Flex Head Ratchet Wrench

A leading manufacturer of hand tools was experiencing challenges with the pivot assembly of its Flexible Head Ratchet Wrench. The design utilized an Ø8 mm Solid Grooved Pin as the pivot axle, allowing the ratchet head to rotate relative to the handle.

A free fit was required in the outer hinge elements (handle), while a large hole tolerance was specified in the center hinge component (head) to achieve the desired fit and function of the rotating joint. However, this tolerance range exceeded the hole size variation that Grooved Pins are typically designed to accommodate. As static fastening components, most Grooved Pins are intended to accommodate a total hole tolerance of approximately 0.05 mm (.002").

The grooves in many Grooved Pins are manufactured using a process that creates crescent-shaped retention features. In the ratchet wrench assembly, the existing hole was slightly larger than recommended. When installed in an oversized hole, the crescent-shaped grooves contacted the hole wall at only a single point, allowing slight movement of the Grooved Pin within the hole. Under the high loads experienced during wrench operation, the Grooved Pin can rock within the hole, gradually reducing its retention until it eventually walks out of the assembly.

Installation of a Grooved Pin requires the displacement of material from either the pin or the hole to create a press fit. As a result, contact between the groove peaks and the hole is limited, making retention more sensitive to hole size variation.

The wrench’s wide hole tolerance also created assembly challenges. When the hole was at the low end of tolerance, excessive insertion force was required, often resulting in time-consuming secondary operations. When the hole was at the high end of tolerance, retention was reduced, increasing the likelihood of Grooved Pin movement during use and cycle testing. Because the wrench carried a lifetime warranty, eliminating this potential failure mode was critical.

In addition to the product performance concerns, the assembly process was labor intensive. Operators manually loaded the Grooved Pin into the assembly, positioned the product in a press, and activated the ram to complete installation. The process required approximately 15 seconds per assembly, and scrap rates approached 5%.

SPIROL Application Engineers evaluated both the product and manufacturing challenges and ultimately recommended a complete fastening and assembly solution that included replacing the existing Grooved Pin with an Ø8 mm Heavy Duty Coiled Spring Pin and automating the assembly process with a Model CR Semi-Automatic Pin Inserter.

Unlike the Solid Grooved Pin, a Coiled Spring Pin has a pre-installation diameter that is larger than the intended hole. During installation, the Coiled Pin compresses to conform to the hole, accommodating normal hole size variation while maximizing radial interference throughout the available engagement length. Unlike a Grooved Pin, which contacts the hole wall only at discrete locations, the flexible coils of a Coiled Spring Pin maintain continuous circumferential contact with the hole while providing uniform radial force along its full length. As a result, the Coiled Spring Pin accommodated the application's wide hole tolerance, provided superior retention, and eliminated the risk of pin walkout. The Heavy Duty Coiled Spring Pin also provided the strength required for the pivot function while retaining sufficient flexibility to absorb shock, impact, and repetitive loading without damaging the surrounding hole.

To address the manufacturing challenges associated with the manual assembly process, SPIROL also supplied a standard Model CR Semi-Automatic Pin Inserter. The operator simply loads the assembly into a fixture and touches the dual optical actuation switches. Once activated, the insertion head advances, installs the Coiled Pin, retracts, and automatically resets for the next cycle.

By combining an engineered fastening solution with automated installation equipment, SPIROL addressed both the product performance and manufacturing challenges. Assembly cycle time was reduced from 15 seconds to 5 seconds, scrap was significantly reduced, and the improved retention eliminated the risk of pin walkout and a critical warranty concern. The reduction in assembly time and scrap paid for the machine in just eight months.

Complimentary Applications Engineering Support: SPIROL Engineers will review your application needs and work with your design team to recommend the best solution.