Aligning sub-assemblies using coiled spring pins
01 December 2009
If you are seeking a very flexible, inexpensive and lightweight method of aligning parts during an automated sub-assembly operation, then the coiled spring pin approach is probably the best way to go, as one automotive component supplier has discovered
Spirol Industries has helped a manufacturer of automotive valve bodies to overcome a problem it had been experiencing when locating the upper and lower parts of these components during a sub-assembly operation. The valve body comprises two die cast aluminium components that are located with the aid of two pre-installed pins. Similar pins are also used in the final assembly to locate the transmission case.
The costs of these assemblies were the subjects of close scrutiny by the manufacturer, whose aim was to reduce the pin hole preparation time, reduce the overall weight of the assembly, minimise installation forces, and simplify the assembly/part feeding process. During the initial assembly tests, machined solid pins and slotted spring pins were used, but these proved difficult and expensive to install.
The solid pin not only required a very precise hole to be machined, but it also increased the overall assembly weight, had a higher installation force and subsequently increased the complexity of the assembly operation. The slotted spring pin, on the other hand, did not provide the level of precision required for this critical alignment task. Moreover, it proved impossible to feed automatically because slotted spring pins are not very straight and the imperfect chamfers on their ends can lead to mating difficulties during installation.
Spirol Industries’ solution to these problems came in the form of its light duty coiled spring pins, which offer the useful balance of strength and flexibility sought for this particular application. Evaluation of the assembly revealed that a pin could be fixed in the lower valve body in order to align the upper body - and provide a clearance fit in the transmission case for ease of manual assembly.
Coiled pins have a pre-installed diameter that is larger than the target hole size. The diameter reduces as the coiled spring pin is inserted, and once installed, it retains its position by exerting a constant radial force against the hole wall as it attempts to recover its pre-installed diameter. This flexibility enables the coiled pin to absorb the widest range of manufacturing tolerances of any other press fit pin, while simultaneously providing the required retention force.
The clean square ends of the coiled pin provide a generous lead-in for insertion as well as assisting in the alignment of mating components. A combination of straightness, square ends, concentric chamfers and gap-free configuration makes this pin ideal for automatic feeding and assembly – an operation that is further assisted by the pin’s low weight and low installation force.
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