Coiled Vs Slotted Pins Key Factors for Optimal Fastening

In mechanical assembly, ensuring reliable connections between components is critical. Spring pins, as common fasteners, are widely used due to their unique elastic properties. However, engineers often face selection challenges when choosing between two primary types: coiled spring pins and slotted spring pins.

Spring pins, also called elastic pins or roll pins, are mechanical fasteners used to secure two or more components. Unlike solid pins, spring pins feature hollow tubular structures typically made from spring steel or other elastic materials. This design provides significant radial elasticity, allowing adaptation to bore diameter tolerances while delivering reliable connection force.

Compared to solid pins, spring pins offer lower radial stiffness. While precision-ground solid pins with matching reamed holes can achieve high accuracy, this approach proves costly. Spring pins' elasticity accommodates larger bore tolerances, reducing machining difficulty and cost. When properly selected, spring pins won't damage host bores, facilitating maintenance and replacement.

Slotted spring pins are characterized by their C-shaped cross-section and axial slot. Though their exact origin remains unclear, they have long been used in modern machinery. Four industry standards currently govern slotted pins: ASME B18.8.2, ASME B18.8.4M, ISO 8752, and ISO 13337.

Coiled spring pins feature a distinctive 2¼-turn spiral cross-section. Their development history is well-documented - invented in 1948 by Herman Koehl to address jet engine rotor fastening under extreme vibration. Unlike rigid solid pins that transfer dynamic loads to bore walls, coiled pins combine strength to withstand operational forces with elasticity to absorb them, protecting bore material integrity.

Available in three load grades for different applications and materials, coiled pins follow standards including ISO 8750, ISO 8748, ISO 8751, ASME B18.8.2, and ASME B18.8.3M - all essentially equivalent.

The fundamental difference lies in cross-section design. Slotted pins use thicker material forming a C-shape, while coiled pins employ thinner material in a spiral configuration. This impacts elasticity significantly.

Slotted pins have limited elastic deformation before becoming rigid tubes, potentially leading to premature failure. Coiled pins maintain near-infinite elasticity post-installation, continuously absorbing vibration and impact to maximize component lifespan.

Slotted pins feature open-ended designs that may cause nesting in automated assembly, potentially jamming feeding equipment. Coiled pins' closed cross-section prevents interlocking and provides smooth rotation surfaces - particularly valuable in hinge applications.

Coiled pins demonstrate uniform strength regardless of load direction, while slotted pins' shear strength varies by approximately 5% depending on slot orientation relative to load.

Some slotted pin standards loosely define chamfer diameter as "less than nominal pin diameter," potentially hindering automatic alignment. Coiled pins feature precisely controlled chamfers with smooth rolled edges for trouble-free installation.

Slotted pins often exhibit non-perpendicular end faces due to manufacturing methods, potentially causing feeding issues in automation. Coiled pins maintain perpendicular ends for automatic alignment with installation tools, ensuring straight insertion and uniform force application.

Coiled pins outperform in dynamic applications involving vibration, impact, or changing loads. Research shows coiled pins significantly exceed slotted pins in fatigue test performance regarding both ultimate load and cycle count at given loads.

Slotted pins exhaust most elasticity during installation, functioning like solid pins afterward by transferring loads directly to host material. Their lifespan decreases by approximately 50% when loaded perpendicular to the slot, while coiled pins maintain direction-independent performance.

For highly automated environments, coiled pins' combination of slot-free design, perpendicular ends, rolled chamfers, and consistent installation force minimizes downtime and ensures reliable assembly.

The choice between coiled and slotted spring pins depends on application requirements, load characteristics, automation level, and budget considerations. Coiled pins generally prove superior for high dynamic loads and automated assembly, while slotted pins may suffice for static, low-vibration applications where cost is primary.