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FASTENINGS & ADHESIVES FEATURE
MAKING THE RIGHT CHOICE EARLY IN THE DESIGN PROCESS
Designers can optimise the performance and
total manufactured cost of a product by looking at the fastener options early in the design stages. The fastener should be simple to
assembly, be good quality to last the lifetime of the assembly, and be low in cost. Press fit pins are a good example. But how do you go about specifying the right one for your application? Jeff Greenwood, product sales engineer at SPIROL International Corporation, explains
P
ress fit pins are the most common types of pins used in modern manufacturing. Among these, there
are two general categories: • Solid Pins These can either have a smooth, uninterrupted, surface (such as dowels); or they may be designed with retention features such as knurls and barbs. These are retained by displacing/deforming the host material. • Spring Pins - available as Slotted Pins and Coiled Pins These retain themselves by exerting a radial force (tension) against the hole wall after installation. Slotted Spring Pins are general purpose, low cost pins, typically recommended for non-critical assemblies. These tend to be used in applications where they are manually installed into mild to hardened steel components. Slotted Pins have a gap designed for the pin to flex during installation, allowing the pin to absorb varying hole tolerance. Coiled Spring Pins are available in light,
standard and heavy duty versions to enable the designer to choose the optimum combination of strength, flexibility and diameter to suit the varying host materials and performance requirements. Coiled Pins have two 1/4 coils of material. These enable the pin to flex during installation to accommodate varying hole tolerance; and after installation to dampen shock and vibration.
SPECIFYING THE RIGHT PIN Many designers will wait until the end of the design to select a fastener, however this can constrict the fastener selection process, limit performance, and force suppliers to use
high-cost manufacturing processes to meet overly complicated specifications. It is therefore important to examine the application early in the design stage as this will facilitate decisions about the design of the host component(s), as well as covering the topics of fastener selection, fastener size, material, duty, etc. So, to begin, the first step is to evaluate the
application and performance requirements by asking a number of questions, such as: • What is the pin’s function? • What are the strength requirements of the pin? • What is the material of the component in which the pin will be used? • What environment will the pin be exposed to? • What is the intended product lifetime and number of cycles?
• How will the pin be installed? • What is the expected volume? It is recommended that manufacturers
consult with pinning technical experts during the early stages of a new design so that the proper pin is selected and the appropriate specifications are applied to the mating components for the application.
PIN FUNCTIONS While there are many different ways to use pins, the most common are shown in Table 1. These guidelines apply the majority of the time, but each specific application should be evaluated for final determination of which pin type is most appropriate.
Hinge: There are two primary types of hinges: 1. A free fit hinge has little to no friction or drag when the latch or handle is rotated. Hinge components are ‘free’
to rotate independent of one another.
2. A friction fit hinge requires interference to prevent free rotation of components relative to one another. Depending on design intent, resistance can vary from a slight drag to a value sufficient to maintain the fixed position of components anywhere in their full range of rotation. All types of press fit pins should be considered when designing a free fit hinge. Solid Pins are often preferred when the pin must travel through multiple clearance holes or when there is limited engagement area in the host component. Coiled Pins are preferred when there is no axial load on the pin and for applications with shock and vibration. Slotted Pins are preferred when cost is paramount (typically at the expense of quality) and performance is sufficient. In general, Coiled Pins are preferred for friction fit hinges because they provide uniform radial tension that creates a ‘resistance’ feel in the hinge. Additionally, Coiled Pins are far more flexible than Slotted Pins or Solid Pins, thereby reducing the risk of damage to the holes during installation and normal product usage. Hub & Shaft: One of the primary benefits of using a Coiled Pin to affix a collar or hub to a
Table 1 FEBRUARY 2022 DESIGN SOLUTIONS 19
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