are three wraps of material, the recovery on the diameter of this special Coiled Pin is limited. Therefore the pin maintains its shape on the portion extending from the holes that provides a consistent diameter for aligning the mating components.

At the end of the life of the injector, the parts are disassembled, rebuilt, and then resold on the remanufactured market.

The problem occurs during disassembly of the injector. During use, carbon occasionally builds up within the fuel injector and binds a nut of the injector casing to the three aligned components. When the injector casing is unscrewed from the base, the bound nut transmits the torque to the pins in the aligned components. The torque is sufficient enough to exceed the shear strength of the pins. When the pins shear, the pins in the tight fit blind holes become lodged, and the sharp edges of the broken pins occasionally mar the mating surfaces of the components. The result is expensive machining to repair the assembly, and at times, the components are irreparable and must be discarded.

The customer requested a pin that had higher shear strength than the 3-coil Coiled 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.



In response to the customer’s needs, SPIROL Engineering developed the Helical Grooved Pin. This unique, special order pin has wide grooves and low profiled peaks that spiral around the body of the pin that creates broad bearing surfaces. This feature eliminates skiving of the holes and deformation of the pin and host during installation, preventing the introduction of slivers into the assembly. In addition, the insertion force is approximately equal to the 3-coil Coiled Pin, and 25% lower than that of a traditional Grooved Pin. Another benefit is that there is minimal variation in insertion force, and the insertion and removal forces are about the same. Finally, the fact that the pin is solid, heat treated 6150 alloy steel means that the shear strength is approximately 30% higher than that of the 3-coil Coiled Pin that will reduce repair and replacement costs of the injector components.

Complimentary Applications Engineering Support: SPIROL Engineers will review your application needs and work with your design team to recommend the best solution.