DS-FEB22-PG19+20_Layout 1 04/02/2022 10:15 Page 2
FEATURE
FASTENINGS & ADHESIVES
shaft is the Coiled Pin’s ability to prevent hole damage. Its flexibility and ability to effectively absorb forces make it the ideal pin for most hub and shaft applications. While all three types of pins can be used to affix a hub/gear to a shaft, the Coiled Pin provides superior performance and extends the lifetime of the assembly compared to the other pins. Locating/Alignment: The desired level of
precision dictates which pin is appropriate. Coiled Pins are preferred for the vast majority of alignment applications because they conform to the holes in which they are installed and remain flexible – therefore, maximum accuracy in alignment can be achieved with a ‘light’ press to seat mating components. Light duty Coiled Pins are especially advantageous for low insertion forces. Wider hole tolerances can be used with Coiled Pins, which reduces the total manufacturing cost of the product. However, the more precision required, the tighter the hole tolerances need to be controlled in each component and in relation to each other. Ground Dowels are preferred for highly critical alignment applications. Unlike Spring Pins, Solid Dowels rely on material displacement between the pin and the host components for the press fit. This requires a considerably higher installation force than either Spring Pin and requires that the holes are precision machined, which increases cycle time and manufacturing costs. Stop: Coiled Pins, Slotted Pins and Solid Pins
are all commonly used to stop the movement of one component relative to another. For example, Coiled Pins are often used to prevent over- rotation of an actuator. When Slotted Pins are used for this purpose, it is recommended that the pin’s slot is oriented opposite from the component interacting with the pin. Conversely, Coiled Pins and Solid Pins do not need to be oriented. Additionally, when Spring Pins are used as a stop pin, at least 60% of the pin’s length must be retained in the static component to ensure retention. Joining/Retention: Coiled Pins, Slotted Pins, and Solid Pins are also commonly used to join components together. Coiled Pins and Slotted Pins hold components together with the frictional force generated from the radial tension of the pin. Coiled Pins and Slotted Pins are serviceable in the same hole. Solid Pins provide superior retention when an axial load is applied and are not removable/ serviceable. This is advantageous when designers do not want users to disassemble
Table 2
their product. For the vast majority of retention applications, external features like knurls or barbs are preferred over ground solid dowels because they often provide cost savings. Each type of press fit pin serves a purpose for
manufacturers. Table 2 compares the common features, advantages and benefits.
CONSIDERATIONS FOR PIN SELECTION Spring Pins are typically preferred over Solid Pins because of their flexibility, lower insertion forces, and their ability to accommodate wider hole tolerances. Here are several common exceptions where Solid Pins are preferred: • When a head is required for a positive stop or to retain a thin member to a thicker member of the assembly.
• When a smooth, uninterrupted surface is required such as when used in conjunction with a pawl or other angular component. • When a hollow pin is not suitable, such as when the designer is looking to plug a hole (i.e. restrict passage of liquids). • When there is a need to manually align several clearance holes. • When increased bending or shear strength is required. • When precise hole locations need to be maintained. Coiled Spring Pins are superior when it
comes to assemblies subject to dynamic loading. Coiled Spring Pins have a unique combination of strength and flexibility, which allows them to dampen forces and vibration, preventing hole damage and prolonging assembly life. While Slotted Spring Pins are used in similar applications as Coiled Spring Pins, Slotted Pins are typically preferred in non-critical, static applications, where cost is prioritised over product lifetime.
Barbs retain this Solid Pin within the plastic component 20 DESIGN SOLUTIONS FEBRUARY 2022
THE FINAL STEPS Manufacturers should perform testing with the fastener(s) that they have specified to determine that the assembly performs as desired in the most extreme conditions. After testing is completed, engineers can compare measured test
results with the performance requirements that were established. Ultimately, the proper pin for the application should satisfy the quality, performance, assembly and cost goals of the manufacturer. The final step is to reevaluate the overall
product design. Often, the pin evaluation process identifies new information about the assembly. Many manufacturers see significant benefits when they stay flexible with their product design while the fastener is finalised. Here are some actual examples of design changes that were implemented after the fastener evaluation process, leading to performance improvements, cost savings, and/or improved quality: 1. New host material Example: A manufacturer changed the material of their plastic housing from polybutylene terephthalate (PBT) to polycarbonate (PC) after seeing improved retention when using barbed Solid Pins. 2. Hole size Example: A company increased the hole size in their hub & shaft from 2.95 ±0.05mm to 3.05 ±0.05mm so that they could utilise a standard off-the-shelf Coiled Spring Pin. 3. Hole tolerance Example: A company was able to eliminate a timely honing operation by using a Coiled Pin for alignment rather than a Ground Solid Dowel. 4. Boss thickness Example: A plastic moulder witnessed cracking during prototype testing of a plastic hinge. They implemented Spirol’s recommendation to increase the boss diameter surrounding the Solid Pin from 1mm to 3mm, which eliminated the cracking problem. 5. Hinge design change Example: A plastic moulder originally designed a friction fit hinge but was unable to achieve the high swing torque requirement overtime with a Solid Pin as the plastic would relax causing the hole diameter to open up. As a result, the swing torque would diminish due to the enlarged hole size. They replaced the Solid Pin with a Coiled Spring Pin and incorporated the associated design changes to the holes to meet the desired swing torque. The redesign resulted in the swing torque being maintained beyond the expected life of the assembly.
SPIROL International Corporation
www.SPIROL.com
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