Jeffrey F. Koehl
Press-fit pins are engineered fasteners held in an assembly by the frictional forces and interference fit between pin and part. They come in two general types: solid and spring pins.
Solid pins, such as ground dowels and grooved or knurled pins, typically deform the host material and require precise and straight holes to work properly — typically with total tolerances of 0.0002 to 0.0005 in. for ground dowels. Spring pins were developed
as a superior alternative to solid pins. Spring pins, as the name implies, compress to conform to a hole that is smaller than the pin body. They come in ISO and ASME slotted versions, as well as coiled spring pins.
Slotted pins are hollow tubes, chamfered on the ends, with a single slot along the length of the pin. The slot lets the pin compress while the spring material provides resistance that keeps the pin in the hole. The pin's spring characteristics permit greater hole tolerances than with solid pins, and holes need not be perfectly straight or round.
ISO slotted pins, the original design, were invented in Germany. They have an inherent problem in that the pins nest and interlock during shipment and, therefore, must be separated before use. Sometimes, the pins lock so tightly they cannot be easily separated and must be discarded. For the same reason, applying coatings using standard barrel-plating methods is nearly impossible with ISO slotted pins. In America, specifications were changed to prevent interlocking. The resulting ASME slotted pins are a significant improvement but did not change the pin's inherent design.
Unfortunately, all slotted pins share another attribute: uneven chamfers and nonsquare ends. Also, the relatively thick material — necessary to ensure pin strength — generates high radial forces. Combined, these characteristics make slotted pins difficult to use in automated assembly systems.
The objective with automated assembly is to dump pins into a hopper or bin and have the system continuously deliver them to the installation site, where they're consistently inserted into parts. Ideally, this should require little manual intervention. So jams in the feed system and feed head, inconsistent installation, and any other machine downtime must be avoided to minimize manufacturing costs.
The largest challenges with automated slotted-pin installation are the nesting during feed and the nonsquare ends. Automated systems typically jam when parts stick together. Even if they do not jam during feeding, when the pins enter the escapement in installation machines, slotted pins tend to catch on adjacent pins in the feed tube, which prevents advancement.
The pin manufacturing process — roll forming — also creates the possibility of bowed or banana-shaped parts. With their nonsymmetric design, pins tend to stretch at the slot and contract 180˚ from the slot. Stresses imparted during heat treating and quenching also tend to distort pins. Pins that are not straight will not pass through the discharge bushing in the feed bowl and, therefore, never make it into the feed tube. In addition, for maximum strength slotted pins must be oriented such that applied force passes directly through the gap. This can be tricky and expensive to automate.
Finally, once the pin is in the assembly, high radial forces can damage the hole. For example, if the assembly uses soft materials such as plastic, holes may enlarge over time and eventually the friction fit between the host part and pin deteriorates, which can cause the assembly to fail.
For all these reasons, coiled spring pins were invented. They overcome the disadvantages of solid pins and slotted versions, permitting high-efficiency automated production and better performance once installed.
Coiled spring pins are hollow tubular parts manufactured from steel strip that is rolled into a spiral spring of 2 1/4 coils. Coiled spring pins also compress when installed in a hole. But unlike slotted pins, the pin coils within itself under compression. Thus, the pin cannot "butt" when installed in an undersized hole, so it permits wider hole tolerances. They work well in common drilled holes — even if not perfectly straight or round — and are the bestsuited pins for cast, molded, and stamped holes. And being able to install these pins in minus-tolerance holes lets manufacturers use standard drills in materials that tend to cut undersized.
Coiled pins have several characteristics that reduce assembly costs. Unlike other spring pins that only bend opposite the slot during installation, the entire coiled pin compresses radially — thus providing the lowest installation force of all press-fit fasteners.
Coiled pins also generate nearly uniform contact stresses on the hole's inside surface, and the radial force is lower which reduces stresses on the host material. This is important in protecting the mating component, because poor insertion and high contact stresses can cause permanent damage. It is not unusual for poorly designed pin applications to skive either the base material or plating from the host part during installation. This leads to poor joint performance and possibly less corrosion protection.
Coiled pins cannot nest or interlock because there are no gaps. They have square, burr-free ends with concentric chamfers on both ends, and are manufactured to tight length tolerances. This eliminates sharp edges and angles that may "bite" into hole walls. The chamfer concentricity allows for easy engagement with the hole and problem-free bowl and tube feeding. These attributes can significantly reduce downtime during production, decrease equipment costs, and yield trouble-free assemblies.
In addition, the pins do not have to be oriented for strength, thereby permitting semi or fully automated assembly in lieu of labor-intensive pinning operations. Manufacturers can vary material thickness of coiled pins to provide the best combination of strength and flexibility. Lighterduty pins require less insertion force. This often permits smaller assembly machines, reducing the cost of automation equipment. In suitable applications, simplicity and ease of automation gives coiled pins the lowest installed cost of any fastener.
Edited by Kenneth Korane
Spirol International, spirol.com