Why Pins Walk and How to Ensure that Doesn’t Happen

There are common causes of walking for both Solid Pins and Spring Pins such as improperly sized holes, insufficient engagement, and asymmetrical loading. There are also mechanisms of walking unique to each product. For example, rigid pins may deform the holes thereby introducing clearance and compromising retention. If properly selected for host material and load, Spring Pins should not deform holes like rigid Solid Pins. However, once installed, a Slotted Spring Pin’s gap is largely closed. If further movement occurs, the Slotted Pin can butt at the gap at which point it functions as a solid tube (exhibiting the same characteristics as a Solid Pin).

SPIROL’s engineered Coiled Spring Pins are designed to address the deficiencies associated with both Solid Pins and Slotted Spring Pins. Coiled Pins are available in a variety of duties to tailor the strength and flexibility of the pin to the assembly in which it is being used. Light and standard duty Coiled Pins can prevent hole damage in soft and brittle materials, which is often the case when Solid Pins or Slotted Pins are used. In addition, unlike Slotted Spring Pins, Coiled Spring Pins cannot “butt” in holes as they possess a seam rather than a gap.

Coiled Pins and Slotted Pins are functional springs. Once installed, the pin is compressed and it is spring tension that provides retention in an assembly. As previously noted, Coiled Pins cannot butt and therefore remain flexible in assemblies. While Coiled Pins provide critical flexibility in rigid assemblies, it is very important to ensure that they are symmetrically loaded to prevent the creation of an angular force vector. If a force vector is created, it can translate radial compression or coiling of the Spring Pin into lateral movement or ‘walking’. (Shown in Figure 1, compressive load on the pin translates into lateral motion when the pin is under compression and asymmetrically loaded.) 

F = compressive load on pin
F_n = force exerted radially as the pin is compressed and it wants to spring back
F_nx and F_ny = the resolutions of F_n
F_f = the force of friction that retains the pin in the hole

To achieve a friction fit hinge in Figure 4, the same rules apply for maximum performance in that all holes should be precision matched if possible. The difference between the two situations in Figure 3 and Figure 4 is where spring recovery of the pin will occur. In Figure 4, free span length is greatest in the outer components so pin recovery would be greatest at the ends rather than in the center. In this diagram, the pin will be retained most tightly in the center hole while it recovers at each end to maintain contact with the outer hole walls. 

Both Figure 3 and Figure 4 would result in a successful design if the Spring Pin’s spring characteristics are carefully considered for each situation. If required, it is possible to empirically derive values for expected spring recovery that are diameter and length specific. 

And if a free fit hinge were desired in Figure 4? Simple – ensure the pin is retained at the ends. The center component offers little length for pin recovery and as such, it need only be slightly larger than the outer holes to ensure clearance over the pin.