Slotted pin inflexibility leads to failure
Written by Jonathan Higgins June 16, 2010
A “spring pin” is appropriately named by its ability
to flex into a hole that is smaller than the pin’s original
(pre-installed) outside diameter.
A spring pin’s tendency to return to its original shape after installation makes the pin self retaining. Retention is established by friction between the pin and host wall and not by deformation as with alternative solid press fit pins. In general, this concept of preserving joining components (both pin and host) during installation increases the life of dynamic assemblies.
The term spring pin commonly describes both coiled pins and slotted pins. While the concept and naming of coiled pins and slotted pins may be interchangeable, there are distinct differences between the two pins.
As a slotted pin is installed, the pin’s “spring like” characteristic is reduced to a narrow seam opposite to the slot. Here, stress concentrations form as a result of the pin’s limited flexibility. This portion of the slotted pin is susceptible to failure if the pin is subject to impact loads.
A second mode of failure is caused by the slotted pin’s rigidity after installation. When a slotted pin is installed, the slot closes and the pin can act like a rigid hollow tube. This tube behaves similarly to a solid pin. As a solid member, impact loads are transmitted to the host wall; causing hole elongation. As the hole load and size increases, impact loads intensify, and failure is accelerated. The pin cracks, falls out of the hole, or does both.
In the coupler shown, the manufacturer used a slotted pin as the stop mechanism. The crack shown in the inset picture is a result of the slotted pin’s limited flexibility.
The solution to these problems is Spirol’s coiled pin. As the coiled pin is installed, stress is distributed throughout the fastener instead of being concentrated along a line. The coiled pin’s design also ensures flexibility after installation. For the life of the assembly, the coiled pin is able to absorb impact loads without causing damage to the host or the pin. Unlike the slotted pin, it remains an active member of the joint, continuously absorbing loads.
Exemplifying the differences between the coiled pin and the slotted pin in demanding, high-impact applications, the following case study could apply to many similar applications.
A quick coupler manufacturer was challenged to design a stop feature that could withstand high impact loads for construction site excavators. The coupler requires a positive stop to control the stroke of the hydraulic lever each time an attachment is released. In the coupler shown Illustration 1, the manufacturer used a slotted pin as the stop mechanism. The crack shown in the inset picture is a result of the slotted pin’s limited flexibility.
Because the slotted pin is not able to easily conform to the hole size, it folds and creates a limited interference fit at 3 points to the hole. This causes stress to concentrate 180° from the slot. Over time, this weak area was exacerbated by impact loads, leading to failure. The slotted pin was at risk to fall out of the hole and the productivity and safety of the construction site was compromised.
In an attempt to correct this issue, the manufacturer inserted a second slotted pin into the first, called composite pinning. While the result is a stronger, more rigid pin, problems normally arise with this configuration.
Initially, in this application, the composite pin was able to withstand more cycles, but, over time, the same cracking problem occurred. Stress concentrations that were inherent to the slotted pin’s design continued to cause failure opposite to the seam. This more expensive and cumbersome design was only a short term solution.
The heavy duty coiled pin is progressively rolled with thinner gauge strip, yet the additional cross sectional material achieves greater strength than the slotted pin.
The coupler manufacturer contacted Spirol for assistance. Spirol’s
Application Engineering Team reviewed the coupler design and the
manufacturer’s performance objectives. A heavy duty coiled pin was
recommended for the pin’s unique combination of strength and
flexibility.
The heavy duty coiled pin
is progressively rolled with thinner gauge strip (Illustration 2), yet
the additional cross sectional material achieves greater strength than
the slotted pin. This creates a strong pin with increased flexibility,
making the pin able to withstand impact loads and provide long-term
joint integrity. There is no single point of stress concentration, and
the hole size is preserved.
The coiled pin saved the
customer on piece price, assembly costs and reduced warranty claims. It
also enhanced workplace safety.
Jonathan Higgins is
Field Applications Engineer, Spirol International Corp. Spirol offers
free samples and free engineering support.
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